Satellite antenna theory

Satellite antenna theory works on the principles of a geometric property of a parabola. When the parabola receives waves parallel to its axis, it reflects them to a single point known as focus. The concentrated waves at this point become a stronger signal. This is the reason why all the satellite antennas are manufactured in the shape of a parabola. The signals in the form of microwaves which are received from the communication satellites are directed towards its focal point. This is done so as to have a very high signal at one point and it is easier to amplify these concentrated signals.
At the focus of the antenna, a device called feedhorn is used. Feedhorn is a part of the waveguide which captures the microwaves coming around the focal point and sends it to the LNB (Low noise block converter). Sometimes the incoming beams after getting reflected from the dish may get obstructed in reaching the focal point due to the waveguide. For avoiding such a situation, the antenna theory suggests to offset the signals.
To obtain the desired offset, the antenna theory suggests raising the amount of signal by an amount equal to the amount of offset desired. Thus, to obtain an offset of 10 degrees, the signals have to be raised by 10 degrees. The feedhorn receives the signals at this focal point and transmit it to LNB which focuses the signals onto a specific small area. The signals received are amplified. During amplification process, some noise is also generated but it is kept at a very low level. This is done to avoid any problems in further transmission.
The antenna theory further states that the signals are then converted from electromagnetic (microwaves are electromagnetic waves) to electrical signals by the Low Noise Block converters. The theoretical or directive gain is proportional to the square of frequency and circular aperture of the parabola. Thus, the theoretical gain increases as the frequency of signal increases. However, practical constraints reduce the theoretical gain by 40-45% and the actual gain obtained is 55-60% of the theoretical gain.
This antenna theory thus makes it clear that the feedhorn does not receive the signals directly. Rather the signal is first reflected from the parabolic dish to the focal point where the feedhorn, the front part of the waveguide, converts the microwaves to electrical signals and transmits them. It also ensures that the noise introduced in the system is of the lowest level. The noise gets amplified in the rest of the stages. The noise may distort the audio and the video quality of the transmission. The formula used for the theoretical gain is:
G=
Where G is the theoretical gain
D is the reflector diameter in same unit as wavelength
? is the wavelength of the signal