PY4SM / PY2DD - Propagação - Propagation - Solar - ANTENA - DX - CW - CQ - RADIOAMADORISMO

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.

SFI - INDEX	CONDITIONS FOR 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.

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

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

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:

=> 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 diâmetro.de 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:

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.

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