Satellites used for broadcasting are usually in a geostationary orbit 37,000 km (23,000 miles) above the Earth’s equator. The advantage of this orbit is that the period of revolution of the satellite is faster than the rotation of the Earth, so the satellite is in a fixed position in the sky. Thus, a satellite dish antenna that receives a signal can be aimed at the satellite’s location at all times, and it does not need to look at a moving satellite. Some systems instead use a highly elliptical orbit with an inclination of +/- 63.4 degrees and an orbital period of about twelve hours, known as the Lightning orbit.
Satellite television, like other communications transmitted via satellite, begins with a transmitting antenna located at the point of the uplink . Satellite uplink antennas are very large, ranging from 9 to 12 meters (30 to 40 feet) in diameter. The larger diameter results in more accurate pointing and signal amplification to the satellite. The uplink dish is aimed at a particular satellite, and uplink signals are transmitted in a specific frequency band so that they can be picked up by one of the frequency bands tuned to that frequency band on board that satellite. The transponder transmits the signals back to Earth on a different frequency (a process known as conversion is used to prevent uplink signals), usually in the 10.7-12.7 GHz band, but some still transmit in the C-band (4-8 GHz), Ku-band (12-18 GHz) or both. The part of the signal path from the satellite to the receiving earth station is called the downlink.
A typical satellite has up to 32 Ku-band or 24 C-band transponders or more for Ku/C hybrid satellites. Typical transponders have a bandwidth of 27 to 50 MHz. Each geostationary satellite must be 2° longitude away from the next satellite to avoid interference; for Ku the distance can be 1°. This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites or 360/1 = 360 geostationary Ku-band satellites. C-band transmissions are sensitive to terrestrial interference, while Ku-band transmissions are rain dependent (since water is an excellent absorber of microwaves on this feature). The latter is even more negatively affected by ice crystals in thunderstorm clouds. Sometimes a solar outage occurs when the sun lines up directly behind the geostationary satellite that the receiving antenna is pointed at.
The downlink satellite signal, rather weak after traveling a long distance (see the law of inverse squares ), is collected by a parabolic receiving dish that matches the weak signal to the focal point of the dish. A device called a horn or collector is mounted on the brackets at the center point of the antenna. The horn is a section of waveguide with an expanding front end that collects signals at or near the focal point and transmits them to a sensor or transducer connected to a low-noise block frequency converter (LNB). The LNB amplifies the signals and down-converts them to a lower intermediate frequency (IF) block, usually in the L-band.
Initial C- A low-noise amplifier (LNB) connected to the signal horn at the center point of the antenna was used in the satellite TV band system. The amplified signal, still at higher microwave frequencies, must be fed through a very expensive 50-ohm impedance low-loss gas-filled rigid coaxial cable with relatively complex N- connectors with an internal receiver or, in other designs, a down-converter (mixer and voltage tuned oscillator with some filter circuitry) to down-convert to intermediate frequency. Channel selection is usually controlled by a voltage-tuned oscillator, with this voltage fed through a separate cable to the headend, but this design evolved.
Designs for microstrip converters for amateur radio adaptations were adapted for the 4 GHz C band. Central to these designs was the concept of a block conversion of the bandwidth to a lower, more easily handled IF.
The advantages of using an LNB cheaper cable to connect the indoor receiver to the satellite TV antenna and the LNB, and signal processing technology in the L-band and UHF was much cheaper than signal processing technology in the C-band frequencies.
