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Setting up a satellite TV ground station; Pt.3 Setting up a satellite ground station is quite straight­forward – once you have the necessary equipment. The main job involves aiming the dish antenna at the desired satellite. By GARRY CRATT There are many satellites visible from Australia, all with varying power levels and program content. In order to set up a satellite earth station that will provide satisfying results, it is important to carefully consider the available satellites. This will determine the required dish size and operating band. There are also some government restrictions in place, pre­venting overseas broadcasters from offering pay TV services direct to the Australian general 40  Silicon Chip public. This situation should change after 1997 when “deregulation” of the industry takes place. As discussed previously, two frequency bands are used for satellite TV delivery – C band (3.7-4.2GHz) and K band (12.25-12.75GHz). C band (3.7-4.2GHz) is mainly used by international broadcasters, while K band (12.25-12.75GHz) is used for domestic satellite transmissions. The main sources of K-band signals are the Optus B1 and A3 satellites, with occasional teleconferencing carried on Panamsat’s Pas-2 satellite. Generally, the Optus satellites are used as a national delivery system. Among other things, they carry B-MAC transmis­sions such as the ABC, SBS, Queensland Television, Imparja and the Golden West network. These transmissions are designed as a service for remote area viewers and are collectively called HACBSS (Homestead and Community Broadcast Satellite Serv­ice). B-MAC signals can only be received using authorised B-MAC receivers. Without one, no intelligible picture or sound can be received. Unfortunately, the cost of a B-MAC receiver (which will also receive PAL signals) is quite high, at around $2000. The commercial TV networks Table 1: Optus B1 Satellite Channels (K-Band) Transponder Pol. User Mode Decoder Table 2: C-Band Satellite Channels IF (MHz) Audio Intelsat (180°E) 1 V Not Allocated 977 TV NZ; BBC; ITN 964MHz 2 V Not Allocated 1041 TV NZ; ABS 983MHz 3 Lower V Network 9 PAL/NTS Not Required 3 Upper V Network 7 E-PAL 4 Lower V Interchange PAL 4 Upper V Network 10 E-PAL 5 Lower V Network 9 PAL Not Required 5 Upper V Not Allocated 6 Lower V Omnicast FM2 Available 1282 6 Upper V Sky B-MAC Not Available 1308 7 Lower V ABC HACBSS B-MAC Available 1344 7 Upper V SBS HACBSS B-MAC Available 1370.5 8 V Network 9 PAL Not Required 1425 9 H CAA Air to Ground SCPC Scanning Receiver 1009 10 H Pay TV MPEG-2 Available 1996 1073 11 H Pay TV MPEG-2 Available 1996 1137 12 Lower H Network 9 E-PAL 12 Upper H Not Allocated 13 Lower H ABC Interchange PAL Not Required 150.5 13 Upper H ABC Radio Digital Not Available 1276.5 14 Lower H ABC HACBSS B-MAC Available 1313 Gorizont 19 (96.5°E) 14 Upper H SBS HACBSS B-MAC Available 1339 CCTV 4 (China) 1320MHz 15 Lower H QTV RCTS B-MAC Available 1376 AZTV (Turkey) 1425MHz 15 Upper H QTV Data B-MAC Not Available 1402 Network 1 (Russia) 1475MHz also use the Optus satellites to distribute regular programming, using an en­crypt­ion system called E-PAL. Considerable effort is required to unscramble E-PAL and, because the material is subject to copyright, there is little point in expending any effort to decode these signals. In addition, there is a third type of programming known as the “news interchange” service. This material is broad­cast in PAL and is designed to be received by regional TV sta­tions for terrestrial redistribution. It includes entire pro­grams destined for subsequent rebroadcast, news feeds from port­able uplink stations or overseas affiliates, and 30 second “promo” advertisements. There are also many hours of direct un-edited programming which is rebroadcast (after standards conversion) from the Intel­ sat 4GHz service. Typically, services such as CNN, Skynet, BBC World News and many others can be received in the course of a single 24-hour period. Not Required 1094 7.38/7.56 NBC; Network 9 1012MHz 1120 7.38/7.56 RFO Tahiti 1100MHz 1156.5 7.38/7.56 ABC; CNBC; NHK Tokyo 1135MHz 1182.5 7.38/7.56 Worldnet 1178MHz 1219.5 7.38/7.56 CNN 1252MHz NBC/CNBC 1275MHz NBC; ITN; Network 10 1385MHz Occasional Use 1431MHz 1245.5 1188 Panamsat PAS-2 (169°E) 7.38/7.56 1214 6.60/6.60 C-band signals can come from a number of satellites, in­ cluding the “Gorizont” class spacecraft carrying Russian and Chinese language broadcasts, the American Hughes HS-601 satel­lites carrying US and Asian originated programming, and the Rimsat series of spacecraft, leased to countries such as India, New Guinea, and China. Tables 1 and 2 list the available satel­ lites and channels. In the case of Optus K-band satellite reception, a 1.6-metre dish, an LNB (low noise block converter), and a feedhorn are required, along with the satellite receiver. For C-band reception, a 3-metre dish will provide good reception of most of the available international satellites. However, there are some instances where a smaller dish can be used; eg, for dedicated single satellite reception. Aiming the dish Connecting up the system is really no more difficult than connecting the components of a typical hifi system. CNBC (USA) 1035MHz NHK Tokyo (Japan) 1110MHz CNN (USA) 1183MHz MTV 1345MHz Rimsat G2 (142.5°E) EM TV (PNG) 1260MHz ATN (India) 1475MHz However, the dish must be correctly aimed at the satellite in order to receive TV programs. For every location in Australia, there is a different set of “pointing co-ordinates” to aim the dish at a satellite. These dish pointing co-ordinates can be calculated using a commercial software program and most equipment vendors will also calculate them on request. (Note: a dish pointing program for PCs is avail­able from Av-Comm for $15 – Cat. S-1000). Most programs require the satellite longitude, the site latitude and longitude, and the magnetic variation from true north to perform the calculations. Typically, they output the magnetic bearing, the true bearing and the angle of elevation. Fig.1 shows the azimuth and elevation “look angles” for the Optus B1 satellite across Australia. Often, the site latitude, longitude and magnetic variation can be obtained from a local airport. If this source is unavail­able, many general aviation supply outlets carry maps July 1995  41 Copyright Warning Satellite TV reception can be a very satisfying hobby, similar in many ways to shortwave listening. Reception is fortu­ itous and you never know what you may see. However, it is always wise to remember that whatever programming is seen is subject to copyright laws. In particular, readers are warn­ed that the commercial use of such programming invites prosecution unless permission has been obtained from the copyright holder. view of the appropriate part of the sky, unobstructed by buildings, trees or any other objects. There are four critical parameters which must be accurately set, in order to align the dish with the desired satellite and to receive signals: (1) elevation above the horizon; (2) the azi­muth; (3) the focal point; and (4) the LNB (low noise block) polarity. The easiest way to correctly point the dish is to set the elevation first. This can be done using a timber batten, a cheap plastic protractor and a plum bob (eg, a nut tied to a piece of cotton). By affixing the cotton to the centre of the protractor and then holding the protractor against the batten, the angle formed will be equal to the angle of elevation – see Fig.2(a). Fig.1: this diagram shows the azimuth & elevation “look” angles for the Optus B1 satellite which is located in geostationary orbit at 160° longitude. (Aussat Network Designer’s Guide). known as WAC charts (World Aeronautical Charts), which show these details. The magnetic variation for the earth station site is important if a compass is to be used to align the dish. For example, for locations around Sydney, magnetic north is 12.6°E of true north. This means that 12.6° must be subtracted from the true azimuth if using a compass to set the heading. Aiming the dish So having decided on a satellite, assembled the necessary system components and obtained (or calculated) the azimuth and elevation co-ordinates, the dish must be pointed in the correct direction. The dish should have a clear PROTRACTOR LEVEL PLACED MIDWAY UP DISH RIM TIMBER BATTEN ANGLE OF ELEVATION PLUMB BOB (a) ANGLE OF ELEVATION (a) Fig.2: this diagram shows two different methods of measuring the dish elevation. In Fig.2(a), the elevation is measured using a plumb bob & a plastic protractor, while in Fig.2(b) the elevation is measured using a protractor level (eg, from a combination square set). 42  Silicon Chip D Another way of measuring the elevation is with a protractor level (eg, from a combination square set). This is placed on the rim of the dish, as shown in Fig.2(b). The magnetic azimuth bearing (as calculated by the pointing program) can be set using a compass, taking care to ensure that it is kept well away from any stray magnetic metal. Alternative­ ly, if a compass is not available or the magnetic variation is not known, the true azimuth figure can be used, provided that the location of true north is known. To find true north, we need to calculate the midpoint of the day on which the dish is to be set – and we need a sunny day! This is done by first obtaining the times for sunrise and sunset (eg, from a local airport or observatory) and calculating the midpoint of the day. Next, position a pole vertically in the ground. At the calculated midpoint, the shadow cast by the stick will be aligned with true north. It’s then simply a matter of measuring the azimuth angle from true north using an inexpensive protractor and marking out the line of direction (eg, using a pegged string line or a cardboard template). The dish can then be pointed along the marked line. While all the foregoing implies that FOCAL POINT VT The receiver can be tuned to individual transponders during the setting up process by setting the voltage on terminal VT of the tuner module. Table 3 shows the tuning voltages for several transponders on the Optus B1 satellite. the dish must be precisely aimed using these techniques, in practice it is not as complicated as that. All that is required initially is to aim the dish in the general direction of the satellite. A series of “fine-tuning” adjustments can then be made later on, when a picture is visible. Once the dish elevation and azimuth are correctly set, the focal point should be determined. Most manufacturers provide this figure but if not, the focal point can be calculated using the formula F = D2/16C, where D is the diameter of the dish, and C is the depth – see Fig.3. By the way, the depth (C) can easily be measured by stretching a piece of string across the front of the dish and then measuring the distance from the string to the deepest part of the dish. The result indicates the degree of curvature of the dish and determines the location of the feedhorn and LNB. Once the result is known, clamp the feedhorn into position at the correct distance from the centre of the dish. Finally, the polarity of the LNB must be set. In practice, this is done Table 3: Tuning Voltages (Optus B1) Transponder IF (MHz) Format Voltage 3 Lower 1094 PAL 2.95V 4 Lower 1156.5 PAL 3.8V 5 Lower 1219.5 PAL 4.2V 7 Lower 1344 B-MAC 6.8V 8 1425 PAL 7.74V 13 Lower 150.5 PAL 5.16V after a signal is acquired and involves rotating the LNB for best reception of the desired transponder (after the front panel controls have been set). At this stage, some consideration should be given to the routing of the cable from the LNB to the receiver. Among other things, the cable includes a low-loss double-shielded 75-ohm coaxial section which is used to carry the converted block of signals (950-1450MHz) and also to carry the DC supply voltage for the LNB. In addition, the cable has separately insulated conduc­tors for the “Skew Out” connections (where required). The cable should be routed so that C 12281.9 12344.5 F (FOCAL LENGTH) F = D 2/16C Fig.3: the focal point (F) of the dish can be calculated by measuring its depth (C) & its diameter (D) & plugging these values into the formula F = D2/16C. VERTICAL HORIZONTAL 2 1 M 12407.1 12469.7 9 3 10 12270.5 12313.2 12375.8 12532.3 12594.9 12657.5 12720.1 5 6 7 8 4 11 12 13 14 15 12438.4 12501 12563.6 12626.2 12688.8 Fig.4: the transponder layout of the Optus B series spacecraft. Note that adjacent transponders have alternate polarities to minimise interference between them. July 1995  43 +18V 1.5k 680 8 SCAN SPEED VR1 1M 68k 6 IC1 566 7 4 1 100 12VW SCAN RANGE VR2 5k Q1 BC548 150  680  SCAN ON/OFF S1 TUNING VOLTAGE VR3 10k 150  150  D1 1N4148 680  12k TUNING VOLTAGE TO VT 0.1 Fig.5: this scanning circuit produces a triangle waveform which is fed to terminal VT of the tuner module. It allows the entire satellite IF block to be scanned for a signal at a selected rate while the dish is being positioned it can not be tripped over, run over by a lawn mower or subjected to other accidents. If buried underground, it should be run through plastic conduit. This offers good protection and, in the event of a fault, allows the cable to be pulled through and replaced. Final adjustments By far the easiest way to make the final adjustments is to have the receiver and TV set (or video monitor) at the dish site. That way, signals can be directly observed as the dish is aligned. However, before optimising the dish alignment, we must first tune the receiver to a transponder. This can be done simply by setting the correct tuning voltage for that transponder on the tuner module. This is the voltage present on terminal VT in the receiver described last month. Table 3 lists the tuning voltages for a number of transponders. If we study the transponder layout of the Optus B series spacecraft (Fig.4), it can be seen that adjacent transponders have alternate polarities. This is done to minimise interference between transponders and thus maximise frequency usage. For example, transponder 13 is adjacent to transponder 5 and these are horizontally and vertically polarised respectively. By adjusting the receiver tuning to the desired voltage, we can use the corresponding satellite transponder as a beacon to align the dish. For Optus B1, we recommend using transponder 7 – a B-MAC signal –to align the dish. This is a strong transponder and even if the LNB polarity is initially incorrect, will be recognisable as an unscrambled B-MAC signal (see photo). Once a signal is acquired (ie, adjust 44  Silicon Chip The LNB is adjusted by backing off the retaining clamp & rotating the assembly for optimum reception. This video printout shows the typical appearance of an unscrambled B-MAC signal. VR4 until VT reads 6.8V), the dish elevation, azimuth, and LNB polarity and can all be adjusted for best reception. By then tuning a PAL transponder, further visual improvements can be achieved. Note that the LNB is adjusted by undoing its retaining clamp so that the entire assembly can be rotated. Make sure that it remains at the correct focal point during this procedure, however. For reception of other satellites, select a suitable trans­ p onder IF frequency from Table 2. By way of example, a common IF frequency used on Gorizont and Rimsat space­­craft is 1475MHz, while 1105MHz can be used for Intelsat 511 and Pas-2. Scanning circuit For those adventurous enough, the circuit shown in Fig.5 can be built. This is a scanning circuit and was brought to our attention by Herb Miller, a reader from Perth. It uses a 566 voltage controlled oscillator (VCO) based on IC1 and this gener­ates a triangle waveform at its pin 4 output. This in turn drives transistor Q1 which produces a triangular ramp waveform at its collector output and this in turn becomes the tuning voltage for terminal VT of the tuner module. VR1 sets the scanning speed by varying the frequency of the VCO, while VR2 sets the scanning range. Switch S1 allows the scanning function to be switched on or off. When off is selected, the receiver can be manually tuned using VR3. Power for the circuit (+18V) can be derived from the receiver. By fitting an extra 6.5mm socket on the rear panel, the scanning voltage produced by the circuit can be fed to terminal VT of the tuner module via a matching plug. Note that the switch contacts inside the socket must be wired so that, when the plug is inserted, they break the existing connection to VT. The scanning circuit can be built into a separate enclo­sure, or internally wired. When the external scanner is unplugged, the tuning voltage from VR4 is automatically reconnected to the tuner module. Altern­ ative­ ly, if the scanner is built inside the satellite receiver, an additional toggle switch can be added. By using the scanning function, the entire satellite IF block can be scanned for a signal at a selected rate while the dish is being positioned. Once optimum performance has been achieved, the dish can be permanently secured in position. You are now ready to begin exploring the exciting world SC of satellite TV.