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BUILD A 4-BAY BOW-TIE UHF ANTENNA This photo shows how the antenna is oriented to receive horizontal TV transmissions while the photo on the facing page shows the orientation for receiving vertical TV transmissions. 32  Silicon Chip BILL OF MATERIALS Thinking about building an antenna to pick up UHF TV in your area? This 4-bay bow-tie array has high gain & covers both UHF bands IV & V. It can be used for horizontal or vertical polarised TV transmissions. By LEO SIMPSON & BOB FLYNN If you can do basic metalwork, you can build this antenna. Your bill of materials will be around $45 and the finished anten­na should give better performance than commercial UHF Yagi anten­nas costing up to a hundred dollars and more. We presented a very similar 4-bay bow-tie design in January 1988. That design used 6mm aluminium tubing, 3mm aluminium rod and 19mm square aluminium tubing. The 6mm tubing proved difficult to obtain at the time (many people used 1/4-inch rod instead) and the 3mm rod was virtually unobtainable as well. This new design uses 4.74mm aluminium tubing for all ele­ ments and the harness, dispensing with the need for a blowtorch to make the harness connectors. As well, the balun box is simpli­fied and the over- all construction is lighter and more straight­forward. The 4.74mm diameter tubing has a wall thickness of 0.9mm. Its diameter is close to the Imperial dimension of 3/16-inch (4.7625mm) and is a neat fit into 3/16-inch holes. Background to bow-tie arrays In Australia, on the UHF (ultra high frequency) TV bands, the Yagi antenna is king. UHF Yagis are now very familiar on Australian roof-tops. 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 Antenna 1.5 metres of 12.7mm square aluminium tubing with 1.6mm wall thickness 14.5 metres of 4.74mm diameter aluminium tubing with 0.9mm wall thickness 330mm x 125mm x 1.6mm thick aluminium sheet 330mm x 40mm x 3mm thick acrylic sheet 26 stainless steel self-tapping screws No.4 gauge x 10mm long 16 stainless steel self-tapping screws No.4 gauge x 6mm long 4 3mm diameter x 20mm long stainless steel metric screws 12 3mm diameter x 16mm long stainless steel metric screws 16 3mm stainless steel metric nuts 18 3mm stainless steel shakeproof washers 2 U-bolts and clamps to suit mast Balun Box 1 83mm x 54mm x 30mm black plastic jiffy box, Jaycar Cat. No. HB-6015 or equivalent 1 printed circuit board, 37 x 39mm, code 02108941 250mm of 0.67mm diameter enamelled copper wire 2 3mm diameter x 16mm long stainless steel screws 3 3mm diameter x 10mm long stainless steel screws 12 3mm stainless steel nuts 4 3mm stainless steel shakeproof washers 6 3mm stainless steel plain washers Miscellaneous 75-ohm semi air-spaced coaxial cable, Delrin plugs for square tubing. 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 July 1994  33 600 46 370 A 46 B 46 E H 46 184 46 F 230 D E H 46 46 46 46 184 46 46 46 46 C 46 46 G 46 46 D 18 REFLECTOR ELEMENTS REQUIRED 600mm LONG AND 16 DIPOLE ELEMENTS 183mm LONG MATERIAL : 4.74mm DIA ALUMINIUM TUBING Fig.1: front & side elevation of the new UHF antenna. The letters A-H indicate the special hardware items that you have to make. These are: (A) the dipole carriers, four required; (B) the dipole mounting clips, eight required; (C) the boom clamp plate; (D) the dipole boom; (E) the reflector boom; (F) the bent harness connec­tors, four required; (G) the straight harness connectors, two re­quired; & (H) the boom tie plates, four required. Also shown on Fig.1 but not labelled as such are the re­flector elements, of which 18 (600mm long) are required, & the dipole elements, of which 16 are required (each 183mm long). Not shown on Fig.1 is the balun box assembly which is mounted at the centrefront of the antenna. The assembly details for each item are shown in a separate diagram. 34  Silicon Chip no-compromise design in that it is not practical to design a Yagi which will cover both UHF bands, particularly if you want a reasonable amount 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 (526-582MHz). UHF Band V covers 46 28 46 46 648 434 202 46 46 28 46 46 46 802 46 46 28 46 46 156 26 46 10 26 81 46 26 46 156 10 26 46 11 28 D E 46 12 FRONT HOLES 2mm DIA 12 SIDE HOLES THROUGH BOTH SIDES 2mm DIA 5 channels 39 to 69 (603-820MHz). 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 by stacking bow-ties, in either two-bay or four-bay arrays. The latter is the design we are presenting. The four-bay UHF 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 UHF Yagis which may measure up to three metres long. Third, it has good frontto-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 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 3.5 12 SIDE HOLES THROUGH BOTH SIDES 4.76mm DIA HOLES ON 26mm CENTRES ARE 2mm DIA 12 REAR HOLES 2mmDIA DIPOLE BOOM MATL: 12.7mm SQUARE x 1.6mm WALL THICKNESS ALUMINIUM TUBE DIMENSIONS IN MILLIMETRES REFLECTOR BOOM MATL: 12.7mm SQUARE x 1.6mm WALL THICKNESS ALUMINIUM TUBE Fig.2: cut & drill the reflector (left) & dipole booms exactly as shown here. July 1994  35 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 more windage. Second, there is probably more work in fabricating a do-it-yourself design such as this. 34 25 9 18 N DOW BEND BEND DOW N 7.5 15 30 B DIPOLE MOUNTING CLIPS 8 REQUIRED MATL: 1.6mm ALUMINIUM HOLES 3.2mm DIA 38 6 26 Performance 6 80 92 BOOM TIE PLATES 4 REQUIRED MATL: 1.6mm ALUMINIUM HOLES 3.2mm DIA 14 40 14 H 15 15 15 15 80 A DIPOLE CARRIERS 4 REQUIRED MATL: 3.2mm ACRYLIC HOLES 3.2mm DIA Fig.3: this diagram shows the fabrication & drilling details for the dipole mounting clips (top), the boom tie plates (centre) & the dipole carriers (bottom). The dipole carriers are made from 3.2mm-thick acrylic sheet (eg, Lexan or Perspex), while the dipole mounting clips & boom tie plates are made from 1.6mm-thick aluminium sheet. Be sure to keep to the exact dimensions shown here & drill all holes to 3.2mm-dia. 36  Silicon Chip While we did not have equipment for measuring the absolute performance of the bow-tie array featured here, we have been 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 present commercially avail­able Yagis and as noted above, it is notably less susceptible to “aircraft flutter”. As well, this new design is easier to make than the design presented in January 1988. Inevitably, we must draw a comparison with the Corner Re­flector design we presented in the June 1991 issue. This new bow-tie array appears to have higher gain than the June 1991 design and it also is less cumbersome to handle. Against that, the corner reflector is probably easier to make. Having said that, our overall preference is for the bow-tie array. Design features Our bow-tie array is similar in appearance to a number of commercial designs which are available overseas. It is construct­ed mainly of 4.74mm aluminium tubing with the two vertical struc­tural members (booms) being 12.7mm square tubing with 1.6mm wall thickness. The four dipoles are effectively vestigial bow-ties, being Vees made of tubing rather than triangular pieces of flat sheet. This cuts down on the windage while keeping the bandwidth essentially the same. The reflector is essentially a large grille 60cm wide and 80cm high. The four dipoles are mounted on a common boom (the dipole boom) which is spaced away from the reflector boom of the grille by 67mm. The two dipole bays near the centre of the antenna are connected as shown in this photograph. The ends of the harness connectors are flattened using a vyce. A B 46 B Z A 100 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 discoloration. (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-tapping screws. These are strong, readily available and corrosion resistant. We strongly recommend the use of stainless steel for all screws used in this project. 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 or in areas subject to industrial fallout. 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. Do not, on any account, use brass or mild steel screws. If you use these, Z Fasteners This view shows one of the dipole bays at one end of the antenna. Note how the ends of the harness connectors are crossed over to provide correct phasing. B B 46 The antenna is shown in front elevation and side elevation in Fig.1. The diagram of Fig.1 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 boom clamp plate; (D) the dipole boom; (E) the reflector boom; (F) the bent harness connec­tors, four required; (G) the straight harness connectors, two re­quired; and (H) the boom tie plates, four required. Not shown on Fig.1 is the balun box assembly. Also shown on Fig.1 but not labelled as such are the re­flector elements, of which 18 (600mm long) are required; and the dipole elements, of which 16 are required (each 183mm long). A Fig.4: the boom clamp plate is attached to the back of the rear boom using self-tapping screws which are also used to secure three of the reflectors. Drill the holes labelled ‘B’ to suit the U-bolts. Z Z 100 C BOOM CLAMP PLATE MATL: 1.6mm ALUMINIUM HOLES A: 3.2mm DIA B: TO SUIT U-BOLTS DIMENSION Z TO SUIT U-BOLTS July 1994  37 you are wasting your time and you will spoil the job. 5 Making your antenna WIRING HARNESS 4 REQUIRED MATL: 4.76mm DIA ALUMINIUM TUBE HOLES 3.2mm DIA F 184 194 WIRING HARNESS 2 REQUIRED MATL: 4.76mm DIA ALUMINIUM TUBE HOLES 3.2mm DIA 115 G 240 30 115 30 5 30 Fig.5 (left): the inner & outer harness connectors are made from 4.76mm-dia. aluminium tube. Use a vise to flatten the end & centre sections as shown & drill all holes to 3.2mm. The text describes how the outer harness connectors are bent. 38  Silicon Chip Most enthusiasts will have the tools needed for this pro­ject. You will need a hacksaw, electric drill, vyce and pop-rivet gun. 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 12.7mm square aluminium tubing. The details are shown in Fig.2. The reflector boom is 802mm, while the dipole boom is 648mm long. Once cut, 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 3.5mm from the front surface as shown on Fig.2. Do not forget the holes for the tie plates or the holes in the back of the rear boom, for the boom clamp plate. Trying to drill these after the antenna has been partially assembled would be a tricky task. Next, cut all 18 reflector elements and the 16 dipole ele­ments. These are made from 4.74mm aluminium tubing with a 0.9mm wall thickness. The reflector and dipole element dimensions 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 then drive in a 4-gauge stainless steel screw from the rear of the boom so that the element is held firmly in place. Do this for all 18 reflector elements. Note that three of these screws are also used to secure the boom clamp plate. Dipole plate & clips Next, make the four dipole carrier plates, as shown in Fig.3. We used TO RECEIVER TO PRI ANTENNA SEC BALUN PRIMARY: 12T, 0.67mm DIA ENAMELLED COPPER WIRE CLOSE-WOUND ON A 3.2mm DIA MANDREL SECONDARY: 6T, 0.67mm DIA ENAMELLED COPPER WIRE CLOSE WOUND ON A 4.76mm DIA MANDREL Fig.6 (above): this diagram shows the winding & termination details for the air-cored balun. Fig.8: here is the full-size pattern for the balun board. Ready-etched boards can be purchased from RCS Radio Pty Ltd (see page 96). Fig.7 (right): the balun coils are mounted on the copper side of the PC board. Note that the secondary coil is simply slid over the primary & has both ends soldered to earth (ie, the track that runs to the cable clamp & the braid of the coax). 3.2mm 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 mounting clips. We cut and bent these from a strip of 1.6mm-thick aluminium, 30mm wide. Again, Fig.3 shows the details. Each clip can be cut with tin snips, flattened with a hammer and then each side bent up in a vyce. That done, you can make up the four dipole assemblies, each requiring a Perspex dipole carrier plate, two dipole clips, four dipole elements plus four stainless steel 3mm machine screws, nuts and lock washers. Next, make the four boom tie plates (Fig.3) which tie the front (dipole) and rear (reflector) booms together. You can also make the boom clamp mounting plate (see Fig.4) at this stage, since it uses the same material (1.6mm thick aluminium sheet). 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. Next, fix the boom clamp plate (and three of the reflectors) to the rear boom using stainless steel self-tappers, then mount the four dipole assemblies onto the dipole boom. Harness connectors Your antenna now looks the part and only lacks the harness and balun box assembly. Make the straight and bent harness Above: the completed balun box assembly. The coaxial cable enters through a grommeted hole in the bottom of the box & is secured using a large cable tie & the earth clamp. When the assembly has been tested, use silicone sealant to seal the case against the weather. July 1994  39 PLASTIC BOX WIRING HARNESS 3mm SHAKEPROOF WASHERS PCB 3mm SCREW 16mm LONG 3mm FLAT WASHERS This close-up view of one end of the reflector boom shows how the reflector elements are held in place using stainless steel self-tapping screws. Make sure that each element is correctly centred on the boom. BOX CENTRE LINE 3mm SCREWS 10mm LONG EARTH CLAMP GROMMET COAXIAL CABLE 7.5 BALUN BOX DETAIL TWO MOUNTING HOLES FOR PCB REQUIRED IN BASE OF BOX 3.2mm DIA. ON 30mm CENTRES 10mm ABOVE BOX CENTRE LINE 7 12.7 20 EARTH CLAMP HOLES 3.2mm DIA. MATL: 0.75mm BRASS Fig.9: this diagram shows how the balun box assembly is attached to the harness connectors using 16mm long screws & shows how the earth clamp is made. 40  Silicon Chip connectors, as shown in Fig.5. Again, these are made from 4.76mm diameter aluminium tubing. This is the trickiest stage in the whole process. The straight connectors are the easiest to make, so we’ll talk about those first. Cut two lengths 240mm long, then squeeze the ends and centre section flat, as shown in the diagram of Fig.5. That done, centre-punch each end and the centre section and drill 3mm holes, as shown. The bent connector requires a few extra steps. First, cut four lengths of 4.76mm aluminium tube 210mm long. Next, drill two 4.76mm (3/16-inch) diameter holes in a block of wood; one hole 72mm deep and one 30mm deep. Clamp the drilled block of wood in your vyce. Put one end of the tube fully into the 72mm deep hole and bend it over at 45°, then place the bent length of tubing into the 30mm deep hole and bend it back 45° so that the short section is parallel to the long section, as shown in Fig.5. Do this for all four 210mm lengths of tube. This done, squeeze the ends in a vyce, centre-punch each end and drill 3.2mm holes, as shown in Fig.5. The six 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. 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 board measures 37 x 39mm (code 02108941) and has a very simple pattern. The balun is made of two coils of enamelled copper wire, as shown in Fig.7. Use wire with self-fluxing enamel for this job. Self-fluxing 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. Incidentally, do not think that the connection of the outer coil of the balun is a mistake, as shown in Fig.7. It is correct, with both ends soldered together. The balun printed circuit board and its accompanying box is tricky to mount. We used a standard black plastic Jiffy box measuring 83 x 54 x 30mm (Jaycar Cat. HB-6015). We suggest the following method for mounting the balun box which is depicted in Fig.9. First, drill the two 3.2mm holes in the rear of the balun box and a 9.5mm hole for the cable grommet which is fitted to one end. Attach the two straight harness connectors to the balun box using two 3mm diameter x 16mm long stainless steel screws, nuts and lock washers. This done, fit three 3mm diameter x 10mm long The front & rear booms are fastened together using four boom tie plates (see Fig.3 for dimensions). You can use either pop rivets or stainless steel self-tapping screws to secure these tie plates. stain­less steel screws and nuts to the balun board for the cable clamp and cable inner conductor terminal. We tinned the copper lands on the board where the nuts bedded down, to make good contact. You can use brass or copper plated steel for the coax cable clamp and it is attached using an additional two nuts on the board screws. Fit a grommet for the 6mm coax cable to the end of the balun box. Now attach the balun box assembly and the four bent harness connectors to the dipole assemblies and the antenna is virtually finished. You will need to bend each pair of bent harness connectors slightly so that there is about 2mm clearance between them. Do not overtighten the dipole assembly screws otherwise the Pers­ pex will distort and possibly crack. Mounting the antenna 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 V-clamps for this job rather than the cadmium-plated and passivated types used for some antenna hardware. The latter have a gold finish and often start to rust prematurely. 42  Silicon Chip This view, taken from the rear of the antenna, shows how the balun box is attached to the harness connectors at the centre of the dipole boom. The coax cable (not shown here) exits through a hole in the bottom of the box. U-bolts and clamps for automotive exhaust systems are 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 from aluminium centres. 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 antennas is dangerous work. The first step is to decide where to mount the 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 from it 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 (the dielectric has a cellular cross-section), type SSC32 or equivalent. At the TV set end of the cable, you will probably need a diplexer to enable you to terminate the cables from your VHF and UHF antennas. A single cable then goes from the diplexer to the 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. Finally, secure the cable to the mast with plastic cable ties to prevent the cable from flapping in the wind and seal the balun box with silicone sealant to weatherproof it. Painting Depending on where you live, painting the antenna can 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 SC British Paints “Silvar”.