A satellite dish is a type of parabolic antenna that receives or transmits electromagnetic signals to and from another location typically a satellite. A satellite dish is a type of microwave antenna. Satellite dishes come in varying sizes and designs, and are commonly used to receive satellite television. Many of the offset type of satellite dishes are sections of a larger parabolic dish. The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB.
A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle was announced by the University of Waterloo in 2004.[1]The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching.With lower frequencies, C-band for example, dish designers have a wider choice of materials.
The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.
A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. For instance, one BBC News countdown shows a "red data stream" being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF.[2]Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band satellite dish of up to 3 metres in diameter for reception of channels from different satellites.
Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. In a single receiver residential installation there is a single cable from receiver to LNB and the receiver uses different power supply voltages (14/18V) to select polarization and pilot tones (22 kHz) to instruct the LNB to select one of the two frequency bands. In larger installations each band and polarization is given its own cable, so there are 4 cables from the LNB to a switching matrix, which allows the connection of multiple receivers in a star topology using the same signalling method as in a single receiver installation.
