HF radio propagation:
Scientific and Technical Concepts You Should Know

This page is part of the project "Understanding HF Propagation."
by Doron Tal, 4X4XM

HF (High Frequency) radio propagation, used for long-distance communication, involves several key concepts and terminology. Knowing this terminology can help you understand long-distance communication via skywaves.

Find below a glossary of basic terms and an extended index of terms, all related to HF radio propagation:

A glossary of basic terms

Waves and radio propagation

Frequency (f) is the number of cycles of a radio wave that occur in one second, measured in hertz (Hz).
Wavelength (λ) is the distance between two consecutive points in a wave, usually measured in meters (m).
The relationship between wavelength and frequency is inverse, with higher frequencies corresponding to shorter wavelengths and vice versa.
Groundwave propagation is the use of radio waves that travel along the earth's surface, providing communication over relatively short distances, typically up to a few hundred kilometers.
Skywave propagation is the phenomenon in which HF radio waves are bounced back to earth's surface, via the ionosphere, allowing long-distance communication beyond the line of sight.
The ionosphere is the region of the earth's upper atmosphere (approximately 50 to 800 kilometers above the ground) where ionization occurs due to solar radiation. This region plays a crucial role in HF radio propagation by bouncing radio waves back to Earth. The ionosphere is continuously changing due to natural storms, as explained below:
A storm represents a disturbed state of the natural environment, characterized by significant disruptions to normal conditions. In this project, we specifically refer to:
1. Solar storms occur on the sun.
2. Space weather storms occur in the space between the sun and Earth.
3. Geomagnetic storms (also known as magnetic storms) are temporary disturbances of the Earth's magnetosphere caused by a solar wind shock wave.
4. Ionospheric storms involve varying densities of energized free electrons in the ionosphere due to geomagnetic storms.
Skip distance is the minimum distance from the transmitter for receiving skywaves, while blind, or dead zone, refers to the region with poor or nonexistent reception.
MUF (Maximum Usable Frequency) is the highest frequency at which skywave propagation is reliably achievable between two points on earth during a specific time and under specific ionospheric conditions.
LUF (Lowest Usable Frequency) is the lowest frequency at which skywave propagation can be used effectively.
Fading (QSB) is the variation in signal strength and quality experienced during radio transmission due to changes in ionospheric conditions, interference, or other factors.
The F-region is the upper region of the ionosphere that consists of two sub-regions, F1 and F2, and is the primary region for HF propagation. The F2 is particularly important for long-distance communication.
The E-region is the middle region located below the F-region. It is responsible for sporadic E propagation, which can enable HF communication at higher frequencies.
The D-region is the lowest region of the ionosphere, located at an altitude of approximately 48 to 90 kilometers. The D-region absorbs HF radio waves during daylight hours, affecting propagation.
The ionosphere bounces a radio wave at vertical incidence below the critical frequency (fc), regardless of transmitter power.
The Optimum Working Frequency (OWF) is the frequency that provides the best propagation conditions for a given HF communication path.
The Solar Flux Index (SFI) is a measure of the sun's radio emissions at a specific wavelength (usually 10.7 cm). It is used as an indicator of solar activity, which affects ionospheric conditions and HF propagation.
The A-index is a daily measure of geomagnetic activity on a scale from 0 to 400, with lower values indicating quieter geomagnetic conditions. Geomagnetic activity can disrupt HF propagation.
The K-index represents short-term (3-hour) geomagnetic activity at a specific geomagnetic station. It quantifies disturbances in Earth’s horizontal magnetic field by comparing geomagnetic fluctuations, measured by a magnetometer, to a quiet day. The K-scale is logarithmic, a scale from 0 to 9.