Satellite Receiving Systems
The idea to use a man-made satellite for communications first appeared at the end of World War II. British mathematician and sci-fi author Arthur C. Clarke proposed to put a communication satellite on a geostationary orbit at a height of 36000 km above the equator from where it can reach up to 40% of the earth’s surface, with television and radio signals which can be intercepted by countless receiving stations within such an area.
A satellite moving in a geostationary orbit above the equator turns around the earth’s axis so that its speed perfectly matches that of the earth’s rotation, and can cover the same part of the earth’s surface with signals all the time. To observers on earth the satellite seems to be motionless at one particular spot in the sky. Such a satellite can replace thousands of local radio or TV transmitters.
Due to the gravitation forces of the Moon, Sun and the other celestial bodies the satellite slightly deviates from its ideal orbit so it is constantly necessary to make some corrections of its position by means of its engines controlled remotely from the earth’s surface. To connect a TV studio on earth with a particular satellite on the geostationary orbit, an “uplink” antenna is used. It concentrates the radio waves and radiates them in the direction of the satellite.
Communication satellites use two sets of frequencies; one set is used to send TV programs or data up to the satellite, and the other for transmitting signals back to earth.
The satellite’s dish antenna captures the signals incoming from earth and passes them into a receiver, where they are processed and transferred to the satellite transmitter, and, via the satellite antenna, sent back to the earth’s surface. TV signals from the satellite are received on the earth’s surface by dish antennas, but the signal strength differs on various parts of the surface. The satellite antenna radiates the signals in a particular pattern called a footprint, which covers a substantial area of the earth’s surface. The signals are strongest at the centre of a satellite footprint and weakest at its edge. Receiving dish antennas at the outside of the footprint must have larger diameters than those at the centre.
Centimeter waves are mostly used for a satellite signal transmission, the frequency range is approx. 3 to 30 GHz. One of the reasons for using these short radio waves is the disturbing influence of cosmic noise for frequencies 1 GHz and lower. For frequencies over 15 GHz the signals are significantly weakened by water vapour in the atmosphere, and by oxygen molecules. Signals or waves of electromagnetic radiation sent from satellites on the orbit have certain permanent spatial orientation. Most frequently the radiation is linearly polarized, either vertically or horizontally. Some satellites also send polarized spiral pattern signals, with electromagnetic waves rotating around the radiation direction in either clockwise or counterclockwise direction.
Table shows the frequency bands used most frequently for satellite communication.
|Band||Frequency range (GHz)|
|L||1.0 – 2.0|
|C||3.6 – 6.5|
|X||7.25 – 8.4|
|Ku||10.7 – 18|
Perhaps the most popular satellites designed for o direct satellite television are Hotbird (at 13.0 E from o Greenwich) and ASTRA (at 19.2 E), using the following frequencies in the Ku band for “downlink” transmission: 10700 MHz – 11700 MHz (so called low band) and 11700 MHz – 12750 MHz (high band). Analogue programmes are mainly transmitted in low band, and digitally coded programmes mainly in high band. As is well known, digital transmission enables compression of broadcast data and, in a given frequency band, it is possible to transfer substantially more TV and radio programmes compared to the classical analog method.
A typical satellite receiving system comprises a dish antenna that reflects the signal from a satellite to a focal point, which is located in front of the antenna. Just over this point is the so called “feedhorn” which captures the signals reflected by the dish and transfers them into another device, a low noise amplifier (LNA). In the LNA the incoming signals are amplified and sent to a down converter, where the incoming signal frequency is changed to an output frequency (so-called intermediate frequency) given by the difference between frequency of a local oscillator of the down converter and that of incoming signals.
The signals of the output intermediate frequency go from the down converter via a coaxial cable to the satellite receiver. The above described parts, i.e. the feedhorn, LNA and down converter, are usually integrated into one block called an LNB. Local oscillator frequencies of currently produced Ku band LNBs are usually based on the universal standard: 9.75 GHz for low band and 10.60 GHz for high band reception. The LNB comprises either one oscillator of 9750 MHz (with such an LNB it is possible to receive only low band signals) or two oscillators, the first of 9750 MHz and the second of 10600 MHz, this is a so called universal LNB that makes it possible to receive both high and low bands. Switching between polarization levels and frequency bands is done from the receiver. The polarization levels are controlled by a 13/17 V DC or by an appropriate DiSEqC command. Bands are controlled by an alternate 22 kHz signal superimposed to 13/17 V DC on the coaxial cable (of amplitude approx. 0.6 V) or by another DiSEqC command. The following Table 2 shows an overview of the main types of Ku band LNBs used on the market and their output frequency (IF), for frequency ranges of 10700-11700 MHz in the low band and 11700-12750 MHz in the high band.
|LNB TYPE||NUMBER OF OUTPUTS||OUTPUTS DESCRIPTION||OSCILLATOR FREQUENCY||OUTPUT IF|
|Single||1||1. LB (V, H)||9750 MHz||950-1950 MHz|
|Single universal||1||1. LB (V, H), HB (V, H)||9750 MHz + 10600 MHz||950-2150 MHz|
|* Dual||2||1. LB (V)|
2. LB (H)
|* Twin||2||1. LB (V, H)|
1. LB (V, H)
|* Twin Universal||2||1. LB (V, H), HB (V, H)|
2. LB (V, H), HB (V, H)
|* Quatro Universal||4||1. LB (V)|
2. LB (H)
3. HB (V)
4. HB (H)
|* Quad||4||1. LB (V, H), HB (V, H)|
2. LB (V, H), HB (V, H)
3. LB (V, H), HB (V, H)
4. LB (V, H), HB (V, H)
V – vertical polarization
H – horizontal polarization
LB – low band
HB – high band
LNBs marked with * are suitable to be used with EMP-Centauri multi-switches.
A receiving system according to Fig. 2, where a single LNB is placed at the focal point, can only be used for reception of TV programmes on one satellite receiver, i.e. usually on one TV set. To connect two or more satellite receivers to one LNB i.e. to one dish antenna, it is necessary to use another LNB type and insert a device called a multi-switch between the LNB and the satellite receiver. LNB types suitable for use with multi-switches are marked with the symbol Q in Table 2. “Dual” and “Twin” LNBs are suitable for distributing low band channels, “Quatro Universal” must be used to distribute both low and high band channels.
All EMP-Centauri switches are usable for distribution of both analog and digital signals. If not stated otherwise all switches are produced for indoor use.For satellite reception of other than the Ku band an LNB for another band can be used, but output intermediate frequency signals must be in a frequency range of 950-2300 MHz.
Operation and utilization of multi-switches will be best explained using the following descriptions of EMP-Centauri products and examples of their application.