All About the Technology Behind Satellite TV

Usually, highly elliptical or geostationary satellites that orbit above the earth’s equator are used for television relays. Satellite television begins by receiving transmission through antenna connected to the uplink facility. The dishes used in uplink satellites are considerably larger in size (about 30 to 40 feet diameter). This increased diameter leads in providing more accurate aiming and enhanced signal strength. Such uplink dish is positioned in a way that its faces toward a particular satellite. Signals from this uplink dish are transmitted within a particular frequency range. This enables the transponders tuned to similar frequency range to receive the signals. These received signals are then retransmitted by the transponder back to the earth with different frequency range. This signal path - retransmitted by satellite and received by earth station - is known as the downlink. A satellite may have up to 32 transponders or 24 transponders for Ku-band and C-band only satellites respectively. A transponder has bandwidth of approximately 36 to 50 Mbits/s. Every geo-stationary C-band satellite is spaced 2 degrees from another satellite for preventing possibility of interference. 1 degree spacing is considered adequate for K band. It is evident from the above that there is a maximum limit of 360/2 = 180 and 360/1 = 360 for geostationary C-band satellites and geostationary Ku-band satellites respectively. Transmission received through C-band can be affected by the terrestrial interference while transmission received through Ku-band is susceptible to the water (rain). A parabolic dish receives the signals transmitted by the downlink and reflects these signals back to the focal point of the dish. A device known as feed horn is mounted at the focal point of the dish. In fact, f the feed horn is a front-end section of a waveguide. This waveguide collects the signals at the focal point, conducts the signals towards pickup connected to an LBN (Low-noise block) down-converter. The main function of LNB is to amplify the signals, filter the frequencies and convert the frequency to a lower frequency range. In earlier days Low Noise Amplifiers connected at dish’s focal point were used in C-Band satellite TV systems. During those periods, 50 Ohm impedance cables were used for sending the amplified signal to indoor receiver. These 50 Ohm coaxial cables were considered very expensive. To overcome the cost factor, micro strip based converters were adapted, for Amateur Radio frequencies, for the 4GHz C-Band. These designs were based on the centralized idea that blocks down-conversion of a frequency, to a lower frequency and technologically easy conversion of block of frequencies can be achieved through this design. To facilitate this, satellite dishes were fitted with an LNBF that can integrate the feed horn with the LNB. By using the latest technology, the conversion of signals to a desired output (data, television, audio etc) can be achieved easily. The satellite receiver also demodulates and converts the transmitted signals to desired form. At times, the receiver may include the responsibility of unscrambling or decrypting. Such receivers are known as integrated decoder/receiver. Usually, low loss type (like RG-6 or RG-10, etc) cables are used for connecting the receiver to LNB or LNBF.