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A corner reflector antenna for UHF TV You can save quite a lot of money by building your own UHF TV antenna and you will generally get better performance into the bargain. This corner reflector antenna covers both UHF TV bands IV and V and should only cost around $30 to $40 to build. By BOB FLYNN & LEO SIMPSON This is not the first UHF antenna we have described. Back in the January 1988 issue of SILICON CHIP. we described a 4-Bay Bowtie Antenna which covered both UHF TV bands. It has proved extremely popular and we believe that many hundreds have been made. So why are we producing a different design and what was wrong with the bowtie approach? Essentially, there is nothing wrong with the bowtie design at all - it works very well although it does require reasonable metalworking skills. No, the real reason for presenting this new design is simply that we wanted to assess a large corner reflector design. We also felt that it should be easier to build than the bowtie design. The corner reflector antenna is not a very common design and is rarely seen in Australia. Essentially, it consists of a single dipole element with a large reflector immediately behind it. The "corner" typically has an angle of 60-90 degrees. Corner reflectors have most of the advantages of bowtie arrays when compared to the most common UHF antenna - the long Yagi. The corner reflector can be designed to cover a wide frequency range, in this case UHF bands IV and V, from 526582MHz and from 603-820MHz. It also has a narrow vertical acceptance angle which is important for reducing interference effects from aircraft (ie, aircraft flutter and ghosting), and it has a good front-to -back ratio. Yet another advantage is that it can be designed for high gain over the full frequency range. The gain of the design presented here is close to 12dB with respect to a simple dipole. The disadvantage of the corner reflector design presented here is that it is relatively large and cumbersome and it does have relatively high wind resistance. It will need a good strong mast to ensure that it does not blow down in high winds. This requirement applies to any large antenna, of course. Solid bowtie This close up view of the antenna shows the triangular dipole elements attached to the balun box. Note the pop rivets attaching the reflector elements to the two booms. Alternatively, you can use stainless steel self-tapping screws. 24 SILICON CHIP In our design, the dipole element is like a large solid bowtie and is made from two triangular sheets of aluminium. The reflector is essentially two large grilles about 65cm wide and 60cm long which are mounted at rightangles to each other. Norie of the parts are difficult to make although you will need some basic metalworking tools. A guillotine would be nice although we did not make use of one when we made the prototype. At the very least, you will need a hacksaw, an electric drill and drill bits, a vise, tape measure or long steel rule, various files, combi- The corner reflector antenna works well on both UHF hand IV and hand V frequencies and gives good reception from line-of-sight transmitters more than 100km distant. At this site in Carlingford (Sydney), good reception of Wollongong stations was obtained. nation square, a centre punch, scriber and a certain amount of patience. A pop rivet gun is also a necessity if you are going to assemble the reflector sections with pop rivets. Fasteners & hardware Based on our own experience, you are wasting your time making an antenna if you don't use the right hardware. The same applies when installing a ready-built antenna too, although it is amazing how often you will see antenna fittings severely corroded after only a year or two of service. Let's talk about fasteners first. Because aluminium is such an active metal, 't he right fasteners must be used otherwise corrosion will be very rapid, especially in seaside areas. Use the wrong types of screws in a seaside area and they will rust out and cause corrosion of the antenna itself in a matter of only a few weeks or even days. (We are especially aware of this corrosion problem since we are situated near Sydney's northern beaches). Therefore, we recommend only four types of fastener for this project: (1) Aluminium pop rivets with alumin- ium mandrels. They may not pull up as tight as those with steel mandrels but steel mandrels rust out; (2) Stainless steel pop rivets; (3) Though often hard to get, aluminium screws are recommended although they are seldom available in self-tapping types and so all screw holes would have to be tapped; and (4) Stainless steel selftapping screws - these are strong, readily available and resist corrosion very well. Now for the junk we don't recommend (unless you are way out in the country where it only rains once every seven years). Avoid using galvanised, bright zinc or cadmium-plated steel screws. These might be OK for roofing but not for aluminium antennas. Also , don 't use brass or mild steel screws. When used in combination with aluminium they corrode extremely rapidly. In fact, forget we even mentioned brass or mild steel screws - we know you're not silly enough to use them! The next item for consideration is the antenna clamps. In the past, muffler clamps have often been used for do-it-yourself antenna installations. The sad fact is that many muffler clamps are not even good enough for mufflers let alone antennas. Try to avoid those with bright zinc or cadmium plating. In seaside areas, the nuts, washers and U-bolts (which are highly stressed) are the first to corrode, followed by the brackets themselves. You have been warned - go for the best antenna hardware you can obtain. We prefer the use of galvanised U-bolts and V-clamps or, if you want to be really fancy, go for ones made of stainless steel. The best place to buy such hardware is often not your local hardware store or automotive accessory outlet but a ships' chandler (ie, boating supplies). At least they know about corrosion problems and are likely to stock suitable hardware, although their prices can be expensive. Making your antenna Making and assembling this antenna is a fairly straightforward process although some steps do require a little patience. 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 listed above. Now let's just explain the various JUN E 1991 25 ✓ (B~ =========~=========...-(Al II II 11 11 II 11 II II I I r--. ---- I _.-(C) II II I _.!1.....1!..._..,J..--,_ _ _ _ _ _ Y<=" T (G) II II '-7 rII 11 II 11 I==== --- /(F) -(F) 11 II lI 11 IQ! Fig.1: this diagram shows the corner reflector antenna in both front and side elevation. All the parts labelled (A) to (F) are detailed in the other diagrams. diagrams you need to refer to. Fig.1 is a diagram showing the completely assembled antenna in front and side elevations. Note that each of the major parts is labelled and is shown in detail on an additional diagram. For example, note the reflector element labelled (A). Twenty-two of these are required and the required dimensions are shown in Fig.2. Also shown in Fig.2 is the reflector boom - shown as (B) in Fig.1. Mast mounting bracket This edge view shows one of the mast mounting brackets which we fabricated from 1.6mm sheet aluminium although they could also be made from angle aluminium. Both pop rivets and screws were used in the prototype because it had to be disassembled on quite a few occasions. 26 SILI CON CHIP Part (C) in Fig.1 is the mast mounting bracket. This is shown in Fig.3 together with the dipole boom and dipole boom bracket. Part (F) is half the dipole which is detailed in Fig.4 together with the balun box drilling diagrams. Fig.5 shows the detail of the dipole boom, balun box and dipole when assembled together. Fig.6 is the circuit of the balun whil e Fig. 7 shows the wiring of the balun PC board. Having obtained all the materials, you can start by cutting all the aluminium elements with a hacksaw. First cut the two reflector booms (see Fig.2) which are made of19mm square tubing. One end of each boom is cut at 45 degrees, so you will need a combination square and a scriber to mark the cut. Centre punch and drill all the holes on both booms. Make sure that all the holes for the reflector elements are positioned precisely as specified, otherwise the reflector elements will be crooked when assembled. The same comment applies to the drilling of the reflector elements themselves. With the drilling of the reflector metalwork complete, you can now pop rivet both reflector grille assemblies. When that is done, put them both aside, well away from your work area and where they are not likely to be snagged as you pass by. Incidentally, a point needs to be made about the pop rivets. Nominally, the aluminium pop rivets you buy over the counter will all be 1/8-inch or the metric equivalent 3.2mm, although some may be labelled as 3mm. By contrast, stainless steel pop rivets are available in 3mm and 3.2mm sizes. Which ever size you buy, make sure that you drill the correct size hole. Do not drill 3.2mm holes for 3mm pop rivets as they will just pull through. "'"' ~1· ·t~ ~ ~ "'"' "' "'"' ~~\ "'"' 4~.. ♦ Dipole elements The dipole is made from two triangular sections of 1.6mm thick aluminium sheet - see Fig.4. Note: you could use thinner material here, say down to 1mm thick but you will still need 1.6mm thick sheeting for the mast brackets (or purchase angle section aluminium). Cutting out the aluminium dipole sections is one of the more tedious steps in making this antenna and this is where having access to a guillotine would be good. Failing that, do it the hard way using a hacksaw and then file all the edges clean and straight. Don't be tempted to use tin snips to cut them out as it is very difficult to avoid buckling the edges. Two holes are required in each dipole half section, drilled at 3.2mm. After doing the dipole sections, drilling the holes in the balun box will be dead easy (see Fig.3 again) although here's a little tip: don't drill at too high a speed otherwise the plastic "'"' ~ 45° -1 ,o I. I s s ~ (A) REFLECTOR 22 REQUIRED MATERIAL:- 3mm x 10mm ALUMINIUM I 19 FRONT SIDE DIMENSIONS IN MILLIMETRES (B) RELECTOR BOOM 2 REQUIRED MATERIAL:· 19mm SQUARE x 1.15mm WALL THICKNESS ALUMINIUM TUBING HOLES:- A AND B: 3.2mm DIA. FOR POP RIVETS OR TO SUIT AVAILABLE STAINLESS STEEL SELF TAPPING SCREWS Fig.2: cutting and drilling details for the reflector booms and elements. Note that one end of each reflector boom is cut at 45 degrees so that it can be correctly clamped in the mast mounting brackets. Make sure you drill the correct size hole for the pop rivets (see text). will tend to melt around the holes. booms. It requires holes to be drilled at the top and sides, as shown in the Next, cut and drill the dipole boom which is made from the same 19mm detail diagram of Fig.3. You have a choice as far as the square tubing as used for the reflector · JUN E 1991 27 -·A ---· --- , 29.5 SIDE 173 TOP (0) DIPOLE BOOM 1 REQUIRED MATERIAL:-19mm SQUARE x 1.15mm WALL THICKNESS ALUMINIUM TUBING 15 25 12 16 33 40 73 40 (E) DIPOLE BOOM TO CONNECTION BOX BRACKET ·2 REQUIRED MATERIAL:· 1.6mm ALUMINIUM (C) ANTENNA TO MAST MOUNTING BRACKET DIMENSIONS IN MILLIMETRES HOLES:· A AND B: 3.2mm DIA. FOR POP RIVETS OR TO SUIT AVAILABLE STAINLESS STEEL SELF TAPPING SCREWS C: TO SUIT AVAILABLE MAST CLAMP U· BOLT 2 REQUIRED MATERIAL:· 1.6mm ALUMINIUM Fig.3: cutting and drilling details for the mast mounting brackets, dipole boom and balun box brackets. Note that the mast mounting brackets could be made from aluminium angle, which would save the need for bending sheet aluminium and give a neater job. mounting brackets are concerned. We made ours from 1.6mm thick aluminium sheet as noted above, but you can save yourself some work by buying a 300mm length of 32 x 32 x 3mm aluminium angle. You will have to modify the drilling dimensions slightly for the mast U-bolts. Also shown on Fig.3 is the small rightangle bracket (2 required) needed to connect the balun box to the dipole boom. These can be made from a scrap of aluminium. Making the balun The balun box provides a correct Two antenna U-bolts and clamps are required for a secure installation on the mast. The cable is passed through the dipole boom and is anchored to the mast with cable ties to stop it flapping in the wind. 28 SILICO N CHIP termination for the antenna dipole and terminals for the 75Q coax cable, all sealed away from the elements for protection. Inside is a PC board which accommodates the air-cored balun and 75Q coax connection points. The PC board measures 40 x 34mm and is coded SC02106911 . It has a very simple pattern. The balun is made of two small coils of 0.7mm enamelled copper wire, as shown in Fig.6 and Fig.7. Another view of the balun box, showing the four screws & nuts attaching the dipole elements. Stainless steel screws are mandatory here, since they attach to aluminium at one end & the copper balun board at the other. BILL OF MATERIALS Antenna 1.4 metres of 19mm square aluminium tubing with 1.15mm wall thickness 16 metres of 10mm x 3mm aluminium flat (length based on purchasing aluminium in 2 or 4-metre lengths) 1 350mm x 150mm x 1.6mm aluminium sheet (see text) 60 pop rivets, 3.2mm x 10mm, aluminium or stainless steel mandrel; or 60 stainless steel self-tapping screws (see text) 2 U-bolts & V-clamps to suit mast {F) HALF DIPOLE 2 REQUIRED MATERIAL:· 1.6mm ALUMINIUM Balun Box 1 plastic zippy box, 80 x 52 x 30mm 1 PC board, code SC02106911 300mm x 0. 7mm diameter enamelled copper wire 6 stainless steel machine screws, 3mm x 12mm 12 3mm stainless steel nuts <at> <at> A A 15 BOX LID DRILLING FOR DIPOLE MOUNTING +A 15 15 <at> 15 <at> (G) Miscellaneous 75Q semi-air spaced coaxial cable (Hills SSC-32 or equivalent), plastic cable ties, silicone sealant, Delrin plugs for square tubing . t}- . cp ~ . DIMENSIONS IN MILLIMETRES BOX BOTTOM DRILLING ~ HOLES:- A. 3.2mm DIA. B: 8mm DIA. 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. Wind the primary coil on a 3.2mm former (such as a drill bit) and then tin the ends. Similarly, wind the secondary coil on a 4.76mm former (a 3/16-inch drill) and then tin the ends. The primary coil fits inside the secondary coil before they are both soldered to the PC board. Incidentally, do not think that the connections to the outer coil, as shown on Figs.6 & 7 are a mistake. They are correctly shown, with both ends soldered to earth. Fig.5 , the dipole and balun box assembly, shows how the PC board is mounted inside the plastic box. Stain- Fig.4: cutting and drilling details for the half dipole elements, and plastic balun box. Note that thinner material could be used for the dipole halves, down to 1mm thick. Cut them out using a hacksaw & then carefully smooth the edges of each dipole section using a file. (G) BALUN PCB (!) (D) (F) Fig.5: this diagram shows the assembly details for the dipole boom, balun box and dipole elements. Note that stainless steel screws must be used for the balun PC board connections. The balun box is attached to the boom using two right-angle brackets. ]UNE 1991 29 BALUN & ___1-----=r"'='=°="'.. _TO RECEIVER ANTENN~-~PR_l ___ ~ SEC PRIMARY: 12T, 0.7mm ENAMELLED COPPER WIRE CLOSE WOUND ON A 3.2mm DIA. "MANDREL SECONDARY . 6T, 0. 7mm ENAMELLED COPPER WIRE CLOSEWOUND ON A 4.76mm OIA. MANDREL Fig.6: the circuit of the balun is typical of commercial practice and involves two concentric air-cored windings with the secondary being a shorted connection. e COILS MOUNTED ON COPPER SIDE OF BOARD SECONDARY SLIPPED OVER ONE END OF PRIMARY AND BOTH ENOS SOLDERED TO EARTH Fig. 7: wiring details for the balun PC board. Good quality semi-air-spaced 75Q coax with a woven copper shield is mandatory here. Aluminium shielded cable is not suitable for this job since you can't solder to it. less steel screws are used to attach the dipole halves and the PC board to the lid of the box. The box itself is attached to the antenna using the two brackets depicted in Fig.3(E). Don't fit the dipole/lid assembly to the box at this stage - that can wait until the antenna is about to be installed. Final assembly Now to put it all together. The two reflector grilles and the dipole boom are riveted or screwed together using the two mast clamping brackets. The idea is to pop rivet (or screw) the two mast clamp brackets to one of the reflector booms first. That done, pop rivet in the dipole boom and finally, the second reflector boom assembly, so that the dipole boom is sandwiched between the two reflector booms. You can seal the ends of the reflector booms with square Delrin plugs which are made for the job. At this stage, you're just about finished. Installation Take a lot of care when installing the antenna. There's no point doing a wonderful job of making the antenna if you end up in hospital after falling off a ladder. Believe us, climbing ladders while carrying an antenna 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 trees and other antennas. It is not really practical to mount this large corner reflector antenna on the same mast as a VHF antenna un- _J Fig.8: the PC pattern for the balun box is quite simple. You can either etch your own board· or buy a ready-made board from the usual suppliers. less it is vertically separated by a distance of at least one metre. In any case, if all goes well, you should be able to dispense with your VHF antenna altogether. Having erected your mast or J-pole (for barge board mounting) you should be ready to take the antenna up and secure it with the U-bolts. But just before you do that, you need to terminate the 750 coax cable. Pass the cable through the dipole boom and through the balun box. Strip the cable and tin the centre wire and shield, then solder it to the relevant pads on the PC board. Finally, place a cable tie around the cable as a strain relief (as shown in one of the photos) and then screw the dipole/lid assembly onto the case. You can now bolt the antenna to the mast. Use good quality cable This view of the balun box lid shows how the antenna cable is soldered to the balun PC board. A plastic cable tie around the end of the cable stops it from being pulled out of the box. 30 SILICON Cf/11' By the way, for minimum signal loss and good cable life, we recomm end Hills semi-air-spaced cable (with holes in the cable dielectric), type SSC-32 or equivalent. It's also a good idea to keep this cable as short as possible to minimise cable losses, so try to site the antenna close to your TV set. Secure the cable to the mast with plastic cable ties to stop .it flapping in the wind. You may also like to seal the balun box and the rear cable entry to the antenna with silicone sealant, to weatherproof it. Depending on where you live, painting the antenna may be worthwhile, particularly in seaside areas or industrial areas where there is 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". SC