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Main Page - Marcus Martins / Minas Gerais / São Paulo / BRASIL - Portuguese
Understanding better the Propagation
(Reproduction of this text is authorized provided the holder is preserved and mentioned - Copyright : Marcus Martins - PY4SM / PY2DD) 


The table above shows the propagation conditions for different bands - HF and VHF. It also shows information and data indexes A, K, Solar flow and other useful information in order to assess the possibilities of contacts to short or long - distance DX. Let us understand the main and most important information and data shown in the table:


SFI : Shows the Solar Flux Index ( amount of incoming energy from the sun ). The higher the value the better the conditions for doing DX.


60 a 120 Bad
120 a 180 Moderate
180 a 240 Good
Above 240 Very Good

SN : Counting sunspots  - This value increases when we are next solar maximum (every 11 years).


A - Index : Indicates the amount of magnetic disturbances in the ionosphere. For values, see table below.

K - Index : Indicates the geomagnetic activity in the ionosphere based on the index and refers to behavior in the last 3 hours.


A K Magnetic Field
0 - 3 0 Quiet (Quieto)
4 - 6 1 Quiet to unsettled (Quieto a perturbado)
7 - 14 2 Unsettled (Perturbado)
15 - 47 3 - 4 Active (Ativo)
48 - 79 5 Minor storm-Pequena tempestade
80 - 131 6 Major storm-Grande tempestade
132 - 207 7 Severe storm-Tempestade severa
208 - 400 8 - 9 Very major storm-Tempestade muito severa


X - Ray: Indicates possible "blackouts" in the shortwave bands.

304A : ultraviolet quantity emitted by the sun.

Ptn Flx and Elc Flx : Protons and Electrons flow. Show the possibilities of occurrence of "solar radiation storms".

Aurora : Possible geomagnetic storms. Values ​​ranging from 5 to 10 and higher, worse for DX.

Conditions of radio communications HF and VHF bands : Poor (bad), Fair (average), Good (good).

Geomag Field : Field Condition Geomagnetic Earth.

Sig Noise Lvl : Relationship noise signal.

Current Solar-Terrestrial Data Atual Solar
Terrestrial dados
Category Categoria Radio Blackouts Rádio Blackouts
Use X-Ray Use X-Ray
Solar Radiation Storms Tempestades de radiação solar
Use Proton Flux Use Proton Flux
Geomagnetic Storms Tempestades geomagnéticas
Use K-Index/K-nT/ Aurora/Solar Wind/Bz Use K-Index/K-nT / Aurora / Solar Wind / Bz
Band Openings Aberturas Banda
Use Solar Flux (SN) Use Flux Solar (SN)
Electron Alert Electron Alerta
Use Electron Flux Use Electron Flux
Extreme Extremo X20 (1 per cycle) X20 (1 ciclo por)
Complete HF blackout on entire sunlit side lasting hours Apagão HF completas sobre horas lado iluminado duradouras inteiras
1000000 (1 per cycle) 1000000 (1 ciclo por)
Complete HF blackout in polar regions Apagão HF completo em regiões polares
K=9 (nT=>500) [Aur=10++] (SW=>800) [Bz=-40 -50] K = 9 (nT => 500) [Aur = 10 + +] (SW => 800) [Bz = -40 -50]
(4 per cycle) (4 por ciclo)
HF impossible. HF impossível. Aurora to 40°. Aurora a 40 °. Noise S30+. Noise S30 +.
200-300 (SN=160-250) 200-300 (SN = 160-250)
Reliable communications all bands up through 6m Comunicações confiáveis ​​todas as bandas através 6m
>1000 Alert > 1000 Alerta
Partial to complete HF blackout in polar regions Parcial para completar apagão HF nas regiões polares
Severe Grave X10 (8 per cycle) X10 (8 ciclo por)
HF blackout on most of sunlit side for 1 to 2 hours Escurecimento HF em mais do lado iluminado durante 1 a 2 horas
100000 (3 per cycle) 100000 (3 ciclo por)
Partial HF blackout in polar regions HF apagão parcial nas regiões polares
K=8 (nT=330-500) [Aur=10+] (SW=700-800) [Bz=-30 -40] (100 per cycle) K = 8 (nT = 330-500) [Aur = 10 +] (SW = 700-800) [Bz = -30 -40] (100 por ciclo)
HF sporadic. HF esporádico. Aurora to 45°. Aurora para 45 °. Noise S20-S30. Noise S20-S30.
Strong Forte X1 (175 per cycle) X1 (175 por ciclo)
Wide area HF blackout for about an hour on sunlit side Ampla área apagão HF por cerca de uma hora no lado iluminado pelo sol
10000 (10 per cycle) 10000 (10 por ciclo)
Degraded HF propagation in polar regions Propagação HF degradadas em regiões polares
K=7 (nT=200-330) [Aur=10] (SW=600-700) [Bz=-20 -30] K = 7 (nT = 200-330) [Aur = 10] (SW = 600-700) [Bz = -20 -30]
(200 per cycle) (200 por ciclo)
HF intermittent. Intermitente IC. Aurora to 50°. Aurora a 50 °. Noise S9-S20. Ruído S9-S20.
150-200 (SN=105-160) 150-200 (SN = 105-160)
Excellent conditions all bands up through 10m w/6m openings Excelentes condições todas as bandas se através de aberturas w/6m 10m
Moderate Moderado M5 (350 per cycle) M5 (350 por ciclo)
Limited HF blackout on sunlit side for tens of minutes Apagão HF limitada no lado iluminado por dezenas de minutos
1000 (25 per cycle) 1000 (25 por ciclo)
Small effects on HF in polar regions Pequenos efeitos sobre HF em regiões polares
K=6 (nT=120-200) [Aur=9] (SW=500-600) [Bz=-10 -20] K = 6 (nT = 120-200) [Aur = 9] (SW = 500-600) [Bz = -10 -20]
(600 per cycle) (600 por ciclo)
HF fade higher lats. HF desaparecer lats superiores. Aurora to 55°. Aurora a 55 °. Noise S6-S9. Noise S6-S9.
120-150 (SN=70-105) 120-150 (SN = 70-105)
Fair to good conditions all bands up through 10m Feira de boas condições de todas as bandas através 10m
<1000 Active <1000 atividade
Degraded HF propagation in polar regions Propagação HF degradadas em regiões polares
Minor Menor M1 (2000 per cycle) M1 (2000 por ciclo)
Occasional loss of radio contact on sunlit side Perda ocasional de contato de rádio no lado iluminado pelo sol
100 (50 per cycle) 100 (50 por ciclo)
Minor impacts on HF in polar regions Impactos menores sobre HF em regiões polares
K=5 (nT=70-120) [Aur=8] (SW=400-500) [Bz=0 -10] K = 5 (nT = 70-120) [Aur = 8] (SW = 400-500) [Bz = 0 -10]
(1700 per cycle) (1700 por ciclo)
HF fade higher lats. HF desaparecer lats superiores. Aurora to 56°. Aurora a 56 °. Noise S4-S6. Ruído S4-S6.
90-120 (SN=35-70) 90-120 (SN = 35-70)
Fair conditions all bands up through 15m Condições justas todas as bandas através 15m
<100 Active <100 atividade
Minor impacts on HF in polar regions Impactos menores sobre HF em regiões polares
Active Ativo C1 Moderate Flare C1 alargamento Moderado
Low absorption of HF signals Baixa absorção de sinais de HF
10 Active 10 visita
Very minor impacts on HF in polar regions Impactos muito menores em HF em regiões polares
K=3-4 (nT=20-70) [Aur=6-7] (SW=200-400) [Bz=0-+50] Unsettled/Active K = 3-4 (nT = 20-70) [Aur = 6-7] (SW = 200-400) [Bz = 0-50] Unsettled / Ativo
Minor HF fade higher lats. Menor HF desaparecer lats mais elevados. Aurora 60-58°. Aurora 60-58 °. Noise S2-S3. Noise S2-S3.
70-90 (SN=10-35) 70-90 (SN = 10-35)
Poor to fair conditions all bands up through 20m Pobre de condições justas todas as bandas através 20m
<10 Normal <10 normal
No impacts on HF Não há impactos sobre HF
Normal Normal A1-B9 No/Small Flare A1-B9 Não / Pequeno Alargamento
No or very minor impact to HF signals Não ou impacto muito menor IC sinais
1 Normal 1 normal
No impacts on HF Não há impactos sobre HF
K=0-2 (nT=0-20) [Aur=<5] (SW=200-400) [Bz=0-+50] Inactive/Quiet K = 0-2 (nT = 0-20) [Aur = <5] (SW = 200-400) [Bz = 0-50] Inativo / Quiet
No impacts on HF. Não há impactos sobre IC. Aurora 67-62°. Aurora 67-62 °. Noise S0-S2. Ruído S0-S2.
64-70 (SN=0-10) 64-70 (SN = 0-10)
Bands above 40m unusable Bandas acima 40m inutilizáveis
<1 Normal <1 Normal
No impacts on HF Não há impactos sobre HF



The radio waves such as light and other forms of electromagnetic radiation usually propagate in a straight line. Of course this happens not always, because the long-distance communications are made beyond the horizon. The way radio waves propagate without being straight, is a complicated subject, but is hardly a mystery.


The radio belongs to the family of "Electromagnetic radiation" includes infrared, visible light, ultraviolet, X-rays, gamma rays and cosmic. The radio has the longest wavelength of the group, and the lowest frequency. Electromagnetic waves result from the interaction between an electric field and a magnetic field. An electric charge which oscillates in a conductor creates an electric field and the corresponding magnetic field. The magnetic field in turn creates an electrical field, creating another magnetic field, and so on. These two fields interact creating an electromagnetic wave that propagates in space. The components electric and magnetic are between them a right angle 90 ° to the direction of propagation. The polarization of a radio wave is generally the same as its electric field.


Speed ​​- Radio waves as all other forms of electromagnetic radiation propagate approximately 300,000 km per second in vacuum. The radio waves travel more slowly through any other means. The decrease in speed in the atmosphere is so slight that it is generally ignored, but sometimes even this small difference is significant. In turn, the propagation velocity of a conductor is about 95 % the speed in vacuum. The speed of a radio wave frequency is always the wavelength of the product, whatever the environment. The relationship can be summarized by:


C = FL  where:
C = speed in meters / second

F= frequency in Hertz

L = wavelength in meters


 => The wavelength ( L ) of any radio frequency can be determined by this formula  


In a vacuum, where speed is 300 000 000 m / s, the wavelength of a radio signal with a 30 MHz frequency is 10 meters. In other propagating means the wavelength decreases because the velocity of propagation is smaller. In a thread, the wavelength of a 30 MHz signal decreases to 9.5m. This factor should be taken into account in antenna design and other applications


Attenuation and Absorption - Radio waves weaken as they propagate, is almost empty in the cosmos or in the atmosphere. The attenuation of the vacuum is a result of dispersion of energy from the source. Attenuation grows rapidly because the signal decreases in the ratio of the square of the distance. If the distance between the transmitter and the receiver to increase from 1 km to 10 km the signal intensity will decrease to a hundredth. The attenuation dispersion is an important factor in signal intensity, but the radio signals suffer from other types of attenuation.


Energy absorption is lost when the radio waves pass through other means than vacuum. Radio waves propagate through the atmosphere or solid materials (such as a thread) exciting electrons that will radiate on the same frequency. This process is not perfectly efficient, so some energy is transformed into heat and retained in the middle. The amount of energy lost in this way depends on the characteristics of the medium and the frequency. The attenuation is negligible in the atmosphere of 10MHz to 3 GHz, but at higher frequencies the absorption due to water vapor and oxygen can be increased. The energy of the electromagnetic waves is also lost during the refraction, diffraction and reflection phenomena for communicating over long distances. In fact, any useful way of spreading is accompanied by attenuation.


Refraction - Electromagnetic waves propagate in a straight line until deflected by something. Radio waves are slightly refracted or bent as they pass from one medium to another. In this respect radio waves behave in the same way those other forms of electromagnetic radiation. The apparent bending a partially immersed in water pencil demonstrates this principle. The refraction is caused by the change in velocity of the wave when it crosses the border between a propagation medium and the next. If this transition is made at an angle of 90° a portion of the wave front accelerates (or slows down ) before another, bending slightly the wave.


Radio waves are refracted when passing obliquely from one medium to another. The lines represent the crests of a wave front, the distance between the rows wavelength. The directions of the wave change because one end of the wave front decelerates before another when crossing the boundary between the two media. The wavelength is shortened, but the frequency remains constant. The degree of refraction is directly proportional to the difference between the indices of refraction. The refractive index is not more than the speed of a radio wave in a vacuum divided by the speed in the medium in question.

Radio waves are usually refracted when they pass through different layers of the atmosphere, whether the ionosphere at 100 kilometers altitude, or the lower layers of the atmosphere. When the relationship between the refractive indices is large enough, the radio waves can be reflected, as with the light a mirror. The Earth is a reflector with high losses, but a metal surface works well if you have some diâ wavelengths in the middle in question.


Dispersion - The direction of radio waves can also be changed by dispersion. The effect observed in a beam of light trying to pass through the fog is a good example of the dispersion of a light wave. Even on a clear night, the cone of light of a headlamp is visible due to a small amount of atmospheric dispersion perpendicular to the light beam. The radio waves disperse the same way when objects are arranged at random, equal to or smaller than the wavelength like pasta electrons or water droplets. When the density of the dispersing object becomes sufficiently large, it behaves more like a propagation medium with a refractive index characteristic.


Two stations need only to point the antennas to a common reflector, whether stationary or mobile. Contrary to common sense dictates the best position for a reflector is not halfway between the two stations. The signal intensity increases as the reflector approaches an extreme of the path, making the most efficient reflectors are close to one of the two stations. The maximum range is limited by line of sight distance (radius) of the two stations for the reflector and the shapes and dimensions thereof. The reflectors should have multiple dimensions of wavelengths and preferably be flat. Long and medium-haul commercial aircraft are good reflectors and offer the opportunity to contact long distance. The theoretical limit for reflection obtained by the use of aircraft is about 900 km, assuming commercial jets do not fly more than 12 000 m, but in reality the achieved contacts are fairly minor.


Diffraction - radio waves can be diffracted or bent around solid objects with sharp edges. The degree of refraction is directly proportional to the difference between the indices of refraction. The refractive index is not more than the speed of a radio wave in a vacuum divided by the speed in the medium in question. Radio waves are usually refracted when they pass through different layers of the atmosphere, the ionosphere is 100 kilometers altitude, or the lower layers of the atmosphere. When the relationship between the refractive indices is large enough, the radio waves can be reflected, as with the light a mirror. The Earth is a reflector with high losses, but a metal surface works well if you have some diameter of wavelengths.


The summit of a mountain range with at least 100 wavelengths long can serve to diffraction at radio frequencies. Sharp peaks, trees Free and horizontal edges give the best, but even rounded peaks can serve as edge diffraction. Only a small portion of the signal energy will be diffracted, but enable communications with a range of 100 km or more. This form of diffraction works both ways, so communications through what may seem insurmountable mountains may be possible.


Terrestrial wave is the result of a special form of diffraction that affects primarily the wavelengths longer vertically polarized. It is most noticeable in bands of 80 and 160 meters, where the range for this method can reach the 200 km. The ground wave expression is erroneously applied to any form of communication short however this mechanism is only our own longer bands. Radio waves are slightly bent when passing by a sharp edge, but this effect is achieved by very rounded edges. In medium and long wave, the curvature of the earth looks like a rounded edge. The bottom of the wave front loses energy due to currents induced in the earth causing a bending of the wave. This phenomenon slows the bottom of the wave, causing the wave to bend slightly toward the ground. This "fold" follows the curvature of the earth, allowing medium and long wave radio signals to propagate beyond the line of sight. This propagation mode is most useful during the day 1.8 and 3.6 MHz, when the absorption caused by the ionospheric D layer becomes more difficult to spread. Vertical antennas with an excellent ground system offer the best results. The losses are considerably reduced on salt water and increase in dry and rocky soils.


The Earth's atmosphere consists essentially of nitrogen ( 78 % ), oxygen ( 21% ) and argon ( 1 % ), and other rare gases. The water vapor can reach to 5 % under certain atmospheric conditions. This proportion of gas is maintained to an altitude of 80 km altitude at which the mixture begins to change. In the higher altitudes is more helium and hydrogen. Solar radiation acts directly and indirectly on all levels of the atmosphere. Near the Earth's surface, solar heating controls all aspects of climate, creating wind, rain, and other phenomena. Ultraviolet radiation creates small concentrations of ozone between 10 and 50 km. Most of the UV radiation is absorbed in this process and does not reach the earth.

The even higher altitudes, UV radiation and X partially ionizes the gases of the atmosphere. Electrons released by gas atoms recombine with positive ions to reconstitute neutral atoms, but this process takes time. In rarefied environment of high altitudes, the atoms are distant from each other, and the gases may remain ionized over time. At lower altitudes, the recombination happens quickly.

The atmosphere, which reaches over 600 km altitude, is divided into zones. The troposphere is between the Earth's surface and 10 km. Between 10 and 50 km are the stratosphere and the ozone layer, an area where ozone reaches higher concentrations. About 99 % of atmospheric gases are situated in these two layers. From 50 up to about 600 km is the ionosphere, noted for its effects on the propagation of radio waves. These altitudes, oxygen and nitrogen predominate at very low pressures.


UV and X -rays ionize the gases by creating a region where ions are at a relative abundance. The ionosphere is divided into three distinct layers D, E and F. The magnetosphere starts around 600 ft. and extends up to 160,000 km. The composition of atmospheric gases will gradually changing oxygen, helium and finally to hydrogen at the highest levels.


Propagation in the atmosphere - Contrary to what happens with radio waves in space radio waves to pass through the Earth's atmosphere suffers many influences on its part. All have experienced problems with radio waves, caused by certain atmospheric conditions; these problems are caused by the lack of uniformity of the earth's atmosphere. Several factors can influence the propagation conditions, both positively and negatively. Some of these factors are altitude, geographic location, and time (day, night, season, and year).


To understand the phenomenon of propagation of radio waves, we have to know the Earth's atmosphere. The atmosphere is divided into three separate regions, or layers. They are the troposphere, the stratosphere and the ionosphere.


Troposphere - Almost all meteorological phenomena occur in the troposphere. The temperature in this region decreases rapidly with altitude. Clouds are formed, and there may be a lot of turbulence due to changes in temperature, pressure and density. These conditions can have a pronounced effect on the propagation of radio waves, as will be explained later.


The stratosphere is located between the troposphere and the ionosphere. The temperature in this region is almost constant and there is very little water vapor. As it is a relatively calm layer and with few temperature variations this layer hardly influences the propagation of radio waves.


Ionosphere - This is the most important layer of Earth's atmosphere for radio communications over long distances. As the existence of the ionosphere depends directly on the radiation emitted by the sun, the motion of the earth relative to the sun, or changes in solar activity can result in variations in the ionosphere. These variations can be classified into two types:


1) those that occur in more or less regular cycles, and thus can be predicted with some accuracy;

2 ) irregular and those which result from an abnormal behavior of the sun , and therefore cannot be predicted.


Both regular and irregular variations have important effects in the propagation of radio waves. As the irregular variations cannot be predicted, we focus on the regular variations.


Regular variations can be divided into four main classes: daily, 27 days, seasonal and 11 years. Let's dwell on the diurnal variation, as they have a great influence on our hobby. Daily variations in the ionosphere produce four layers of gas electrically charged atoms called ions, which enable radio waves to propagate over long distances around the earth. The ions are produced by a process called ionization.


In the process of ionization, ultraviolet rays of high energy coming from the sun periodically reach the ionosphere, electrically collide with neutral gas atoms, and remove one or more electrons for each atom. When these electrons are free, the atoms are positively charged (positive ions) remain in the space and, together with the free electrons. The free electrons absorb some ultraviolet energy that freed them and form an ionized layer.


As the atmosphere is bombarded with ultraviolet rays of different frequencies, multiple layers are formed at different altitudes. The highest frequency of ultraviolet rays penetrates deeper producing ionized layers in the lower part of the ionosphere. Conversely, the lower frequency ultraviolet rays penetrate less, and form ionized layers in the higher regions of the ionosphere.


An important factor in determining the density of these ionized layers is the angle of sun rise. As this angle varies with frequency, altitude and thickness of the layers ionized varies depending on time of day and the season. Another important factor in determining the density of the layer is known as recombination.


Recombination is the opposite process to ionization. It occurs when free electrons and positive ions and free electrons collide, combine, resulting electrically neutral atoms. As ionization, recombination depends on the time of day. Between the early morning and late afternoon, the rate of ionization exceeds the rate of recombination. During this period the ionized layers reach the maximum density and exert the greatest influence on radio waves. However, in the evening, the recombination rate exceeds the ionization, causing a decrease in the density of ionized layers. Throughout the night, the density continues to decline, reaching its lowest point just before sunrise. It is important to understand that this process of ionization and recombination varies depending on the ionospheric layer and the time of day. The following paragraphs explain the four layers (or regions) of the ionosphere.


Layers Ionosphere - The ionosphere is composed of three distinct layers, designated by D, E and F D being the lowest latitude lies, as seen in the figure. F layer is further divided into two layers F1 (lower) and F2 (highest). The presence or absence of these layers in the ionosphere and its altitude varies with the position of the sun. At noon, the radiation in the ionosphere is maximum, while at night is minimal. When radiation disappears most of the particles that were ionized recombine. In the time between these two conditions, the position and number of ionized layers of the ionosphere change. As the position of the sun varies daily, monthly and annually for a given point on Earth, the exact number of layers present is extremely difficult to determine. However, the following propositions on these layers may be made.


D Layer - This layer is present between 50 and 90 km above the earth. The ionization in the D layer is low because being the lowest layer is the one that receives less radiation. For very low frequencies, the D layer and the soil acts as a giant waveguide, making it possible to communicate through use of large antennas and powerful emitters. The D layer absorbs the medium and low frequencies, limiting the diurnal range to about 400 km. From 3 MHz, the D layer begins to lose absorbent characteristics. Long distance communication is possible for frequencies up to 30 MHz Radio waves with frequencies above this value pass through the D layer being, however attenuated. After sunset, the D layer disappears because of the rapid recombination of ions. Communications at low and medium frequency become possible. It is for this reason that the stations on AM and medium waves behave differently at night. Signals traveling through the D layer are not absorbed but are propagated through the layers E and F.


Layer E - lies between 90 and 140 km altitude about. The ionosphere recombination is quite fast after sunset, causing their disappearance in the night. The E layer allows the average distance communications to frequencies located in the range between low and high. For frequencies above 150 MHz, radio waves pass through the layer E. Sometimes solar flares cause ionization night this layer on certain areas. The spread provided by this layer in these conditions is called "Sporadic -E". The range provided by Sporadic -E sometimes exceeds 160 km, but the range is not as great as through the layer F.


Layer F - lies between 140 and 390 km altitude. During the day, the F layer separates into layers days, F1 and F2. Generally overnight F1 layer disappears. The F layer produces maximum ionization after noon, but the effects of the daily cycle are less pronounced than in the layers D and E. F layer of ionized atoms remain for a long time after the sun set, and during the peak solar activity may remain ionized all night. Since the F layer is the highest of the ionosphere, it is also the one that allows greater range. For horizontal waves, the range obtained from a single jump (hop) may be 5000 km. For the signals to propagate to larger distances, multiple hops are required. The F layer is responsible for most HF communication distance. The maximum frequency that the F layer reflects depends on the point of the solar cycle we are in. At the peak of the solar cycle, the F layer can reflect signals up to 100 MHz. During the foot of the solar cycle maximum usable frequency can go down to 10 MHz.


Atmospheric propagation - In the atmosphere, radio waves can be reflected, refracted, and diffracted. In the following chapters discuss these forms of propagation.


Refraction - A radio wave transmitted through ionized layers is always refracted, or bent. This bending of radio waves is called refraction. Note the radio wave in the figure below, crossing the Earth's atmosphere with a constant speed. As the wave goes in and denser layer ionized, the upper part of the wave moves faster than the bottom. This abrupt increase in speed from the top of the wave causes the wave to be diverted toward Earth. This shift is always towards the propagation medium where the propagation speed is lower. The amount of refraction a radio wave suffers depends on three main factors:


1. The layer ionization density;


2. The frequency of the radio wave;


3. The angle of incidence on the layer;


Layer Density - The figure shows the relationship between radio waves and ionization density. Each ionized intermediate layer has a region where the ionization is the densest between two regions where the ionization is reduced. A radio wave when entering a region where the ionization is progressively larger, the speed increases causing the gap toward Earth. Within the more densely ionized, refraction occurs at a slower rate because the density is uniform. When the wave reaches the upper region minus ionized, the speed of the upper part of the wave decreases and the surge is diverted from the earth.


Frequency - The lower the frequency of a radio wave, the wave is refracted faster for a given degree of ionization. For any ionized layer, there is a frequency at which the transmitted power is lost vertically in space. The frequency immediately below this is often critical. The frequency criticism of a layer depends on its density. If a wave passes through a given layer can be further refracted by an upper layer, if the frequency is lower than the critical frequency of the top sheet.


Incidence and Critical Angle - When a radio wave encounters a layer of the ionosphere, that wave is returned to the earth at an angle (approximately) equal to the angle of incidence. Any wave at a given frequency, which let the antenna at an angle of incidence greater than the critical angle, will be lost in space. The critical angle for radio waves depends on the density of the ionized layer and the signal wavelength. As the frequency of a radio wave increases, the critical angle must be reduced so that refraction occurs.


Area and distance of Silence (SKIP) - When issuing a radio wave, this will spread in two ways, by ground wave and ionospheric wave. Armed with this concept we can discuss the outer limits of the zone of silence (skip distance) and the silence zone (skip zone).


Exterior limit of the Zone of Silence (Skip Distance) - The Limit Exterior Zone of Silence (Skip Distance) is the distance from the transmitter and the point where ionospheric wave hits the earth the first time. This distance depends on the frequency and angle of incidence, and the degree of ionization. The quiet zone is the area between the point where the ground wave is too weak to be received and the point where ionospheric wave back to earth the first time. The outer boundary of silence zone varies considerably, depending on the frequency, time, the season, solar activity, and the direction of transmission. At very low frequencies, low, and medium, the quiet zone is never present. However in HF, the silent zone is always present. As the frequency increases, the silence zone increases to a point where the outer limit of the zone of silence can be thousands of kilometers. For frequencies above a certain value leaves even be spread by F. zone Sometimes ionospheric wave returns to Earth within the area covered by ground wave. In this case, we can experience a fading (QSB) quite large, caused by the phase difference between the two waves (the ionospheric wave goes a long way).


Reflection - Reflection occurs when radio waves are reflected by a flat surface. Basically we can consider two types of reflection: Earth and ionosphere. Under normal conditions, the radio waves reflected in phase produce stronger signals, if reflected staggered produce a weaker signal or variable. The ionospheric reflection occurs when certain radio waves reach a thin, highly ionized layer of the ionosphere. Although in reality the waves are refracted, some of them return to earth so quickly that it seems to be reflection. For ionospheric reflection occurs, the thickness of the ionized layer cannot be greater than one wavelength. As the ionized layers generally have a thickness of several km, ionospheric reflection happens with long waves.

Diffraction - Diffraction is the wave capacity circumvent obstacles and "bend corners". If the wavelength is larger than the diameter of the obstacle is readily circumvents the same. However as the wavelength decreases, the attenuation increases until VHF appears a shadow area. The shaded area is an area opposite the obstacle in the direction of the transmitter. Diffraction can extend the range beyond the horizon. Using high-power and low frequencies, radio waves follow the Earth's curvature diffraction.


Effects of the atmosphere in the spread - As mentioned before, changes in the ionosphere can produce dramatic changes in the propagation of radio waves. In some cases, the range is extended in an extraordinary way. Other times, the range is reduced or null. The following paragraphs attempt to explain the problems caused by fading and selective fading.


Fading - One of the most frustrating problems in reception of radio signals is the change in signal intensity, a phenomenon known as fading (or in Portuguese fading). There are several conditions that produce fading. When a radio wave is refracted by the ionosphere or reflected by the earth's surface, there can be random changes in the polarization of the wave. Antennas mounted horizontally or vertically, respectively are designed to receive horizontally polarized waves or vertically. Therefore, changes in polarization cause changes in the intensity of the received signal. The absorption of RF in the ionosphere also causes fading. Most of this absorption occurs in the lower part of the ionosphere where ionization density is higher. To cross the ionosphere, radio waves lose some of its energy to the free electrons and ions therein. Since the degree of absorption varies depending on the density of the ionized layers, there is no definite relationship between the distance and the signal intensity in the ionospheric propagation. The fading caused by absorption extends over a longer period than for other types of fading, as the absorption occurs slowly. Under certain conditions, the absorption is so high that communications beyond the line of sight become very difficult. Although fading caused by absorption is the most severe; the fading on the ionospheric propagation results mainly spread by multiple routes or English multipath.


Multipath Fading - multipath is a term used to describe the multiple pathways that a radio wave may go between the sender and the receiver. These propagation paths include ground wave, ionospheric refraction, re -radiation by the ionospheric layers, terrestrial ionospheric reflection or multiple layers, and so on. Radio waves received in phase reinforce each other and produce a stronger signal, while incoming out of phase produce a weak or fading signal (fading). Small changes in the pathway can change the phase relationship between two signals, causing periodic fading.


Selective Fading - The resulting fading propagation through multiple paths (multipath) varies with frequency because each frequency reaches the receiver via a different route. If a set of different frequencies are transmitted simultaneously in each of the fading is different. At this variation is called selective fading. When this happens, the transmitted signal frequencies do not maintain the original phasing and relative amplitudes. This type of fading produces a serious signal distortion.


Other phenomena that affect communications - Although the daily variations in the ionosphere have the most pronounced effect in communications, other phenomena also affect communications, both positively and negatively. These phenomena are briefly discussed in the following paragraphs.


Seasonal variations in the ionosphere - Seasonal variations result from the Earth translation movement around the sun because the sun's relative position changes with the seasons. Seasonal variations of the layers D, E, and F1 are directly related to the inclination of the sun's rays, the maximum ionization of these layers during the summer. With the F2 layer succeeds the opposite. Its ionization is maximum during the winter. So frequencies propagated by the F2 layer are higher in winter than in summer.


Sunspots - One of the most notable occurrences on the surface of the Sun is the appearance and disappearance of dark, irregular patches known as sunspots. It is believed that sunspots are caused by violent solar flares and are characterized by causing strong magnetic fields. These stains cause variations in the degree of ionization of the ionosphere. Sunspots tend to appear in two cycles, every 27 days and every 11 years.


27 days cycle - The numbers of spots in each time changes constantly longer disappear and appear each other. As the sun has rotational movement, these spots are visible at intervals of 27 days, which is about the time it takes the sun to rotate about its axis. During this period, fluctuations of ionization vary more pronounced in the F2 layer.


Cycle Eleven years - Sunspots can occur at any time, and the lifetime thereof is variable. The eleven-year cycle is a regular cycle of solar activity with a minimum and a maximum activity occurring every eleven years. During the period of maximum activity, the density of ionization of all layers increases. Because of this, the absorption layer D is increased and the critical frequencies for the layers E, F1 and F2 are greater. At this point, the higher frequencies should be used for communications over long distances.


IRREGULAR VARIATIONS - irregular variations are unpredictable changes in the ionosphere that can profoundly affect our ability to communicate via radio. The most common variations are: sporadic E, ionospheric disturbances and ionospheric storms.

E sporadic - sometimes form in the layer and irregular spots with an unusually high ionization, this phenomenon is called E sporadic (sporadic E). The exact cause for this phenomenon is not known and its occurrence cannot be predicted. However, this phenomenon varies with latitude. In latitudes further north appears to be related to the aurora borealis phenomenon. This layer can be so thin that radio waves to penetrate easily and are refracted by the upper layers, or can be strongly ionized and extend for hundreds of kilometers. These conditions may be beneficial or detrimental to the propagation of radio waves. On the one hand you can completely eliminate the use of the upper layers of the ionosphere, or cause additional absorption of the radio signal in some frequencies. It can also cause problems by multipath propagation. On the other hand, the critical frequency of this layer power twice as often criticizes the normal layers. This can allow long distance abnormally high frequency communications. You can also enable communications to locations that would normally be in silence zone. This layer can come and go quickly during the day or night.


Ionospheric disturbances Sudden - This type of disorder can happen without warning and its duration varies from a few minutes to a few hours. When these disorders happen, HF communications over long distances become virtually impossible. Sometimes it seems that the receiver broke ... This phenomenon is caused by a solar flare that produces an abnormally high amount of ultraviolet radiation that is not absorbed by the layers F1 , F2 or E. Unlike this, because increased ionization density layer D. As a result, frequencies above 1 MHz or 2 cannot penetrate the layer D and are completely absorbed.


Ionospheric storms - These storms are caused by disturbances in the Earth's magnetic field. They are associated with solar flares and 27 days of the cycle, i.e., with the rotational movement of the sun. The effects of ionospheric storms are a turbulent ionosphere and erratic ionospheric propagation. These storms affect mainly the F2 layer, reducing the ionization density and making the lower critical frequencies than normal. The effects on communications are that the range of usable frequencies is lower than normal and it is only possible using lower frequencies.


WEATHER - The wind, air temperature, and humidity can combine to increase or decrease the range of the radio communications. Rainfall influences, especially the higher frequencies. The frequencies of HF and below do not suffer too much about it.


RAIN - The attenuation caused by raindrops is greater than that caused by any other form of precipitation. It may be caused by absorption, where the raindrop acts as a dielectric poorly absorbs the energy of the radio wave and transforms this energy into heat; or dispersion. The attenuation caused by rain due more to the dispersion of the absorption at frequencies above 100 MHz. For frequencies above 6 GHz, the dispersion is even greater.


FOG - The attenuation caused by fog depends on the amount of water per unit volume and the size of the droplets. The attenuation caused by fog at frequencies below 2 GHz is negligible, but above this frequency attenuation by absorption can be increased.


SNOW - Since the snow has about 1/8 the density of the rain, and because of the irregular shape of the flakes, losses due to scattering and absorption are difficult to calculate, being however smaller than those caused by rain.


HAIL - The attenuation is caused by hail stones determined by size and by their density. The attenuation the attenuation caused by hail dispersion is less than the skin caused rain.


TEMPERATURE REVERSE - When forming layers of hot air over the cold air layers sets up a condition known as inversion temperature. This phenomenon causes the formation of channels or the cold air ducts, between earth and a hot air layer or between two layers of hot air. If an antenna station is within that channel, or a radio wave then enter a very low angle of incidence, emissions of VHF and UHF can propagate beyond the horizon. These long distances are possible due to the different densities and refractive properties of hot and cold air. The sudden change in density when a radio wave enters the warm air above the duct causes the wave is refracted back to earth. When the wave reaches the earth or a layer of hot air duct below the opposite happens and the wave proceeds along the conduit.


TRANSMISSION LOSSES - All radio waves propagated through the ionosphere suffer power losses before they reach the receiver. As stated earlier, absorption and effects of lower atmosphere cause of most of the attenuation. There are also two other types of losses that also affect the spread. The combined effect of loss by reflection on earth, and losses in free space, produces most of the ionospheric attenuation.


LOSS FOR REFLECTION ON THE GROUND - When the spread is through refraction in multiple steps, rf energy is lost each time it is reflected by the earth's surface. The amount of energy lost depends on the frequency, the angle of incidence, soil roughness, and soil conductivity in the reflection point.


LOST IN SPACE FREE - Normally, most of the energy and lost because of the spreading of the wave front. With increasing distance, the area of the wave front increases as the beam of a flashlight. This means that the amount of energy contained in a wave front area decreases as a function of distance. When the energy arrives at the receiving antenna the wave front is so widespread that the antenna reaches only a small portion of the wave front.


FREQUENCY SELECTION TO USE - Selecting the correct frequency to certain conditions requires a good knowledge of the propagation mechanisms. For the success of radio communications between two points at a certain time of the day, there are a maximum frequency, minimum and optimum that can be used.


MAXIMUM USABLE FREQUENCY (MUF) - The higher the frequency of a radio wave, the lower the degree of refraction caused by the ionosphere. So, for a given angle of incidence and time of day, there is a maximum frequency that can be used in communication between two points. This frequency is known as MAXIMUM USABLE FREQUENCY (MUF). Radio waves above the MUF frequencies are refracted more slowly and return to Earth at a point beyond the desired place or are lost in space. Variations in the ionosphere can raise or lower the expected muffle at any time. This is especially true in the F2 layer.


MINIMUM USABLE FREQUENCY (LUF) - As there is MUF there is also a minimum frequency that can be used, known as RATE MINIMUM USABLE ( LUF ). By decreasing the frequency the degree of refraction increases. Then a wave whose frequency is lower than the LUF back to earth at a point lagging behind. By decreasing frequency, RF energy absorption increases. A wave, whose frequency is very low, is absorbed to the point of being too weak to be received. Atmospheric noise is also higher for low frequencies. The combination of these two effects can result in unacceptable signal noise ratio. Therefore in determining the LUF it is necessary to take into account these factors.


EXCELLENT WORK FREQUENCY ( FOT ) - The best operating frequency is the one that lets you communicate with fewer problems. Must be high enough to avoid problems of multipath fading , absorption, and noise found in the lower frequencies ; but not so high that it can be affected by sudden changes in the ionosphere . A frequency that corresponds to these requirements is the GREAT WORK FREQUENCY ( FOT ). FOT is short for "frequency optimum de travail". The FOT is about 85 % of the muffle, but this percentage will vary and may be much less than 85 %.



Near-Real-Time MUF Map (with SSN & A-index)

Near-Real-Time MUF map

This image courtesy of  Solar Terrestrial Dispatch



Images SUN

The images below are "live" and show the SOL seen by different "wave widths" taken by the SOHO satellite and Yohkoh soft- Xray telescope. Generally, the brightest regions of the solar disk indicate greater solar activity. Click on any of the pictures to see a larger image.

SOHO - 17.1nm

SOHO 17.1nm

SOHO - 19.5nm

SOHO 19.5nm

SOHO - 28.4nm

SOHO 28.4nm

SOHO - 30.4nm

SOHO 30.4nm

Free images provided by Solar Data Analysis Center of NASA Goddard Space Flight Center



Layers of the Ionosphere

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F2 layer - This is the highest of ionosphere layers, and there are between 200 and 400km. This layer is the primary means of ionosphere reflection to communications on shortwave at very high distances, this distance that can vary throughout the day, with the time of year and with the solar cycle. These variations are caused by the ionization degree as well as with altitude layer. The F2 layer appears around sunrise when the F layer decomposes to give the F2 and F1. At this point it is clear a sharp increase in critical frequency of work. The reflection characteristics reach maximum approximately when the sun reaches the highest elevation on the horizon, at which the trend reverses and then begins to decrease its reflective characteristics through a reduction of ionization that occurs within this fuse F1 layer and give rise to a single layer, "F" as already stated.


F1 Layer - This layer exists just below the F2 layer and as the F2 also it exists only during daylight hours. Sometimes this layer may serve as a reflective at certain frequencies. But usually electromagnetic energy passing through the layer and also runs through this and eventually is reflected in the F2 layer. The most usual cause this layer is an additional absorption at the crossing signals "E" before reflecting F2.


And Layer - This layer lies beneath the F2 and F1 layers and exists only during daylight hours, disappearing almost overnight. However and very rarely can be seen traces of it overnight. The height of this layer is between 80 and 100km. The maximum activity is the layer when the sun rays impinge perpendicularly to the surface thereof. This layer enables communications in HF medium distances, and is also responsible for the spread of frequencies below 1.5 MHz overnight at considerable distances. However this layer is famous among amateurs by enabling communications at frequencies above 50 MHz at distances that can easily exceed 2000km. In this case we will say that we are in the presence of a "sporadic E". Sporadic E occurs when during a certain time (especially at the time of the spring) are strongly ionized areas for abnormal conditions of solar activity, enabling the reflection of very high frequency signals. The altitude at which is situated the ionized cloud and the ionization density determine the jump distance for a given angle of incidence. One of the simplest ways to check that this is a sporadic "E", and as such a good chance to make a far more grid is the appearance of broadcasting stations in FM that are sometimes located more than 1500km. During the existence of a sporadic is normal hearing in Portugal FM stations Italian French and German among others.


Layer D - This is the lowest of all levels reaching between 50 and 80 km and apparently has more absorption of radio-electronic energy during the period of its existence and it lies almost only during the daylight hours with disappearing the sunset. It is also most of all unknown ionosphere layers and the least degree of ionization features. It is believed that this layer is responsible for the absorption of radio waves MF and HF during the daylight hours.


Ionosphere disturbances - All variations that occur in the ionosphere are more or less predictable and depend mainly on solar activity and the degree of ionization that solar radiations cause the ionosphere. This way you can with current knowledge predict propagation conditions within certain limits. But sometimes normal ionosphere behavior is changed by certain phenomena that occur on the solar surface such as solar flares that release "flares" with many thousands of km long and at an amazing speed that can reach 2000km per second. These bursts emit X rays and particularly a large amount of proton particles. If flares occur in the direction of Earth, then this rain proton reaches the earth causing serious disturbance of the ionosphere layers ionizing them in the region of the poles giving rise to the spectacular Northern Lights. During the period in which the land is exposed to these anomalies characteristics of the various layers is changed and severe disorders occur in communication systems, even causing complete blockage of communications by ionosphere reflection. At this point the receiver seems to be broken, because only you hear a slight background noise. It will only be possible to listen to stations that are closest and therefore use propagation by terrestrial wave. These disorders are divided into two categories: sudden ionosphere disturbances and solar storms.


Sudden ionospheres disturbance comes from solar flares more or less sudden and strikes the earth about 15 minutes after having given the rash and only affects the surface of the earth that faces the sun. These disorders have a limited duration and mainly affect the frequency 0s between 2 and 30 MHz. The attenuation of signals during phenomenon can achieve 40dB and normally takes less than one hour.


On the other hand solar storms although not as pronounced, present greater problems for communications because of its length are greater. During a solar storm, the signals using the ionosphere to propagate drastically decreases in intensity and may even extinguish during several days and occurs throughout the world, even in places where it is night. Under these conditions the F layer abnormally increases its altitude and the attenuation of the waves is increased giving rise to what is called a "flutter fading".



Spread over 30MHz

The propagation at frequencies above about 30 MHz usually not used ionosphere layers to be reflected. But there are exceptions when these frequencies propagate through more or less common phenomena that abnormal conditions occur in the usual propagation characteristics such as propagation sporadic "E". Thus the main propagation modes at frequencies above 30 MHz are as follows:


Visual range for two seasons at the same altitude - When two stations are at the same altitude is possible through a simple mathematical formula to calculate in terms of a standard atmosphere (K = 1.33) these stations are considered in line of sight. This calculation is very important when trying to establish communications in a microwave. Theoretically the maximum distance at which these stations may be, is when the beam that unites both play at the point of common horizon.


To calculate the distance to the horizon, from the elevation of one station and knowing the factor K can utilize the following formula.


DR = Km distance on the horizon

Hs = Altitude acima do nível do mar

K = Fator do raio efetivo da Terra (K = 1, 33 numa atmosfera standard)

Dr = SQR (12, 75 x Hs x K)

SQR = raiz quadrada

Refractive line of sight - This is the most common way of spreading at high frequencies and is only affected by the obstacles lying in the path as well as between the existing weather stations. Theoretically communications in VHF and higher frequencies should not go beyond the visual horizon, but as we all know. In practice this does not happen and can communicate beyond the visual horizon even in normal conditions. This is due to the existence of reflections and refractions in the path of electromagnetic energy. Because the relative altitude air becomes thinner electromagnetic waves along the earth's surface tend to move more slowly produces the effect of refraction down, thus causing the radio wave accompanying somehow routes in not too long the curvature of the Earth.


In conditions of large temperature inversions can be given exactly the opposite, namely next to the surface there is a layer of warm air, then overlaid with a cooler air . In this way electromagnetic waves, because of the greater density of cold air moving faster to the earth's surface thus causing upward curl. The degree of refraction can be greater or smaller and depends mainly on the factor "k" . Factor k has a value of 1.33 in a temperate climate and in normal pressure and temperature conditions. These phenomena cause much confusion in some hams unfamiliar with these issues because it produces a phenomenon known in our jargon as unilateral spread, i.e. listen well a station that a high area but not get there or come and listen and evil.


Another factor that has influence in the spread beyond the line of sight is the fact that electromagnetic waves reflect on objects they find on their way thereby changing its direction. This is a common way of spreading and widely known to all.

  • Refractive line of sight by temperature difference

Refraction line of sight by temperature difference - in this case the lowest season arrives and the highest station but listens badly. If the temperature conditions in refractive zone is then reversed to lower station and listen to reach higher but poorly.


Speculating point - Point speculate is the point on the horizon where it reflects the radiation of a real antenna transforming this radiation imaging radiation. This point can pose a problem at high frequencies since the radiation caused this point will offset 180 ° and reach a receiving antenna can override the signals that arrive without reflection. The specular reflection occurs mostly in a horizontal polarization reflective surfaces with good characteristics such as the large surfaces of water. When the calculations are carried out with a connection SHF, usually have in mind the large water surfaces on the route and sets up the system so that the illuminator of the parabolic reflector not see this surface, often using a cylindrical metallic sheath that is placed on the perimeter of the satellite.

Reflection protection speculate on a satellite

Determination of specular point between two stations with a surface

 reflecting (e.g. a lake ) there between

Tropospheric refraction - This is the most common of refractions and almost always occurs in more or less: is what allows in most cases the contacts on VHF and higher frequencies contacts beyond the visual horizon. The troposphere is not a homogeneous medium such as the temperature at the pressure degree of humidity and air composition varies widely especially above 10 km. The air becomes less dense, the temperature and pressure also lowers making the radio waves to penetrate this less dense medium is refracting and lean back to Earth. When the angle of refraction is equal or greater than the Earth's curvature is spread by conduct.


Won by obstacle - When between two stations is an obstacle that may be a mountain, under certain conditions can be given a phenomenon known as "razor's edge effect ", or won by obstacle. When electromagnetic energy progresses in space and reaches the summit of a hill, the bottom part of the electromagnetic beam suffers a downturn and tend to refract down, thus allowing plants which are beyond the obstacle to be heard with signs strong. This effect is the more pronounced the thinner the top of the obstacle. Hence the name "razor's edge" or "knife - edge diffraction".

Diffraction by razor wire effect ( Knife- edge diffraction )

Propagation lunar reflection (EME) - As the name implies, this is a way to communicate over long distances at very high frequencies using the moon as a reflector. It is a way of communicating used by experienced amateurs and engaged in communications techniques, since it is a difficult way to communicate, not only because the equipment is usually specially designed for that purpose, but also because due to the enormous distance (between 707 000 and 806 000 km) and also the mitigation suffering electromagnetic energy on reflection, it takes many hundreds of watts and high gain antennas to achieve practical results. By way of example, at 144 MHz the attenuation path in EME communication is about 225dB. Currently in Brazil (and the world) a few amateurs are dedicated to this interesting and difficult mode of communication. However, some have achieved considerable success and international impacts this way, even at frequencies as high as 10GHz.


Propagation "of conduct effect" - This type of spread is mainly due to the variation of the refractive indices in the zone that divides a layer of warm air and cold air . It has a certain analogy with what has been said about the refractive line of sight, but here the air masses will be much larger and higher altitudes. The size of these masses and the altitude of the same will depend on the distance at which communication is possible. A temperature inversion can occur from of a weather surface with extensions that can exceed 1500 km, enabling this communication so very interesting distances. Usually this type of propagation happens over the sea and coast areas. This type of spread is rare to happen for frequencies below 144 MHz.


Propagation behavior

Propagation "Sporadic E" - This type of spread is well known for amateurs, by enabling contacts in frequencies above 144 MHz at distances greater than 2000 km As the name implies, this type of propagation occurs when there are areas of layer "E" strongly ionized, able to reflect electromagnetic energy at very high frequencies. Because of its altitude (between 80 and 100 Km) you can make contact by radio at high frequencies at distances that would normally be impossible. This type of propagation, occurs more on the continental shelf, and is usually of short duration. The ability to make contacts by "sporadic E" may vary between a few minutes and about an hour, but rarely extends beyond this time. It is very common in these conditions extremely strong signals in the band of 144 MHz stations located at distances greater than 1500 km. Go doing some tapping into 144.300 SSB (USB) mainly during the months of May and June.

Propagation by sporadic E

Propagation Trans equatorial dispersion - This type of propagation occurs especially in the lower frequencies of the VHF band, in the case of amateurs in the 50 MHz allows communications at very large distances, which could reach 10,000 km or more.  This type of spread is caused by defects in the F layer over the area of the magnetic equator and allows double reflection without the radio wave will reflect on the Earth. To make communications through the spread of this type it is necessary that both stations are in a position in which the angle of incidence at the reflection point is great, and the conditions usually are only possible when the radiation reaches direction perpendicular to the reflection area, or either NS or SN and both stations are the same distance from the geomagnetic equator.

Trans equatorial spread


Propagation via trope-Dispersion - This type is caused by propagation irregularities in certain areas of the atmosphere in which the refractive index as well as the humidity and temperature conditions differ from the surrounding zones. The area of ​​this dispersion is called "common volume dispersion". The signals received by tropospheric scatter are generally weak and variable because the signal is reflected and scattered, so that only a small part reaches the receiver. For such systems that use this type of propagation are equipped with antennas of high gain and narrow beam and usually issuers are high-power. However they can be more stable signals at distances beyond 500 km. This propagation medium has little use these days and it is not very common in amateur means. It was this process that the few dozen years ago, before the advent of satellites used to provide telephone communications between countries, so it used complex antenna systems and also high power transmitters operating at about 900 MHz.

Propagation trope – dispersion 

Aurora propagation - As the name implies this type of propagation makes use of the reflective properties of the Aurora Borealis to communicate at distances that can reach 3000 km in frequencies ranging from 100 MHz to 400 MHz the reflection characteristics. Auroras vary very quickly which causes a lot of distortion in the telephony signals, and it is very used contacts in high speed CW. Auroras Earth occur in areas where the air is thin such as the poles. To these areas are called maximum occurrence aurora zones. In the Northern Hemisphere this zone is between Norway, Greenland and Central Canada, returning through Alaska, Siberia to Northern Europe. This type of phenomenon affect communications on shortwave substantially, not only by the noise it causes but also by the strong attenuation that produces the HF signals that cross. Only hams which are located in aurora’s areas or on the outskirts of the same may use this type of propagation.


Propagation "meteoscatter" - This type of spread is caused by the entry into the atmosphere of meteors per small they may be. Around the trajectory of these celestial bodies when passing through the layer "E" forms a highly ionized zone cylindrically shaped , narrow but very long , which allows the reflection of high frequency signals . This ionized zone is short , but when a large amount of meteorites entering the same time in the atmosphere , such as rain of meteorites , it creates quite reflective areas allowing the establishment of radio contact albeit short-lived . This mode of communication is quite common among amateurs. These amateurs are experienced people and are always aware of meteoritic rain warnings in order to point the antenna at the right time, and make announcements very interesting.


Final notes - Finally just want to comment that in addition to these types of most common spread, there are others of less importance. However, and by way of example, the reflections in aircraft or in frontal surfaces with temperature differences in their progression. These last often occur in parts of the world with certain characteristics, and can occur at any time of year. An example of the latter are the conditions that occur with some frequency in the Mediterranean Sea Area, when the hot, dry winds, called the winds "Siroccos" originating in the Sahara Desert, meet cooler and more moist surfaces on the Mediterranean Sea. These surfaces may extend from the island of Madeira even the Middle East. I assume that was one of these types of propagation that in 2000 allowed some contacts in 144.300 between distant countries.


Other factors that have much influence on the spread, especially above 1 GHz, are the rain, the fog, and especially the low clouds. In case of heavy rain can happen even in sign Hertz beams, operating at frequencies of several gigahertzes ​​to check the communications cut.


I hope that with this simple article without mathematical complications and easy to understand have contributed to that colleagues understand a little more about the phenomena of propagation.



Signal x Noise


Radio waves are able to achieve huge distances. This journey relief and the atmosphere can favor them or prevent them, weakening or strengthening its natural enemy called NOISE. The noise will always exist and the fight is to beat it with something useful, audible, called SIGNAL. When there is not enough signal will hear only noise and the release will not take place.

Improve the S / N ratio, that is the challenge facing... and win!

1 - More power at the transmitter will not necessarily lead the signal further; will just get stronger in the destination if there the receiver is able to distinguish it.

 "better a sensitive receiver than a powerful transmitter"


2 - What makes the signal is further spread to certain frequency that moment.


We’re not so a commercial radio station, which operates in MW with 100,000 watts of power, would cover the whole world with its signal. But this does not occur because during the day there are no propagation conditions for this signal at that frequency, and not for lack power in transmission. As the evening significantly improves the propagation conditions for the low bands, and so there is no mutual interference stations operating on the same frequency, the international law provides that such stations (broadcasting) to reduce transmitter power up to a tenth of that radiated during the day.


In most cases 10% of the power daytime cover larger distances at night, due to propagation. 


As we hams usually work only tens of watts, the goal to pursue the authentic experiencer is based on the slogan of operators QRP (low power operators):


"achieve the longest distance imaginable with the lowest possible power"


3 - So the trick to optimize the performance of a station consists of signal radiation dose to the propagation conditions that frequency at that time. The most logical alternative BEFORE INCREASING POWER, is to "focus the radiated energy”, avoiding waste, so...


A - narrow the pass band width; for example, migrating from FM 10 KHz wide and electing AM to 6 kHz, the 2.5 KHz SSB, CW with 0.5 KHz or other digital modes with bandwidth up closer. The price you pay is the drop in sound fidelity as you narrow the bandwidth.


B - point the signal to the intended recipient through directional antennas of higher gain than omnidirectional or in which they were using. The cost of this is the performance drop in the other quadrants.


4 - Increase power is expensive. To increase a unit "S" in the receiver tuning meter that is necessary is to quadruple (multiplied by 4) power. Do not forget that the difference between units in a properly calibrated DB meter is equal to 6 dB. In practice this means...


S-7 with 10 watts; S -8 to S- 40 watts and 160 watts with 9


Know decibels (one tenth of the Bell) measure a logarithmic relationship (rather than linear) between magnitudes, and twice (2 times) this scale is equal to 3 dB.


5 - A radiant system does not add to watts delivered to him. On the contrary, mismatched impedance, coaxial cables, filters, coils, baluns or any random ingredient added to the antenna steal energy used by the transmitter. Therefore, an antenna with higher relative gain, which is receiving 100 watts of equipment, radiate this energy (RF ) to an equally greater distance, but will not add any more to the watt delivered to him.


6 - The antenna coupler neither improves nor adjusts the irradiating system only adjusts the transmitter output impedance (usually 50 ohms) to the impedance reading at the end of the coaxial that connects to the equipment. In practice, fools the transmitter so that it - seeing one SWR of 1 : 1 - release all its power, as there is in it a protection circuit that reduces proportionately the amount of RF as increases Stationary Wave Ratio (ROE ).


7 - In summary, about power and propagation, live with the three situations described below:

In the situation "A" the Propagation (P) for that frequency at that time and the power or the concentration of radiated energy (W), are accordingly adjusted, being allowed to contact in distance point 10 on an imaginary straight.


In situation "B" the power or the concentration of radiated energy (W) is used below the propagation conditions ( P) for that frequency at that time. So contact at most could happen in point 5 of the imaginary line. The transmission could be better utilized (refer to item 3 above) so that the contact occurred in paragraph 10 of the straight.


In the position "C", the power or concentration of the radiated energy ( W ) used is beyond the propagation conditions ( P ) for that frequency at that moment . So contact also not happen beyond the point 5 of the imaginary line. In this case some Watts were being wasted.


So do not ever forget:

=> No transceiver will yield more than the antenna where it is connected <=



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Amateur Chirp Project- Amateur projects to investigate radiowave propagation - G3PLX\'s chirp project, ionosonde database
An introduction to HF Propagation- This article will help to unravel some of the mysteries as well as giving useful operating
Current solar images- Images from the Solar Dynamics Observatory and Atmospheric Imaging Assembly
D-Region Absorption Prediction- This map represent the current position of the sun and how a solar flare influence an HF
Geomagnetic and Solar Indices- NOAA / Space Weather Prediction Center, Geomagnetic K-indices and Running A-indices
Geophysical Alert Message wwv.txt- Broadcast via WWV and WWVH at 18 and 45 minutes past the hour.
High Frequency Active Auroral Research Program- A Scientific endeavor aimed at studying the properties of the ionosphere.
Hourly STD Solar and Geophysical Report- Boulder K-Indices, Planetary K-Indices, 10.7 cm solar radio flux updated every 30 minutes
Near-Real-Time F2-Layer Critical Frequency Map- This map can be used to determine the frequencies that will always be returned to the Earth.
NOAA Space Weather Scales- The NOAA Space Weather Scales were introduced as a way to communicate to the general public
Propagation Planning for DXpeditions- 6 Steps for a More Successful Trip by Carl Luetzelschwab K9LA
Radio Wave Propagation- A PDF document, used for ham radio training, that cover all aspects of radio wave propagation
Report of Solar and Geophysical Activity- The Report and Forecast of Solar and Geophysical Activity is the primary daily report prepared by SEC.
RF Skywave Propagation- Excellent presentation on RF skywave propagation by Greg, N6LYU
SEC's Radio User's Page- Provide radio operators with current data on the state of the ionosphere
The Ground Wave- About ground wave, article by
Today's Space Weather - SEC- Another interesting reports page with plots and graphs of solar flux.
VHF Propagation Maps- Experimental maps that show real-time VHF propagation derived from analyzing data gathered from the APRS-IS network.
VHF/UHF Propagation updated- In the past few years there has been a growing movement from VHF frequencies (50-300 MHz) to UHF (300-900 MHz)
WSPRnet Weak Signal Propagation Reporter Network

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