Satellite dishes. Purpose

Why do we need an antenna?

Uninformed people believe that to receive a signal is to amplify this signal to a level sufficient for human perception or any device. In fact, the task of receiving any signals is to distinguish a weak signal against the background of strong interference. Indeed, if it was only a gain, then a direct gain receiver could be used to receive arbitrarily weak signals. Weak signal - we put the amplifier, a little - we put another one. We install several amplifiers “in cascade” - and now the voices of our brothers in the mind are heard ... It was not there: instead of voices, we would hear a roar and a roar. The signals we need are hundreds and thousands of times weaker than the "unnecessary" signals of numerous radio and television stations, the spurious emissions of electric cars, the electromagnetic radiation of the sun and stars. Amplification does not solve the reception problem, the amplifier equally amplifies both the useful signal and harmful noises. The receiving system should carry out the selection of the signal, its separation from noise and interference by some specific features. Two main features: frequency (wavelength) and direction. The frequency selection is performed by the receiver (receiver). Practically all modern receivers, from a pocket radio receiver to a radar receiver, are heterodyne. With the help of a tunable local oscillator (local oscillator) the required signal is transferred to some intermediate frequency. The signal at the intermediate frequency is passed through a band-pass filter, which extracts only the band occupied by the signal from the entire frequency spectrum. Noises and unnecessary signals at other frequencies are cut off. However, even in this band, the noise power can be many times greater than the power of the useful signal. In this case, the receiver is powerless, he has already done everything he could. It will only help selection for another sign - in the direction. It provides a directional antenna. The task of the antenna is to receive the signal coming from one particular direction with maximum efficiency, and at the same time to receive as little as possible unnecessary signals and noises coming from all other directions. The receiving properties of a directional antenna are concentrated in a small solid angle around the direction of the signal source due to the attenuation of these properties in all other directions. Thus, the signal power increases, and the noise power decreases in comparison with the same values ​​at the output of the omnidirectional antenna.

Why are special antennas used for satellite television?

In television, a unit of time requires transmitting hundreds of times more information than is required for transmitting, for example, sound. Therefore, with the same transmission methods, the television signal occupies a much wider frequency band than the radio signal. The television signal bandwidth in the Russian standard D is 8 MHz, while the AM signal band of a radio station is approximately 0.01 MHz. For television, long, medium and short waves were unsuitable: in the DV bands (30-300 kHz) and CB (0.3-3 MHz) it is simply impossible to accommodate even one television channel, but in the entire KB band (3-30 MHz) they fit there would be only three television programs. For terrestrial television, the meter (30–300 MHz) and decimeter (300–3000 MHz) wavelength ranges are used. The higher the frequency, the more the properties of radio waves approach the properties of light. The meter and decimeter waves "do not know how" to bend around the round surface of the Earth, they extend only in a straight line, at a distance of direct visibility. For this reason, the antennas of terrestrial telecentres climb high towers, the greater the height of the television tower, the greater the range of the television transmitter. However, even the television tower of large cities provide reception at a distance of no more than 100-200 km. In today's world, television programs need to be broadcast to entire countries and continents. This problem is solved by satellite television. If you place a television transmitter and antenna at a huge height above the Earth, in its direct visibility will be almost half of the earth's surface. Here a number of problems arise. For this transmitter to work, it needs electricity. Wires from the Earth to the satellite can not be reached, it will be prohibitively expensive to bring fuel to it, there is nothing to divert heat from a nuclear reactor. Therefore, all satellite equipment is powered by solar panels, and the power of satellite television transmitters is small, usually 100-150 watts. For comparison: terrestrial television transmitters in large cities can have a power of from 1 to 25 kW. Another problem: if the satellite rotates around the Earth, sooner or later it turns out to be on its reverse side, it goes beyond the horizon - like the Moon, for example. This is unacceptable, because television programs are needed around the clock. Therefore, the satellites used for television are launched into the so-called geostationary orbit, which is located exactly in the equatorial plane at a distance of 35,786 km from the Earth's surface. At this distance, the force of gravity is such that the satellite can orbit with an angular velocity exactly equal to the angular velocity of the Earth. Therefore, the geostationary satellite rotates with the Earth, remaining stationary relative to its surface, as if it was planted on a needle passing through the center of the Earth. 35768 km - a huge distance, it is almost three times larger than the diameter of our planet! Thus, the low power transmitter is located very far from the receiver, therefore the satellite television signals are very weak. To provide reception, a number of technical solutions are used. In analog satellite television, instead of the usual amplitude modulation, frequency is used, which made it possible to reduce the signal-to-noise ratio necessary for reception by more than 100 times. The payoff for the gain was an increase in the occupied frequency band: for the signal with the FM instead of 8 MHz, it took 36 MHz. Even the UHF range for such signals has become cramped, so satellite television uses the range of centimeter waves (3-30 GHz), or rather, its two sub-bands, the so-called C-Band (“band band”, 3400-4200 MHz) and Ku- Band (Kei Band, 10700-12750 MHz).

Another solution is the use of highly directional antennas, both on the satellite itself and in the receiving system. Satellite transmitting antenna amplifies the transmitter signal, concentrating power on the desired direction. However, it is impossible to make the gain of the transmitting antenna too large - then all the power will be concentrated on too small a portion of the earth's surface. The more territory you need to provide with broadcasting, the less is the gain of the transmitting antenna. The power of the satellite transmitter is distributed over a certain area. The distribution is uneven. At the point of the service area, which is located on the axis of the transmitting antenna, the maximum signal strength is created. This point is called the aiming point. In general, the farther from the aiming point, the weaker the signal. To assess the signal strength for each satellite in different geographic points, special maps are used, called footprints or coverage zones. In Russian, such cards are sometimes referred to as more harmonious: "service area". On them, the isolines connect the points of the earth's surface, which correspond to the same EIRP, the equivalent isotropically radiated power (Equivalent Isotropic Radiated Power, EIRP). This is the power that a satellite transmitter would have to have if it worked on a non-directional (isotropic) antenna in order to create at a given point on the earth’s surface a signal of the same power that a real transmitter with a directional antenna creates. In other words, the eirp is a value that takes into account the power of the satellite transmitter and the directional properties of the antenna for a particular direction. The eirp of modern satellites in the center of the service area can be several hundred kilowatts. Many satellites have not one, but several transmitting antennas for broadcasting to different countries, for example, to Europe and South Africa. In this case, a separate map is drawn up for each "beam" (beam).

Now that we know that the power of the satellite signal is distributed over the area, it is easy to understand the purpose of the satellite receiving antenna. Her task is to “collect” this power from a certain area and concentrate at one point where it will be converted into an electrical signal. It is clear that the larger the area of ​​the antenna, the greater will be the power of the useful signal at its output. A very important conclusion follows from this: the amplifying properties of the antenna depend only on its size, and practically do not depend on the design, shape, material, etc. In practice, the quality of the antenna is also of great importance - the “curve”, mechanically damaged or simply made roughly, will work worse.