Satellite Dish
When the signal reaches the viewer’s house, it is captured by the satellite dish. A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam.
 The curved dish reflects energy from the feed horn, generating a narrow beam. |
The dish on the receiving end can’t transmit information; it can only receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point, just like a concave mirror focuses light onto a particular point.
 The curved dish focuses incoming radio waves onto the feed horn. |
In this case, the point is the dish’s feed horn, which passes the signal on to the receiving equipment. In an ideal setup, there aren’t any major obstacles between the satellite and the dish, so the dish receives a clear signal.In some systems, the dish needs to pick up signals from two or more satellites at the same time. The satellites may be close enough together that a regular dish with a single horn can pick up signals from both. This compromises quality somewhat, because the dish isn’t aimed directly at one or more of the satellites. A new dish design uses two or more horns to pick up different satellite signals. As the beams from different satellites hit the curved dish, they reflect at different angles so that one beam hits one of the horns and another beam hits a different horn.
The central element in the feed horn is the low noise blockdown converter, or LNB. The LNB amplifies the radio signal bouncing off the dish and filters out the noise (radio signals not carrying programming). The LNB passes the amplified, filtered signal to the satellite receiver inside the viewer’s house.
A satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and/or receiving from satellites. A satellite dish is a particular type of microwave antenna. Satellite dishes come in varying sizes and designs, and are most 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. 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. (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. [1])
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.
Motorised satellite dishes are still popular with enthusiasts. There are three competing standards, which often are all supported by a set-top box, DiSEqC, USALS, and 36v positioners. They can only supply one receiver.
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. See, for instance, this BBC News 24 [2] countdown that 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.
In Europe the frequencies used by DBS services are 10.7 - 12.75 GHz on two polarisations H and V. This range is divided into a “low band” with 10.7 - 11.7 GHz, and a “high band” with 11.7 - 12.75 GHz. This results in two frequency bands, each with a bandwidth of about 1 GHz, each with two possible polarizations. In the LNB they become down converted to 950 - 2150 MHz, which is the frequency range allocated for the satellite service on the coaxial cable between LNBF and receiver. Lower frequencies are allocated to cable and terrestrial TV, FM radio, etc. Only one of these frequency bands fits on the coaxial cable, so each of these bands needs a separate cable from the LNBF to a switching matrix or the receiver needs to select one of the 4 possibilities at a time.
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.
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.
The dish is a reflector antenna and almost anything that reflects radio frequencies can be used as a reflector antenna. This has led to dustbin lids, woks and other items being used as “dishes”. Coupled with low noise LNBs and the higher transmission power of DTH satellites, it is easier to get a usable signal on some of these “dishes”.
Another common satellite dish is the VSAT. This provides two way broadband communications for both consumers and private networks for organisations. Today most VSATs operate in Ku band, C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka band satellites operating at higher frequencies, such as Spaceway offering greater performance at lower cost. These antennas vary from 74cm to 120cm in most applications though C-band VSATs may be as large as 2.4m.