Fullscreen HTML5Med Res (??MB)HTML4

The $1000 satellite TV ground station is based on a 1.8-metre pressed steel dish as shown in the foreground of this photo. Unlike other dishes which use a steel mounting pole cemented into the ground, this unit uses a simple but effective "stand". The author's older 1.8-metre segmented dish is in the background. Tuning in to satellite T\T, Pt.3 This Ku-band ground station can receive TV pictures from Aussatfor less than $1000. In this article, we take a look at the equipment and describe how it's set up to tune into Aussat 1. By GARRY CRATT Satellite enthusiasts will no doubt be impressed with the price tag of this satellite ground station. Imported by Av-Comm Pty Ltd, the system comprises a 1.8-metre pressed steel dish , a magnetic polariser and feedhorn, a low noise block converter (LNB), a 99-channel remote controlled receiver, and the cable to connect the components at the dish "head" to the receiver. 4 SILICON CHIP Installing the system is no more difficult than plugging in the components of a typical hifi system. The assembly of the dish is uncomplicated and the complete receiving station can be assembled in about 30 minutes, despite the fact that the only instructions are two dish assembly diagrams printed on the outside of the shipping carton. The dish comes packed in a single heavy duty carton and weighs about 50kg. Unlike other dishes which use a steel mounting pole cemented into the ground or bolted to the top of a building, this unit uses a simple but effective "stand". This allows the dish to be used on balconies or in courtyards, where the installation of a 2metre length of 100mm steel pipe would be impractical. It's also means that you don't have to worry about cementing a steel pipe into the ground. Siting the dish The siting of the dish is most important. Under no circumstances must the "view" that the dish has towards the satellite be obstructed by nearby buildings or trees. Any obstruction will prevent reception. Naturally, the dish must be adjusted for correct elevation and azimuth and this is all part of the installation procedure. For example, the Aussat 1 pointing co-ordinates provided by the supplier for our location were: elevation 50.07°, azimuth 8.89°. The elevation is easily set using a timber batten, a plastic protractor and a plumb bob (eg, a nut or bolt tied to a piece of cotton). By affixing the cotton to the centre of the protractor (either using glue or by drilling a small hole and then tying the cotton at that point) and holding the protractor against the batten, the angle formed with reference to the horizontal will be equal to the angle of elevation. Fig.1 shows the details. The azimuth can be set using a magnetic compass and this procedure will be described later in this article. Of course, the compass and protractor method are only used to determine the initial pointing directions. We'll describe how to precisely adjust the dish for best reception later on. You should also consider the expected reaction from neighbours when this 1.8-metre dish appears in your backyard. Even though the dish is powder coated a pleasant neutral grey colour, try to site the dish to minimise the impact on your neighbours. Where possible, the dish should be mounted so that it is out of sight (eg, behind a fence, a screen or bushes), whilst still maintaining a clear view of the satellite for good reception. The mounting stand Before installing the dish, you need to have a level mounting surface to accept the stand. This can easily be Fig.2: the elevation of the dish is adjusted by sliding a square section of steel tubing hinged to the apex of the dish inside another square section affixed to the base of the stand. This heavy duty thumbscrew then clamps the two sections together. Fig.3: after the dish is removed from the carton, you have to remove two steel locking tubes like the one shown here, so that the dish can pivot on the hinges at the front of the stand. · checked by using a spirit level and then packing the site with bricks or timber blocks to ensure a level mounting surface. Due to the unorthodox, but highly adaptable mounting system used, some explanation regarding assembly is appropriate. In this design, the elevation of the dish is adjusted by sliding a square section of steel tubing hinged to the apex of the dish inside another square section of tubing affixed to the base of the stand. When the dish is elevated to the correct angle, a heavy duty thumbscrew clamps both sections together - see Fig.2 After removal of the dish from the shipping carton, two steel tubes, used to lock the frame to the dish for protection, must be removed and discarded. Fig.3 shows the location of one of these steel tubes. Once this is done, the dish is free to pivot on the two hinges at the front of the stand. Next, an "L" shaped section of steel tubing, which is supplied taped together with the three feedhorn support arms to the mounting stand, must be released and fitted between the apex of the dish and the base of the mounting frame. Note: this L-shaped section contains the elevation adjustment thumbscrew - see Figs.4(a) and 4(b). The dish can now be tilted to a suitable angle (say 45°) and locked into position, allowing easy access to the mounting points for the three feedhorn support arms. These arms must be attached under three of the -TIMBER BATTEN Fig, 1: the angle of elevation can be set by using a protractor to measure the angle formed between a plumb bob & a wooden batten fixed across the face of the dish. Fig.4(a): this view shows the fully assembled 1.8-metre dish. Note the L-shaped section at the rear. This contains the elevation adjustment thumbscrew. AUGUST 1991 5 Fig.4(b): the L-shaped section is attached to the base of the stand via a hinged joint. This allows the L-shaped section to move as the dish is tilted to achieve the correct elevation. be fitted together, to form one assembly. The feedhorn and polariser are supplied together in a single box, together with the necessary screws, so these two items should be assembled first. The LNB, which is supplied with a rubber gasket, is then secured by four stainless steel screws to the feedhorn/polariser assembly. Because the aperture at the mouth of the LNB is rectangular, it is easy to align this correctly with the corresponding aperture of the polariser body. Fig.6 shows the complete LNB/ polariser/feedhorn assembly while Fig.7 shows the assembly after it has been clamped in position. At this stage, some consideration should be given to the routing of the cable from the LNB to the receiver. Among other things, this cable includes a low-loss double-shielded 75ohm coaxial section which is used to carry a DC supply voltage from the receiver to the LNB and to carry the converted block of frequencies (from 12.25-12.75GHz) to the receiver. In addition, the cable has three separately insulated conductors: polarity, ground and signal strength (normally used for remote dish adjustment). The cable should be routed so tha·t it can not be tripped over, run over by the lawn mower, or subjected to other stress. If buried underground, the cable should be run through plastic conduit. This offers good protection and in the event of a malfunction, allows the faulty cable to be pulled through and replaced. The cable is terminated with "F" connectors and is small enough to be run through floorboards quite discreetly! The receiver Fig.5: the three support arms for the feedhorn are bolted to three of six bolts around the circumference of the dish. Mount the arms at intervals of 120°, then attach the feedhorn mounting plate to the arms at the centre of the dish. six bolts around the circumference of the dish and spaced 120° apart. This is easily done by undoing each of the three bolts and passing the h ead of each bolt through the corresponding hole on each support arm. Fig.5 shows the detai ls. After affixing all three arms, which may droop slightly at this stage of partial assembly, the feedhorn mounting plate must be attached to the three arms. This plate is a galvanised piece of steel with three recessed locating 6 SILICO N CHIP tabs at 120° intervals, each having the same shape as the ends of the support arms. The arms are now secured using the galvanised bolts and nuts supplied. In addition, there is a galvanised clamp which holds the throat of the feedhorn in position, as shown in Fig.7. Feedhorn assembly Prior to mounting the feedhorn in this clamp, the LNB (low noise block), feedhorn and magnetic polariser must At the other end of the cable, mounted indoors, is the receiver. It supplies the DC voltage (18V) to the LNB and decodes the IF block converted by the LNB. In addition to IF tuning, the receiver also features variable IF and audio bandwidth , adjustable audio subcarrier tuning, adjustable polarity for the LNB, and a programmable digital readout. The receiver is supplied pre-tuned to all 15 A us sat transponders, to make initial operations easier. It's difficult enough to adjust both dish elevation and azimuth at once, without having to also tune the receiver for an active transponder. Once the dish is aligned and the system optimised, the receiver Fig.6: this close-up view shows the complete LNB/ polariser/feedhorn assembly, prior to installation on the mounting plate. The two leads shown emerging from the polariser are run to the receiver via the coaxial cable. tuning can be trimmed up. As all the receiver information is stored in an EEPROM, the units can be shipped around the country pre-programmed, without fear that a memory back up battery will go flat in transit. The receiver features video and audio outputs, and a modulated RF output (channel 3/4). Hence there are a number of methods of connecting the unit to a televi- Fig. 7: the feedhorn is held in place at the centre of the dish by a galvanised clamp attached to the feedhorn mounting plate. Adjust the feedhorn position & orientation for optimum signal as described in the setting up procedure. sion set. The modulator output can be fed directly to the TV set and this is satisfactory if the set is being used solely for satellite TV use . If it is also used to watch terrestrial television, a switch should be installed in the antenna input lead. These items are commonly sold as "TV game switches" for less than $10 in most electronics stores. Alternatively, the audio and corn- posite video outputs could be fed to a VCR, and thence to the television receiver, or to a video monitor and separate audio amplifier. Of course, the audio can be connected to the AUX input of any hifi system, although the inputs to the amplifier must be tied together so that both audio channels are driven. Tandy stores carry a suitable adaptor, which splits a single RCA female to twin RCA male connectors. The ultimate distribution method is to purchase a "video sender", which is a small, low-powered TV transmitter operating on a UHF TV channel. These units have a range of around 15 metres or so and enable satellite TV signals to be viewed on any suitable UHF-equipped TV set in the house. These units are also available from Tandy for around $100. Final adjustments Fig.8: the low noise block (LNB) is connected to the receiver via low-loss 75Q coaxial cable. Route the cable so that it is well out of the way or, better still, bury it in plastic conduit. Once the dish has been positioned as closely as possible 'to the specified azimuth and elevation settings, the cable should be connected to the receiver. For initial dish set up, nothing beats having the receiver and the TV set at the dish. This can normally be achieved by using a long 240 volt extension cord to power the equipment at the dish site. Remote indicators such as receiver audio and signal strength metering are impractical at such an early stage of the installation. We initially set the receiver to transAUG UST 1991 7 Fig.10: the receiver supplies the DC voltage (18V) to the LNB and decodes the IF block converted by the LNB. In addition to IF tuning, the receiver also features variable IF and audio bandwidth, adjustable audio subcarrier tuning, adjustable polarity for the LNB, and a programmable digital readout. The receiver is supplied pre-tuned to all 15 Aussat transponders & features video & audio outputs, plus a modulated RF output. ponder 7 which is a strong B-MAC signal. A good indication that the system is working is to connect the LNB to the receiver and watch the impulse noise increase. Once this is verified, the dish can be aligned. Because the elevation has been set as described previously and because the receiver is pretuned to a known active trans- ponder, the azimuth of the dish can now be set. To do this using a magnetic compass involves taking the magnetic variation at the location of the Earth sta. tion into account. In our case, the azimuth for Aussat 1 was given as 8.89° but to compensate for the magnetic variation for Sydney, we had to Fig.11: view inside the receiver. When a watchable picture has been obtained, trimpot RV6 (at the end of the pen) is adjusted so that the S-meter reads halfscale as described in the text. After that, the feedhorn polarity & focal length are carefully adjusted for maximum reading on a digital voltmeter connected between the receiver's S-meter terminal (on the rear panel) & ground. 8 SILICON CHIP subtract 11.5° (ie, the compass bearing was 357.39°). All this sounds very complicated. In reality, all we had to do was take a rough reading in the general direction of the satellite and then gently rotate the dish until a picture was received. However, knowing the approximate compass reading certainly helps. When some kind of picture is visible, a series of "fine tuning" adjustments must be made. Feedhorn adjustment Apart from the elevation and azimuth which can both be adjusted for best reception, the position of the feedhorn must also be adjusted so that the focal length and polarity are correct. This can be done by backing off the pressure on the feedhorn mounting clamp so that the entire assembly can be rotated, and also so that the distance from the dish surface to the mouth of the feedhorn can be adjusted. Once the picture and sound are watchable, further adjustments must be more scientifically monitored. To do this, we connected a digital voltmeter between ground and the "Sm et er" terminal at the back of the receiver. The cover is then removed from the receiver and meter drive potentiometer RV6 adjusted so that the receiver S-meter reads half scale and so that a reading of about 70mVis obtained on the DMM. Fig.11 shows the layout of the receiver circuit board. The feedhorn polarity and focal length are now adjusted for a maximum reading on the DMM. Often, this exercise must be repeated several times for optimum performance. A2 A3 156°E 164'E SATELLITE ENillUSIASTS! • • Fig.12: the dish can easily be moved to view each of the three Aussats by cementing three sets of galvanised bolts into the ground. Aussat 1 is at 160°E longitude, Aussat 2 at 156E & Aussat 3 at 164E 0 • 0 1.5M Dishes complete $445 High performance Ku Band Hemt LNC's 2 yrs warranty .. $370 • Complete B-Mac Systems .. $2895.00 • Complete C Band Systems • Dishes Small through to Large • Pal Receivers ... $595.00 • Complete range of cables, connectors and accessories Call now for FREE Catalog, Information Bulletin and SPECIALS .. (02) 489 5474f Note that these latter adjustments may not result in any visible improvement to the received picture. This is because the satellite uses FM and once the receiver is in limiting, no noticeable improvement can be made to the received signal. However, by making these fine adjustments to squeeze the maximum performance from the dish, we are building up a performance margin which will compensate for rain attenuation, or a reduction in satellite transmit power. Securing the dish Once the maximum performance level has been achieved, the dish can be secured using the galvanised "U" brackets provided. However, if more than one satellite is to be received, the dish cannot be locked down permanently. On our system, we used heavy duty steel tent pegs obtained from the local disposals store to anchor the dish. Of course, a more permanent method would be to cement Fig.13: if space is a problem, the mounting locations for the dish can be overlapped. Don't forget to subtract the local magnetic variation when taking compass bearings. galvanised bolts into the ground in the appropriate locations, so that the dish could be moved from one set of mounting bolts to another for different satellites. Fig.12 shows how this can be achieved. Once successful reception has been achieved, most enthusiasts will want to explore the potential of their satellite receiving system. Besides television services, Aussat also carries a number of SCPC (single channel per carrier) radio programs. These can be found by connecting a scanner, capable of reception in the 950MHz to 1450MHz band. Experience has shown that most of these radio channels are horizontally polarised and are located around the 1200MHz area, using wideband FM as the mode of transmission. It would be improper to list such frequencies here but any scanner fitted with a search function and capable of covering the appropriate frequency range will uncover these transmissions. To couple the LNB feed to the antenna input of a scanner, a splitter without DC feed capacity must be used. If the correct splitter is not used, the DC supply voltage from the receiver to the LNB will damage the scanner. Suitable splitters are available from most satellite hardware suppliers. Finally, a note of caution. While there is no harm in viewing unencoded programming from Aussat to technically improve reception equipment, or out of interest in international affairs, or because of curiosity as to how TV networks operate, enthusiasts Videosat Pty. Ltd. [ 83Alexandria Pde. Wabroonga NSW 2076 Fax (02) 489 3557 _ \ ~ VJOEQS~T_. J ... 1 ._ Mr/Mrs/Ms .......................... ............ . Address .......... ................... ............... . Telephone .................................... sq"191 * For all your Satellite Needs - Nationwide * should be reminded that program material is protected by copyright laws and that some program material may be proprietary. Also, the transmissions described in this and previous articles are not designed to be received by parties other than those who have paid the appropriate fee to do so, such as the transponder lessee, etc. This means that enthusiasts are in a privileged position to be able to receive such transmissions without cost. Any use of material broadcast through Aussat for commercial gain would therefore be absolutely illegal. SC Equipment Availability Footnote: the satellite receiving system described in this article is available from AV-Comm Pty Ltd, phone (02) 949 7 417 (see adverhsement this issue). Similar satellite TV equipment is also available from .Videosat Pty Ltd, 83 Alexandria Pde , Wahroonga 2076. Phone (02) 489 5474. AUGUST 1991 9