Ionosphere D,E,F,

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The ionosphere is divided into the following layers, according to their height: D, E, F.

The D layer extends from 50 km to 90 km, and is available only during the day. At night this layer disappears due to rapid ionic reconnection. Layer D refracts only low frequency radio waves however, it also causes disturbance to the waves, reducing or even absorbing them.

The E-layer extends from 80 km to 125 km, having the highest ionization. It exists only during the day. The frequencies refracted from this layer are greater than those refracted from layer D. Occasionally, this so-called sporadic E. occurs in this layer. This can be configured at any time, having an unexpected duration and extent. It can also be shaped at night and change the course of the moving waves, which would deflect to a lower altitude.

The F layer consists of two substrates, F1 (from 100 km to 200 km) and F2 (from 200 km to 400 km). At night, or even during the day (depending on the solar cycle), F1 and F2 are confused, resulting in a single layer, F. The ionization level in these layers is the highest, because the atmosphere it is less dense and ionic reconnection takes place slower. Therefore, an ionized layer is always present. These layers are responsible for long-distance transmissions over the HF band.

Variations in the ionosphere

The ionosphere, as observed, is not homogeneous nor stable, exhibiting changes over time, or by other means:

* changes during the day: Solar radiation modifies the electron density of the ionosphere. That is, ionization varies with solar radiation, which gradually increases in the morning to a maximum at noon, and decreases in the afternoon, and virtually attenuates at night (when there is no solar radiation).

* Seasonal variations: Maximum operating frequencies vary seasonally. This is one of the main reasons for frequency changes at least twice a year, usually in March and October.

* changes caused by the solar cycle: Solar activity is not constant, it obeys an eleven-year cycle, called solar cycle, which can be observed by the activity of the sunspots (the violent eruptions that occur on the solar surface). When the number of sunspots is high, the ionosphere exhibits a higher electron density and therefore the propagation improves at higher frequencies. Monthly values ​​for sunspot numbers are calculated by various research centers to determine the most appropriate frequencies used.

* latitude variations: Due to the lower incidence of solar radiation near the ground poles, the electron density is lower in the ionosphere than in the equatorial regions.

Apart from the factors mentioned above, there are other factors that can modify the ionosphere. For example the so-called "E-dispersion", which occurs in layer F and when it causes the wave to diffuse and when to add different refracted waves from different heights and positions in the ionosphere.



Types of transmitted waves



There are basically two types of transmission of electromagnetic waves: the terrestrial wave and the celestial wave. The first consists of three distinct waves: direct wave, reflected wave, and diffracted wave.

Ground wave

The surface wave propagates on the surface of the earth. Due to the terrestrial conductivity, an amount of energy from this wave is absorbed by the surface. The loss varies in a manner inversely proportional to the ground conductivity: the higher the conductivity the lower will be the loss, resulting in a greater distance being achieved for the surface wave. For example, transmissions over seas and oceans reach far greater distances than transmissions over land. The direct wave propagates along an almost straight line from the transmitter to the receiver. The direct wave is slightly affected by the tropospheric refraction, which causes the inclination to the ground surface. Also called tropospheric wave.

The reflected wave is the amount of ground wave reflected from the surface of the earth. The way the wave is reflected depends on the reflection coefficient of the surface and the associated angle. Although this angle and the reflection angle are the same, there is a phase variation between the associated and reflected waves, with a phase difference of 180 ?. This type of wave is considered undesirable in some cases. It can result in the wave being completely canceled at the receiver if the direct wave and the reflected waves are received at the same size. However, generally the cancellation is partial because the phase difference is not exactly 180 ? and the reflected wave is obtained at a lower magnitude due to delays and differences in path length.

Rainbow wave

The celestial wave is the wave that radiates in one direction so that the angle to the surface of the earth is large enough to direct the wave into the ionosphere. From there it refracts back to Earth and is reflected back to the ionosphere again, repeating the process. This is the mode of propagation used by shortwave transmissions and favors long distances.

The ionosphere has a decisive influence on the propagation of the celestial wave.

It usually acts as a conduit that absorbs some of the energy transmitted, but also acts as a "radio mirror", deflecting the wave back to earth. The ability to return a radio wave depends on factors such as ion density, radiation angle, and transmission frequency. Refraction may not even take place.

The distance between the transmitter and the point at which the radio waves return to Earth also depends on the angle of radiation. This distance is limited by frequency. The higher the refraction, the harder it is, though the distance can be greater. Each layer in the ionosphere can refract the radio waves up to a frequency called MUF (maximum usable frequency). There is also an "optimum frequency", OWF (optimum operating frequency), which represents part of the MUF.

It is known that above a certain frequency the radio waves no longer refract and penetrate the ionosphere. However, if the radiation angle is reduced, the radio waves can return to the earth's surface. The higher angle that still permits the refraction of a transmitted wave is called the critical angle for that frequency in particular.

The radiation angle can be roughly determined according to the frequency and distance between the transmitter and the receiver:

* 1.5-3.0 MHz: Low angle for long distances. A high radiation angle can lead to a weakening of the propagation of the waves.

* 3.0-7.0 MHz: Good return of the rainbow for any angle. High radiation angles can be used for short or moderate distances, while a low radiation angle can be used for long distance transmissions.

* 7.0-12.0 MHz: Radiation angle from 45o to 30o for short distances. Low angles for long distances.

* 12.0-30.0 MHz: This frequency range is not useful for short distances. The maximum useful angle for frequencies between 12 and 16 MHz is about 30o. For higher frequencies, the angle decreases. Above 28 MHz the angle is less than 30o. The path from the transmitter antenna to the point where the refracted wave reaches the earth is called a "jump". Depending on the distance to the receiver, a wave can make one or more jumps (the wave is reflected back to the earth's surface in the refracted ionosphere, and so on). The term "jump distance" is used to indicate the distance between the point of transmission and the return to earth, or the distance between each jump. An amount of irradiated wave can be propagated by the ground wave, however it reaches a short distance from the transmitter. The area on the terrestrial surface extending from the boundaries of the terrestrial wave to the return of the celestial wave to the earth receives no signal, and is called a "jump zone".

The layers that make up the ionosphere undergo particular changes in their height, density and thickness, mainly due to changes in solar activity, as we mentioned earlier. During the period of maximum solar activity, the F layer is denser and appears at higher altitudes, which change the jump distance of the radio waves. At night, in the absence of solar activity, the waves that would deflect from layers D and E are now deflected from layer F, resulting in a longer jump distance.

Signal degradation factors.

The transmitted signal does not reach the receiver with the same power as when it was irradiated by the antenna. Propagation causes loss of power and consequently, along with fading, absorption and noise.

Fading

The attenuation refers to any fluctuation or change in the signal intensity displayed on the receiver during their route from the transmitting antenna. Weakening can occur any time where the ground wave and the sky wave are received. In this case, the two waves can reach a phase difference, causing the signal to be canceled. In regions where only the celestial wave arrives, the attenuation can be caused by two celestial waves following different paths, reaching a phase difference between them.

Changes in absorption and path length in the ionosphere can also cause attenuation. Occasionally, a sudden disturbance can lead to the complete absorption of all the energy of the rainbow. Weakening also occurs when the receiver is near the boundary of the jump zone or when the working frequency is near the MUF.

The reduction in signal intensity can occur at levels almost negligible. The main reason for the occurrence of ionospheric attenuation is multi-path propagation, when the received signal is a synthesis of two or more signals arriving at the receiver following different paths. If the waves are received in phase, a weaker signal will be received. On the other hand, if the waves are received in the same phase, a stronger signal will be received. Small differences in path length can cause changes in the phase difference between the received waves.

Noise

There are various sources of noise that affect the reception. Noise can come from natural sources or from artificial sources. The first case involves atmospheric noise, which is usually the main source of noise in the HF zone. It is higher near equatorial regions and decreasing as latitude increases. Also included in this case is the cosmic noise from the stellar space, which affects the higher frequencies.

In the latter case all kinds of noise caused by ignition, transmission lines, electronic lamps, electric motors in general can be included ... This noise is directly related to the technological development and the density of electrical and electronic devices. in the areas where the signal is received. Artificial noise tends to be polarized vertically and the use of a horizontal antenna helps to reduce the impact of noise.

Ionospheric reduction

The D layer causes a decrease in the waves passing through it. The loss varies according to the solar cycle, and is about its highest. It also varies seasonally and throughout the day, with the highest impact in summer and around midday. In conclusion, it can be said that the loss varies with the ionization density in the D layer.

Weather conditions

Weather is one of the main factors affecting the spread. Depending on the weather, radio waves can be transmitted over longer distances, or even dramatically reduced. Unfortunately, there is no rule that predicts the effects of weather on transmission because the weather variables are complex and subject to frequent changes.

The fading due to raindrops is higher than the reduction caused by other types of water. The decrease can be caused by the raindrops acting as a poor dielectric, on the power of the electromagnetic wave. The reduction is significant for frequencies above the VHF band.

The fall due to snow is similar to the fall due to raindrops. It is more decisive for frequencies above 2 GHz above the UHF band. The reduction due to hail loss is determined by the size of the pieces and their density. Since ice has a lower refractive index, the reduction due to hail loss is much lower than the reduction due to raindrops.



Ionospheric waves



Propagation affected by the presence of a gas-plasma electron high in the earth's atmosphere.

The transmission of radio signals over long distances is generally limited to the high frequency (HF) part of the spectrum (approximately 3 - 30 MHz), although ionospheric transmission can occur at other frequencies in unusual circumstances.

The HF signals are refracted by the ionization regions that are high in the earth's atmosphere. These regions are known as the ionosphere and consist of layers of ionized gas molecules and free electrons. It is the electron gas plasma that can deflect electromagnetic waves and prevent them from escaping into space. Ionized gas molecules are not considered to play an important role in radio frequencies.

Much of the early research work on ionospheric propagation was done by Professor E.V. Appleton FRS in the 20s at Cavendish Laboratories in Cambridge.

The exact radiofrequency that can be refracted depends on the electron density (increasing with the height from the ground) and the angle of incidence of radio waves in the ionosphere. These still depend on the distance between the regions, the types of antennas in use and the ionosphere limit.

The amount of ionization (and hence the density of electrons in plasma) depends on the flow of radiation (solar ultraviolet) from the sun. Separation of electrons from gas molecules creates plasma and ions. The level of radiation from the sun depends on the time of day, the time of year and the phase of an 11 year (approx.) Cycle of activity. A good estimate of the level of solar activity can be found by calculating the number of spots visible on the surface of the sun. These correlate well with the solar radiation level and are easy to observe with a solar telescope.



During the day, the solar flux increases as the sun's rays enter the atmosphere and with the density of electrons (ionization extends down through the atmosphere). This allows high frequencies (perhaps around 20 MHz or higher) to be propagated over long distances. The signals refract from the ionosphere and return back down to Earth. Long distances can be supported in this way. At night when the solar flux is low, electromagnetic radiation is less absorbed and lower frequencies are propagated better.



The ionosphere is divided into layers as follows:

Layer D. This is formed during the day at a height of about 50 - 90 kms from the ground. Since the density of the atmosphere at this low altitude is quite high, a lot of absorption is made especially at low frequencies (up to 7MHz). High frequencies are unaffected and can penetrate to higher levels with relatively low reduction. During the night, layer D dissolves rapidly and affects the very bottom of the spectrum.

Layer E. This layer is about 80 -125 kms from the ground.