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PART '1 Satellites are now used to distribute a raft of TV signals into & around Australia. This series discusses the basics of satellite TV reception, looks at the signals available, and shows you how to tune into these signals using a low-cost commercial ground station. By GARRY CRATT ' The ;;./;~I{) i~l~1I8:~~i~;); This 1.8-metre dish is sited in a Sydney suburban backyard & receives good TV pictures from Aussat 1. The programming carried includes American news feeds (eg, CNN - Cable Network News) plus various interstation feeds between local stations. Note the low-noise block (LNB) at the focal point of the dish. 14 SILI CON CIIII' first man-made satellite launched into orbit was the Russian" Sputnik 1" in August, 195 7. It circled the Earth in a low polar orbit that required constant tracking adjustment to receive the Morse code signal it transmitted. Later, in December 1958, the US Airforce launched a satellite named "Score" which transmitted a pre-recorded message from the President. It too was launched into a polar orbit. The world 's first geostationary (or geosynchronous) satellite was designed and built by the Hughes Aircraft Corporation in 1963. It was called "Syncom" and had the capacity to relay either a single TV channel or 50 separate telephone conversations from its orbit high above the equator be- --- axis at a fixed rate. This spin stabilisation technique was subsequently used by most other satellites. Subsequent Intelsat satellites launched in 1966 and 1967 also used omni-directional '\ antennas but were placed \ I in positions around the \ I globe as shown in Fig.1. \ I This enabled the \ I Intelsats to illuminate I I I more than 40% of the I Earth. This technique , · I I I now known as a global \ \ I beam, allowed Intelsat to I \ extend communications I \ services to locations both / '\ north and south of the equa/ tor. / / In 1965, the USSR launched a / / domestic satellite communications / ...... system called Molniya (Russian for lightning). It connected isolated villages and towns throughout the Soviet Union with radio and TV broadFig.1: the Intelsat series of satellites launched in 1966 & 1967 were placed in casts originating from Moscow. But geostationary orbit at intervals around the globe so that they covered more than instead of using a geostationary orbit, 40% of the Earth's surface. They employed large omni-directional antennas & carried 240 voice circuits & one TV signal. the Russians deployed several satellites in an elliptical orbit around the lite as it passes overhead (ie, the sateltween Africa and South America. This Earth. As a result, ground stations lite is fixed with respect to the Earth). position allowed it to relay the first had to constantly track the satellite as it passed overhead. real live TV transmissions between If we were to populate the geostationary orbit with satellites spaced North America and Europe. The Molniya system is still used By 1965, the scientific community at 2° intervals, we would only be able today, distributing TV and FM radio programs from Moscow to remote had realised that TV sign als could be to accommodate 180 satellites. This is the limit of the geostationary syscommunities in Russian-influenced easily distributed over vast distances tem. To date, there are 89 satellites in regions of the northern hemisphere. by using satellites in geostationary orbit as relay stations. A geostationary geostationary orbit and orbital "slots" Global coverage are allocated by the ITU (International satellite is one which remains above Telecommunications Union), based on the same spot on the Earth as it orbits. By the late 1960s, Intelsat had deThis means that the time taken for a the needs of member countries. The vised a system where spin stabilised 2° separation limit, by the way, is the geostationary satellite to complete one satellites were given orbital assignsmallest that can be used by small orbit in space is the same as the time ments over the Atlantic, Pacific and Earth terminals on a non-interference taken for the Earth to rotate once on Indian Oceans to cover the entire basis. its axis. globe. By this tim e, the telephone caThe distance above the Earth that a pacity had been increased to 1500 Intelsat 1 satellite must reach to remain in simultaneous voice circuits and one In 1966, the International Telecom- TV circuit. In addition, those satelgeostationary orbit is about 36 ,800 munication Satellite Organisation kilometres (22,280 miles). This aplites carried directional antennas so plies to all geostationary satellites. (Intelsat) launched Intelsat 1 (also that the full available power was radi known as Early Bird) into a geoLogically, the higher an object is , the ated towards Earth. longer it takes to orbit the Earth; and stationary orbit over the Atlantic Subsequent Intelsat satellites dethe lower the orbit, the shorter the Ocean. It had the capacity to transmit veloped in the 1970s carried steerable 240 voice circuits or a single TV sigtime. This is why satellites used for spot beam antennas which were able navigation purposes are located in a nal but, partly because it employed to concentrate transmitted energy into an omni-directional antenna, a large low polar orbit where they can orbit powerful beams on desired areas of proportion of the available energy was the Earth every 90 minutes. the globe. Also, by the late 1970s, the radiated in directions where it could A geostationary orbit has two major capacity of the Intelsat satellites had not be used . advantages over a polar orbit: (1) the been improved to a level whereby distance between the satellite and the In order to keep the satellite an3,750 telephone circu its and two TV receiving station is constant; and (2) tenna oriented in the correct plane for channels co uld be carried simultanetransmitting towards Earth, the cylinthere is no need to continuously move ously. Of course, these signals were drical body ofintelsat was spun on its the ground antenna to track the sate!designed for reception by large Earth '' " --- "' ''', ---- - - ]UNE 1991 15 stations having a parabolic dish antenna measuring 10-20 metres in diameter. Home satellite TV It wasn't until late 1975 that a British experimenter, Steve Birkill, intercepted satellite TV transmissions using a small Earth station. The signals came from ATS-6, a satellite loaned to India by NASA and located in a temporary orbital slot over the Indian Ocean. It was all part of an experiment to show how satellite technology could deliver educational television to small communities scattered · throughout India. Using a 1.5-metre antenna made from wire mesh, Birkill was able to receive transmissions from the satellite in his back yard in Britain, thousands of kilometres away from the centre of ATS-6's footprint. It is largely due to his pioneering efforts that private individuals realised that the reception of satellite TV was •within their means. But Birkill wasn't alone in his efforts . Several innovative American enthusiasts, including Bob Cooper and Taylor Howard, were also busy developing reception techniques based on surplus military equipment. Their efforts were directed at amateur radio operators and experimenters alike. By 1979, popular American electronics magazines carried designs for do-it-yours elf Earth stations, primitive by today's standards, but neverthel ess capable of receiving satellite TV signals. Because of the large US population , there was some commercial motivation for networks in the USA to utilise satellite technology to deliver TV programming and, later, exclusive pay-TV services. In Australia, however, the Intelsat series of satellites was mainly used to provide a link to the Northern Hemisphere. It was not until 1980 that Intelsat IV was used by the ABC to relay programming from Sydney to outback locations in Western Australia. These signals were received by dedicated large Earth stations and re-transmitted terrestrially using the PAL system. In addition, as Australian TV networks developed, they increasingly relied on satellites to relay US news programs. As a result, several networks leased transponders on a series of Intelsat satellites which broadcast TV signals into Australia. These signals were also designed for reception by large Earth stations and sometimes used scrambling techniques to ensure privacy. By 1980, a few satellite television enthusiasts in Australia had developed techniques which allowed them to monitor these TV transmissions. In addition, it also became possible to receive a later generation of Russian Sqtellites named Gorizont, as well as domestic satellites serving Indonesia (Pala pa). And as technology improved, the cost of the receiving equipment dropped, thus making access increasingly easier. Receiving systems Generally speaking, there are two frequency bands used for satellite television reception. The oldest system operates in the "C band" which covers from 3.8-4.ZGHz. This system re- Fig.2: this map shows the signal footprint from a 12W transponder on one of the Aussat satellites. The contours indicate lines of equal signal strength. Note how the signal level decreases as the distance from the beam centre (or boresight) increases. -15 ·20 I I I L ____ i__, ·25 I I ·30 .35 ·40 115 16 120 125 SILICON CHIP 130 135 140 145 150 155 ANGLE OF ELEVATIONPERPENDICULAR TO EARTH 'S SURFACE LINE PARALLEL TO EARTH 'S SURFACE (a) Fig.3(a): the single pole fixed mount technique is the easiest way of mounting a dish if only one satellite is to be viewed. The elevation is adjusted using a turnbuckle or threaded rod arrangement. ANGLE OF ELEVATIONPIVOT MANUAL _- Em~11g~ - MANUAL AZIMUTH ADJUSTMENT (b) Fig.3(b): if more than one satellite is to be viewed, the alternative El/Az (elevation/azimuth) mount can be used. The arrangement shown here allows manual adjustment of both elevation & azimuth. quires a large dish for good reception - about 3 to 6 metres, depending on the satellite signal or "footprint" level. The "footprint" is simply the illumination pattern from the satellite as it falls on the Earth and this is often depicted as a map with contours showing the signal strengths. But no matter what type of beam pattern is transmitted by the satellite, the footprint provides the strongest signal in the centre of the pattern. As the distance from the beam centre (or "boresight" as it is often called) increases , the signal level progressively decreases. When planning a ground station, the local signal strength is obviously an essential piece of information. This information is available in various specialist publications which show the orbital assignments and footprint coverage for all international satellites. A satellite footprint map indicates the performance that satellite engineers expect at a particular. In some cases the level may be higher than indicated but more often it is lower, especially as the output power of the satellite drops with age. The contours of a footprint map are expressed in dBW (decibels referenced to one watt power). Fig.2 shows the footprint over Australia from a 12W transponder on one of the Aussat satellites. To obtain the boresight EIRP (effective isotropic radiated power) level, the gain of the transmitting antenna must first be added to the power level of the spacecraft transmitter. It's then a matter of subtracting any losses caused by the feedline and multicouplers on board the satellite, and the path loss which is of the order of 200dB or so. Because of these losses, the signal intensity on the Earth is often below the level of ground noise. This is why particular attention must be paid to dish accuracy and size , as these two parameters play a critical part in determining the performance of an Earth station. For example, a 6-metre dish typically has a gain of about 45dB as opposed to about 40dB for a 3-metre dish. Obviously, a 6-metre dish installation would be impractical in a typical backyard. However, a 3-metre dish, using suitable electronics to provide reasonable results, could be accommodated. System components This close-up view shows the general arrangement used for the single pole fixed mount technique. Note the long threaded rod which is used for making azimuth adjustments. The dish is clamped to the top of the pole using U-bolts. A satellite system comprises a dish, feedhorn , LNB (low noise block), receiver, video monitor and audio amplifier. The dish, the most obvious component of a satellite system, is normally parabolic in shape and made from steel, aluminium or fibreglass sheeting impregnated with a reflective coating. Some manufacturers use mesh instead of sheeting, to achieve a reduction in wind resistance. The dish must also be coated with some kind of weatherproofing material to prevent corrosion and pitting of the reflective surface. The most important parameter of the dish is its gain, and this is dependent on the accuracy of the parabolic surface of the dish. Any imperfections or deviations of 2mm or more from a perfectly parabolic surface can mean a significant drop in efficiency. Smaller dishes (up to 2 metres in diameter) may be produced in one piece, either spun from aluminium or, if fibreglass, made in a mould. However, one-piece construction is impractical for the larger dish sizes which are often made up of a number of identical "petals". This makes transportation to the site far easier. There are several methods of mounting a satellite dish, the method selected depending on the us er's needs . If the obj ective is to view one satellite only, then a simple single pole "fixed mount" will be acceptable (Fig.3a). The dish is fix ed on top of a pole and the elevation (above the horizontal plane) adjusted using a turnbuckl e or length of threaded rod. Somet imes a car jack can be used under the front of the dish as the elevation mechanism. Th e pole is cement ed into th e ground, so that it is strong enough to support the dish and also to prevent any movement in strong winds. This is important, as any movement of th e dish in either the horizontal or vertical planes by more than 25cm can caus e degradation of the picture. For users interested in observing more than one satellite, the "El/ Az" mount is suitable (Fig.3b). This mounting metliod allows th e dish to be moved from one satellite to another, by readjusting the elevatio n and the azimuth. Although this can be tedious, the El/Az mount is simpl er to construct than th e "polar" mount, which allows geosynchronous satellites to be tracked with only azimuth ad justment to the dish. ]UNE199 1 17 F • F=D'/16C Fig.4: the focal point of the dish can be calculated by measuring its depth (C) & its diameter (D) & plugging these values into the formula F = D2/16C. In operation, the dish must be positioned so that it has a clear view of the satellite. The view must not be blocked by trees, buildings or any other objects, as this will eliminate all signals. In addition, the bottom front lip of the dish should ideally be mounted one metre or so above gro und level. to minimise ground noise. Different dishes The purpose of the dish is twofold: (1) to collect the maximum available signal; and (2) to focus this signal on the feedhorn. It is therefore important to appreciate the different types uf dishes. A parabolic dish can have either a shallow or deep parabolic curve. This in turn will determine the focal point, which is important for correct placement of the feed horn components. Every dish has a design "focal length to diameter" ratio. This simply means that the amount of curvature built into the dish establishes a relationship between the diameter and the distance from the back of the dish to the focal point. This ratio can easi ly be calculated by measuring th e diameter across the front of th e dish and th e distance from the back of the dish to the point of intersection across the front of th e dish. In practice, this can be easi ly done using a piece of string stretched across 18 SILICON CHIP the front of the dish, and a rigid tap e measure. Fig.4 shows the formula for calculating the focal point. Once this has been calculated, the F I D ratio can be derived. One confusing aspect of all this is that a dish having an F ID ratio of 0.5 is actually shallower than one having an F/D of 0.25 , although the magnitude of the ratio might seem to indicate otherwise. The advantage of using a deep dish is that it produces better side lobe rejection, thereby reducing the effects of unwanted terrestrial signals. This is often quite an advantage for C-band users, as there are often terrestrial microwave links operating in the same band that can cause interference. There are still many Telecom links operating around Australia on this band, each operating at a signal level hundreds of times more powerful than the satellite signal. It is important that the feedhorn illuminates the entire dish, not just part of it, to achieve maximum efficiency. Feedhorns are manufactured to suit dishes having a particular F/D ratio , so the choice of feedhorn is important. There are several different feedhorn arrangements. The most simple and widely used is th e "prime focus " feed, where th e feedhorn is placed at the focal point of the dish . Whilst this is a simple arrangement, alignment is critical. Fig.Sa shows the details. Another type of feedhorn arrangement is the "cassegrain" feed (Fig.Sb) . This system uses two reflectors - the dish itself and a smaller second reflector at the focal point. This second reflector has Lhe shape of a hyperbola, and reflects the signals through a hole in the centre of the dish to the LNB. This system has a higher gain/noise ratio and is superior in performance to the prime focus system, but only on large dishes where the dish size corn- pensates for the aperture blockage caused by the sub-reflector. Just as a terrestrial TV antenna must be connected to a receiver, the feedhorn of a satellite system must be connected to the antenna. The dish is really only a reflector, and the energy from the feedhorn must be connected to the antenna which is actually a "probe" mounted in the mouth of the COAXIAL CABLE TO RECEIVER (a) SIGN AL FROM / / SATELLITE / (b) Fig.5: the two different feedhorn arrangements. The "prime focus" feed shown at (a) uses a feedhorn at the focal point of the dish while the "cassegrain" feed shown at (b) uses a second small reflector to reflect the signal through a hole in the centre of the dish to the LNB. FROM LOW NOISE BLOCK AT ANTENNA 1450-950MHz 70MHz IF TUN ING VIDEO PROCESSING VIDEO OUTPU T AUDIO PROCESSING AUDIO OUTPU T DEMODULATOR AFC LO CAL OSCILLATOR Fig.6: block diagram of a typical satellite receiver. Frequency conversion from either 4GHz or 12GHz to the first IF (1450-960MHz) is carried out in the LNB & then applied to the receiver for conversion to a second IF at 70MHz. I \ -10 RECEIVER THRESHOLD POINT ii, 40 ~ -20 ~ >:::, ; ~ -30 > ~ ffi c:, ~ ,!,_ '-' w cc :::, 30 1 - - - - ->-- t c c - - - - - - - - - - - - - - fuc:, > cc -40 ,........ 20 ~~~-~~~-~~~-~~~-~~~ 4 -60 20 ~ ,_,. . .~- -50 30 40 50 60 70 80 90 100 110 120 5 6 1 .. POOR FREQUENCY (MHz) Reception techniques These days, frequency conversion from either 4GHz or 12GHz to the first IF (1450-950MHz) is done in the LNB (low noise block converter) which is essentially a low noise amplifier and frequency converter in one package. This converter produces a block of output frequencies 500MHz wide, which is then fed via a coaxial cable to the receiver. The receiver contains a second frequency converter and a local oscillator which is either manually tuned or stepped using a synthesised PLL (phase lock loop) circuit. This provides a second IF at 70MHz which is 8 , ,. FAIR 9 1 .. 10 11 GOO□ 14 15 16 EXGELLENT VIDEO RECEPTION QUALITY Fig.7: typical 70MHz SAW filter response. The steep skirts ensure that signals interfering with the 70MHz IF are substantially rejected. LNB. To make this connection we use a waveguide, which is far more efficient than coaxial cable because it uses air as the dielectric. The physical dimensions of the waveguide determine the impedance. Because the probe is fixed when the LNB is manufactured, it cannot be moved to allow reception of different polarity signals. Nor is it convenient for the user to have to rotate the LNB every time a transponder having a different polarity is selected. To counter this problem, various mechanical rotation devices have been used over the years with varying degrees of success. One "no moving parts" solution is to use a dual polarisation feed. This system uses the principle of Faraday rotation where an axial magnetic field is applied to a waveguide containing ferrite material. A current carrying coil is wound around the ferrite and the resulting magnetic field changes the polarisation of the incoming signal. 7 Fig.8: the video quality drops markedly when the relationship between the C/N (carrier to noise) & SIN (signal to noise) ratios becomes non linear. then amplified, filtered and detected using a balanced demodulator to provide a baseband output. This baseband output contains all the video and sound subcarrier information. After further filtering to remove certain video components from the audio subcarriers, the video is processed using a standard video detector. The audio is demodulated using either a PLL capable of covering 57MHz or a quadrature detector operating at 10.7MHz. This second approach allows the use of standard wideband FM filters but does not allow any flexibility when detecting either very wide or very narrow audio subcarriers. Fig.6 shows the block diagram of a typical satellite receiver. To maximise satellite use, a number of transponders or satellite channels are allocated within the downlink passband. These may be either full or half transponders, having a bandwidth or either 36MHz or 18MHz respectively. Because the signals are FM, every effort should be made to obtain a signal level that's sufficient to take the receiver into limiting. This means making the system as efficient as possible. One part of the receiver circuit that plays an important role is the IF filter. The IF filter is normally a SAW (surface acoustic wave) type, with very steep skirts to ensure that interfering signals to the 70MHz IF are rejected by a substantial amount. Fig. 7 shows a typical 70MHz SAW filter response. This is necessary due to the large volume of interfering signals on low band VHF. Obviously, if the banrlwidth of the filter is 36MHz and the bandwidth of the signal being received is 18MHz, a considerable amount of noise will also pass through the filter, degrading the carrier to noise ratio (C/ N ratio) of the receiver. If a filter with a bandwidth of 18MHz is used instead, there will be in improvement of 3dB in the C/N ratio. In practical terms, a half transponder signal can be received using an IF filter having a bandwidth as narrow as 10-lZMHz. The trade off is less intense colour and slightly noisy audio against what can be up to a 6dB improvement in C/N. This is considered very worthwhi le, as most users can tolerate a video signal of less than This view shows the feedhorn & LNB used at the focal point of the 1.8-metre dish. The LNB boosts the signal & provides frequency conversion. JUNE 1991 19 days, noise temperatures of 25°K and 115°K are achievable in C and Kuband LNBs respectively (each costing less than $500). Fig. 9 shows the C/N improvement that can be achieved using an LNB having a lower noise temperature. ...;z ~ > 1.5 g:: 1 0 ~ ~ 0.5 0 ---~--~------'------' 60 50 70 80 90 LNB TEMPERATURE (°K) Fig.9: C/N vs LNB temperature for LNBs with a 120°K (top) & 100°K noise temperature. Note that for a 20°K drop in LNB noise temperature, there is a 0.BdB improvement in carrier to noise (C/N) ratio. broadcast quality, particularly if it is to be viewed on a standard TV set. Even so, it is important to realise that Earth stations must be designed to maximise the incoming signals, whilst keeping external and internal (semiconductor) noise to a minimum. This relationship is expressed as the system carrier to noise ratio, which is calculated by adding the system noise level an d the signal carrier level, and dividing the sum by the noise level alone. Every receiver has a threshold point expressed in dB C/N. As the C/ N falls below threshold, the video becomes increasingly noisy. Fig.8 shows the video reception that can be expected as the receiver drops below threshold. The threshold of the receiver is defined as the point at which the relationship between the carrier to noise ratio (C/N) of the incoming signal and the signal to noise ratio (S /N ) of the resulting video becomes non linear. Above the threshold point, each ldB increase in C/N will cause a corresponding 1 % increase in S/N for the video signal. However, when operating below threshold, a ldB increase in C/N can cause several dB improvement in the video SIN. Polarisation Another important factor to be taken into consideration is the polarisation of th e satellite downlink. In order to achieve some degree of frequency reuse, dual polarity is often used on satellites. As there is sufficient isolation for opposite polarity signals to co-exist (normally 30dB or so), this effectively doubles the number of transponders that can be carried within the satellite bandwidth. While the Australian Ku-band system operated by Aussat uses both horizontal and vertical polarisation , Intelsat IVA and Intelsat V both use circular polarisation. This requires a modification to the feedhorn, to convert from circular to linear polarisation. A righthand circularly polarised signal possesses an electromagnetic fi eld that rotates in a clockwise direc- VERTICAL HORIZONTAL tion while a lefthand circularly polarised signal rotates anticlockwis e. The standard scalar feed designed for linear polarisation can reduce the margin of any Earth station by 2dB unless it is modified. One modification technique is to insert a dielectric plate into the throat of the feedhorn at a 45° angle relative to the orientation of the LNB. probe (Fig.10). When the dielectric insert is positioned on the righthand side of the probe, the feedhorn will intercept right hand circularly polarised (RHCP) signals. Similarly, when the dielectric is placed on the lefthand side of the probe, the feedhorn will intercept lefthand circularly polarised (LHCP) signals. But while this is the simplest and most commonly used method of converting circularly polarised signals to linear polarisation, it is not the most efficient. For C-band reception, where signals are very weak indeed, the use of a "hybrid mode" feed is recommended. This kind of feed uses a series of adjustable screws or, in later models, transitional steps inside the waveguide to perform the polarity RIGHT HAND CIRCULAR LEFT HANO CIRCULA.!1 Fig.10: one technique for converting from circular to linear polarisation is to insert a dielectric plate into the throat of the feedhorn at a 45° angle relative to the orientation of the LNB probe. THIS END FITS INTO SLEEVE ATTACHED TD WAVEGUIDE f SCALAR HORN SLIDES OVER THIS END I Noise temperature As one of the contributing factors to C/N is the internal noise generated by the semiconductor amplifiers in the LNB , by far the most critical parameter when selecting an LNB is its noise temperature. The lower the LNB noise temperature, the higher the performance. In fact, for every 20°K drop in noise temperature, there is a co rresponding 0.6dB improvement in C/N. These 20 SILICON CHIP 1 50mm DIA. COPPER TUBE Fig.11: for C-band reception, the "hybrid mode" feed is used for polarity conversion. This type of feed typically consists of a copper tube fitted with a series of adjustable screws. conversion. Fig.11 shows the details of a screw-type hybrid mode feed. Transmission modes The three major video standards in the world today - PAL, NTSC and SECAM - are also used for international satellite TV transmissions. NTSC is used by the USA and Japan; PAL by the UK, various other European countries , Australia, New Zealand and China; and SECAM by the French and the USSR. Whenever one of these video formats is uplinked to a satellite, the downlink format uses precisely the same standard. This means that it is necessary to use an NTSC monitor to observe NTSC pictures in colour, a PAL monitor to receive PAL pictures in colour, and a SECAM monitor to receive SECAM pictures. Alternatively, for international ~eception, a multi-standard video monitor is ideal as often all three video standards can be carried on the same satellite. In addition to the three world video standards, Australia has also adopted B-MAC as a transmission standard for the Ku-band Aussat system. B-MAC signals use a time multiplex system to create an audio data baseband consisting of a multilevel code that is transmitted during the video signal horizontal blanking period. The maximum data rate of a B-MAC transmission is 1.8Mb/s, while the overall bandwidth of a ·B-MAC signal is just over 6MHz. The B-MAC system is also user addressable and this is a great advantage for pay TV operators who can switch off any clients who fall behind with their payments. Unfortunately, B-MAC is considerably more expensive than PAL, at least as far as the user is concerned. This is considered by many to be the reason Australia's Aussat system has not been as popular as predicted. Audio services Although television reception may often be the prime motive for establishing an Earth station, there are also many single channel per carrier (SCPC) audio services that can be received. Many radio networks transmit SCPC signals by satellite to regional stations around the country. In addition, this system is used on Aussat by the Department of Transport and This US news program was received from Aussat 1 using a 1.8-metre dish linked to a low-cost satellite receiver. In addition to TV signals, the Aussat satellites also carry numerous audio signals from radio networks. Communications to provide communications between aircraft and ground based control zones. The Indonesian Pala pa series of Chand satellites also use the SCPC technique for transmitting FM radio and TV sound signals. SCPC signals are usually located at evenly spaced intervals across the transponder bandwidth. To maximise the number of signals that can be carried, compander circuits are often used to compress the peak deviation level of the audio prior to frequency modulation. This means that expander circuits must be used on the ground to restore the audio signals to their original state. Because an SCPC signal only has a relatively narrow bandwidth, a single transponder can conceivably carry hundred's of different audio signals. In practice, the bandwidth is totally dependent on the amount of deviation. A voice grade SCPC signal may occupy no more than 5kHz, while an audio channel might occupy 60kHz or so of bandwidth. Although modern satellite receivers can usually receive SCPC signals, older receivers cannot. However, this does not preclude the reception of SCPC audio services. A scanner can easily be used for this job by connecting it directly to "tap" off the incom- ing block of frequencies from the LNB. These days, scanners can easily cover the 950-1450MHz band and if fitted with a "search" facility, as most are, can tune SCPC signals with relative ease. Naturally, the IF feed to the scanner must have the DC component removed prior to connection. Another method used to multiplex voice and data signals onto a satellite circuit is FDM (frequency division multiplex). This method is primarily used for voice grade telephony circuits and utilises SSB. In practice, 12 individual SSB signals are multiplexed together to form a composite baseband signal called a "group", each group containing telephone signals spaced 4kHz apart. Several groups can then be multiplexed together to form a "supergroup " which can contain as many as 3600 separate voice channels. To decode FDM transmissions , a scanning receiver can be ~onnected to the baseband output of the satellite receiver. Telephone signals can then be received by tuning from 500kHz to 11MHz or so, using the SSB mode. That's all we have space for this month. Next month, we will discuss the difference between C-band and Ku-band reception, and look at the programming that's available off the satellites. SC JUNE 1991 21