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Introduction To Almost every day, in some way, satellite communications affect all Australians. Direct television links across the world are now commonplace and satellite TV usage in the Pacific area is set to expand at an astounding rate. All of this is good news for the electronics enthusiast. Many of the satellite signals covering our country are receivable using inexpensive equipment including our new satellite receiver kit, as this series of articles will explain. Satellite TV has grown enormously since the 1970s, linking millions of people around the world. In 1957, the world’s first satellite (Sputnik 1) 16  Silicon Chip was launched in a low Earth orbit, several hundred miles above the Earth. Sputnik orbited the Earth at a faster velocity than the Earth’s rotation and consequently had to be tracked by ground stations – quite a cumbersome operation. By 1965, the scientific community had realised that three satel­ lites placed strategically around the Earth in geostationary orbit could be used to relay TV and telephone signals. Geostationary orbit A geostationary orbit places a satellite above the equator at an altitude of 37,000km, at a rotational velocity the same as that of the Earth’s. The two main advantages are that the path between satellite and Earth remains constant and there is no need to track the satellite. As the theoretical minimum spacing between satellites is 2 degrees (based on achievable dish beam­width), the maximum number of satellites that can be placed around the equator in geostationary orbit is 180. In practice this is not the case and already there are several orbital locations where more than one satellite is locat­ed. At longitude 19.2 degrees west (over the UK), there are four satellites co-located and operating on a non-interference basis. In the Pacific, there are two satellites co-located at Using a pre-aligned module, you can build a satellite TV receiver to pick up signals from around the world. There are a great many satellite signals to receive & with a dish antenna & an LNB, you can take your pick from a plethora of programs. By GARRY CRATT* *Av-Comm Pty Ltd o Satellite TV May 1995  17 INTELSAT 507 (708 95) INTELSAT 604 INTELSAT 602 INTELSAT 704 (803 1996) RIMSAT (RESERVED) PANAMSAT PAS-4 GORIZONT 16 APSTAR 2 (1995) INTELSAT 501 GORIZONT 19 (STATSIONAR 14) ASIASAT 11 (1995) STATSIONAR 21 PALAPA C2 (1995) ASIASAT 1 PALAPA B2R PALAPA C1 (1995) PALAPA B2P PALAPA B4 JSAT 3 (1995) RIMSAT 2 RIMSAT 1 APSTAR 1 GORIZONT 18 (STATSIONAR 7) RIMSAT 3 G2 STATSIONAR 16 OPTUS A3 (BS 1995) OPTUS B1 OPTUS A2 (A3 1995) PANAMSAT PAS-2 RIMSAT (RESERVED) INTELSAT 701 (801 1996) INTELSAT 703 (802 1996) INTELSAT 511 INTELSAT 503 = C (3.6-4.2GHz) = K (12.5-12.75GHz) = K (11.2-11.5GHz) 57ø 60ø 63ø 66ø 70øE 72ø 80ø 87.5ø 91.5ø 96.5ø 100.5ø 103ø 104ø 105.5ø 108ø Fig.1: this diagram shows the large range of C and K-band satellites which can be received in Australia. 183ø 180ø 177ø 174ø 170.75øE 169ø 113ø 118ø 128ø 130ø 134ø 138ø 164ø 140ø 160ø 142.5ø 156ø 145ø (801) 130 de­grees east longitude. By careful adjustment of the satellite transmit dish “footprint”, power levels and down­link signal polarity, it is possible to operate in this way without causing interference In some cases, extended frequency coverage is also used, requiring wide­ band LNBs and feedhorns at the Earth station. Fig.1 shows the satellites visible in Australia. Since 1966 when Intelsat 1 (carrying 240 telephone circuits or one TV channel) was launched into a geostationary orbit over the Atlantic ocean, there has been a tremendous amount of devel­ opment in launch vehicles. There are presently four countries offering sat- ellite launch facilities and all of these operators have a significant backlog. The 1994 World Satellite Yearbook lists 108 operational satellites distributing television and radio around the world. In addition to this figure, there are scientific, military, weather and navigation satellites. The capacity of satellites has also increased dramatically. The latest Hughes HS-601 spacecraft operated by Optus has the capacity for 30 half transponders of analog television and up to 120 channels of digitally compressed television signals. The satellite is 3-axis stabilised, weighs 3000kg, has 6kW of battery capacity, 50 watt transmit power capacity, steerable antennas covering Australia and New Zealand, and a design lifetime of 13 years, a far cry from the late 1970s when the maximum capacity of any Intelsat spacecraft was two television channels. History of TVRO Small dishes, such as this 1.6-metre prime focus K band dish are usually made of pressed metal or spun aluminium, while larger dishes (eg, the 3.7-metre dish on page 17) are usually of mesh construction to cut down on wind resistance. 18  Silicon Chip Electronics enthusiasts and amateur radio operators have long played an important role in the development of home satel­lite TV equipment or TVRO (television receive only) equipment as it is sometimes known. In 1975 a British experimenter, Steve Birkill, pio- ITU 1 ITU 2 ITU 3 Fig.2: Australia is located in ITU region 3, while the frequency range band in use for K-band satellites in this region is 12.25-12.75GHz. neered the construction of home-made dishes, mi­ crowave amplifiers and receivers for satellite TV reception. In the USA, enthusiasts Bob Cooper and Taylor Howard were busy developing techniques and modifications to allow surplus military equipment to be pressed into service, and subsequently a group of 30 or so radio amateurs began the “Home Satellite TV” industry in the UK and USA. Australia had its pioneers too, like Victor Barker VK2BTV who pioneered reception of Intelsat 3 in the mid 1970s using low-threshold receiving techniques which the experts thought impossi­ble. In fact, it wasn’t until 1980 that satellite distribution was used in Australia to relay ABC programming from Sydney to outback locations in Western Australia. These signals were transmitted by Intelsat 4 and received initially using large spherical anten­ nas, for re-transmission terrestrially to local communities. By 1981, a handful of satellite enthusiasts had developed techniques for receiving these signals. Not only were the ABC transmissions available but so were signals from Japan and the American AFRTS (Armed Forces Radio and Television Service). As technology improved, receiving equipment became more affordable. A microwave amplifier purchased in 1980 cost 10 times the current price and was only 10% as efficient as those available today. After a feasibility study using a Canadian “Anik” satel­lite, specially moved above Australia for various experiments in 1982, the Australian government formed AUSSAT, the body responsi­ble for design, purchase and operation of Australia’s domestic satellite system. The “A” series satellites (Hughes type HS 376) were launched in 1984, 1985 and 1987. The first of the “A” series satellites was re­placed in August 1992 with the B1 satellite, a Hughes HS 600, A1 having exceeded its mission life. The B2 satellite was destroyed at launch in December 1992, while the final satellite in the B series (B3) was successfully launched in August 1994. Fig.3: this diagram shows the coverage of the Optus B1 satellite. Optus is the only operator using K band in our part of the world. For good reception of K band signals, dishes up to 2-metres are required in fringe areas, whilst a 1.6-metre dish will perform adequately along the east coast of Australia. May 1995  19 SOUTH POLAR AXIS ALIGNED WITH EARTH'S NORTH/SOUTH AXIS NORTH DECLINATION OFFSET ELEVATION ANGLE POLAR AXIS ANGLE Fig.4: the geometry of a polar mount. Polar mounts are used where a number of geostationary satellites must be tracked in the azimuth axis. Polar mounts are equivalent to the “equatorial” mounts used by astronomers to make a telescope track the motion of the stars. To date, the B3 satellite remains as an in-orbit spare, the B1 satellite carries most domestic TV traffic, the A3 satellite carries most itinerant traffic, and the ageing A2 “bird” is now in an inclined orbit to save precious station-keeping propellant. The latter satellite is used as a backup to the 0.5 0.4 0.3 0.25 f/D RATIO DIAMETER (D) f f f Frequency bands There are two frequency bands used for satellite television delivery. The oldest system operates in the 3.7GHz4.2GHz range and is known as C band. This band is used internationally and, depending on the satellite power, may require a dish from 3m-5m for good reception. The other band used is known as K band and the frequency allocation depends on the ITU region. Australia is located in ITU region 3 and the frequency band in use is 12.25-12.75GHz. Fig.2 shows the ITU boundaries. FOCAL POINT The only operator using K band (f) in our part of the world is Optus Communications. For good reception of K band signals, dishes up to 2m are required in fringe areas, whilst a 1.6m dish will be adequate along the east coast of Australia. Fig.3 shows the Optus B1 satellite national beam. Fig.5: deep dishes have a shorter focal length than shallow dishes. This allows the feedhorn to be shielded by the dish itself, providing some rejection for terrestrial interference or “TI”. This diagram shows the difference between a shallow & a deep dish. Note the location of the focal point. 20  Silicon Chip Optus fibre optic network across the country. Equipment The most obvious piece of equipment needed is a dish. For C band operations a polar mount dish is desirable, so that geosta­ tionary satellites can easily be located using a single motor drive unit or actuator operating in a single axis. When used on telescopes, this is known as an equatorial mount. It was original­ly devised last century by astronomers who realised it would be much easier to keep a telescope aimed at a particular planet if it could be swivelled around a single axis to exactly counteract the Earth’s rotational motion. The polar axis of the Earth lies parallel with a line drawn through the North and South geographic poles. To achieve this orientation, the axis of the dish is set to an angle which is a function of the site latitude and the difference between the satellite and site longitude. For example, for an Earth station at the equator, where the latitude is zero, the polar axis angle equals zero because the arc of satel­lites can be tracked by moving the dish along a circle directly overhead. Fig.4 shows the geometry of a polar mount. Polar mounts are used where a number of geostationary satellites must be tracked in the azimuth axis. Inclined orbit Some ageing satellites, kept in orbit due to the backlog of launch bookings for new satellites, have been deliberately put into an “inclined” orbit, to prolong their useful life. In this situation, a certain amount of station keeping tolerance is acceptable to ground stations. By accepting the resultant effects of the gravitational pull of the Sun and the Moon, the radiation force of sunlight and the pull of the Earth’s gravitational field, and allowing the satellite to drift within a target “box” in space, a significant amount of propellant can be conserved. However, this does mean that ground stations must track the satel­lite in both azimuth and elevation. To track these satellites, a modified polar mount must be used, having bearings or bushes in both axes, and a declination angle set to zero. As the inclination of these satellites can reach 4 degrees, compared with the geostationary inclination of 0.1 degrees, significant movement of the dish is necessary to maintain the downlink. There exists a patented manoeuvre, called the Comsat Manoeuvre, which cleverly conserves satellite station-keeping fuel. Part of an excerpt from the patent reads “a conventional satel­lite uses an average of 37 pounds of station-keeping fuel for each year of design life . . . approximately 34 This is the view inside the completed Satellite TV Receiver to be described next month. It’s based on a pre-aligned module which makes it easy to assemble & get going. pounds of fuel is used for north/south correction, whilst only 2 pounds is used for east/west correction, and 1 pound for attitude control.” So it is obvious that any kind of manoeuvre that can minimise the amount of fuel used in north/south station keeping can prolong the life of the satellite. On the ground, an Earth station must be equipped with both a mechanical dish mount capable of moving in both axes and a satellite tracker capable of reading the incoming signal level and controlling two motors to pivot the dish. This is necessary in order to track the satellite. Dish construction Dishes can be made from fibreglass, steel, aluminium, and perforated sheet or mesh (where wind resistance is likely to be a problem). The larger dishes are used on the weaker satellites and these generally operate on the C band. Compared to K band, the requirement for surface accuracy is considerably relaxed and a C band dish can tolerate up to 10mm in surface inaccuracies without noticeable performance degrada­tion. On the other hand, K-band dishes must be very accurate and so are normally fabricated from either spun aluminium or hot pressed steel. Using either of these fabrication techniques, surface accuracy of a few millimetres is achievable. The size of the dish required is determined by the satel­lite “footprint” and signal level on Earth. From the centre of the “footprint”, called the “boresight”, where the signal is at the highest level, signal contours radiate outward at decreasing levels. The lowest signal contour is known as the “beam edge”. Mathematical formulas are used to calculate what is known as the “link budget”, and these formulas take into account path loss, satellite EIRP, available dish gain, dish noise temperature and signal bandwidth. Several computer programs are commercially available to perform these link calculations. In practice, the size of a dish required to receive a par­ticular satellite could be determined on a subjective basis. Whilst this technique will provide some results, there is normal­ly no margin allowed for rain fade or a drop off in satellite power as the spacecraft ages. In addition, broadcasters can change the direction of the satellite footprint and power level. For this reason, it’s always wise to use a dish larger than the calculated minimum. The shape of the dish is also important. There are two basic types of dish: (1) prime focus; and (2) offset. A prime focus dish is perhaps the most recognisable, May 1995  21 SECTION OF PARABOLA USED FOR OFFSET ANTENNA OFFSET ANTENNA SECTION FOCAL POINT FOCAL POINT Fig.6: the compact size of offset dishes has made them popular with enthusiasts, despite their mechanical instability for sizes over 90cm. Offset dishes have very good sidelobe performance & no aperture blockage, unlike the prime focus dish. This diagram shows the relationship between prime focus & offset dishes. as it is used almost exclusively on C band and more often than not for larger K band installations. Prime focus dishes Prime focus dishes can be made to various degrees of “deep­ness”. Deep dishes have a shorter focal length than shallow dishes. This allows the feed­ horn to be shielded by the dish itself, providing some rejection of terrestrial interference or “TI”. Fig.5 shows the difference between a shallow and a deep dish. Note the location of the focal point. and ice pooling on the dish degrades the incoming signal. Because the offset dish is actually only a section of a larger prime focus antenna, the offset angle means that the actual angle of the reflector with respect to the horizontal plane is much higher than that of a prime focus dish, ensuring that rain, ice and snow easily fall off the reflector. In Australia, we generally avoid such problems, due to our climate. However, the compact size of offset dishes has made them popular with enthusiasts, despite their mechanical instability for sizes over 90cm. Offset dishes have very good sidelobe per­ formance and no aperture blockage, unlike the prime focus dish. Nevertheless, the prime focus dish is much easier to align and point. Fig.6 shows the relationship between prime focus and offset dishes. For reception of a single satellite in geostationary orbit, a simple ground mount can be used. If the mounting location demands a pole supported mount, such as a wall bracket, an “Az/El” mount can be used. This type of mount allows adjustment of both elevation and azimuth, normally using a piece of threaded steel rod and lock nuts, for each axis, but has no facility for tracking through the polar arc, in order to view other geostation­ ary satellites. This is the main difference between a polar and an Az/El mount. Satellite receiver We have presented several articles in the past dealing with the equipment necessary for satellite television reception. Most readers would find Offset dishes Offset dishes were developed primarily for use in high latitude countries, where the effect of water, snow These photographs show some of the many foreign satellite TV programs which are available at any given time. Some of these are broadcast in NTSC format & you will need an NTSC VCR or standards converter to watch them in colour. Alternatively, they can be displayed in black & white on a standard PAL TV receiver. 22  Silicon Chip difficulty in “home-brewing” a dish or the microwave components required for satellite reception but few will have difficulty with the re­ceiver to be described. A typical satellite television receiving system comprises a dish antenna, microwave feedhorn, low noise block down-converter, cable and a receiver. Each of these components performs a vital function, and the interconnections are shown in Fig.7. A commercial satellite designed to carry television pro­gramming can operate on either (or both) of the two internation­ally agreed satellite bands: (1) C band (3.7-4.2GHz); and (2) K band (12.25-12.75GHz in our part of the world). A parabolic dish antenna is commonly used to provide a significant degree of gain, normally in the region of at least 40dB. Depending on the band used, this equates to an approximate dish diameter of 3m for C band or 1.2m for K band. LOW NOISE BLOCK DOWNCONVERTER (LNB) Home construction of a 3m C band dish is quite an undertak­ing considering the physical size. Similarly, the con­struction of a 1.2m dish for K band is also quite difficult, considering the surface accuracy required (3-4mm over the entire dish). Equally daunting is the prospect of constructing a mi­cro­wave feedhorn, polariser and low noise amplifier and, consid­ering the price concessions to be offered on these items to SILICON TELEVISION RECEIVER DISH VHF CH3 OR 4 950-1450MHz SATELLITE RECEIVER Fig.7: a typical satellite television receiving system comprises a dish antenna, microwave feedhorn, low noise block down-converter, cable & a receiver which feeds the TV set. CHIP readers in conjunction with the receiver kit, the incentive to build these items is minimal! A satellite receiver is the one component of a system that can easily be constructed, saving around 50% over the price of commercial units. As noted above, the incoming satellite signal “block” has a frequency of either 4GHz or 12GHz, depending on the band used. This signal is collected by the dish antenna and directed through a piece of waveguide called the feedhorn to a quarter-wave dipole antenna, an integral part of the LNB (low noise block converter). The LNB performs two vital functions: (1) it amplifies the incoming signal whilst maintaining a very low noise figure (typically around 50°K for K band and 20°K for C band); and (2) it converts the incoming 500MHz wide block of signals to a much more manageable range, normally 9501450MHz. This allows the use of inexpensive RG-6/U 75Ω coaxial cable to connect the LNB to the receiver. There is sufficient output from a typical LNB, and sufficient AGC range on our receiver, to allow cable runs of up to 100 metres without additional line amplifiers. Next month, we will present the circuit and assembly details of a complete satellite receiver, along with some special offers on dishes and LNBs for readers of SILICON CHIP. The off-screen photos included with this article are just a sample of what can SC be received. May 1995  23