Satellite antenna construction

Satellite antenna construction involves the following equipments: a communication satellite which would send and receive the electromagnetic waves, a parabolic or spherical reflector which would receive the signals sent from the communication satellite and send them to the feedhorn, the front end of the waveguide. The radiowaves or the electromagnetic waves are converted into electrical signals by this device and then transmitted through the coaxial feeder to the sets. The noise levels are also kept in check by this same device.
The reflector is designed in the shape of a parabola because of the property of this geometrical shape. It reflects all the waves coming along the axis to the focal point. Thus the reflector receives the electromagnetic waves from the communication satellites and sends it to the feedhorn at its focus. The reflector is made up of a metal and converted to form a paraboloid of revolution. This forms the diameter of the antenna and this paraboloid has a distinct focal point. The waveguide is placed here.
If lower frequency is used, the designer gets more flexibility to choose its material. The large size dish is typically made out of a metal mesh on a metal framework. Some designs have solid dish with perforations. The feedhorn is a low gain type i.e. it has a broad electromagnetic wave beam width. This is required so that the transmitted signals can overcome even the rough terrain. The very wide beam also helps in reducing the noise and is more reliable. Some of the more complex antennas like Cassegrain antenna have got a secondary reflector which gets illuminated by the energy fed into it and reflects the beam back into the main reflector which then forms the desired beam onto the focus. These sub reflectors are located away from the focal point and helps in the better support of the focal device. The signals are then fed to the radio frequency transmitting devices through coaxial feeders or cable transmission lines.
The directive gain or theoretical gain for a parabolic reflector is given by
G=
Where G is the gain
D is the reflector diameter, units same as wavelength
? is the wavelength of the signal
The above equation shows that as the signal frequency increases (or wavelength decreases) the theoretical gain also increases. Also, the larger the aperture of the reflector, the better the gain achieved. However, due to practical considerations, the actual gain obtained is always less than the theoretical gain. It is roughly 35-55% of the total directive gain.
Practically when the above formula was used to calculate the gain for a 2500 centimeter diameter antennas found in Very Large Array(VLA) and Very Long Baseline Array(VLBA) radio telescopes which are operating at frequency of 1.42 GHz, the gain obtained was of the order of 0.14 million times. The actual gain also depends on other factors like accuracy of shape of reflector, its surface finish and feeedhorn matching.