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14 SILICON CHIP By LEO SIMPSON & BOB FLYNN If you can do basic metalwork you can build this antenna. Your bill of materials will be around $25 and the finished antenna should give better performance than commercial UHF Yagi antennas costing up to a hundred dollars and more. In Australia, on the UHF (ultra high frequency) TV bands, the Yagi antenna is king. UHF Yagis are now very familiar on Australian rooftops. They have a long boom, up to 1.8 metres or more, with many short elements arranged along it. The Yagi design for UHF has many advantages. It is easy to mass produce, uses a modest amount of material, has relatively low windage (ie, force due to wind acting on it), good directional characteristics and good gain, depending on the number of elements. The Yagi does have a number of drawbacks though. It must be made with considerable precision if it is to perform well, so it is not so easy for the enthusiast with basic metalworking facilities to build. It is also a no-compromise design in that it is not practical to design a Yagi which will cover both UHF bands, particularly if you want a modicum of gain. You can have band IV or band V but not both. In Australia, by the way, UHF band IV covers channels 28 to 35, 526 to 582 Megahertz. UHF band V covers channels 39 to 69, 603 to 820MHz. Each channel occupies a 7MHz slot. In Europe and other parts of the world, there are common alternatives to the Yagi design. One is a Yagi with a corner reflector, another is a bow-tie with corner reflector, while a third is the most common, the bow-tie array. This is essentially a dipole (shaped like a bow tie) with a plane reflector close behind it. Higher gain is obtained .. .. .. :(l 1 REFLECTOR ELEMENTS 6mm x 1mm WALL THICKNESS ALUMINIUM TUBING 600mm LONG 17 REQUIRED Fig.1: front and side elevation of the new UHF antenna. This diagram labels all the special hardware items that you have to make except for the reflector elements. Below is a close-up view of two of the dipole bays. Thinking about building an antenna to pick up UHF TV in your area? This four-bay bow-tie array has high gain and covers UHF bands IV and V without modifications. JANUARY 1988 15 THIS ROW OF HOLES ALL 6mm DIA. r ... "'"' N :!l + .,... BILL OF MATERIALS I .... "' N -+-t-+--+- ~ ., ;;; t .,... .,... l t I I "' "' I l l FRONT t "' "' I SIDE FRONT VERTICAL BOOM 19mm SQUARE x 1.6mm WALL THICKNESS ALUMINIUM TUBING BACK SIDE REAR VERTICAL BOOM 19mm SQUARE x 1.6mm WALL THICKNESS ALUMINIUM TUBING Fig.2: cut and drill the front and rear booms exactly as shown here. 16 SILICON CHIP by stacking bow ties, in either twobay or four-bay arrays. The latter is the design we are presenting. The four-bay bow-tie array antenna has a number of advantages over typical Yagis. First, it can cover both bands IV and V without modification. Second, it has better gain than all except the highest gain Yagis which may measure up to three metres long. Third, it has good front-to-back ratio and a much narrower acceptance angle, in both the vertical and horizontal planes . (Note: the 18-element TC-18 from Hills is a combination of a long Yagi with a small corner reflector. The corner reflector gives it slightly higher gain and a narrower acceptance angle. For those who do not wish to build their own antenna, it Antenna 1 .6 metres of 1 9mm square aluminium tubing with 1.6mm wall thickness 13.8 metres of 6mm aluminium tubing, 1 mm wall thickness 1 .4 metres of 3mm dia. solid aluminium rod 270mm x 130mm x 1.6mm aluminium sheet 4 1 0mm x 50mm x 3mm thick Perspex 50 pop rivets (3mm x 6mm , aluminium mandrel); or 50 stainless steel self-tapping screws (see text) 2 U-bolts and clamps to suit mast 16 stainless steel screws, 3mm dia. x 16mm long, nuts and shakeproof washers Balun Box 1 80mm x 52mm x 30mm plastic box 1 printed circuit board, SC2-1-0188 3 6BA x 12mm screws with nuts 2 6mm spacers 3 stainless steel self-tapping screws, 1 5mm long Miscellaneous 7 50 semi-air spaced coax cable (Hills SSC32 or equivalent), plastic cable ties, silicon sealant, Delrin plugs (for square tubing) is a good choice in fringe areas. It is available in Band IV and Band V versions). The narrow acceptance angle of a four-bay bow-tie array is important, particularly if your location does not have a good line-of-sight to the transmitter and if you are often over-flown by aeroplanes. This combination of circumstances can lead to a phenomenon known as "aircraft flutter". When this occurs, the signal reflected from the aircraft to your antenna can be stronger than the more direct signal received from the transmitter. This causes very strong ghosting on the screen and a slowly fluctuating vertical bar on the screen which is the ghost of the horizontal sync pulse. The picture flutters because the plane is moving at high velocity relative to your antenna and so the path of the strong reflected signal is changing rapidly. In severe cases. aircraft flutter can cause the picture to lose horizontal synchronisation. Where the bow-tie array has a considerable advantage over the Yagi is that it has a much narrower vertical (and horizontal) acceptance. This is about half that for a Yagi of equivalent gain; ie. about 27° versus about 40°. This means that the bow-tie array will pick up much less reflected signal from high flying aeroplanes and therefore interference is much less. Well, what about the disadvantages of the bow-tie array versus the Yagi. Yes, it does have some. First, because it is a vertical rather than horizontal array, it has considerably more windage. Ser:;ond, there is probably more work in fabricating a do-it-yourself design such as this. While we did not have equipment for measuring the absolute performance of the bow-tie array featured here, we were able to make a lot of direct comparisons with commercial UHF band IV and band V Yagi designs. These were essential to optimise the performance for both band IV and band V. After a lot of trial and error, we are pleased to present a design which is very competitive with pre- The front and rear booms are fastened together using four tie plates (see Fig.6). Bend the cross-coupled harness connectors slightly so that they do not touch each other. sent commercially available Yagis and as noted above, it is notably less susceptible to "aircraft flutter". Design features Our bow-tie array is similar in appearance to a number of corn- mercial designs which are available overseas. It is constructed mainly of 6mm aluminium tubing with the two vertical structural members (booms) being 19mm square tubing. The four dipoles are effectively vestigial bow-ties, being Vees made of tubing rather than JANUARY 1988 17 tapping screws. These are strong, readily available and corrosion resistant. We do not recommend galvanised, bright zinc or cadmium plated steel screws as these do not stand the test of time. Often they will start to rust within a few days' exposure in seaside areas. They may be OK for roofing work but in combination with aluminium they rust. If you live away from the sea and decide to use these types of screw anyway, we recommend that you paint the antenna. We'll talk about that later. Don't, on any account, use brass or mild steel screws. If you use these, you will spoil the job. 207 Fig.3: the 16 dipole elements are all made from 6mm aluminium tubing. Cut the dipoles to a length of 207mm. Q "' 18 26 26 18 100 Fig.4: either 3mm thick Perspex or clear Lexan can be used for the dipole carriers (four required). Making your antenna DIPOLE MOUNTING CLll'S 1.6mm ALUMINIUM 8 REQUIRED + + Fig.5: the eight dipole clips are cut out using tin snips and then bent up in a vice. ◄ 87 triangular pieces. This cuts down on the windage while keeping the bandwidth essentially the same. The reflector is essentially a large grille about 60cm wide and 80cm high. The four dipoles are mounted on a common vertical boom which is spaced away from the vertical boom of the grille by about 50mm. The antenna is shown in front and side elevation in Fig.1. This diagram labels each special hardware item you will have to make. These are: (A) the dipole carriers, four required; (B) the dipole mounting clips, eight required; [C) the main mounting plate; (D) balun box assembly; (E) dipole elements, 16 required; (F) the boom tie plates, four required; (G) front boom; [H) rear boom; (J) o'!lter harness connectors, four required and [K) inner harness connectors, four required. Also shown on Fig.1 but not 18 SILICON CmP Fig.6: cut and drill the four aluminium tie plates as shown here. These tie the front and rear booms together. labelled as such, are the reflector elements, of which 17 are required. Fasteners After a few years' exposure to the elements, many antennas are in a poor state. Because aluminium is such an active metal, the right fasteners must be used otherwise corrosion will be very rapid, especially in seaside areas. We recommend three types of fastener for this project: (1) Aluminium pop rivets with aluminium mandrels. Those with steel mandrels are not recommended. Eventually, their mandrels will rust and while this may not harm the antenna it will cause unsightly discolouration. (2) Though often hard to get, aluminium screws are recommended although they are not available in self-tapping types and so all screw holes would have to be tapped. (3) Stainless steel self- Most enthusiasts will have the tools needed for this project. You will need a hacksaw, electric drill, vice, pop-rivet gun, blow-torch or LPG cylinder and torch. Apart from a pair of antenna clamps [U-bolts ), no special hardware or fittings are needed as we will detail how every part is made. Making and assembling this antenna is a fairly straightforward process although some steps are a little tedious. You must first obtain all the aluminium and hardware listed in the Bill of Materials, and make sure you have access to all the tools we have listed above. Having assembled together all the raw materials, you can start work by cutting all the aluminium elements with a hacksaw. Cut the two booms first, which are made of 19mm square aluminium tubing. The details are shown in Fig.2. The rear boom is 812mm, while the front boom is 720mm long. Then centre-punch and drill all the holes in both booms. Make sure that all the holes for the reflector elements in the rear boom are precisely in line and that their centres are 4.6mm from the front surface as specified on the diagram of Fig.2. Do not forget the holes for the tie plates or the holes in the back of the rear boom, for the main mounting plate. Trying to drill these after the antenna has been partially popriveted together would be a tricky task. The mounting plate is rivetted to the back of the rear boom and carries two U-bolts to mount the antenna to the mast. Next, cut all 17 reflector elements and the 16 dipole elements. These are made from 6mm aluminium tubing with a 1mm wall thickness. The dipole element dimensions are shown in Fig.3 while the reflector lengths, all 600mm, are shown in Fig.1. Assemble each reflector element into the rear boom, one at a time. The method we used was to thread one element through the boom, centre it precisely and centre-punch on the front of the boom, at the intersection of the centre-lines of the boom and the reflector element. Drill a 3mm (or 1/8-inch) hole through the front of the boom and element and then pop rivet the two together. Do this for all 17 reflector elements. Dipole plate & clips Next, make the four dipole plates, as shown in Fig.4. We used 3mm thick white Perspex but you can use clear Lexan or Perspex as they stand the weather equally well. When drilling, do not use too high a speed otherwise the Perspex will tend to melt and congeal on the drill. Now, make the eight dipole clips. We cut and bent these from a strip of 1.6mm thick aluminium, 38mm wide. Fig.5 shows the details. Each clip can be cut with tin snips, flattened with a hammer and then each side bent up in a vice. That done, you can make up the four dipole assemblies, each requiring a Perspex dipole plate, two dipole clips, four dipole elements plus four stainless steel 3mm screws, nuts and lockwashers. Next, make the four tie plates which tie the front and rear booms together. You can also make the main mounting plate at this stage, since it uses the same material, 1.6mm thick aluminium sheet. The details are shown in Fig.6 and Fig.7. Now assemble the front and rear booms together. using the four tie plates. You can use pop rivets or stainless steel self-tapping screws for this job. Fix the main mounting plate to the rear boom, using pop rivets or stainless steel self-tappers. Mount the four dipole assemblies onto the front boom. Use pop rivets or stainless steel self-tappers. Your antenna now looks the part and only lacks the harness and balun box assembly. JANUARY 1988 19 Make the inner and outer harness connectors, as shown in Fig.8. These are made from 3mm diameter aluminium rod. This is the trickiest stage in the whole process. After cutting to length, the ends of each connector must be hammered flat. To do this satisfactorily, you will have to anneal each end with a blow torch (or LPG torch). Unfortunately, there is no easy way of judging how much heat to apply but if you overheat the end it will suddenly melt and fall on the floor. The way to do it is to place each end in the flame for a few seconds and then hammer it flat. If necessary, reheat the end to finish the job. In fact, aim to do the flattening in two steps of hammering and annealing otherwise you will inevitably melt it. That done, centre-punch each end and drill 3mm holes. The eight connectors are then ready to be attached to the four dipoles but before you can do that you need to prepare the balun box assembly. Incidentally, note that when the ends of the harness connectors are hammered flat, they spread and stretch quite a bit. This accounts for the fact that the outer connectors are cut to 200mm long but when the ends are hammered flat, the hole centres for the connector screws can be drilled 199mm apart. Balun box assembly The balun box provides a correct termination for the antenna harness and terminals for 75-ohm coax cable, all sealed away from the elements for protection. It takes the form of a black plastic box with a small printed circuit board inside. This mounts the air-cored balun and the terminations. The printed circuit measures 30 x 70mm (code SCZ-1-0188) and has a very simple pattern. The balun is made of two small coils of enamelled copper wire, as shown in Fig.9 and Fig.10. Use wire with selffluxing enamel for this job. Selffluxing enamel melts easily in a solder pot or with a soldering iron and is much easier to work with than high temperature wire enamels which must be thoroughly scraped off before the wire can be tinned with solder . .20 SILICON CHIP ~ ~ • 4 -I--. • ., I ♦ • ~ -$ 15 35 0 • 15 35 116 Fig.7: drill the mounting plate to suit the Ubolts and clamps used. CE==========================~ 199 OUTER CONNECTORS 3mm DIA SOLIO ALUMINIUM 4 REQUIRED CUT PIECES 200mm LONG, ANNEAL ENDS ANO HAMMER FLAT I 75 I { ~ 130 INNER CONNECTORS 3mm DIA SOLID ALUMINIUM 4 REQUIRED CUT PIECES 132mm LONG, ANNEAL ENDS AND HAMMER FLAT BEND AS SHOWN BEFORE MARKING HOLE POSITIONS Fig.8: dimensions for the inner and outer harness connectors. Use a blow torch to anneal the ends before hammering them flat. Incidentally, do not think that the connection of the outer coil of the balun is a mistake, as shown in Fig.10. It is correct, with both ends soldered to earth. The balun printed circuit board and its accompanying box is tricky to mount. We suggest the following method. First, attach the four harness inner connectors to the printed board using stainless steel screws and nuts. The aluminium conductors must not make physical contact with the copper side of the board. You can use brass or copper plated steel for the coax cable clamp. We suggest you solder brass nuts to the copper side of the board to secure the cable clamp and the screw to terminate the inner conductor of the coax cable. Fig.11 shows the details of the balun box and how it is mounted. Use one self-tapping screw to secure the box to the front vertical boom. Use two spacers and two self tapping screws to secure the printed board to the case bottom. The latter two screws should penetrate the boom. Now attach all eight harness connectors to the dipole assemblies and the antenna is virtually finished. Do not over-tighten the dipole assembly screws otherwise the Perspex will distort and possibly crack. You will need a pair of antenna clamps or U-bolts to mount the antenna to the mast or J-pole (for barge-board mounting). We prefer the use of galvanised U-bolts and Vclamps for this job rather than the cadmium-plated and passivated types used for most antenna hardware. The latter have a gold finish and often start to rust prematurely. U-bolts and clamps for automotive exhaust systems are 0 300{) ANTENNA __ 7~{l T _,. ~ BALUN TRANSFORMER PRIMARY 12T, 0.67mm ENAMELLED COPPER WIRE CLOSE-WOUND ON A 3.2mm DIA. MANDREL SECONDARY SLIPPED OVER END OF PRIMARY AND BOTH ENDS SOLDERED TD EARTH BALUN COILS MOUNTED ON COPPER SIDE OF BOARD SECONDARY 6T, 0.67mm ENAMELLED COPPER WIRE CLOSE-WOUND ON A 4. 76mm DIA. MANDREL Fig.9: winding and termination details for the air-cored balun. Fig.10: the balun coils are mounted on the copper side of the PCB. Note that both ends of the secondary are soldered to earth. tThe completed balun board. Brass nut-s are soldered to the copper pattern to secure the screws for the cable clamp, coax cable inner conductor, and harness connectors. generally quite suitable and have good corrosion resistance. Or, if you want to be really fancy, go to a ship's chandlers and buy stainless steel U-bolts and clamps. They're costly but good. We suggest that the ends of all the reflector and dipole elements be stopped up with silicone sealant. This will stop them from whistling in the wind. You can do the same with the booms although, for a neater result, you can buy square Delrin plugs. Installing the antenna Take a lot of care when installing your antenna. There's no point doing a fine job of assembly and saving all that money if you end up in hospital because you fell off the ladder. Climbing ladders with anten- 0 PLASTIC BOX BOX CENTRE LINE 00 00 ,- II -l- 1 N 0 0 (J en This is the actual size pattern for the balun board. nas is dangerous work. The first step in installation is to decide where to mount your antenna. For best results mount it as high as possible and well clear of other antennas. It is not really practical to mount this bow-tie array on the same mast as a VHF antenna unless it is vertically separated by at least one metre. Having mounted your mast, take the antenna up and secure it with the U-bolts. Then terminate the coax cable. For minimum signal attenuation and good cable life, we recommend Hills semi-airspaced cable, type SSC32 or equivalent. At your TV set end of the cable, you will probably need a diplexer to continued on page 95 View inside the balun box assembly. It should be sealed against the weather. WIRING HARNESS (BENO HARNESS TO CLEAR '--..._ ~ - - - - - + - - - - - - / " - - , , C A B L E CLAMP) ' FRONT BOOM 0 1 ,- I COAX 6.6mm DIA. HOLE FOR COAX IN ONE ENO DNL Y Fig.11: details of the balun box assembly. It is secured to the front boom at one end using a selftapping screw and at the other end by two selftapping screws which pass through the PCB and 6mm spacers. JANUARY 1988 21 Ribbon or coax: which is best? All of the information I have read about UHF antennas in Australia recommends the use of 75-ohm coax cable and most of the installations I have seen in this country have used coax. But I have been overseas to the US and Japan on a number of occasions and there they seem to use ribbon cable frequently. Why is tha~? Why is there such a preference in Australia for coax? I can't believe that our reception conditions for UHF would be any more difficult than in cities in Japan and the US. • We doubt whether UHF reception conditions are any more difficult in Australian cities than overseas. Based on typical attenuations figures at UHF for ribbon cable and coax cable, you might think that ribbon would be the more desirable. After all, most coax cables have losses of 10dB/30m or more at UHF which is considerably more than the nominal losses of la.dder-type ribbon cable but we would not recommend ribbon cable on that basis. It is possible that in some situations, 300-ohm ribbon cable might give comparable reception and three 7-segment displays), you will need to run 12 separate data lines plus the supply lines. That makes it messy. We don't think UARTs (Universal Asynchronous Receiver/Transmitter) would be practical or cheap since you would need two 8-bit UARTs to send and another two to receive, plus all the timing circuitry required. Nor would A-D and D-A conversion be practical or cheap because that also implies quite a lot of supporting circuitry. The only practical way seems to be to send all 12 data lines via RS232 receivers and transmitters. We suggest you use the Motorola MC1488 quad line driver and MC1489 quad line receivers, both of which are quite cheap. You would need three to transmit and quality to 75-ohm coax but you could only justify its use in areas away from the sea which don't have high rainfall. Once ribbon cable becomes wet or coated with a salt film, it starts to produce much higher losses. It deteriorates quickly too and is subject to signal pickup on the cable itself, producing leading ghosts and smeary pictures. As far as we are aware, coax cable is universally used throughout Europe for UHF reception and the same can now be said for the USA and Japan. It is also true that many if not most installations in apartment buildings overeases would not feed UHF signals via distribution system to each tenant. The signal losses, particularly in older installations using inferior cables, would make it impractical. Rather, it is standard practice to feed the UHF signals to a down-converter so that the signals are distributed at VHF. The message is: forget ribbon for UHF. If coax cable losses are .likely to be a problem, you should use a masthead amplifier to boost the signal before it is distributed. three to receive. You will also need ± 12V supplies for the transmitters in addition to 5V for the remote display. We admit that our suggested solution does not look simple, but it is probably the cheapest way. Corrections Digital Fundamentals, Dec. 1987: Fig.6 on page 92 has been reproduced incorrectly. The type down the left-hand edge of the diagram should read INPUT A, INPUT B, INPUT C and OUTPUT D. In addition, the second last paragraph on page 92 should read as follows: At times t 1 through ta the three inputs are never high at the same time. However, beginning at time ta and ending at time t 9 the three inputs are all high so that output D goes high. Subcarrier Adaptor continued from page 67 setting up procedure is relatively simple. First, make sure that VR1 is set so that its wiper is turned toward the LM565. This will provide maximum signal level. Now adjust VR2 so that there is audio signal. Find the extreme settings of VR2 where the audio signal drops out, then set VR2 halfway between the two extremes. VR1 is used to minimise noise from the audio signal when the FM signal level is poor. Adjust the trimmer until the sound becomes distorted and then back off the adjustment until distortion is no longer audible. If you have a strong FM signal, adjustment of VR1 will have no effect on the noise level and so it should be left at its maximum resistance setting. UHF Antenna continued from page 21 enable you to terminate cables from your VHF and UHF antennas. A single cable goes from the diplexer to your TV set. Alternatively, the diplexer output may be fed to a splitter and then to various TV wall plates around your home. Tune your TV to the local UHF station(s) and then orient the antenna for best reception. Secure the cable to the mast with plastic cable ties to prevent the cable from flapping in the wind. Seal the balun box with silicone sealant to weatherproof it. Painting Depending on where you live, painting the antenna cart be worthwhile, particularly in seaside areas or near industrial areas where there may be a lot of fallout. In these cases, we suggest painting the antenna with an etch primer and then finishing with an aluminium loaded paint such as British Paints "Silvar". As a final comment, if you are building the antenna to receive stations right at the top of band V, say between channels 59 and 69, a small improvement can be gained by shortening the dipole elements by 5%. JANUARY 1988 95