home Understanding HF Skywave Propagation
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Tutorial, Applications & Tools for Radio Amateurs

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By Doron Tal, 4X4XMQRZ |
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Use CTRL-F5 to refresh. Updated on 2024-07-23 05:39z
This page offers radio amateurs (beginners and experienced) a useful overview and tutorials on skywave propagation. Find here basic and advanced explanations, practical methods, charts, maps, calculators, real-time reports 1, 2, 3, 4, 5, 6, 7, 8, models 1, 2, 3, table of contents, index of terms, and references.
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The entire website contains additional pages that each cover a sub-topic. See the sitemap. Perform search.

Understand and Predict Skywave Radio Propagation
Table of contents

Practical Approach

Real-time Band Activity
DXview screenshot
Real-time QSOs on 11 bands

Assessing band conditions
PSKR screenshot
by PSK-reports

Real-time HF propagation conditions at a glance

Recent observations

Tools & Applications
    analysis and forecast

Banners and widgets

Recent / Forecasts
Recent Solar Events
Recent Space Weather Events
Predict solar flux
Space Weather Forecast

Theory-based tutorial

1. HF Radio Propagation
2. Real-time Band Conditions
    DX Clusters, PSKR, WSPR, WebSDRs, Beacons

3. Forecasting Propagation Conditions
HF propagation basics
4. HF Propagation Modes
5. Impact of the Sun (preface)
6. The Ionosphere (preface)
Propagation Factors & Conditions
7. Ionosphere's Impact on HF
  7.1 Ionospheric Regions
  7.2 Skywave multi-refractions
  7.3 Long range skywave
  7.4 Critical frequencies
  7.5 NVIS Propagation
  7.6 Greyline propagation

8. Regional HF Conditions
  8.1 Ionosonde
  8.2 Ionogram
  8.3 Diurnal changes
  8.4 Seasonal phenomena
  8.5 Real-time propagation charts

9. Ionospheric Instability
  9.1 Ionospheric "Clouds"
  9.2 Ionospheric Disturbances

10. Total Electron Content (TEC)
11. Global HF Conditions
  11.1 Banners & Widgets
  11.2 Solar Indices: SSN, Solar Flux
  11.3 Geomagnetic Indices K, A, HPo

The Sun and Space Weather
12 Solar Phenomena
  12.1 Quiet sun
  12.2 Active Sun
  12.3 Sunspots
  12.4 Solar storms
  12.5 Solar Cycle
  12.6 Current solar events
  12.7 Predict solar flux

13. Space Weather Illustration
  13.1 Space Weather Explained
  13.2 Solar Wind
  13.3 The Magnetosphere
  13.4 Geomagnetic storms
  13.5 Space Weather Reports
  13.6 Predict Space Weather

14. Radio Blackouts
15. Summary
  * Epilog
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↑   Chapter 1. HF Radio Propagation - An introduction

What is a Radio?
Radio is a form of electromagnetic-radiation that propagates as a wave at the speed of light.

How are Radio waves classified?
Radio waves are classified based on their frequency and wavelength; higher frequencies have shorter wavelengths.

What is Radio Propagation?
Radio propagation is the process of transmitting radio waves from one location to another.

What are Propagation Conditions?
The term "propagation conditions" refers to the quality and dependability of radio waves transmitted between two points on Earth.

What is HF Radio?
HF is a high-frequency radio wave ranging from 3 MHz to 30 MHz (100 to 10 meters), sometimes known as "shortwaves".

The HF bands assigned to Radio Amateurs
Hover over a line to read about band's characteristics.
  1. 160 m (1.800-2.000 MHz) is officially part of the MF range but is also referred to as HF
  2.  80 m (3.5–3.8 MHz Region 1; 3.5–3.9 MHz) Region 3; up to 4.0 MHz, in the Americas, Region 2
  3.  60 m (5.3305-5.4069 MHz), five 2.8 KHz USB channels. In some countries unavailable or limited
  4.  40 m (7.0-7.2 MHz) Regions 1&3, up to 7.3 MHz in the Americas, Region 2
  5.  30 m (10.100-10.150 MHz) 1979 WARC; only CW and digital transmissions.
  6.  20 m (14.000-14.350 MHz) The popular band
  7.  17 m (18.068-18.168 MHz) 1979 WARC
  8.  15 m (21.000-21.450 MHz)
  9.  12 m (24.890-24.990 MHz) 1979 WARC
  10.  10 m (28.000-29.700 MHz), the widest HF ham band

This page focuses on skywaves.

The Rebirth of HF Radio - Decline and Comeback

HF communications via skywaves are making a comeback after 45 years of decline (1975–2020). Using satellites for global communications is too expensive, and the space systems are vulnerable. Technological advances like digital voice, ALE, and spread spectrum improve reliability at a lower cost. This is why there is a growing interest in skywaves.

The advantages of skywaves:
  1. Skywaves travel longer distances and can reach places not covered by satellites.
  2. No Infrastructure is required.
  3. Low-power wireless transmitters are sufficient over very long distances.

HF bands play a vital role in various fields, including aviation, emergency services, maritime communication, and military operations. These frequencies enable communication over vast distances, often surpassing thousands of kilometers, making them indispensable in situations where traditional communication methods are unavailable or unreliable.

During emergencies, such as natural disasters or large-scale accidents, traditional communication networks may be severely damaged or overloaded. In these situations, the HF band becomes crucial for emergency services to establish communication links and coordinate rescue efforts. HF radios can be quickly deployed to affected areas, allowing first responders to communicate and provide assistance to those in need, regardless of the distance or terrain.

How does HF radio wave propagate?
HF radio can propagate in variety of modes: Line of sight, Ground wave and Skywave.

What are HF band conditions?

The term "HF band conditions" refers to the quality and dependability of HF radio signals transmitted between two points on Earth by skywaves and how the ionosphere's dynamic affect them.

You may find here answers to questions, such as:

  1. What characterizes each of the HF amateur bands?
  2. How are propagation conditions affected by time of day, season, and ionospheric state?
  3. How and why are skywaves used for long-range communication?
  4. What differs ionospheric regions, and how are HF radio waves bent from the sky back to Earth?
  5. There is a separate page titled HF Propagation Overview

↑   Chapter 2. Real-time Band Conditions
This chapter provides an overview of various methods and tools for obtaining real-time band conditions.

One of the following methods may provide real-time propagation conditions:

2.1 Listen on the bands using your own receiver;
2.2 Listen on the bands using WebSDR / KiwiSDR accesible via internet;
2.3 Monitor worldwide Beacons;
2.4 Watch internet services (real-time maps and DX clusters);
2.5 Watch real-time MUF maps accesible via internet;
2.6 Simulate the current ionospheric state, using offline and online applications and tools;
      learn its effect on HF radio waves using applications based on models
      and data collected from recent Solar Activity, Space Weather reports, and remote-sensing of the ionosphere and its regions.

Utilize more than one option to gain a comprehensive understanding of radio communication conditions, as no single method can provide all the necessary information.

↑   2.1 Assessing Band Conditions by using your own receiver

Years ago, we manually scanned HF bands using analog receivers. It took us a long time to determine the communication conditions in each band.

Nowadays, FT8 allows real-time data viewing on monitors on active stations within a specific band.
Platforms like PSKReporter provide insights into band openings.

For example, see below an online report-map, based on signals received by:

Malachite DSP v3 RX conected to K-180WLA 70cm diameter Magnetic Loop antenna (MLA)

Signals were analyzed by WSJT-X v2.6.1 software (running on a PC) and reported online to PSKReporter that generated the map shown below:

PSKReporter map generated on April 2, 2023
PSKReporter demo
A screenshot of PSK-Reporter with manualy added notes.
Receiver: Malachite DSP v3, Antenna: Mag Loop, Sofware: WSTJ v 2.6.1

↑   2.2 Real-time HF Bands Monitoring using Receivers accessed via internet: WebSDR / KiwiSDR

The majority of these stations offer waterfall and spectrum displays, as well as the option to record audio.

For example watch the entire LF-MF-HF spectrum at a glance using a  Wideband WebSDR at the University of Twente, Enschede, NL

entire shortwave spectrum

Alternatively choose a remoted receiver from WebSDR / KiwiSDR lists or the KiwiSDR Map (below)


↑   2.3 Real-time HF Band Monitoring using Beacons

It's a good idea to listen to NCDXF Beacon Network whenever you plan to hunt DX stations.

Eighteen worldwide beacons shown on the map below share five bands 20, 17, 15, 12 and 10 m and take it in terms.
All these stations use a standardized antenna and output power levels.

Eighteen beacons map

Click to see which beacon is now transmitting on what frequency?

All 18 beacons use the same five frequencies: 14.100, 18.110, 21.150, 24.930, and 28.200 MHz. It is recommended that you spend some time on one of these frequencies, listening to transmissions from all around the world. This option may indicate where the bands are now open.

The IDs of all the 18 stations are callsigns in CW and then a short carrier decreasing in four power levels: 100, 10, 1, and 0.1 Watts. If you can hear the 0.1W that means that propagation is really good or you're in a really low noise location. Put these frequencies in your receiver's memory channel and you'll be able to flick quickly between them.

While you're on 28 MHz tune up between 28.2 and 28.3. There's a lot of additional beacons all on their own frequencies operating full time.

↑   2.4 Real-time Band Conditions - Using online / internet services

The following real-time maps and DX clusters can help you find which bands are open now:
  1. Real-time ham activity map Jon Harder, NG0E
    Click on the map above to see the global radio ham activity for the previous 15 minutes on 11 ham bands (1.8–54 MHz).
    Data is gathered from online sources: WSPRnet, Reverse Beacon Network (CW, FT4, FT8), and DX Cluster.
    JavaScript is required to view the graphics.
  3. DXMAPS Shows real-time QSOs on specified bands.
    DX Clusters are nodes that gather information from radio amateurs about DX stations and/or real-time propagation conditions.
  5. DXWatch alerts (using a filter) when a specific DX station is on the air.
  6. DXZone is an amateur radio internet guide.
Reporters of digital modes:
  1. PSK Reporter is an amateur radio signal reporting and spotting network
  2. WSPR is a weak-signal radio communication protocol used for sending and receiving low-power transmissions to test propagation paths on the MF and HF bands. Links: WSPRnetmap, WSPR Rocks, WSPR Live, etc.

↑   2.5 Real-time Band Conditions using online propagation maps

Real-time forecasting of HF band conditions using maps on Maximum Usable Frequencies (MUF) optimizes long-distance communication, minimizes interference, and ensures reliable and efficient utilization of the HF spectrum.

↑   2.6 Simulate Band Conditions using offline and online applications and tools

Such apps and tools can simulate the current ionospheric condition and its effect on HF radio waves using mathematical models, data collected from recent Solar Activity, Space Weather reports, and real-time sensing of the ionosphere. All these topics are covered below.

Multiple methods and tools should be used and their outputs compared to gain a more complete understanding of the propagation conditions, because no single method or tool can provide all of the information needed to make an accurate assessment.


↑   Chapter 3. Forecasting HF Propagation Conditions

How are propagation conditions estimated?
HF propagation conditions are evaluated using MUF determined from Ionosondes measurements.

How can we predict propagation conditions?
Predicting propagation conditions for HF radio involves collecting physical parameters and using mathematical models, considering the time of day, season, Sunspots, and ionospheric conditions.

HF propagation prediction is a technique used to assess the quality of radio transmissions between Earth-specific locations via the ionosphere.

There are some general rules for predicting at what time you might find band openings and in what direction:

  • Pre-dawn: 40 and 80 meters (for ambitious hams)
  • Sunrise: 20 meters usually opens to ranges of 3,000-6000 km
  • Late Morning: 10 and 15 meters may open to ranges over 10,000 km
  • Afternoon: 20 meters may open to trans-equatorial destinations
  • Evenings: 40 meters (this can be a real opportunity for DX)
  • Late Evenings: 80 meters (openings can last awhile allowing for rag chewing)
  • 160 meters opens even later for those with large full-size antennas
Alternatively, you can watch real-time world-wide ham activity.

Forecast vs Prediction of HF Band Conditions

The difference between forecasting and prediction is that forecasting explicitly adds a temporal dimension.
Forecast is a time-based prediction, so it is better suited to dealing with time series data.

Why do we need propagation forecasts?

We need forecasts of HF radio propagation because the state of the ionosphere changes constantly and affects the performance of HF radio communication. By predicting the Maximum Usable Frequency (MUF) and other factors, we can choose the best frequency bands and times to communicate with a desired location. Forecasts may help prepare for situations where HF radio signals are blocked or distorted by space weather events associated with solar flares or geomagnetic storms.

The forecasts are used to select the best radio communication frequencies and times, plan antenna systems, and optimize communication link coverage.

How has the process of predicting HF propagation conditions changed over the past thirty years?
Incredible advances in space technology, SDR (Software-Designed Radio), and the internet have enabled us to study radio wave propagation in ways we never thought possible. Prior to the 1990s, propagation reports and charts were published in amateur radio magazines, but computer programs and online internet tools can now display propagation using real-time solar indices.

HF propagation basics

↑   Chapter 4. HF Propagation Modes
This chapter reviews the primary modes of high frequency (HF) radio propagation.

There are three main modes of HF Radio Propagation: LOS, Ground wave and Skywave

Propagation Modes
Illustration of HF Propagation Modes

1. LOS - Line of Sight propagation.

  • Line of Sight occurs when two stations are directly visible to each other.
  • Non-LOS-propagation could be complex reflections by conductive surfaces for example: ElectronicsNotes, Wiki.
2. Ground wave or surface wave propagation
  • During the day, AM radio broadcast stations use ground wave propagation.
  • A vertically polarized surface wave travels parallel to and adjacent to the Earth's surface and can cross the horizon.
  • Geologic discontinuities like mountains, rivers, and deep gorges, as well as RF absorption by the earth, attenuates ground wave transmission.
  • Ground waves are most effective at frequencies below 1 MHz above salty seawater or conductive ground, but are practically useless over 2 MHz.
3. Skywave (or skip) radio waves propagate around the globe due to "reflections" (a result of multi-refractions) from the ionosphere.

  1. Skywaves are effective at frequencies 3-30 MHz, and are used for long-range communication.
  2. There are several ionospheric regions E, F1, and F2 that commonly refract HF radio waves.
  3. During the day, the D-region absorbs frequencies below 10 MHz, allowing higher frequencies to reach the upper regions.
  4. The ionospheric regions may vary in thickness and altitude and may form in clouds of charged particles.
  5. The typical ionization charge density may fluctuate in a gradient, with the highest density in the middle.
  6. Ducting effects can occur at times.
  7. "Skip distance" is a zone of silence (dead zone, see the illustration below) between the ground wave and sky wave where there is no reception.

    Where h is the height, fMUF is maximum usable frequency and fc denotes the critical frequency.
  8. NVIS is a mode of skywave with effective frequencies ranging from 2 to 8 MHz. NVIS can address the issue of a "dead zone" by using very low horizontal antennas.

    This page does not cover the following propagation modes:
  1. Aurora and/or Meteor Scatter.
  2. Backscatter
  3. Low VHF 30-150 MHz - In late spring or early fall radio waves can be unpredictably "reflected" back to Earth via Sporadic E(Es).
  4. VHF-UHF-SHF "Scatter Propagation" due to Troposphere irregularities, offering communication between 160 km to 1600 km.

↑   Chapter 5. How does the sun affect radio communications?

Solar activity drives HF propagation by the ionosphere that bounces radio waves thus enabling propagation beyond the horizon.

  1. All forms of Solar Emissions affect "HF Propagation Conditions".
  2. The communication conditions depend on the sun's position and orientation, i.e. time of day, season, and geographic location.
  3. When solar activity is high, the ionosphere becomes more ionized, leading to improved propagation conditions, particularly in the higher HF bands.
  4. Sunspot number and solar flux serve as indicators for assessing global propagation conditions.
  5. Solar storms, including flares and coronal mass ejections (CMEs), can cause rapid and significant changes in the ionosphere. Such disruptions can severely impact global communications.
Find below the full chapter on solar phenomena and their impact on radio communications.


↑ Chapter 6. The ionosphere (preface)

This chapter serves as an introduction, laying the basis for a deeper study of the ionosphere's role in HF radio communication.

Earth's weather and space weather both affect the ionosphere, a spectacle of charged particles. "Ionospheric clouds" move at heights between 150 km and above 400 km at different speeds and directions, with irregularities in conductivity. See below a beautiful clip made by NASA.

Welcome to the Ionosphere courtesy NASA Goddard

The ionosphere is always changing.

The clip above illustrates the dynamic atmosphere—the dance of radio waves within a vibrant airglow.
Solar storms intensify the ionosphere's beauty, while Earth's weather below adds to the unique destination.

The term "ionosphere" refers to the region of the atmosphere (60-700 km above the Earth's surface) where Solar radiation ionizes gases.

Ionosphere (Thermosphere) is part Earth's Atmosphere

The Thermosphere is characterized by very high temperatures ranging
from 550 to over 1100 degrees Kelvin, due to the ultraviolet solar radiation.
The correlation with SF - Solar Radio Flux F10.7 (SF=50 yields 550 K, SF=200 yields 1150 K)

How does the ionosphere affect HF Radio Propagation?

The Solar radiation generates free electrons in the ionosphere. The process is named "Ionization".

Generation of free electrons

HF radio waves from Earth to the ionosphere cause free electrons to oscillate and re-radiate, resulting in wave refraction with a refractive index similar to geometric optics with a refractive index.

The refraction of radio waves in the ionosphere is characterized by the critical frequency. This is the highest frequency at which radio waves reflect back to Earth. Higher frequencies escape into space.

Refraction of radio waves in the ionosphere
The left two paths of signals demonstrate higher-frequency radio waves lost in space.
Read explanations in the next chapter.

Propagation Factors and Conditions

↑ Chapter 7 - ionosphere's Impact on HF

The ionosphere refracts HF skywaves, enabling long-distance communication by bouncing signals off different ionospheric regions.

  7.1 Regions (layers)   7.2 Multi-refractions   7.3 Long range skywave   7.4 Critical frequencies   7.5 NVIS   7.6 Greyline Propagation

↑   7.1 - Ionospheric Regions

The D, E, and F "layers" are commonly used to depict the structure of the ionosphere, but this is not entirely accurate. Ionization exists throughout the ionosphere, with its level varying with altitude. There is also a C region below them which has low ionization levels that don't affect HF radio propagation. The term "region" is more appropriate than "layer".

It’s common to present the order of ionosphere regions affecting HF skywaves from the highest region downwards.
D-E-F regions Day/Night
The ionospheric regions
  • The F-region (180-600 km) is responsible for long-distance HF propagation.
  • This region consists H+, He+ (ionized Hydrogen and Helium ) has the highest free-electron density up to 1012 electrons per m3 excited by solar radiation 10-100 nano-meter EUV. It splits during the day into sub-regions F1 and F2. It slowly dissipates after sunset.
  • The E-region (90-150 km) dissipates a couple of hours after sunset.
  • This region consists O2+ (ionized Oxygen) up to ~1011 electrons per m3 excited by solar radiation 1-10 nano-meter EUV.
    During intense Sporadic E(Es) events (particularly near the equator) it reflects frequencies above 50 MHz.
  • The D-region is the lowest at 50-90 km above ground. It is effective only during daytime, blocking radio frequencies below the LUF from reaching the higher E and F regions. It dissipates at sunset.
  • Absorption
    The D-region is ionized by solar radiation (at 121.6 nanometers UVC of the Hydrogen spectral line), which ionizes Nitric Oxide, NO+ up to approximately 1010 electrons per m3. Bursts of solar flares (0.1–1 nm X-ray) may cause blackout events that can last minutes to hours.
These ionospheric regions differ in terms of gas compositions and free-electron densities:

An overview of the ionospheric regions

When Present Typical
by UV Solar
F  180-600 km HF Super Reflector Splits at daytime
into F1 and F2
15 - 30 1011/m3 1012/m3 10-100 nm H+ He+
E*   90-150 km Medium-Frequency and
Sporadic* VHF Reflector
Negligible at night 7 - 10 109/m3 1011/m3 1-10 nm O2+
D    48- 90 km Daytime Attenuation Daytime only 2 - 6 108/m3 109/m3 121.6 nm NO+
N2+ O2+
* See notes about Sproadic E-region.

The typical distribution of free electrons in the ionosphere

Plasma Density and height
The above graph is based on a review from U.C.Berkeley Bob Brown, NM7M (SK), Ph.D.

Free-electron densities change throughout the day and night, the seasons*, and are influenced by a number of factors such as Sunspots solar cycle, geomagnetic storms, and lightning storms, all of which may affect radio propagation conditions.

Why does the density of free electrons increase sharply with height?

The density of free electrons is affected by the balance of two opposing processes, ionization and recombination (the capture of a free electron by a positive ion). The F-region gets most of the UV radiation compared to the lower E and D regions, while the rate of electron-ion recombination is much faster at the lowest D region (due to the higher gas density). As a result, the free-electron density of the highset F-region (at noon) is significanly higher than that of the E and D regions.

At most, only one thousandth (1/1000) of the neutral atmosphere is ionized.

↑   7.2 - Multi-refractions

Skywave can be bounced in a variety of modes by the ionosphere

The free electrons in the ionosphere refract radio waves as they move through the ionospheric regions, where the free-electron density gradually varies; numerous refractions are what create the frequency-dependent reflections of ionosphere skywaves.

See below an illustration of complex propagation modes such as F Skip / 1F1E, E-F Ducted, F Chordal, E-F ocasional and sporadic E.
Complex HF Propagation Modes
An illustration of Complex Propagation Modes D-region ignored
Provided by the Australian Space Weather Services

Sporadic E (Es) is an unusual form of radio waves' propagation using the E-region of the Earth's ionosphere, which normally does not refract radio waves. It reflects radio waves off relatively small "plasma clouds" in the lower E region, located at altitudes of about 95~150 km.

Equatorial sporadic E (within ±10° of the geomagnetic equator) is a regular midday local time. At polar latitudes, however, sporadic E can accompany auroras and associated disturbed magnetic conditions and is called auroral E. At mid-latitudes, the Es propagation often supports occasional long-distance communication during the approximately 6 weeks centered on the summer solstice at VHF bands, which under normal conditions can only propagate by line-of-sight.

↑   7.3 - Long range skywave

The figure below illustrates skywave reflections from ionospheric regions at various angles

Ionosphere Reflection vs Angles

Long-range skywave propagation typically uses a low transmission angle, which corresponds to a high incident angle
Transmission angle incident angle

The highest MUF occurs at the lowest transmission angle. This results in the longest range, meaning the transmitted ray is nearly horizontal.

As a rule of thumb, the MUF is approximately 3 times the foF2, as explained above: MUF factor = 1/cosθ.

See real-time worldwide 3000km MUF map

↑   7.4 - Critical frequencies relevant to skywave-propagation: foF2, MUF, OWF and LUF

7.4.1   The critical frequency (foF2) is the highest frequency below which a radio wave is reflected by the F2-region at vertical incidence.

vertical incidence
Vertical reflection from F2 region (layer)

The critical frequency is depndent on the collision frequency of the free-electrons and their density:

Where fc is the critical frequency and Nmax is the electron density.
If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal.

Statistically, between 2005 and 2007, the critical frequency varied from 1.8 MHz to 11 MHz, with an average of 7.5 MHz.
The daily critical frequencies range from 6.8 MHz to 12 MHz.

7.4.2   The  Maximum Usable Frequency (MUF) is the highest radio frequency reflected by the ionosphere at a given incident angle.

MUF illustration
MUF illustration
  • The MUF sets the conditions of propagation between certain places.
  • The MUF is a function of foF2: MUF = foF2 / cosθ, where θ is the incident angle.
  • As a rule of thumb, the MUF is approximately 3-4 times the critical frequency .
  • Ionosondes determine the MUF.

See near real-time maps of MUF and foF2.

7.4.3   The  Optimum Working Frequency  (OWF) is usually 85% of the MUF.

7.4.4    Lowest Usable Frequency (LUF) is the frequency where signal strength rapidly decreases due to D-region absorption.

The LUF is a soft frequency limit, unlike the ionospheric skip MUF.

* Typical diurnal changes of MUF, OWF and LUF.

↑   7.5 - Understanding NVIS Propagation

NVIS - Near Vertical Incidence Skywave is a unique communication mode using skywaves directed almost vertically.

NVIS provides the solution for the dead zone (between groundwave and skip). It is the only solution for communication coverage in hilly and/or jungle areas over short distances of a few hundred kilometers.

An illustration: How NVIS provides communications within a hilly area.
  • Typical operating frequencies are 2-4 MHz at night and 4-8 MHz during day.
  • NVIS requires suitable antennas (like a low dipole at hight of 0.1-0.25 wavelengths) to improve vertical radiation and reduce lower-angle radiation, contrary to what is customary for long-range communication.
  • NVIS offers enhanced resistance to fading (constant signal level), and minimal attenuation, making it suitable for low transmit power levels and omnidirectional coverage, allowing flexibility in setup and placement.

The NVIS map shows the recent global distribution of critical frequency (foF2).

↑   7.6 - Greyline Propagation?

The greyline is a narrow band around the Earth that separates day and night.
Improved long-distance communications are possible on the lower HF bands for a brief period at dawn and dusk.

Why is radio propagation better along the greyline?

The absorbing D region is gone, but the reflecting F-region remains. This is because the F-region receives sunlight while the D-region does not.
Note: Because of the denser air at lower altitudes, the D-region dissolves before sunset, and ion recombination is faster.

Greyline illustration
The height of the F and D regions
is exaggerated in comparison to Earth dimensions.
  Greyline map
An example of a grey-line map

Some radio operators use specialized greyline map to predict when the grey line will pass over their location, as well as the best frequencies and modes of propagation to apply at that time. Overall, greyline propagation is a fascinating and useful phenomenon that has the potential to open up exciting opportunities for long-distance radio communication.


↑   Chapter 8. Regional HF Propagation Conditions

Regional HF Propagation Conditions provide a detailed picture of the conditions that individual operators are likely to experience.
The regional conditions are based on the LUF, MUF and OTF between two locations.

Sub-topics:   8.1 Ionosondes   8.2 Ionograms   8.3 Diurnal changes   8.4 Seasonal phenomena   8.5 Current maps of MUF and foF2

↑   8.1 Ionosonde

The ionosonde (invented in 1925) is an "HF radar" that emits short pulses of radio radiation into the ionosphere in order to determine optimal frequencies for HF communication.

How does it work?

The highest frequency that the ionosphere can reflect vertically is known as critical frequency (foF2).

Vertical Ionosonde
Ionosonde probes the F2-region

Typical ionodonde modes are vertical and oblique:
Typical ionosonde
The ionosonde, also known as a chirpsounder, sweeps the HF range from 2 to 30 MHz. The transmitted (Tx) frequency increases at a rate of about 100 kHz per second and is digitally modulated in 25 kHz increments. Matching receivers (Rx) detect and analyze echo signals.

The ionosonde measures the time it takes for reflected pulses to arrive after being transmitted. The height (calculated from the time delay) is plotted versus frequencies. This recording, known as an ionogram, shows the plasma density distribution in ionospheric regions at various altitudes (48–600 km).

Every 15 minutes, real-time data from ionosonde stations throughout the world is reported via the internet.

Map of GIRO ionosonde stations

Some stations aren't always active, and significant regions of the globe are uncovered yet with ionosonde stations, as shown on the above map.

Readings of foF2 from several sites can be combined to build a propagation map for foF2.

↑   8.2 Ionogram

An ionogram is a visual representation of the height of the ionospheric reflection of a specific HF radio frequency.
Typical ionogram
A typical ionogram
E, F1 and F2 indicate ionospheric regions

Ionograms usually contain a dual representation:

  1. A series of (more or less) horizontal lines indicating the virtual height,
    at which the (amplitude modulated) pulse will be echoed as a function of the operating frequency;
  3. A curve in vertical direction representing the density of free-electrons per cubic centimeter,
    as a function of height.

↑   8.3 Day/Night Cycle - Diurnal changes

The MUF/LUF changes during time of day, the seasons, and the sunspot number VS solar cycle.
Typical diurnal changes in frequencies relevant to skywave
MUF and Time
MUF - maximum usable frequency
OWF - optimum working frequency
EMUF - E-region maximum usable frequency
LUF - lowest usable frequency
A simulation of parameters for a 5KW transmitter link
between San Francisco, CA and Honolulu, HI (Oct 2002),
a presentation of Naval Postgraduate School.

  • MUF, OWF, and LUF explained
  • Real-time MUF conditions

↑   8.4 Seasonal and diurnal phenomena

Free-electron densities are greater and higher in summers compared to winters and near the equator vs. the poles, due to increased solar EUV radiation. As a result, HF propagation conditions on the bands above 10 MHz are better in the summer and closer to the equator, whereas propagation conditions on the bands below 10 MHz are better in the winter and at high latitudes.

The dynamics of ionospheric regions near the equator and mid-latitudes

↑   8.5 Near real-time maps of MUF and foF2

    See below six online maps of regional propagation conditions, all based on recent Ionosonde measurements:
  1. Greyline map with a few regional MUF & solar indices updated every 3 hours; Provided by N0NBH
  2. MUF 3000 Km map - information about HF propagation conditions at a glance
    provided by KC2G updated every 15 minutes | There is also an animated version.

  3. NVIS Map shows wolrdwide distribution of foF2 provided by KC2G updated every 15 minutes
  4. The next 3 NVIS maps are provided by the Australian Space Waether Service (AGSWS) updated every 15 minutes
  5. Map of NVIS (foF2) AGSWS
  6. Map of T index foF2 AGSWS
  7. Map of foF2 Anomaly compared to the monthly median AGSWS

  8. Charts of Current foF2 Plots as measured at various locations Australian Space Weather Forecasting Centre

↑     Greyline / Grayline map showing a few regional MUF values and global indices is updated every 3 hours (by Paul L Herrman, N0NBH ).
Solar indices and Regional MUF
It shows day/night, 13 local MUF reports, and the Global Indices: SFI, SN, A&K indices, 30.4 nm, Geomag, Sig Noise.

↑    Real-time MUF 3000 Km Propagation Map updated every 15 minutes

The map below was designed by Andrew, KC2G for amateur radio operators, and is updated every 15 minutes.
A radio path of 3,000 Km is being considered for unification. This map was developed between 2018 and 2021 to assist radio amateurs in finding the best times and frequencies for contacts by displaying HF Propagation conditions at a glance. However, this tool is limited; see notes below

MUF3000 map *** KC2G server does not respond ***
MUF 3000 Km Propagation Map

↑ How to use this map?

The colored regions of this map, which are rebounded by Iso-Frequency contours, illustrate the Maximum Usable Frequency that is expected to bounce off of the ionosphere on a 3000 Km path. The grey line position is also included.

The ham bands are designated by iso-frequency contours: 5.3, 7, 10.1, 14, 18, 21, 24.8, and 28 Mhz.
For example, if a given area on the map is greenish and lies between the contours labeled "10" and "14," the MUF in that location is around 12 MHz.

The raw data is MUF calculated from data collected by ionosondes, which are represented by numbered colored discs that show their location.
A number inside a disc indicates the calculated 3000km MUF from the critical ionospheric frequency, foF2. The information from the stations is compiled by Mirrion 2 and GIRO, and processed by the International Reference Ionosphere (IRI) model (produced by a joint task group of COSPAR and URSI.

The MUF along a path between any two locations shows the possibility of long-hop DX between those points on a given band.
For example, if the MUF is 12MHz, then 30 meters band and longer will work, but 20 meters band and shorter won't.
For long multi-hop paths, the worst MUF anywhere on the path is what matters. For single-hop paths shorter than 3000 Km, the usable frequency will be less than the indicated MUF. As one gets closer to vertical, i.e. NVIS, the usable frequency drops to the Critical ionospheric frequency, foF2 (as shown in the next map).

Additional Notes:

  1. The MUF(3000km) map shows the estimated MUF, calculated from ionograms. Inaccuracy can result from the limited coverage of innosonde stations. Therefore the map is based on data interpolation. The algorithm aims to find the MUF (or FoF2) at scattered points globally, but achieving accurate extrapolation from few data points is challenging. The guessing process is better in areas close to measurement stations, but uncertainty increases in distant areas.
  3. The accuracy of commercial radio services is insufficient, because:
    1. Stations may provide inaccurate or disagreeing data, leading to peculiar results in an attempt to align measurements. Stations may go off-line or re-appear, causing unexpected changes in the model's global picture due to the limited number of data points initially used.
    3. Real-time ionosonde data sharing has been discontinued by countries like Russia, China, Japan, the US Space Force, and NOAA in recent years. Some ionosondes are only available through NOAA, and if GIRO outages occur, maps on this site may stop updating.
    5. Additionally there is uncertainty associated with predicting the ionosphere's state using vertical sounding data.
    7. The effects of geomagnetic storms and Blackouts due to Elevated X-Ray flares and/or Proton Events are implicitly included in the results of ionograms. But it is impossible to predict band conditions for the next few hours. Geospace dynamic models are still being developed.
  4. The "MUF(3000km)" project is the result of research and development by Andrew D Rodland - KC2G, which is based on an earlier work by Matt Smith - AF7TI. WWROF financing and data from ionosonde operators all over the world, provided by GIRO and NOAA made it feasible. See also Acknowledgments.
  6. Read more about this open source project.
  8. Read more about the open source software and models.
  10. Roland Gafner, HB9VQQ extended the static presentation with an excellent Animated Map showing the last 24 hours, in 15 minutes steps. ↑

↑     NVIS real-time worldwide map (critical frequency: foF2) provided by Andrew D Rodland, KC2G updated every 15 minutes

foF2 map - if not displayed KC2G does not respond ***

The colored regions of this map, which are rebounded by Iso-Frequency contours, illustrate the critical frequency that is expected to bounce off of the ionosphere at near vertical angle. The ham bands (160, 80, 60, 40, 30 ,20m) are designated by iso-frequency contours: 1.8, 3.5, 5.3, 7, 10.1, and 14 Mhz.

foF2, as measured by ionosondes, is the raw data that powers the site.
Colored discs indicate the location of stations. A number inside each disc represents the critical frequency, foF2.

↑  Another NVIS map provided by the Australian Government Space Weather Services is updated every 15 minutes. It displays contours of the critical ionospheric frequency - foF2. There are a few differences between this map and the KC2G map (above), mainly due to the choice of frequencies for the contours. The KC2G map highlights ham bands. The following map, however, is designed for commercial use.
foF2 WW Map

Click on the map to view the source page. There is further information.

↑   T Index Map - foF2 is provided by Australian Government Space Wheather Services

The T index is intended to correct inconsistencies between sunspot number and solar flux, as well as to account for geomagnetic storms that may influence the foF2.

This index can be any value, however it is usually limited to a range of -50 to 200. Low numbers suggest the usage of lower HF frequencies and vice versa.

↑  Current Anomaly Map of critical frequency, foF2, compared to the monthly median

foF2 WW Anomaly Map

The plot above shows a near real-time foF2 anomaly map. The anomalies are calculated by subtracting the median foF2 for the last days from the currently observed foF2. The current foF2 and dataset are used to calculate the median foF2 the identical time of day and geographical attributes. The anomaly differences are in units of MHz. The regions in red indicate significantly lower frequencies compared to the last 30-day medians.
One may choose an animated display that shows changes in anomalies for the last 1-7 days.

Recent foF2 measurements at various locations of Australia, New Zealand and East Antarctica
Current foF2 Plots
foF2 Plots courtesy Australian Space Weather Forecasting Centre

↑ Chapter 9. Ionospheric Dynamics - instability, disturbances and storms

This chapter is under preparation. At this point, two topics are briefly covered.

Sub-topics:   9.1 Ionospheric "Clouds"   9.2 Ionospheric Disturbances

↑   9.1 Ionospheric "Clouds"

The ionospheric regions are not uniform. They consist of "plasma clouds", as illustrated below.

Ionospheric clouds

The moving plasma clouds are travelling disturbances of electron density

How do "ionospheric clouds" affect HF propagation?

The dynamic ionosphere causes signal fading over time. Small-scale irregularities in the ionosphere are observed at all levels, with periodic motions attributed to neutral atmospheric waves interacting with ionized components in the upper atmosphere. While understanding is limited, the research promises short-term changes.

Additionally the ionosphereic regions are disrupted by (1) The chaotic solar activity and (2) The tropospheric weather from far below.

What effect does tropospheric weather have on the ionosphere?

Storms, hurricanes, and strong wind patterns can all temporarily alter the TEC caused by EUV solar radiation.

In other words, the ionosphere and troposphere are coupled by a variety of mechanisms.

For instance, a lightning storm can cause electrodynamic interaction, as shown in the following figure.

Electrodynamical Coupling of the Troposphere with the Ionosphere
Ionospheric Clouds due to Troposphere-Ionosphere coupling


Sprites - Transient Luminous Events (TLEs)
The different forms of Transient Luminous Events. Credit: NOAA

There are other complex mechanisms that couple the troposphere to the ionosphere. We won't go into detail at this point.

In conclusion, "Ionospheric clouds" that develop as a result of the coupling between the troposphere and ionosphere may affect skywave HF propagation.

How are ionospheric "clouds" detected?

The Digisonde Directogram was developed to identify ionospheric plasma "clouds".

Digisonde Directogram

It consists of multi-beam ionosondes, which measure echoes coming from various locations.
Seven ionosonde beams (one vertically and six diagonally) are used to generate the ionograms.
The end result is an extended ionogram of "plasma clouds" as they drift over a Digisonde station.

Sample directogram for Cachimbo station from 12 UT Oct 10 to 12 UT Oct 11, 2002.
Blue color means ionospheric motion from west to east.

Probing traveling F region ionospheric disturbances

The Super Dual Auroral Radar Network is an international network of 35 HF radars (8 MHz–22 MHz) located in the northern and southern hemispheres.

SuperDARN site in Holmwood SDA, Saskatoon

The radars are designed to study F region Ionospheric dynamics, instability, disturbances and storms. The research covers geospace phenomena, including field-aligned currents, magnetic reconnection, and mesospheric winds. It tests theories of polar cap expansion and contraction under changing IMF conditions, observing large-scale responses to substorms. The collaboration includes various institutions.

↑   9.2 - Ionospheric Disturbances

The free-electron density of the ionosphere is always changing. It may be local "ionospheric clouds", or even global at times.

Other ionospheric disturbances include "Sudden Ionospheric Disturbances" (SID) and "Traveling Ionospheric Disturbances" (TID):

9.2.1 SID (Sudden Ionospheric Disturbances) is any one of several ionospheric perturbations, resulting from abnormally high ionization/plasma density in the D region of the ionosphere and caused by x-ray solar flares and/or solar particle events. SIDs result in a sudden increase in radio-wave absorption that is most severe in the range of 1.8–10 MHz, and as a result, they often interrupt or interfere with telecommunications systems. These events are more likely to occur around solar maximum.

9.2.2 TID is (Traveling Ionospheric Disturbances) a wave-like structure passing through the ionosphere, altering the altitude and angle of refraction of sky waves. TIDs travel horizontally at 5–10 km/minute, with varying phases, amplitudes, and angles of arrival. Some originate in auroral zones or weather disturbances. Read also about probing TIDs.


↑   Chapter 10. Total Electron Content (TEC)

TEC is an important descriptive quantity - number of free-electrons integrated between two points along a one-meter-squared-cross-section tube. It is calculated from real-time foF2 data measured by ionosondes. It is a reliable indicator of how the ionization level of the ionosphere influences the propagation conditions of radio wave transmissions. TEC is strongly affected by solar activity.

Tropospheric weather may affect TEC:
The troposphere and ionosphere are separate atmospheric regions with distinct functions. However they do interact through various processes research model.
Tropospheric lightning may induced changes in total electron content, and consequently affect propagation conditions. Thunderstorms can also worsen the signal-to-noise ratio, in particular the lower HF bands i.e., tropospheric weather may affect propagation conditions of the HF bands, especially in the tropical regions.

Thus, monitoring and modeling TEC patterns and variations allows us to better understand and prepare for the constantly changing band condtions.

TEC data is gathered from thousands of ground-based GPS receivers around the globe. It is characterized by observing carrier phase delays of received radio signals transmitted from satellites located above the ionosphere, often using GPS satellites.

TEC distributions between seasons can be compared to each other. Data analysis showed qualitative trends relating the spring and fall equinoxes as well as the summer and winter solstices.

*** If you don't see TEC Map it may be due to outage of the DLR website ***
TEC/TECU provides the number of free-electrons per square meter (x1016) for a shell height of 400 km. This map is based on measurements collected from ionosonde stations around the world, it provides near real-time information and data service for the current state of the ionosphere, related forecasts, and warnings.
Credit: Ionosphere Monitoring and Prediction Center (IMPC) of the European Space Agency`s network of space weather services.

See additional references on this topic.


↑ Chapter 11. Global Propagation Conditions

Sub-topics:   11.1 Banners & Widgets   11.2 Solar Indices SN, SF   11.3 Geomagnetic Indices K, A and HPo

Global HF propagation conditions are the overall factors, such as Solar Activity, the ionosphere's current condition, and in particular the average global Ionization level of the F2-region that influence HF radio waves on a global scale.

Please keep in mind that regional conditions could vary significantly from the global conditions.

↑   11.1 Global conditions » Banners and Widgets

The following Banners / Widgets were created by Paul L Herrman (N0NBH).
They were designed to help in monitoring global variations in HF peopagation.

Ham bands global conditions

The Basic Solar indices
SFI, SN, A and K

SFI correlates with F2-region ionization (see below).
A and K indices indicate geomagnetic instability.

Solar-Terretrial Data, N0NBH
  Glossary of the Terms provided by N0NBH, Paul L Herrman
SFI - Solar Flux, 2800MHz (10.7cm) (SFU units) correlates with F2-region ionization; higher value better HF conditions.
SN - Daily Sunspot Number is a measure of the solar activity; correlates with better HF conditions
Instability of Earth Magnetic field A-Index; good conditions A < 10K-Index: good conditions K < 3
X-Ray; Scales range from A0.0 to X9.9; affect D-region absorption
30.4 nm: Total Solar Radiation SEM (Solar EUV Monitor) wavelenth: 30.4 nm that affects F-region ionization
Pf - Proton Flux Density   |   Ef - Electron Flux Density in the Solar Wind
Aurora indicates the strength of the ionization of the F-region in the polar regions
Bz - Magnetic Field z perpendicular to Earth's Ecliptic Plane   SW - Solar Wind
Aur Lat - Aurora Latitude: Calculation from NOAA - estimate the lowest latitude auroa is observed
EsEU - Sporadic E - Europe. Updated every ½ hour.
EsNA - Sporadic E - North America. Updated every ½ hour.
EME Deg - Earth-Moon-Earth Degradation/attenuation. Updated every ½ hour.
MUF - Maximum Usable Frequency (MHz), updated every 15 minutes.
MS - Meteor Scatter Activity colored bar (updated every 1/4 hour).
GeoMag - Calculated - Earth`s Geomagnetic based on K-Index. Updated every 3 hours.
Sig Noise - Background noise due to Solar Wind and geomagnetic activity, calculated every ½ hour: S-units

Read more about The Current Solar Images visual comparison of the solar activity at four EUV wavelengths

↑   11.2 - Global communication conditions versus Solar Indices SN and SF Explained

The Extreme Ultra Violet radiation - EUV, creates the ionosphere, notably the F2-region.

However EUV is completely absorbed by the inonosphere and therefore never reaches the ground.
That is why ground-based devices cannot measure the solar EUV directly.

Before the space age, scientists used two indirect markers to determine the ionization levels of the F2-region: 1. Sunspot Number and 2. Solar Flux.
The Greater levels of both may suggest better propagation conditions.

  1. SSN - Sunspot Number is a count of the number of dark spots seen on the sun.
    Higher SSN values indicate improved conditions on 14 MHz band and above: SSN <50 poor propagation     SSN >150 ideal propagation.
  3. SF - F10.7 cm Solar Radio Flux

    Solar Radio Flux SF refers to 2800 MHz (10.7 cm) radio emissions from the solar corona. Higher flux correlates with increased ionization of the E and F regions, enhancing HF radio propagation.

Approximate Correlation between SSN and SF

Typical values of SF from poor to good propagation conditions

< 90< 15MHzfairly poor
> 100> 15MHzfairly good
> 120> 25MHzgood
> 150> 28MHzvery good
> 200> 50MHzextremely good

See the recent SSN

Supplementary information:

  1. The undisturbed solar surface, developing active regions, and short-lived enhancements above the daily level all contribute to 10.7 cm radio flux. Levels are determined and corrected to within a few percent.
  2. SF is quoted in terms of Solar Flux Units (SFU) = 10-22 Watts per metre2 per Hz.
  3. The amount of solar radiation varies around the world. Even with correction factors added, it is difficult to obtain a consistent series of results. To overcome this, the reading from the Penticton Radio Observatory in British Columbia, Canada, is used as the benchmark. As a result, these numbers are extremely useful for predicting ionospheric radio propagation.
  4. Radio telescopes in Ottawa (February 14, 1947-May 31, 1991) and Penticton, British Columbia, have routinely reported solar flux density at 2800 megahertz since June 1, 1991. Levels are determined every day at local noon (1700 GMT in Ottawa and 2000 GMT in Penticton) and then corrected to within a few percent for factors such as antenna gain, air absorption, bursts in progress, and background sky temperature.
  5. Sunspot Number records been traced back to the 17th century. However, these records are open to subjective observation and interpretation. The 10.7 cm wavelength (2800 MHz) also coincides with the daily number of sunspots. Both databases are interchangeable. The 10.7 cm Solar Flux data is more stable and reliable.

↑   11.3 - The geomagnetic indices K, A, and HPo

Geomagnetic activity is a solar-induced phenomenon that causes auroras and may disrupt the electrical power grid, spacecraft operations, and worldwide radio propagation. The geomagnetic indices measure this activity.

The K-index quantifies disturbances in the horizontal component of Earth's magnetic field.

It measures the maximum fluctuations of horizontal magnetic components observed on a magnetometer at a specific geographic locations, compared to a "quiet day", during a three-hour interval.
    The K-scale is quasi-logarithmic.
  • 0 to 1: The Best Conditions for contacts from 20 meters and up
  • 2 to 3: Good Conditions
  • 4 to 5: Average Conditions
  • 5 to 9: Poor Conditions
The A-index is a daily average level for geomagnetic activity.
    The A-scale is linear:
  • 1 to 5: Best conditions on 10-20 meters
  • 6 to 9: Average conditions exist
  • 10 and above: Poor conditions on 10-20 meters
  • A=400 indicates an extreme geomagnetic storm.
Conversions between Ap and Kp

The K and A indices are averaged over places and time, as follows:

  1. K-index indicates a short-term (3 hours) instability of Earth Magnetic field, compared to a “quiet day”
  2. Kp-index is a weighted place average of K from a network of 13 mid-latitude stations.
  3. A-index indicates a longer-term (24 hours) instability of Earth's geomagnetic field (eight 3-hour values of K-indices)
  4. Ap-index is 24 hours average (eight 3-hour values) of Kp.

The HPo (GFZ) indices:

The half-hourly Hp30 and hourly Hp60, developed at GFZ (German Research Centre for Geosciences), offer improved time resolutions compared to the three-hourly Kp. Together with the linear versions Ap30 and Ap60, they are collectively known as the HPo index, providing near-real-time data from about 13 geomagnetic observatories.

Solar flux (SF) enhances wave propagation on one side, and on the other side chaotic solar activity can lead to disruptive geomagnetic storms, causing instability in the earth magnetic field, as indicated by the K and A indices.

The combined effect of Solar Flux (SF) and K-index
"Making Sense of Solar Indices" courtesy VK2FS, https://3fs.net.au/making-sense-of-solar-indices/
Solar Indices for best conditions

Good Propagation Conditions are when SF>200 (high free-electron density) and K<2 (weak geomagnetic activity).

See the recent K and A indices, as provided by NOAA / NWS Space Weather Prediction Center

See the recent Terrestrial Data

The Sun and Space Weather

↑ Chapter 12 - Solar phenomena

Sub-topics:   12.1 Quiet sun   12.2 Active Sun   12.3 Sunspots   12.4 Solar storms   12.5 Solar Cycle   12.6 Solar events   12.7 Predict Solar Flux

Solar events are classified into two types: quiet and active. Both affect HF skywave propagation conditions.

↑   12.1 - Quiet sun

The sun emits Electromagnetic Radiation across a wide spectrum from Gama-rays to ELF (long radio waves).

Ionosphere formation is due to Solar EUV
The spectrum of solar electromagnetic radiation

The EUV (Extreme Ultra Violet) is the most significant solar emission affecting terrestrial radio communications.

see below the measured spectral lines of a "quiet Sun" at Extreme Ultra Violet - EUV:

Solar Spectrum SDO (Fig.11)
The EUV spectrum of the Sun, as measured by the SDO flown aboard a rocket in April 2008 (solar minimum between cycles 23 and 24)

Solar Spectra in the Extreme UV (10-120 nm) are capable of ionizing molecules (of earth ionosphere) by a single-step energy transfer.
This EUV light is emitted from the solar chromosphere Courtesy NASA, UCAR.

Peak (He II) EUV radiation at a wavelength of 30.4 nm is the most important solar emission contributing to half of the Ionospheric F-region ionization.

Lyman series-alpha Hydrogen-spectral-line at a wavelength of 121.6 nm ionizes Nitric Oxide (NO) at the D-region causing mostly absorption of HF bands below 10 MHz.

↑   12.2 - Active Sun

Solar activity is driven by the sun's magnetic field. It is caused by a helical dynamo in the sun's core and a chaotic dynamo near the surface.

    The main solar phenomena associated with HF radio propagation on Earth are:
  1. Sunspots (last from a few days to a few months); the number of spots varies in 11-year solar cycle (a deterministic chaos)
  2. Flares (radiation bursts that last from tens of seconds to several hours)
  3. Solar wind propels energetic particles.
  4. CME - Coronal mass ejections

All these affect space weather, as explained below.

↑   12.3 - Sunspots

  • Sunspots are dark, cooler regions of the Sun's surface created by local magnetic activity.
  • These local magnetic fields inhibit heat transport, resulting in lower surface temperatures.
  • Sunspots can take on a variety of shapes, change size and last from a few hours to several months.
Two images of the Sun, that were taken at the same time (February 3, 2002)
by NASA`s Solar and Heliospheric Observatory (SOHO) satellite
courtesy European Space Agency and NASA.
Compare Sunspots and Flares
on the left: Sunspots in visible light             on the right Extreme Ultra Violet (EUV 30.4 nm)

Q. What is the reason for analyzing sunspots in both visible and ultraviolet light?

A. "Visible light" is what we see with our bare eyes. Magnetic disturbances are only detectable in EUV light.

The Current Solar Activity

Near real-time views of the Sun shown below were taken by SOHO telescope at various EUV wavelengths.
Brighter areas show higher levels of solar surface activity, i.e. higher Solar Flux Index.

Real-time SOHO images at EUV
EIT (Extreme ultraviolet Imaging Telescope)
images of the solar activity at several wavelengths
SOHO 17.1nm
SOHO 19.5nm
SOHO 28.4nm
Helium II
SOHO 30.4nm
Solar Images courtesy of NASA, Solar Data Analysis Center
Click on a thumbnail to view a larger image (opens a new window).
Sometimes you may see the text "CCD Bakeout" instead of the images.
For a technical explanation, read NASA CCD Bakeout explanation.

The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft, used to obtain high-resolution images of the solar corona. The EIT is sensitive to EUV light at different wavelengths: 17.1, 19.5, 28.4 nm produced by ionized Iron, and 30.4 nm produced by Helium. The four images show the intensity distribution at these wavelengths, originating in the solar chromosphere and transition region.

The average and local intensity may vary by orders of magnitude on time scales of minutes to hours (unpredictable solar flares), days to months (predictable solar rotation), and years to decades (predictable Solar Cycle).

The ionospheric free-electron density varies over similar magnitudes and time ranges as the EUV radiation.

The following video chronicles solar activity from Aug. 12 to Dec. 22, 2022, as captured by NASA’s Solar Dynamics Observatory (SDO). From its orbit in space around Earth, SDO has steadily imaged the Sun in 4K x 4K resolution for nearly 13 years. This information has enabled countless new discoveries about the workings of our closest star and how it influences the solar system.

133 Days on the Sun - Courtesy NASA Goddard

↑   12.4 - Solar storms

For centuries, people have been observing sunspots without knowing what they are.

We now understand that these are symptoms of solar magnetic storms, which can cause geomagnetic storms that disrupt skywave communication.

The major components of solar storms are: (1) radiation - solar flares and (2) ejected particles - Coronal Mass Ejections; see the illustration below.

(A) X-ray Solar Flare are bursts of energeric radiation (X-ray and EUV, 1-10Å) from the Sun.

  1. This radiation enhances the ionization of the lower ionosphere, specifically the D-region at 50-90 km altitude.
  2. The enhanced D-region absorbs HF radio, causing radio signals to blackout (fadeout) events.
  3. Solar flares can last from tens of seconds to several hours. with flux levels classified A, B, C, M, or X on a logarithmic scale.
  4. The D-Region absorption model is used as a guide to understand the possible fadeout events. See reports of recent flux levels.

(B) Charged Particle Events are CME, SPE, and SEP, explained as follows.

  1. Coronal mass ejections (CME) are huge flows of matter released from the sun with a larger mass compared to the quasi-constant solar wind.
    Solar storms may generate CME
    Image of coronal mass ejection (CME) captured by NASA and ESA's SOlar and Heliospheric Observatory (SOHO)
    Credit: NASA / GSFC / SOHO / ESA
  3. Solar Particle Event (SPE) refer to protons ejected by the Sun during a solar flare or a Coronal Mass Ejection (CME).
  5. Solar Energetic Particles (SEP) include electrons, protons, and alpha particles that are ejected from the Sun at high speeds.
    Reaching Earth they interact with the magnetosphere.
    Guided by the earth magnetic field, these particles are attracted by the north and south magnetic poles.

Fadeouts and blackouts (of radio communication) can occur when highly charged particles reach Earth.
Energetic protons penetrate all the way down to the D-region, boosting ionozation levels close to the poles. Polar Cap Absorption (PCA) occurs because enhanced ionization significantly increases the absorption of radio signals traveling through the region. In extreme cases, HF radio signal absorption levels may reach tens of decibels, which is enough to absorb the majority (if not all) of transpolar transmissions. These blackout events often last between 24 and 48 hours.

Sunspots, unlike flares and CMEs, are statistically predicted.
Sub-topic 12.5 discusses the Solar Cycle.
Sub-topic 12.7 presents long term prediction for Radio Flux at 10.7 cm.

↑   12.5 - Solar Cycle

Sunspots change in eleven year cycles. There are many sunspots during solar maximum and few during solar minimum.

Sunspot Number Progression since 1750
Solar Cycle Sunspot Number Progression
Source: The International Space Environment Service (ISES)

An animated overview of the Solar Cycle; published by NASA in May 2013

Solar magnetic flips are associated with solar maximum, when the number of sunspots is near its maximum, but it is often a gradual process that can take up to 18 months. The reversal will most likely take three to four months to complete.

The sunspot cycle begins when a sunspot appears on the Sun's surface at roughly 30 degrees latitude. The formation zone then travels toward the equator. At its maximal intensity, the Sun's global magnetic field has its polar regions reversed, as if a positive and negative end of a magnet were flipped at each of the Sun's poles.

There have been 24 (11-years) solar cycles since 1749. The magnetic field of the sun totally flipped every 11 years or so. In other words, the sun's north and south poles switched places. After two reversals (22 years), the solar magnetic field returns to its former orientation. This is known as "Hale cycle".

Understanding the complex interactions between solar magnetic fields, sunspots, and the solar cycle is crucial for comprehending the Sun's dynamic behavior and its impact on Earth, specifically HF propgation conditions.

The Current 25th Cycle began in 2020. The number of sunspots observed far exceeds predictions (see the graph below).

Sunspot Number Progression
Sunspot Number Progression during Solar Cycles 24 and 25
The Recent Sunspot Number Progression
The recent Estimated International Sunspot Number (EISN)

Solar flux like sunspot number can be also used to show the observed and predicted Solar Cycle.
Sunspot Number Progression

  1. Solar Cycle Notable Events

  2. In the 19th century, more than 150 years ago, extreme events had been observed.

    The Carrington Event was the most intense geomagnetic storm, recorded on 1 to 2 September 1859 during solar cycle 10.
    SSN progression 1845-65
  3. Notable Events in the Recent Years

  4. The sun is getting angry; Should you really worry about solar flares? (August 2023)
    Misteries behind explosions on the Sun

    A glob of plasma and radiation that was blasted by an unexplained explosion on the sun's far side is expected to crash with Mars.
    According to researchers, if the solar storm strikes the Red Planet, it may cause weak UV auroras and even shred a portion of the Martian atmosphere.

    It missed us by 9 days (April 2022)

  5. Comparison of the recent Solar Cycles:
    The current solar cycle (25) is stronger than the previous cycle (24) but weaker than the three previous cycles (21-23).
    Comparison of Solar Cycles
    Graph presented by Jan Alvestad, https://www.solen.info/

  6. North-South Sunspot Asymmetries

  7. Previous research has found north-south asymmetries for solar activity. These data point to some decoupling between the two hemispheres during the evolution of the solar cycle, which is consistent with dynamo theories. So yet, only little data are available for the two hemispheres independently for the most important solar activity metric, sunspot numbers. Below see an example:
    Hemispheric Sunsopt Number 1950-2021 provided by SIDC - Solar Influences Data Analysis Center, Royal Observatory of Belgium

  8. Solar Cycle - radio emissions may indicate complex processes

  9. Multi-frequency (VHF-SHF) radio bursts superimposed on a persistent background characterize solar flares:
    Solar Radio Emission
    Picture Source: Patrick McCauley Mccauley.pi, CC BY-SA 4.0
  10. Solar flares are also characterized by radio wave radiation at different frequencies

  11. Different sunspot cycles can have different radio burst distributions at 245 MHz.
    That is to say that the sunspot cycles can vary and that they may not be considered identical.

    See an article covering Burst Comparisons, Probabilities, and Extreme Events:
    Solar Radio Burst Statistics in 8 Bands and Implications for Space Weather Effects
    by O. D. Giersch, J. Kennewell, M. Lynch (2017)
  12. Solar Interference with Terrestrial Services

  13. Solar radio bursts from the Sun can interfere with communication, radar, and navigation systems (e.g., GPS).
    The forecast of future solar events will be an underestimate of the true burst rate due to the deficiencies in the data archives.
    1 Solar radio emission as a disturbance of radio mobile networks (June 2022).
    2 What a Solar Flare *Sounds* Like When It Reaches Earth (2013)
    3 An analysis of solar noise outbursts and their application to space communication (1971)

↑   12.6 - Observations of Solar Events

Events include: observed sunspots, measured radiation (solar flux, X-Ray) and ejected particles.
  1. Observed Solar-Flux / Sunspot-Number (1d 1w 1m 3m 1y) courtesy Andrew D Rodland, KC2G | GIRO, NOAA NCEI, WWROF
  2. Current Sunspot Regions Space Weather Live Belgium
  3. Solar Synoptic Map
    Forecasters at the NOAA Space Weather Prediction Center use synoptic maps to view the various characteristics of the solar surface on a daily basis. They create a snapshot of the features of the Sun each day by drawing the various phenomena they see, including active regions, coronal holes, neutral lines (the boundary between magnetic polarities), plages and filaments, and prominences. This map is a valuable tool for assessing the conditions of the sun and making the appropriate forecast for those conditions.
  4. Recent Hours Solar Events:
    1. Recent Solar Watch NOAA
    2. Views of the Sun taken by SOHO and the Yohkoh soft-Xray telescope at various EUV wavelengths
    3. CME - Corona Mass Ejection, monitored by LASCO
    4. The sun today NASA SDO's AIA - Atmospheric Imaging Assembly
    5. Solar Data Analysis Center - serves Solar Images, Solar News, Solar Data, and Solar Research NASA
    6. Monitor Solar Active Regions - search by date Peter Thomas Gallagher, Irland
    7. EarthSky A private initiative to advertise solar events

Solar emissions during recent days:
    Radiation (X-Ray)
  1. Recent X-Ray Flares from 2 hours to 3 days Spaceweather-live, a non-profit organization in Belgium
  2. Recent X-Ray Flux from 6 hours to 7 days NOAA - GOES, SWPC

  3. Particles (Solar wind)
  4. Proton Flux from 6 hours to 7 days NOAA - GOES, SWPC
  5. Real Time Space Weather 4 minutes span Rice University
  6. Near-Earth solar wind forecasts (EUHFORIA) provided by ESA
Recent Month observations:
  1. Recent Month Sunspot Number SILSO, Royal Observatory of Belgium
  2. Recent Month Daily Sunspot Number MET Malaysia
  3. Recent Month Solar Activity Plot Australian Space Weather Services
  4. Recent month Solar and geomagnetic data Table copied from Institute of Ionosphere, Kazakhstan Solen-Jan Alvestad

Recent Year: Solar Terrestrial Activity (Graphs and links) Solen-Jan Alvestad
Historical reviews:
  1. Archive of remarkable solar activity and space weather events (select month and year)
  2. Historical innovation RHESSI NASA
  3. Historic Notable Events List of solar storms Wikipedia

↑   12.7 - Predict Solar Flux

The NOAA Space Weather Prediction Center predicts monthly sunspot number and 10.7 cm radio flux. The Sunspot Number indicates the number of sunspots visible on the solar surface, whereas the 10.7 cm Radio Flux quantifies the solar radio emission at a 2800 MHz. A combination of observational data, analytical methods, and AI techniques contributes to predicting solar flux.

There are two recommended sources:
  1. Predicted sunspot number and Radio Flux at 10.7 cm are provided by NOAA / NWS Space Weather Prediction Center

    This is a multi-year (2022-2040) forecast of the monthly Sunspot Number and the monthly F10.7 cm radio flux.

    The Predicted values are based on the consensus of the Solar Cycle 24 Prediction Panel.

  3. 27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices
    prepared by the US Dept. of Commerce, NOAA, Space Weather Prediction Center.
  4. The 27-day Space Weather Outlook Table , issued Mondays by 1500 UTC, is a numerical forecast of three key solar-geophysical indices; the 10.7 cm solar radio flux, the planetary A index, and the largest daily K values. A complete summary of weekly activity and 27-day forecasts since 1997, plus an extensive descriptive, are online as the weekly.


↑   Chapter 13. Space Weather

Space weather refers to "events in space" beginning with solar storms that affect the geospace environment and induce geomagnetic storms.

Sub-topics: 13.1 definitions and explanations, 13.2 Solar Wind, 13.3 Magnetosphere, 13.4 Geomagnetic storms, 13.5 Space Weather Reports, 13.6 Space Weather Prediction

Why is Space Weather relevant to HF propagation?
HF communications can be disrupted by Solar Flares and Geomagnetic storms, which alter the ionosphere. Five major events are related to Space Weather:
  1. Solar Wind is a continuous stream of energetic charged particles flowing away from the sun.
  2. Solar Flares - flashes of radiation
  3. CME - Coronal Mass Ejections - A huge amount of charged particles flowing away from the sun at speeds faster than Solar Wind.
  4. The Magnetosphere protects Earth from the Solar Wind.
  5. Geomagnetic storms are caused by enhanced solar wind carrying energetic charged particles.
Space Weather Illustration
An illustration of the Space Weather environment
The Lagrange Mission was designed to monitor hazardous CMECoronal Mass Ejections headed toward Earth.
Credit: European Space Agency. Baker, CC BY-SA 3.0 IGO AGU - Advanced Earth and Space Science; Titles added by webmaster (4x4xm)

On the right side (of the above picture), you may see an illustration of the Magnetosphere, which protects Earth from Solar Wind. The magnetosphere is a part of a dynamic, interconnected system that responds to solar, planetary, and interstellar conditions. It is disturbed when solar wind interacts with the space environment surrounding Earth.

The Lagrange point L1 is of interest because a sattlite placed at L1 can keep pace with the Earth as it orbits the Sun, always staying on a line between the Earth and the Sun.

Animation of a satellite trapped at the L1 point
of the Sun-Earth-Moon gravitational system.
Published by Space Weather Live

↑   13.1 - Space Weather definitions and explanations

Space Weather refers to changes in space conditions, beginning with Solar Activity that disrupt the geospace environment and Earth Magnetic field (geomagnetic storms).

  1. What kind of weather events occur in space?
  2. When are they likely to strike?
  3. Why doesn't space weather just torch us?
  4. What are the effects of space weather on Earth?
  5. How do scientists monitor space weather?
  6. Can individuals prepare for space weather events?
Space Weather Scales NOAA | pdf
The NOAA RSG Scales describe three event types, with numbered levels (0-5) for severity and possible effects. R1-5   Radio Blackouts
S1-5   Solar storms
G1-5   Geomagnetic storms

Reports of Space Weather Conditions

↑   13.2 - What is Solar Wind?

The solar wind is a stream of charged particles emitted by the sun's corona to outer space. Solar winds cause charged particles to move along a magnetic field to Earth's poles, resulting in the aurora.

Solar Wind
The figure above is an illustration of the solar wind reaching the Earth's magnetosphere. It shows how the solar wind rearranges the magnetosphere, compressesing the magnetic field on the side facing the sun, while elongating it on the opposite (far) end.

It is impossible to exactly predict when the solar wind will increase or decrease, but there is some correlation with sunspots and solar flares.

Strong solar winds known as CME (Coronal Mass Ejection) can trigger geomagnetic storms, as seen in 1989 in Canada, with aurora visible in southern regions like Texas and Florida.

How long does it take for Solar Wind to reach earth?

The charged particles escape the sun's gravity at speeds ranging from 200 to 800 kilometers per second. They are made up of electrons (~47%), protons (~47%), and alpha particles (~5%). The electrons are the first to reach Earth. During a coronal mass ejection (CME), however, energetic charged particles travel at higher speed. Electrons may arrive 20 to 30 minutes after the storm begins, whereas heavier particles such as protons arrive in a day and alpha particles can take up to four days. See the recent solar wind.

Variations in solar wind velocity are associated with waves and turbulence, with higher-speed streams undergoing larger fluctuations. Scientists classify solar wind based on a variety of variables, including speed, density, pressure, energy per particle, and other characteristics.

Understanding the characteristics and behavior of the solar wind is crucial for studying space weather and its effects on HF radio propagation.

↑   13.3 - The Magnetosphere and Earth Magnetic field

Geomagnetic conditions refer to the state of the magnetosphere created by the Earth's magnetic field as affected by solar activity.


The magnetosphere is a "magnetic bubble" that surrounds Earth and protects us from the Solar Wind.

Its shape depends on the pressure of the solar wind and the orientation of the Earth’s magnetic field.


Earth Magnetic field
known also as the
geomagnetic field

The Earth Magnetic field is generated by currents caused by the circulation of molten iron in the inner core
and nickel convection currents in the outer core.

↑   13.4 - Geomagnetic storms affect HF propagation

A Geomagnetic Storm is a disturbance in the magnetosphere that may trigger auroras.

Illustration of Geonagnetic storms as seen from earth close to the polar regions:

Public-domain photographs show electrical discharge in ionospheric regions near Earth's polar regions.

What causes geomagnetic storms?

A CME is a shock-wave of highly charged particles emitted by the sun.

When a CME enters the magnetosphere, it causes a Geomagnetic Storm

Major magnetic storms may block HF propagation (3-30 MHz) by modifying the distribution of free-electrons in the ionosphere.

↑   13.5 - Reports of Space Weather Conditions

Space weather reports may help to understand variations in HF propagation conditions.
  1. The recent 3 days RSG Scales by NOAA SWPC services
  2. Solar Wind and Interplanetary Magnetic Field by Rice Space Institute
  3. The last 24 hours Space Weather Plots by Solar Terrestrial Dispatch
  4. Recent Kp index by Europen Space Weather Service
  5. Recent Solar-Terrestrial Data by N0NBH, Paul L Herrman
    The NOAA Space Weather Scales describe three event types:
  1. Radio Blackouts caused by solar flares
  2. Solar storms characterized by charged particles flux
  3. Geomagnetic Activity that causes Geomagnetic Storms
Recent 3 days of Space Weather - RSG Scales
Published by NOAA SWPC services
Reference: Space Weather images at (US) NOAA SWPC
↑ Real-time Solar Terrestrial Probe, provided by Rice Space Institute see there "Real Time Dials"
The Solar Terrestrial Probe monitors Solar Wind and Interplanetary Magnetic Field.
Color schemes: Green indicates that values in this range are unlikely to disturb the near-Earth space environment. Yellow indicates that values in this range may contribute to disturbances, and Red indicates that values in this range are likely to drive disturbances.
Put the cursor over each image to see an explanation.
sw speed sw density sw pressure SW temperature
Real-time Solar Wind (SW)
IMF magnitude IMF Clock IMF Azimuth
Real-time Interplanetary Magnetic Field (IMF)


↑ Real-time Space Weather Plots
published by Solar Terrestrial Dispatch

Click on the image below
Recent Space Weather Parameters

↑ Kp index
provided by Europen Space Weather Service Network


↑ Recent Solar-Terrestrial Data
Provided by N0NBH, Paul L Herrman

On the right last 30 days graph
Recent Solar Terrestrial Dara, N0NBH
On the left: the recent values
SFI - Solar Flux, 2800MHz (10.7cm)higher value better HF conditions.
SN - Daily Sunspot Number is a measure of the recent solar activity
30.4 nm: EUV Radiation that affects F-region ionization
EVE: SDO EUV Variability Experiment; SEM: SOHO Solar EUV Monitor
K-Index: Geomagnetic Disturbance; good conditions K < 3
A-Index: 24 hours Average; good conditions A < 10
Ptn - Proton Flux / Elc - Electron Flux Density, both impact E-region
Aurora indicates the strength of the ionization of the F-region in the polar regions.
Aur Lat - Aurora Latitude estimated lowest latitude
Solar Wind speed
Mag Bz - Magnetic Field z perpendicular to Earth's Ecliptic Plane
S Noise S-units: This value shows how much noise, in S Units, is being generated
by the interaction between the solar wind and the earth’s geomagnetic activity.
The higher the numbers, the greater the noise.
GeoMag Earth`s Geomagnetic Field stablility based on K-Index. Levels:
Very Quiet, Quiet, Unsettled, Active, Minor Storm, Major Storm, Severe Storms, or Extreme Storms

↑   13.6 - Predict Space Weather

ACE project NOAA - Space Weather Prediction Center (SWPC)

Advanced Composition Explorer (ACE) collects data from a satellite that is located at the L1 Lagrange point, which is a point in space where the gravitational forces of the Earth and the Sun balance each other.

The data collected by ACE includes measurements of solar wind, Earth Magnetic field, and energetic particles that are crucial for predicting space weather events like geomagnetic storms and solar flares. ACE SWPC processes this data and produces alerts and forecasts for space weather events that could potentially impact satellites, power grids, communication systems, and other technological infrastructure on Earth.

The military, airlines, power companies, and telecommunications providers all use ACE SWPC data and predictions to make decisions and take precautions to protect their assets and operations from the effects of space weather.


↑ Chapter 14. Radio blackouts (fadeouts)

A radio blackout occurs when high solar activity causes ionospheric storms that disrupt radio communication.

X-ray solar flares and solar proton events enhance D-region absorption, causing radio blackouts.

The D-Region Absorption Prediction (D-RAP) Model (by SWPC-NOAA) analyzes how solar phenomena affect HF radio communication, as illustrated below:

See for example the unusual solar storms and geomagnetic activity occured during 10-19 May 2024.

* Search the term "Blackout" at NOAA website.

↑ Chapter 15. Summary

The ionosphere enables HF radio waves to propagate over large distances. Solar radiation (EUV) forms the D, E, and F regions at heights from 60 to 700 kilometers above the Earth. The outermost F-region supports long-distance HF transmission (skywave). The inner D-region mostly absorbs the lower HF bands (160, 80, 60, and 40 m). Ionization during the day and dissipation at night affect all the HF bands, as well as the seasons and solar conditions, such as the 11-year sunspot cycle.

Review the essential points explained in the previous chapters:
  1. HF radio is a form of electromagnetic radiation that travels as waves at the speed of light. HF radio waves can travel very long distances without the need for infrastructure like satellites or cables.
  3. There are three main modes of HF radio propagation: line of sight (LOS), ground wave, and skywaves that involve wave reflections from the ionosphere.
  5. NVIS is a unique communication mode used for local coverage in hilly and/or jungle areas, over short distances (a few hundred kilometers).
  7. The greyline refers to the Earth's twilight zone between day and night, that is considered a "sweet spot" for HF radio communications.
  9. Forecasting HF propagation conditions is the process of predicting how radio waves will travel through the upper atmosphere. It may help in selecting communication frequencies, planning antenna systems, and optimizing coverage.
  11. Real-time Band Conditions can be assessed using various methods, such as: monitoring band activity through DX clusters, using personal receivers, accessing remote receivers via internet (WebSDR or KiwiSDR), monitoring CW beacons, and watching real-time charts or maps. It's recommended to use a combination of tools to get a complete picture of propagation conditions.
  13. The Solar EUV radiation ionizes the upper atmosphere, forming the ionosphere. The free electrons in the ionosphere bend and reflect radio waves back to Earth.
  15. The amount of Solar emissions and the sun's position affect HF propagation conditions. Sunspot numbers and solar flux serve as indicators for assessing global propagation conditions. Higher solar activity improves propagation conditions, while solar storms can disrupt communications.
  17. During the day four ionospheric regions are active. The D-regions vanishes at sunset and the higher regions E and F dissipate at night.
    Ionospheric D-E-F Layers
  19. Diurnal changes in the day/night cycle are due to the Earth's rotation around its axis, which causes periodical changes in the free-electron density of all regions.
  21. The F-region is the dominant "reflector" of Skywaves. Higher electron density of the F2-region enables bauncing of higher frequencies.
    The effective range for communication is a function of the incident angle:
    incident angle
    and the MUF is affected by the free-electron density in the ionosphere:
    Ionospheric electron density - typical profiles

    Day/Night, and Solar Cycle Min/Max variations
    are based on 1987 research of Arthur D. Richmond

  23. Seasonal and diurnal phenomena: HF propagation conditions are better in the summer and closer to the equator. This is because direct solar radiation generates more free-electron in the summer and near the equator.
  25. Local anomalies: The ionospheric regions are not homogeneous, rather composed of moving "plasma clouds".
    Ionospheric clouds
  27. Regional anomalies:
  29. The critical frequency is an important characteristic that defines short wave propagation conditions. It correlates with Sunspot Number.
    The graph below illustrates variations in the critical frequency as a function of years and seasons.
    Correlation of SSN and Critical Frequencies and Time
    The critical frequency and Sunspot Number and Time, published by AGSWS
  30. The left vertical axis denotes critical frequency - the highest frequency reflected at noon at near vertical angles from the F2, F1, and E leyers.
    The information was derived from ionograms collected at noon in Canberra, Australia.
    The highest F2 region has greater relative fluctuations in electron density than the lower F1 and E regions, because it is more influenced by the solar activity.

    The right vertical axis shows sunspot sumber represented in the graph by the redish line.
  31. Communication conditions can be unexpectedly disrupted due to solar storms, which affect the D-Region. This region may block signals in all the HF bands amd in particular below 10 MHz.
  32. Blackouts:
    1. The radio blockouts associated with solar flares occur during daytime and are most intense when the sun is directly overhead.
    2. In a typical solar flare, X-Ray penetrate to the bottom of the ionosphere (to around 80 Km) and enhance the ionization of the D-region, which absorbs the lower HF bands.
    3. Solar protons can also disrupt HF radio communication. These energetic protons are guided by Earth magnetic field, such that they collide with the upper atmosphere near the north and south poles. The fast-moving protons have an effect similar to the X-ray flares and create an enhanced D-region thus blocking HF radio communication near polar caps.
    4. During auroral displays, the precipitating electrons can enhance other layers of the ionosphere and have similar disrupting and blocking effects on radio communication. This occurs mainly on the night side of the polar regions of Earth where the aurora is most intense and most frequent.
  34. Forecasting HF propagation conditions requires mathematical models utilizing solar solar indices (SSN, SF), geomagnetic indices (K and A), frequency of operation, time of day, and season.
  36. Alternatively, you can view the current skywave propagation conditions at a glance.


↑ References

The references below are organized by topic, as follows:
  1. Links to websites from which online data is collected
  2. Monitor Band Activity of Radio Amateurs Real-time watching of worldwide hams' activity
  3. Propagation modes Basic principles and models
  4. Skywave Propagation via Ionosphere > Propagation > Ionospheric Intro & Model > Layers/Regions > MUF-OWF-LUF > Seasonal & Anomalies > Probing
  5. NVIS unique mode of a skywave
  6. Greyline
  7. Solar Phenomena
  8. Solar Indices
  9. Space Weather What is it? Solar Indices | Geomagnetic storms - Impact on HF radio Propagation | Space Weather Agencies & Services
  10. Recent Observations of The Sun, Space, Terrestrial, TEC Total Electron Content, MUF from ionosondes, Propagation Charts
  11. Forecast HF band conditions based on recent events
  12. Tools and Applications for analyzing and forecasting HF propagation
  13. Misc. References Definitions, cross-disciplinary research etc.

  1. This page collects online data from the following websites:
    1. hamqsl.com, Paul L Herrman, N0NBH
    2. prop.kc2g.com, Andrew D Rodland, KC2G
    3. hb9vqq.ch, Roland Gafner, HB9VQQ
    4. hf.dxview.org, Jon Harder, NG0E
    5. qrzcq.com, QRZCQ
    6. solen.info, Jan Alvestad, Retired from FMC Kongsberg Subsea AS, Norway
    7. spacew.com, Solar Terrestrial Dispatch
    8. services.swpc.noaa.gov, and www.swpc.noaa.gov, Space Weather Prediction Center, NOAA
    9. sohowww.nascom.nasa.gov, the international cooperation between ESA and NASA
    10. swe.ssa.esa.int and new.swe.ssa.esa.int, The European Space Agency (ESA)
    11. mms.rice.edu, Rice Space Institute
    12. impc.dlr.de, German Aerospace Center
    13. sidc.be, The Royal Observatory of Belgium
    14. sws.bom.gov.au, The Australian Space Weather Alert System

  2. Monitor HF Band Activity of Radio Amateurs ↑ Real-time watching of worldwide hams' activity

      SDR - Software-Defined Radio is a technology in which analog hardware components are replaced by software on a computer or embedded system.

    1. SDR - Software Designed Radio Wikipedia ↑
    2. There are two worldwide networks of remote public SDR receivers↑

    3. WebSDR list of public stations
      1. Wideband WebSDR at the University of Twente, Enschede, NL
      2. WebSDR.org background information
      3. FAQ about the WebSDR project
    5. KiwiSDR map of public stations
      1. KiwiSDR list of public stations
      2. Introduction to using the KiwiSDR
      3. KiwiSDR design review

    Activity Map, DX Maps and DX Clusters

    1. Real-time Ham Band Activity Map Jon Harder, NG0E
    2. Sites for Checking Signal Propagation and Band Activity SPARC (W6SPR)
    3. DXMAPS Gabriel Sampol, EA6VQ
      Use DX Maps to understand HF Propagation conditions
      DXMAPs website shows on maps and lists the most recent contacts and reception reports in the different amateur radio and SWL bands. It’s aim is to provide a detailed view of the propagation conditions in real-time in various bands and modes.
    4. Real-time propagation and band conditions QRZ online
    5. HamDXMap : MUF, foF2, live radio frequencies weather Christian Furst, F5UII
      Map for amateur radio uses (HamRadio) : In the shape of a terrestrial world globe, the MUF and Aurora Borealis layer. This gives the distances and directions of antennas between amateur radio stations, the position on the Maidenhead Locator grid.
    6. DXWatch filter Spot Search and Create Your Filter
    7. DXZone Amateur Radio Internet Guide; incl. curation of 51 DX clusters nodes.
    8. Reporters of digital modes

    9. Digimode PSK-Reporter
    10. HF Signal Propagation Reporter, PSK/JT65/FT-8/CW/JT9 HamRadioConcepts KJ4YZI
      APRS-IS - Automatic Packet Reporting System-Internet Service
    12. Find Real-time Contacts, DX Cluster, Spotter Network, APRS  HamRadioConcepts KJ4YZI
    13. VHF Propagation Map APRS-IS real-time radio propagation from stations operated near 144 MHz
    14. WSPR - Weak Signal Propagation Reporter ↑

    15. WSPRnet website
    16. WSPR Rocks
    17. WSPR Live
    18. Weak Signal Propagation Reporter Wikipedia
    19. WSPR - An Introduction for Beginners | WSJT-X Ham Radio Ham Radio DX, 7-Jan-2022
    20. WSPR Explained: How to Get Started With One-Way Ham Radio ExtremeTech
    21. Average propagation conditions: The recent WSPR reports on 80-10m Ham Bands up to 60 days WSPR Rocks
    22. Beacons

    23. NCDXF Beacon Network see above ↑
    24. International Beacon Project NCDXF
    25. Beacons IARU
    26. International Beacon Project (IBP) Wikipedia
    27. High Frequency Beacons and Propagation VU2AWC
    28. Amateur Radio Propagation Beacon Wikipedia
    29. Ham Radio Beacon List Google
    30. Types of Radio beacon HF Underground
    31. Investigating Radio propagation using beacons HF Underground
    32. Reverse Beacon Network (RBN) | History | Online Activity
    33. Beacon monitoring programs DXZone
    34. Detect Changes in Propagation Conditions by RBN, WSPR, PSKR etc.

    35. Ham Radio Reporting Networks are useful to assess propagation conditions. HamSCI
    36. Using the WSPR Mode for Antenna Performance Evaluation and Propagation Assessment on the 160-m Band 2022 Jurgen Vanhamel et al.
    37. Ionospheric Sounding Using Real-time Amateur Radio Reporting Networks (2014) Nathaniel A. Frissell, W2NAF et al.
    38. Reverse Beacon Networks – PSK Reporter And WSPR 2013 Fred Kemmerer, AB1OC
    39. Interpreting WSPR Data for Other Communication Modes 2013 Dr. Carol F. Milazzo, KP4MD

  3. Radio Waves Propagation ↑

    Waves, EM Radiation, Radio waves - Basic principles

    1. Electromagnetic Radiation Wikipedia
    2. Electromagnetic Spectrum Wikipedia
    3. Lyman series-alpha hydrogen radiation at a wavelength of 121.6 nm Wikipedia
    4. Gama Ray Wikipedia
    5. Radio Waves Wikipedia
    6. Radio propagation Wikipedia
    7. Introduction to RF Propagation John S. Seybold
    8. Radio Propagation Tutorial Basics Electronics-Notes
    9. Propagation Overviews

    10. The Rebirth of HF Rohde & Schwarz
    11. Course Overview: Atmospheric Effects on Electromagnetic Systems Naval Postgraduate School
    12. All-In-One Overview: There is nothing magic about propagation José Nunes – CT1BOH (2021)
    13. Overview: Understanding HF / VHF / UHF / SHF Propagation Paul L Herrman N0NBH
    14. Propagation of Radio Waves Basu, VU2NSB principles and methods
    15. Propagation Modes

    16. LOS - Line Of Sight propagation Wikipedia
    17. Ground Wave
    18. Ground Wave Propagation Wikipedia
    19. Ground Wave Propagation Tutorial Electronics-Notes
    20. Ground wave MF and HF propagation AGSWS Part of key topics within ionospheric HF propagation
    21. Ground Wave Propagation BYJU’S Tuition Centre
    22. Skywave / Skip↑
    23. Skywave or Skip Propagation Wikipedia
    24. Skywaves & Skip Zone Electronics-Notes Key topics within ionospheric HF propagation
    25. Path length and hop length for HF sky wave and transmitting angle AGSWS
    26. Skip zone Wikipedia
    27. Atmospheric Ducting Wikipedia
    28. Tropospheric Ducting Wikipedia

  4. Skywave Propagation via ionosphere ↑
            Propagation > Refractive Index > Ionospheric Intro > Model > Layers/Regions > MUF-OWF-LUF > Seasonal & Anomalies > Probing

      Skywave Propagation

    1. HF Progagation: The Basics - QST, December 1983 Denis J. Lusis, W1JL/DL
    2. The HF Bands for Newcomers (An Overview), ARRL (2007) Gary Wescom, N0GW
    3. Introduction to HF Propagation (33 pages presentation, Nov 2018) Rick Fletcher, W7YP
    4. An introduction to HF Propagation Sean D. Gilbert Mipre, G4UCJ
    5. An Introduction to HF propagation and the Ionosphere ZL1BPU
    6. Propagation of radio waves explained PA9X
      Radio waves; Earth’s atmosphere (from Troposphere to Ionosphere); Main Propagation modes; Ionospheric layers; Solar Activity; Sunspots and Solar Flux; Solar Wind; Earth’s Geomagnetic Field; Solar Flares; Coronal Holes; CME; The 27-day cycle; The sunspot cycle; The Earth’s seasons; How HF propagation is affected by solar activity: Flares, Coronal holes, CME; Unique propagation effects: Sporadic-E, Backscatter, Aurora, Meteor scatter, Trans-Equatorial, Field Aligned Irregularities.
    7. When is the best time to make an HF contact? Propagation Prediction tools Ria's Ham Shack Ria Jairam, N2RJ, 7 April 2022
      When is the ideal time to make HF contact with a specific region of the world?
      A general talk of about 18 minutes, without demonstrations or definitions of basic concepts.
    8. Ionospheric propagation Basics Electronics-Notes
    9. Introduction to Ionospheric HF Radio Propagation AGSWS
    10. Understanding HF Propagation Rohde Schwarz
    11. Understanding HF Propagation Steve Nicols, G0KYA, RSGB
    12. Radio Propagation 101 - Why should you be interested in propagation? Dan Vanevenhoven
    13. Ward Silver On Radio Wave Propagation Ham Radio Crash Course
    14. The Ionosphere, Shortwave Radio, and Propagation MIT Film & Video Production club
    15. The Effects Of The Ionosphere On Radio Wave Propagation An Excellent Presentation made more than 86 years ago!!! Art Bodger
    16. Ionospheric Propagation University of Toronto
    17. Regional and Long Distance Skywave Communications Ken Larson, KJ6RZ
    18. Transequatorial Radio Propagation CO8TW
    19. Ionospheric Research

    20. Welcome to the Ionosphere NASA Goddard
    21. Ionization (basics) Wikipedia
    22. Plasma (basics) Wikipedia ↑
    23. Plasma recombination Wikipedia ↑
    24. The Ionosphere UCAR
    25. Ten Things to Know About the Ionosphere NASA
    26. Ionosphere Electronics-Notes
    27. Refractive index in general Wikipedia ↑ The refractive-index-of-the-ionosphere
    28. Refractive Index of Ionosphere Calculator Calculator A to Z
    29. The refractive index and the absorption index of the ionosphere Research notes
    30. Ionosphere and Radio Communication Saradi Bora, Kamalabaria College, North Lakhimpur, Assam, India
      The ionospheric refractive index P.126
    31. Refractive index of ionosphere Plasma Physics
    32. Ionospheric Radio Wave Propagation Richard Fitzpatrick, University of Texas at Austin
    33. The Complex Refractive Index of the Earth's Atmosphere and Ionosphere Ernest K. Smith, University of Colorado
    34. The Earth's Ionosphere (book 1989) Plasma Physics and Electrodynamics Michael C. Kelley
    35. Ionospheric model

    36. Ionosphere (basics) Wikipedia
    37. Introduction to the ionosphere Anita Aikio
    38. Ionospheric model Wikipedia
    39. Ionospheric Radio (book 1990) Kenneth Davies
    40. Layers / Regions

    41. Mesopause Wikipedia ↑
    42. Distribution of ionospheric free-electrons Bob Brown, NM7M (SK), Ph.D.
    43. The Ionosphere and the Sun Naval Postgraduate School
    44. Layers of Ionization Wikipedia
    45. Ionospheric D, E, F, F1, F2 Regions Electronics-Notes
    46. D Layer Wikipedia
    47. Ionospheric D Region Britannica
    48. D-region absorption of radio signals Ham Radio School
    49. Day vs Night Ionospheric Layers Northern Vermont University Lyndon Atmospheric Sciences
    50. E-region Wikipedia
    51. Sporadic E propagation Wikipedia
    52. Sporadic E-region ScienceDirect
    53. Sporadic E Propagation in 2 minutes VK3FS
    54. Sporadic E Propagation VK3FS
    55. Understanding Sporadic E Propagation for VHF DX Ham Radio DX
    56. Understanding Sporadic E Rohde Schwarz
    57. F-region Wikipedia
    58. MUF, OWF, and LUF - Explanation of the concepts; see below How is MUF determined?

    59. HF Radiation - Choosing the Right Frequency Naval Postgraduate School
    60. MUF Maximum usable frequency Wikipedia
    61. The critical frequency Wikipedia
      The critical frequency as a Function of: Free-electron density, MUF, Plasma frequency, Index of refraction
    62. Critical frequency, MUF, OWF, and LUF Electronics-Notes
    63. How to use Ionospheric Propagation? Electronics-Notes ↑
    64. Regular Ionospheric variations

    65. Sunspot Number and critical frequencies and Time (Years and Seasons) AGSWS
    66. Season Rollover – Why do shortwave frequencies have to change? Neale Bateman, BBC
    67. The Seasonal Behavior of the Refractive Index of the Ionosphere over the Equatorial Region Turkish Journal of Science & Technology
    68. Ionospheric anomalies

    69. Persistent anomalies to the idealized ionospheric model Wikipedia
    70. Effect of Seasonal Anomaly or Winter on The Refractive Index of in Height of The Ionospheric F2-Peak International Journal of Basic & Applied Sciences
    71. Ionosphere Probing Principles | Ionosondes | Ionograms | Stations | Charts | R & D

    72. Radio Techniues for Probing the Terrestrial Ionosphere (book 1989) R.D. Hunsucker
    73. Introduction To Ionospheric Sounding (2006) Bruce Keevers, National Geophysical Data Center, NOAA
    74. Principles - Theoretical and Methodolical Aspects
    75. Chirping Explained - Passive Ionospheric Sounding and Ranging Peter Martinez, G3PLX
    76. Chirp reception and interpretation (2013) Pieter-Tjerk de Boer, PA3FWM
    77. Software-Defined Radio Ionospheric Chirpsounder For Hf Propagation Analysis (2010) Nagaraju, Melodia (NY State Univ); Koski (Harris Corporation)
      A prototype description of Software Defined Radio (SDR) chirpsounder system, based on a commercially-available SDR platform, that dynamically select the best channels to be used in an HF link, to maximize communications capacity and reliability.
    78. International Reference Ionosphere model IRI
      IRI is an international project that formed a Working Group in the late 1960s to produce an empirical standard model of the ionosphere, based on all available data sources. They provide monthly averages of the electron density, electron temperature, ion temperature, and ion composition in the ionospheric altitude range.
    79. Ionosondes ↑
    80. Introduction to Ionospheric Sounding for Hams Dr. Terry Bullett. W0ASP - University of Colorado
    81. Ionosonde Wikipedia
    82. Ionosonde HF Underground
    83. DIGISONDE®: Simultaneous Ionospheric Observations Around The Globe Lowell Digisonde International (LDI)
    84. Ionograms ↑
    85. Ionogram Wikipedia
    86. Understanding HF Propagation and Reading Ionograms  Bootstrap Workbench
    87. Digisonde Directogram UMass Lowell Space Science Lab website, MA, US
    88. Digital Ionogram DataBase Global Ionosphere Radio Observatory (GIRO)
    89. Mirrion 2 - Real Time Ionosonde Data Mirror Space Weather Service at NOAA
    90. Ionogram Data Info GIRO, UML
    91. The DST Group High-Fidelity, Multichannel Oblique Incidence Ionosonde (2018) DOI AGU
    92. Remote sensing of the ionosphere Google Search
    93. ICON - Ionospheric Connection Explorer Wikipedia
    94. Probing ionospheric disturbances by Auroral Radar Network ↑
    95. Super Dual Auroral Radar Network (SuperDARN) Wikipedia
    96. First Observations of Large Scale Traveling Ionospheric Disturbances Using Automated Amateur Radio Receiving Networks (2022) Nathaniel A. Frissell, W2NAF et al.

  5. NVIS a unique mode of a skywave: real-time map↑, explanation↑
    1. Understanding NVIS  Rohde Schwarz
    2. HF NVIS  Military HF Radio
    3. NVIS Wikipedia
    4. NVIS Propagation: Near Vertical Incidence Skywave Electronics-Notes
    5. Near-Vertical Incidence Sky-Wave Propagation 36 pages Presentation for radio hams Gerald Schuler, DU1GS / DL3KGS
    6. Near Vertical Incidence Skywave (NVIS) by W8BYH, Fayette ARES
    7. Near Vertical incidence Skywave Propagation NVIS Antennas  80, 60, 40m bands KB9VBR Antennas
    8. NVIS Overview  David Casler, KE0OG
    9. Ham Radio NVIS for Regional Communications  Radio Prepper
    10. NVIS - Near Vertical Incidence Skywave What is it? advantages; antennas; links Jim Glover, KX0U (ex WB5UDE)
    11. Near Vertical Incidence Skywave (NVIS) Ham Radio School, W0STU
    12. NVIS Propagation Dave Lawrence, VA3ORP (2007)
    13. NVIS Explained - part 1 NCSCOUT
      NVIS Explained - part 2
      NVIS Explained - part 3
      NVIS EXPLAINED citing the above 3-parts publication AmRRON
    14. NVIS Antennas Dale Hunt, WB6BYU
    15. Extended Research papers

    16. Radio communication via Near Vertical Incidence Skywave propagation: an overview Telecommun Syst (2017) DOI, Ben A. Witvliet, Rosa Ma Alsina-Pagès
    17. Analysis of the Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Communications Channels Sensors (Basel)
    18. NVIS HF signal propagation in ionosphere using calculus of variations Geodesy and Geodynamics, Umut Sezen, Feza Arikan, Orhan Arikan

  6. Greyline Propagation ↑
    1. Grey Line HF Radio Propagation Electronic Notes
    2. Identifying Gray-Line Propagation Openings DXLab
    3. Greyline Propagation G0KYA
    4. Gray-line Propagation Explained Radio Hobbyist
    5. Round the world echoes G3CWI
    6. An introduction to gray-line DXing Rob Kalmeije
    7. Greyline Map DX QSL Net
    8. Greyline Map DXFUN

  7. Solar Phenomena affecting HF Propagation ↑

    Solar Physics

    1. Solar Physics (Heliophysics) Youtube playlist
    2. Heliophysics Wikipedia
    3. Heliophysics NASA
    4. Heliophysics and amateur radio: citizen science collaborations for atmospheric, ionospheric, and space physics research and operations Nathaniel A. Frissell, W2NAF et al.
    5. Quiet Sun Radiation

    6. Solar Radiation / Sunlight Wikipedia
    7. Extreme Ultraviolet (EUV) Wikipedia
    8. Active Son is it chaotic?

    9. Overview of Solar phenomena Wikipedia - Sunspots (Solar Cycle), flux (SF), solar wind, particle events, flares, CME
    10. Links to types of Solar storms Wikipedia
    11. Sunspots↑
    12. Sunspots Wikipedia
    13. Sunspot Number AGSWS
    14. The Lifetime of a Sunspot Group AGSWS
    15. Effective sunspot number: A tool for ionospheric mapping and modelling URSI General Assembly 2008
    16. Solar Cycle↑
    17. Solar Cycle Wikipedia
    18. Solar minimum Wikipedia
    19. Solar maximum Wikipedia
    20. Understanding the Magnetic Sun NASA
    21. Solar Cycle Progression NOAA
    22. Sunspot number series: latest update SILSO, Royal Observatory of Belgium
    23. North-South Asymetry of Monthly Hemispheric Sunspot Numbers SILSO, Royal Observatory of Belgium
    24. Solar Cycle AGSWS
    25. Solar Magnetic Storms (referred as "solar radiation storms") ↑

    26. Solar Raditaion storms Proton Events NOAA
    27. Solar Radiation Storm Space Weather Live
    28. Solar Flares Wikipedia
    29. Classification of X-ray Solar Flares or Solar Flare Alphabet Soup Spaceweather.Com
    30. Understanding how solar flares affect radio communications Barrett Communications, Australia
    31. Solar Particle Event (SPE) Wikipedia
    32. Solar energetic particles (SEP) Wikipedia
    33. Solar Proton Events Affecting the Earth Environment 1976 - present NASA
    34. Next-Generation Solar Proton Monitors for Space Weather Eos
    35. The Difference Between CMEs and Solar Flares NASA
    36. CME ↑
    37. What is Coronal Mass Ejection Wikipedia
    38. Coronal Mass Ejections - CME NOAA
    39. The result: Particle Precipitation
    40. Particle Precipitation ScienceDirect
    41. Particle Precipitation in the Earth and Other Planetary Systems: Sources and Impacts Frontiers
    42. Energetic particle precipitation Laboratory for Atmospheric and Space Physics, Univ. of Colorado
    43. Solar Observation reports
    44. SDO Mission NASA - The Solar Dynamics Observatory
    45. The Active Sun from SDO: 30.4 nm NASA - The Solar Dynamics Observatory
    46. EVE Overview Solar Phys. - The Solar Dynamics Observatory The EVE project (real-time high-resolution EUV measurements) was designed to improve understanding of the evolution of solar flares
      and extend the related mathematical models used to analyze solar flare events.
      The article is an Overview of Science Objectives, Instrument Design, Data Products, and Model Developments.
      Published in Solar Phys. - DOI 10.1007/s11207-009-9487-6
    47. May 2024 Solar storms Wikipedia
    48. Geomagnetic storms May 2024 Duckduckgo
    49. Analysis
    50. Responses and Periodic Variations of Cosmic Ray Intensity and Solar Wind Speed to Sunspot Numbers (2020) Hindawi - Collaborative work Advances in Astronomy: Relationship of sunspot numbers with cosmic ray intensity and solar wind speed.
      Article ID 3527570 | https://doi.org/10.1155/2020/3527570

  8. Solar Indices↑ as a measure of Global HF & VHF Radio Propagation↑
    1. Beginners Guide to Propagation Forecasting Ed Poccia, KC2LM
    2. The history of the 10.7 cm solar flux Government of Canada
    3. The 10.7 cm solar radio flux K. F. Tapping, AGU
    4. Penticton/Ottawa 2800 MHz Solar Flux NOAA
    5. Solar Indices: SFI, SN, A, K, Kp Electronics-Notes
    6. K-index Wikipedia
    7. Understanding Solar Indices ARRL
    8. Solar Indices - Glossary of Terms HamQSL , Paul L Herrman, N0NBH
    9. What are Solar Flux, Ap, and Kp Indices? VK3FS
    10. Hp30 and Hp60 vs. Kp index GFZ (German Research Centre for Geosciences)
    11. Solar Index and Propagation Made Easy - HF Ham Radio The Smokin Ape
    12. Current Ham Radio Propagation Conditions HR4NT - Ham Radio for Non-Techies
    13. Solar Resource Page Mark A. Downing, WM7D

  9. Space Weather What impact does it have on HF radio propagation? ↑
    1. Space Weather Wikipedia
    2. A Media Primer for the Solar Cycle and Space Weather NESDIS
    3. Solar-terrestrial science CSA
    4. Solar Wind Wikipedia
    5. Space Weather Naval Postgraduate School
    6. The Magnetosphere Wikipedia
    7. Magnetosphere (MS) NASA
    8. The Interplanetary Magnetic Field (IMF) - Sun’s magnetic field, B(t)x,y,z, Earth’s magnetosphere Space Weather Live
    9. Mathematical Models of Space Weather NASA
    10. How does Space Weather impact HF radio communication? NOAA
    11. Space Weather and Radio Communications AGSWS
    12. NWS Space Weather Prediction Center NOAA
    13. Space weather: What is it and how is it predicted? SpaceCom
    14. How to Improve Space Weather Forecasting (2020) Eos, AGU
    15. How to Assess the Quality of Space Weather Forecasts? (2021) Eos, AGU
    16. Space Weather Highlights AGU
    17. The Sun and HF radio propagation Electronic Notes
    18. Space Weather and Propagation (A presentation 2019) Martin Buehring, KB4MG
    19. Solar Activity and HF Propagation (A presentation) Paul Harden, NA5N © QRP-ARCI – 2005
    20. Ionospheric Disturbances ↑

    21. Sudden Ionospheric Disturbance (SID) Wikipedia
    22. Sudden Ionospheric Disturbance (SID) Draft: WFD (23 March 2014) William Denig, National Centers for Environmental Information-NOAA
    23. Sudden Ionospheric Disturbances An overview National Centers for Environmental Information-NOAA
    24. Sudden Ionospheric Disturbances (SIDs) -- Polar Cap Absorption (PCA) events AGSWS
    25. Travelling Ionospheric Disturbances (TIDs), AGSWS
    26. Geomagnetic storms ↑

    27. Geomagnetic storms Wikipedia
    28. Geomagnetic storms NOAA
    29. Geomagnetic storms Maine Emergency Management Agency
    30. A presentation: Solar Activity and HF Propagation Paul Harden, NA5N © QRP-ARCI – 2005 Pages 85-88 focus on the impact of geomagnetic storms on HF propagation
    31. The impact of geomagnetic storms on HF propagation Bing Search
    32. Space weather impact on radio wave propagation Norbert Jakowski, German Aerospace Center (DLR), Institute for Solar-Terrestrial Physics 2 Feb 2023
    33. Monitoring and forecasting of ionospheric space weather - effects of geomagnetic storms J. Lastovicka, Institute of Atmospheric Physics, Czech Republic
    34. Effect of magnetic storms (substorms) on HF propagation: A review D. V. Blagoveshchenskii, Geomagnetism and Aeronomy volume 53, pages 409–423 (July 2013)
    35. High-Frequency Communications Response to Solar Activity in September 2017 as Observed by Amateur Radio Networks AGU
    36. Effect of Weak Magnetic Storms on the Propagation of HF Radio Waves Kurkin, V. I. ; Polekh, N. M. ; Zolotukhina, N. A. (Feb 2022)
    37. HF Propagation during geomagnetic storms at a low latitude station Physics & Astronomy International Journal 2020
    38. Enhanced Trans-Equatorial Propagation following Geomagnetic storms Oliver P. Ferrell, Nature volume 167, pages 811–812 (1951)
    39. Space Weather agencies and their services

    40. The International Space Environment Service ISES
    41. National Oceanic and Atmospheric Administration NOAA
    42. NOAA Space Weather Prediction Center SWPC NOAA
        SWPC services:
      1. Real-time Space Weather Conditions R-S-G, EUV, CME, Aurora, GOES Flux (X-Ray, Proton), K-index
      2. Index of NOAA SWPC services: products, experimental, images, text data format, and json data format
      3. Solar Wind predicted, 3-hours to 7-days
      4. 3 day forecast R, S, G - Space Weather Scales
      5. SWPC Forecasts, Reports, Models, Observations, Summaries, Alerts, Experimental
      6. Weekly Highlights and Forecasts of Solar and Geomagnetic Activity
      7. Predict space weather for the next 24 hours based on recent indices
    43. About NOAA Space Weather Prediction Center Wikipedia
    44. A list of international space weather providers NOAA
    45. Canadian Space Agency CSA
    46. Space Weather Canada SWC
    47. The Embrace Program Brazil
    48. European Space Agency - Space Weather Service ESA
    49. Space Weather - Met Office UK
    50. Solar Influence Data Analysis Center Royal Observatory of Belgium
    51. Australian Space Weather Forecasting Center - Space Weather Services AGSWS
    52. Overview of the Australian Space Weather Alert System 2022 AGSWS
    53. Australian Bureau of Meteorology, Space Weather Services AGSWS
    54. Korean Space Weather Center RRA/KSWC
    55. Taiwan Space Weather Operational Office Central Weather Administration (CWA)
    56. South African National Space Agency (SANSA) SANSA
    57. Mission Space LEO
    58. Space Weather Canada
    59. World Meteorological Organiztion WMO
    60. American Commercial Space Weather Association ACSWA
    61. Space Weather Forecast National Institute of Information and Communications Technology, Japan NICT, ISES, RWC
    62. China-Russia Consortium Global Space Weather Center
  11. Recent Observations
                Solar > Space > Terrestrial Geomagnetic Indices, TEC Total Electron Content > Propagation Conditions

      Recent radio-ham records related to HF propagation

    1. Recent records of eSSN / eSFI ( 1y / 3m / 1m / 1w / 1d ) Andrew D Rodland, KC2G courtesy GIRO, NOAA NCEI, WWROF
    2. Live Ionospheric Data Paul L. Herrman, N0NBH presented by Meteorscan.com
        EVE - SDO EUV Variability Experiment; SEM: SOHO Solar EUV Monitor
    3. Ionogram Information Hamwaves - Serge Stroobandt, ON4AA
    4. Recent solar data and HF propagation QRZCQ
      • Effects of Solar Activity (SFI, SN, X-ray, A, Kp) in the last 36 hours
      • Solar Data History in the last 30 days
      • Current Propagation based on WSPRNet
      • Current Propagation based on DX Clusters
      • Propagation data in the last 30 days
    5. Live Solar Events Andy Smith, G7IZU
    6. Current Solar Conditions and Ham Radio Propagation W5MMW
    7. The K7RA Solar Update ARRL (Google Search)
    8. SolarHam Latest Imagery of Solar Watch and Alerts of Space Weather VE3EN
    9. Recent Solar Observations↑

    10. Solar and Heliospheric Observatory - SOHO ESA & NASA
    11. Extreme ultraviolet Imaging Telescope (EIT) Wikipedia
    12. Yohkoh Soft X-Ray Telescope Wikipedia
    13. ACE Real-time Solar Wind NOAA
    14. Recent 3 days: X-ray, Proton Flux, and Geomagnetic Activity NOAA
    15. R6 Army MARS: Consolidated Solar Weather Real-time Terrestrial indices due to Solar Weather Region 6 Army MARS
      Compare fof2 today, yesterday, 5 fays ago, at Austin TX, Eglin AFB, and Boulder, CO
      Unusual D-Region Absorption Patterms
      Recent 3 days K-index. Solar X-Ray Flux, GOES Magnetometer, Electgron and Proton Flux
    16. Space Weather Prediction Center - Index of images NOAA
    17. Analysis of Solar Activity affecting Space Weather Conditions

    18. Dr. Tamitha Skov - Space Weather Woman; Wikipedia, Youtube channel, facebook, Homesite
      Dr. Tamitha Skov is a space weather physicist that reviews solar storms and anlyzes how they affect spacewetaher. She specializes in forecasting and analyzing space weather processes in the heliosphere and exosphere. Her work extends to both traditional media and social platforms. As a credentialed space weather forecaster, she helps the public understand the effects of space weather.
    19. Real-time Space Weather Conditions

    20. Space Weather Conditions NOAA
    21. Space Weather Conditions AGSWS
    22. Live Space Weather Andy Smith, G7IZU
    23. Terrestrial - Geomagnetic Indices

    24. Current Space Weather Parameters Solar Terrestrial Dispatch
    25. Station K and A Indices for the last 30 days NOAA
    26. Magnetospheric MultiScale (MMS) Rice
    27. Real-time TEC - Total Electron Content (calculated) ↑

    28. Total Electron Content (TEC) Wikipedia
    29. TEC - Recent Theories, Methods and Models
    30. Near-real-time TEC maps ESA - Europen Space Weather Service
    31. TEC at Ionosphere Monitoring and Prediction Center ESA
    32. One-hour Forecast Global TEC Map DLR (ESA)
    33. Station List DLR (ESA)
    34. Archive of TEC DLR (ESA)
    35. North American TEC NOAA
    36. Near Real-time Global TEC Map AGSWS
    37. Global Ionosphere Map (GIM) SpringerLink
    38. Real-time MUF estimations using ionograms at different locations

    39. Ionosonde Station list UML - University of Massachusetts Lowell
    40. GIRO - Instrumentaion GIRO, UML
    41. About GIRO UML, Center for Atmospheric Research
    42. Real-time foF2 - Plots for Today, Yesterday and the past 5 days (more than 100 links to Inonosonde stations)NOAA
    43. Real-time Ionograms
    44. Recent Ionograms (Cyprus) University of Twente, Enschede, Netherlands
    45. Animated Ionograms Latest 24-Hour GIRO
    46. Ionosonde Stations Connected to NOAA NGDC, NOAA
    47. Ionogram Information Hamwaves - Serge Stroobandt, ON4AA
    48. Ionograms Research Development
    49. Small Form Factor Ionosonde Antenna Development Tyler Erjavec, The Ohio State University
    50. Observations of pole-to-pole, stratosphere-to-ionosphere connection MIT Haystack Observatory
    51. Ionospheric Density Irregularities, Turbulence, and Wave Disturbances during the Solar Eclipse over North America 21 August 2017 MIT Haystack Observatory
    52. Modeling Amateur Radio Soundings of the Ionospheric Response to the 2017 Great American Eclipse Nathaniel A. Frissell, W2NAF et al.
    53. HF Propagation Charts (Products of Ionograms)
    54. Current foF2 (NVIS) Propagation Map updated every 15 minutes Andrew D Rodland, KC2G
    55. Current MUF 3000 Km Propagation Map updated every 15 minutes Andrew D Rodland, KC2G
    56. Ionospheric Maps - Current foF2 Plots (Global) Australian Space Weather Forecasting Center (AGSWS)
    57. Hourly Area Predictions (HAP) Charts of selected regions AGSWS
    58. Current foF2 Plots (Asia & Australia) AGSWS
    59. Amateur Radio Usable HF Frequencies & Forecast Charts refresh every 20 minutes Remarkable Technologies, Inc.
    60. Global HF Propagation Andy Smith, G7IZU

  12. Forecast HF band conditions based on recent events
    1. Space Weather Forecast Discussion SWPC NOAA
    2. Radio Communications Dashboard SWPC NOAA ↑
    3. Predicted sunspot number and Radio Flux at 10.7 cm NOAA / NWS Space Weather Prediction Center ↑
    4. 27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices NOAA ↑
    5. Space weather for the previous and upcoming 24 hours, compiled from reports and dynamic propagation data Doug Brandon, N6RT
    6. Solar Flare Probabilities VE3EN
    7. 3 Day Geomagnetic and Aurora Forecast VE3EN
    8. Propagation Links eham.net
      NOAA Alerts, Observations, Scales Activity; DX Cluster WWV announcements, Propagation Links
    9. Radio blackout ↑
    10. Communications blackout Wikipedia
    11. Solar flares (radio blackouts) NOAA
    12. D-Region Absorption Prediction (D-RAP) SWPC NOAA

  14. Tools and Applications for analysing and forecasting HF propagation

    Apps Categories: Real-time Activity / Band Monitoring, real-time maps & Charts, Prediction Software, Mathematical models, etc.

    1. App-Category: Online Activity / Band Monitoring - gathering information of Real-time Hams' Activity and/or beacons
      1. Real-time Ham Band Activity Map Jon Harder, NG0E
      2. Analyzing Propagation From Active DX Stations DXLab
      3. Radio Propagation Maps Based on established contacts Andy Smith, G7IZU
    3. App-Category: Online Tools for HF Propagation Real-time Conditions (Charts, Raw Data)

        Real-time HF Propagation Tools

      1. HF-START - HF Simulator Targeting of All-users, Regional Telecommunications NICT, Japan
        HF-START - High Frequency Simulator Targeting for All-users’ Regional Telecommunications - is HF propagation simulator that is developed to meet the needs of space weather users for, but not limit to telecommunications: real-time info, web tools, about
      2. HF Propagation Tools Hamwaves - Serge Stroobandt, ON4AA
        Real-time online dashboard of solar activity influencing HF propagation on Earth.
      3. Real‑time HF propagation space weather Hamwaves - Serge Stroobandt, ON4AA
        Real-time online dashboard of solar activity influencing HF propagation on Earth.
      4. Propagation Banners

      5. Add Solar-Terrestrial Data to your Website HamQSL , Paul L Herrman, N0NBH
      6. Real-time Maps & Charts

      7. MUF 3000 Km Map based on Real-time measurements Andrew D Rodland, KC2G ↑
        * Read more about the MUF (3000 km) project
        * Read a review titled: "Developing an Open-Source HF Propagation Prediction Tool".
        Roland Gafner, HB9VQQ extended this project with an excellent Animated Map viewing the last 24 hours, in 15 minutes steps.
      8. HamDXMap for the DXer, radio propagation concepts Christian Furst, F5UII
    4. App-Category: Prediction Software (Calculators using various models)
      1. An Open-Source IRI-based Nowcasting Tool for Ionospheric Electron Density and HF Propagation Andrew D Rodland (2022 Harvard Abstracts)
        An overview of the software and the models behind prop.kc2g.com, a website using the IRI-2016 model, conditioned on near-real-time ionosonde data, to provide global maps of MUF(3000) and foF2. While primarily designed for radio amateur use, this system is useful for nowcasting of F region ionospheric density and mesoscale low elevation HF propagation characteristics.
      2. The Advanced Stand Alone Prediction System (ASAPS) AGSWS
        Australian Space Weather Forecasting Center offer three software products to predict HF propagation:
        1. GWPS - designed for HF operators working in defence and emergency services
        2. ASAPS Kernel - The Advanced Stand Alone Prediction System designed for government, defence and emergency services
        3. Consultancies - designed for industry, defence and emergency services
      3. S/N HF Propagation Forecast Calculator for the current month DL0NOT
      4. DR2W - Predict Propagation Conditions DK9IP (Winfried), DH3WO (Wolfgang), DJ2BQ (Ewald), ZS1AO/DJ2HD (Mathew)
        A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies
      5. VOACAP Online Application for Ham Radio Jari Perkiömäki, OH6BG / OG6G
        VOACAP forecats monthly average of the expected reliability with diurnal and seasonal variations.
        A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies.
      6. VOACAP Quick Guide Jari Perkiömäki, OH6BG / OG6G
      7. VOACAP Shortwave Prediction Software Rob Wagner VK3BVW
      8. How to use VOACAP - Part 1: Overview, Part 2, Part 3 Jari OH6BG & OH7BG Raisa
      9. VOACAP Charts for RadCom VOACAP
      10. Proppy Online - HF Propagation Prediction James Watson, M0DNS
      11. RadCom online Propagation Prediction Tools RSGB
      12. Ionospheric Characterisation Analysis and Prediction tool (IOCAP) SANSA
      13. IOCAP Application Introduction Video SANSA
        The South African National Space Agency (SANSA) created i/o cap Primary Work Surface, an operational HF communication solution.
        It's a modern, user-friendly HF frequency prediction tool that's simple to use and accurate. In a software program, it blends space weather research and practical HF experience.
      14. DX Toolbox - Shortwave / Ham Radio / HF Radio Propagation Black Cat Systems
        This is a software application that provides a range of tools for HF radio operators, including propagation forecast based on the Solar Terrestrial Dispatch (STL) model.
        It also includes a real-time solar data display and a grayline map.
      15. HF Propagation (Google Play) Android Package Kit
      16. HF Propagation (Microsoft Apps) Stefan Heesch, HB9TWS
      17. Proplab-Pro v3: Review eHam Manual spacew.com
        Proplab-Pro 3.2 (Build 45, March 2023) Three-dimensional ray-tracing ionosphere; can run as standalone; not free.
      18. PROPHF v1.8, HF Propagation predictions Christian, F6GQK
      19. W6ELProp (2002) Sheldon C. Shallon, W6EL Rated: 7.56 by DxZone
        Predicts skywave propagation between any two locations on the earth on frequencies between 3 and 30 MHz
      20. HamCAP (VOACAP interface) by Alex Shovkoplyas, VE3NEA. Rated 8.93 by DxZone
      21. The Propagation Software Pages A collection of links AC6V
    5. App-Category: Overviews and Reviews of prediction software
      1. What can we expect from a HF propagation model? Luxorion Dynamic processes relevant to HF radio propagation are simulated using mathematical models, and numerical procedures.
        Interactions between the Sun's surface and the Earth's surface are considered using sun, space weather, ionosphere,
        and atmosphere models, all of which can aid in the prediction of HF radio propagation.
      2. Evaluation of various models for HF propagation prediction SANSA Space Science
      3. Review of HF Propagation analysis & prediction programs Research Oriented Luxorion Some of these propagation programs are only accessible via the Internet via a web interface and provide graphical solutions.
        Amateurs have also created small applications that simulate various ionospheric effects.
        Using either near-real-time data or well-known functions, the majority of them achieve extremely high accuracy.
      4. Review of Propagation prediction programs - VOACAP-based Luxorion The VOACAP propagation prediction engine is the result of decades of US government-funded HF propagation research
        stretching back to the dawn of computing. While VOACAP's forecasting capability has been continuously improved
        as knowledge about HF propagation has increased, its software technology is firmly rooted in the 1980s.
      5. Predicting and Monitoring Propagation DXLab
        * Solar terminator display and prediction - shows greyline at any specified date and time
        * Propagation prediction - provides a graphical view of openings by frequency and time using your choice of the included
        VOACAP, ICEPAC, and IONCAP forecasting engines.
      7. PropView DXLab Rated 8.27 by The DXZone PropView uses the included VOACAP, ICEPAC, and IONCAP propagation prediction engines to forecast
        the LUF and MUF between two locations over a specified 24 hour period.
        Results are rendered in an easy-to-understand color-graphic display.
        You can specify locations via direct latitude/longitude entry.
        Alternatively, PropView interoperates with DXView to allow location selection via DXCC prefix entry
        or by clicking on locations on a world map. It can:
        (1) build schedules for the IARU/HF beacon network and automatically QSY your transceiver to monitor each scheduled beacon.
        (2) monitor the NCDXF/IARU International Beacon Network to assess actual propagation and compare it with forecast propagation.
        Beacon schedules can be assembled by band, by location, or by bearing from your QTH.
        PropView interoperates with Commander and DXView to automatically QSY your transceiver
        to hear each beacon in your schedule, and to display the location of the current beacon.
      8. Propagation prediction software for ham radio DxZone RF prop, Radio Propagation & Diffraction Calculator, W6ELProp, PropView, HamCAP
      9. Radio Propagation Forecasting (2019) Basu, VU2NSB Beacons, VOACAP, CCIR and URSI Models
    7. App-Category: Mathematical models / Numerical procedures based on solar activity, space wather, Earth Magnetic field, ionospheric models, ray-tracying taking into account the time of day:
      1. ITU-R Directory
      2. ITU-R P.533 model
        This is an ITU table links to Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise in a wide range of propagation conditions. The ITUR HF Prop experimental software, was written by G4FKH and HZ1JB, and is based on the ITU-R P.533 method.
        It uses a probabilistic approach to estimate radio coverage with algorithms that are supposed to be more accurate than other similar programs.
      3. Mathematical Models of Space Weather NASA
      4. Space Weather Modeling Framework (SWMF)
      5. Global Assimilation of Ionospheric Measurements (GAIM) model
      6. Advanced D-region Ionosphere Prediction System (ADIPS)
    9. App-Category: Ray-tracing models based on frequency, angle of incidence, and electron density profiles of the ionosphere.
      1. IONCAP - Ionospheric Communications Analysis and Prediction HF transmission prediction program for US military and other applications since 1986.
        It was based on Automatic Link Establishment (ALE) Frequency Selection for a Ten-Node Australian High-Frequency Network.
        This program was clumsy, slow, and complicated, as it only allowed users with a sufficient background in ionospheric physics
        and computer data entry experience to use it. A new version was developed to fix these flaws while also improving capability
        to the point where it could be used by a layperson.
      2. ITUR HF Prop
        Prediction of HF circuits based on Recommendation ITU-R P.533 model - an improved (2017) point-to-point propagation prediction tool, based on an ITU engine, developed by Gwyn Williams, G4FKH.
      3. VOACAP (Voice of America Coverage Analysis Program)
        VOACAP forecats monthly average of the expected reliability with diurnal and seasonal variations,
        but it does not account for unpredicted ionospheric and magnetic disturbances or anomalies,
        i.e. what are the expected variations of A-index, K-index, and energy densities of solar proton / electron flux, etc.
    11. App-Category: Neural network models:
      These models use machine learning techniques to predict the behavior of radio waves based on input data.
      They may take into account factors such as solar activity, geomagnetic conditions, and the time of day.
      The HF Ionospheric Prediction and Solar Terrestrial Data Center uses machine learning algorithms to predict ionospheric conditions.
      1. Neural Network Ionospheric Model (NNIM)
    12. App-Category: Hybrid methods: Integration of several methods to provide predictions
      1. Application of Machine Learning Techniques to HF Propagation Prediction Richard Buckley, William N. Furman - Rochester, NY

  16. Misc. References Definitions, cross-disciplinary research etc.

      Our hobby

    1. Amateur Radio Wikipedia
      The name of the hobby "Amateur radio" refers to a non-commercial communication, wireless experimentation, self-training, private recreation,
      radiosport, contesting, and emergency communications activity that may use radio transmitters and receivers.
    2. Amateur radio station Wikipedia
      Read about different types of stations used by amateur radio operators.
    3. Radio Amateur Wikipedia
      "Radio Amateur" is the person usualy a licensed operator who communicates with other radio amateurs on amateur radio frequencies.
    4. Shortwave listening (SWL) Wikipedia
      Shortwave listening, or SWLing, is the hobby of listening to shortwave radio. See for example OfficialSWLchannel a dedicated youtube channel for shortwave listeners.
    5. Shortwave radio Wikipedia
    6. AI applications used to revise this website

    7. Initial chats with ChatGPT 3.5
    8. ChatGTP 4o
    9. Quillbot
    10. The HF Bands assigned for Radio Amateurs

    11. Amateur Radio Band Characteristics Ham Universe
    12. Ham Radio Bands DXZone
    13. Special articles by Bob Brown, NM7M (SK), Ph.D. U.C.Berkeley

    14. The Little Pistol's Guide to HF Propagation (1996) Bob Brown
    15. HF Propagation Tutorial Bob Brown (SK), NM7M
      "There is a lot of information out there on the Internet; but what about understanding?" ↑
    16. WARC bands Wikipedia
    17. Communication Modes and Techniques

    18. FT8 Wikipedia
    19. Digital Voice (DV)
    20. Digital Voice the Easy Way 2023 QST
    21. FreeDV: Open Source Amateur Digital Voice 2023FreeDV
    22. A Guide to Digital Voice on Amateur Radio April 2021 VK3FS
    23. How to Use FreeDV Digital Voice Over HF Ham Radio Dec 2020Ham Radio Crash Course
    24. Using FreeDV To Talk On Digital HF 80M Oct 2019 Tech Minds
    25. RSGB 2018 Convention lecture: FreeDV - Digital Voice for HF and other low SNR channels Sept 2019 RSGB
    26. Digital Voice on HF 2013 G4ILO
    27. Will digital voice (on HF) ever be a thing? 2018Dan, KB6NU
    28. International Digital Audio Broadcasting Standards: Voice Coding and Amateur Radio Applications 2003 QEX
    29. Practical HF Digital Voice June 2000 G4GUO, G4JNT , QEX
    30. Automatic link establishment (ALE)

    31. Automatic link establishment (ALE) Wikipedia
      ALE is a (military or commercial) feature in an HF communications radio transceiver system that enables the radio station to make contact, or initiate a circuit, between itself and another HF radio station or network of stations. The purpose is to provide a reliable rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference, and shared spectrum use of busy or congested HF channels.
    32. Automatic Link Establishment (ALE) Demo with SCS P4Dragon Modem Commsprepper (Nov 2021)Wikipedia
    33. Automatic Link Establishment Overview 2018
    34. HF Automatic Link Establishment (ALE) Kingston Amateur Radio Club (2009)
    35. ALE HF Network Ham Radio Amateur Radio Bonnie Crystal, KQ6XA, HFLINK (2007)
    36. ALE - The Coming Of Automatic Link Establishment Ronald E. Menold, AD4TB, QST 1995
    37. Spread Spectrum
    38. Spread Spectrum Wikipedia
    39. Frequency-hopping spread spectrum Wikipedia
    40. Science and technology-related terms

    41. Ecplictic Plane Wikipedia ↑
    42. Geometrical Optics Wikipedia ↑
    43. Secant Wikipedia ↑
    44. Earth Magnetic field Wikipedia ↑
    45. Physical Coupling Wikipedia
    46. Satellite Wikipedia
    47. The Lagrange Mission Wikipedia ↑
    48. Lagrange points (Google Search) ↑
    49. Aurora
    50. Aurora NOAA / NWS Space Weather Prediction Center
    51. Aurora forecast NOAA / NWS Space Weather Prediction Center
    52. Aurora Wikipedia
    53. The Auroral E-region is a Source for Ionospheric Scintillation EOS
    54. The auroral E-region ionization and the auroral luminosity Omholt, A. (1955)
    55. Auroral Effects on the Ionospheric E-Layer Omholt, A. (1965)
    56. Diffuse Auroral Electron and Ion Precipitation Effects on RCM-E Comparisons With Satellite Data During the 17 March 2013 Storm JGR Space Physics 2019 - Chen, Lemon, Hecht, Sazykin, Wolf, Boyd, Valek
    57. 7 Magical Places to View Auroras National Geographic
    58. Deterministic Chaos ↑
    59. Deterministic Chaos The Exploratorium, 1996
      Deterministic chaos refers in the world of dynamics to the generation of random, unpredictable behavior from a simple, but nonlinear rule. The rule has no "noise", randomness, or probabilities built in. Instead, through the rule's repeated application the long-term behavior becomes quite complicated. In this sense, the unpredictability "emerges" over time.
    60. Deterministic Chaos Principia Cybernetica 2000
      A system is chaotic if its trajectory through state space is sensitively dependent on the initial conditions, that is, if unobservably small causes can produce large effects.
    61. Concepts: Chaos New England Complex Systems Institute
      A chaotic system is a deterministic system that is difficult to predict. A deterministic system is defined as one whose state at one time completely determines its state for all future times.
    62. HF Propagation Research 1958-1990

    63. Basic Radio Propagation Predictions for September 1958, Three Months in Advance National Bureau Of Standards
    64. Ionospheric Radio Propagation 1965 (replaced an obsolete pubication of 1948) Kenneth Davies, National Bureau Of Standards
    65. Solar-Terrestrial Prediction Proceedings | Solar-Terrestrial Prediction Proceedings 1979 Richard F. Donnelly, Space Environment Lab, NOAA
    66. Ionospheric Radio (book 1990) Kenneth Davies
    67. HF Propagation - Novel Research and Analysis

    68. Monitoring and forecasting of ionospheric space weather - effects of geomagnetic storms Jan Lastovicka (2002)
    69. Short and long term prediction of ionospheric HF radio propagation J. Mielich und J. Bremer (2010)
      A modified ionospheric activity index AI has been developed on the basis of ionospheric foF2 observations.
      Such index can be helpful for an interested user to get information about the current state of the ionosphere. Using ionosonde data.
    70. Spread-F occurrences and relationships with foF2 and h′F at low and mid-latitudes in China (2018) Wang, Guo, Zhao, Ding & Lin (Chaina)
       appear as scattered echoes in high-frequency (HF) band ionograms that are known as spread-F events that include frequency spread-F (FSF), range spread-F (RSF), and mixed spread-F (MSF) events.
    71. Long-Term Changes in Ionospheric Climate in Terms of foF2 Jan Lastovicka (2022)
      There is not only space weather; there is also space climate. Space climate includes the ionospheric climate, which is affected by long-term trends in the ionosphere.
    72. Ionospheric Monitoring and Modeling Applicable to Coastal and Marine Environments Ljiljana R. Cander and Bruno Zolesi (2019)
    73. Statistically analyzing the ionospheric irregularity effect on radio occultation M. Li and X. Yue, Atmos. Meas. Tech., 14, 3003–3013, 2021
    74. Analysis of Ionospheric Disturbance Response to the Heavy Rain Event Jian Kong, Lulu Shan, Xiao Yan, Youkun Wang - Remote Sens. 2022, 14(3), 510
    75. A simplified HF radio channel forecasting model Advances in Space Research, Volume 69, Issue 6, 15 March 2022, Pages 2477-2488, Moskaleva, Zaalov
      Estimating the maximum electron density in the ionosphere in relation to the critical frequency of the ionospheric F2 layer (foF2). The suggested forecasting model estimates the electron density profile one day ahead. Comparisons of observed and predicted vertical ionograms indicate the technique's utility.
    76. Ionospheric current Upper Atmospheric Science Division of the British Antarctic Survey
    77. Radio Propagation Prediction for HF Communications (2018) Dept. of Appl/ Physics & Tel., Midlands State Univ., Gweru, Zimbabwe
    78. A Preliminary Systematic Study of HF Radio Propagation from a Source in the Subarctic
      Using HAARP and the Ham WSPR Network (2018)
      Citizen Space Science, Fallen, C. T.
    79. The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocation M. Warrington, A.J. Stocker, N. Zaalov (2002)
    80. Analyzing the current ionospheric conditions Google search
    81. Recent Theories, Methods and Models

    82. Develop ionosphere computer models to enhance HF radio propagation Military Aerospace 2022
      Develop new ways to model the ionosphere in real time to help predict the propagation of high-frequency (HF) radio waves for improved communications and sensing.
    83. Recommendation: Ionospheric Characteristics And Methods Of Basic MUF, Operational MUF AND Ray-Path Prediction ITU 1995
    84. Recommendation: Propagation Factors Affecting Frequency Sharing In HF Terrestrial Systems ITU 1994
    85. Recommendation: HF propagation prediction method ITU 2001
    86. Comparison of observed and predicted MUF(3000)F2 in the polar cap region Radio Science AGU (2015)
      Comparison of ICEPAC, VOACAP, and REC533 models reveal diurnal and seasonal variations.
      Summer diurnal variation is not represented by the VOACAP or ICEPAC models.
      REC533 surpasses VOACAP during the winter and equinox months.
      ICEPAC performs poorly during periods of low solar activity.
    87. Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication
      Based on Oblique- and Vertical-Incidence Sounding Data (2022)
      atmosphere MDPI
      MOF were compared to predicted MUF. The INGV model outperformed for MUF prediction over Beijing
      and its adjacent mid-latitude regions, according to the root-mean-square error comparison.
    88. Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication
      Based on Oblique- and Vertical-Incidence Sounding Data (2022)
      atmosphere MDPI
      MOF were compared to predicted MUF. The INGV model outperformed for MUF prediction over Beijing
      and its adjacent mid-latitude regions, according to the root-mean-square error comparison.
    89. Use of electron density profiles in HF propagation assessment: part 1- Requirements, prediction and forecasting (1991) Advances in Space Research Journal
    90. ITM Processes

    91. Terrestrial Atmosphere ITM (Ionosphere, Thermosphere, Mesosphere) Processes NASA Visualization (2018)
    92. Detection of Rapidly Moving Ionospheric Clouds H. Wells, J. M. Watts, D. George (1946)
    93. Three-dimensional simulation study of ionospheric plasma clouds S. Zalesak, J. Drake, J. Huba (1990)
    94. Nonlinear Three-Dimensional Simulations of the Gradient Drift and Secondary Kelvin–Helmholtz Instabilities in Ionospheric Plasma Clouds Almarhabi, Lujain & Skolar, Chirag & Scales, Wayne & Srinivasan, Bhuvana (2023)
    95. Articles about "Ionospheric Plasma Clouds" Google search
    96. Vertical Coupling (Troposphere - Ionosphere)

    97. Sprite (lightning) Wikipedia
    98. Upper-atmospheric lightning Wikipedia
    99. Transient Luminous Events: Lightning above our atmosphere AccuWeather
    100. NASA ScienceCasts: Observing Lightning from the International Space Station NASA
    101. Severe Weather 101: Lightning Types NOAA
    102. Transient Luminous Events (TLEs) SKYbrary
    103. Investigations of the Transient Luminous Events with the small satellites, balloons and ground-based instruments Safura Mirzayeva 2022 Master Thesis
    104. Stunning jellyfish sprite seen in the night sky during Texas storm Aug 2020 The Weather Network
    105. Solar cycle changes to planetary wave propagation and their influence on the middle atmosphere circulation (1997) Arnold & Robinson, Annales Geophysicae, vol. 16, no. 1, pp. 69–76, 1998
    106. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2011) International Journal of Geophysics
    107. A review of vertical coupling in the Atmosphere-Ionosphere system:
      Effects of waves, sudden stratospheric warmings, space weather, and of solar activity
      (2015) Journal of Atmospheric and Solar-Terrestrial Physics
    108. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2020) University of Lucknowת India
    109. A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms (2012) NASA
    110. Influence of lightning on electron density variation in the ionosphere (2015) University of Cape Town, Dsc Thesis
    111. TEC variations detected over southern Africa due to lightning storms published by South African National Space Agency
    112. Statistical Study of Global Lightning Activity and Thunderstorm-Induced Gravity Waves in the Ionosphere (2023) Swati Chowdhury, New Zeeland
    113. Eyes on the Earth Earth's Weather / Tropospheric Weather NASA/JPL

↑ Index of Terms

  1. A glossary of basic terms
  2. Index of terms referred to this website


↑    Last but not least:

The world is changing as the radio amateur spectrum is being sold off to commercial users since few amateurs operate SHF and above.

We did, however, gain new narrow bands in the short, medium, and long wave bands. It may not be enough, but it opens up new avenues for communication improvement that do not rely on commercial infrastructure.

If you have comments, questions or requests please e-mail.

73 de Doron

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