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Build this 5-element FM antenna How's your FM reception? Not up to scratch? You can greatly improve matters by building this 5-element Yagi antenna which is designed specially for the FM band. By LEO SIMPSON & BOB FLYNN It's amazing isn't it. So many people spend thousands of dollars on their hifi equipment and then just string up the simple dipole antenna that came with their receiver or tuner. It really is a waste. After all, most people know that to obtain 28 SILICON CHIP good TV reception, they need a good antenna installation. Well, the same applies for FM. If you want the best sound quality, you need a good antenna. Sure, if you live in a strong signal area, you might get away with just a tap off your existing TV antenna but a separate FM antenna will always give a better result. The problem is: what antenna to buy? There are very few available and of those that are, few are suitable for fringe areas. To be specific, the most common FM antenna available is a 3element Yagi which gives, at best, about 6dB gain with respect to a simple dipole. These are OK in strong signal areas but we wanted more gain. We also wanted more directivity to cope with poor recepFig.1: this diagram shows all the details of the 5-element antenna. At top is a plan view while the other diagrams show hardware and element mounting details. ► . . E E B ~ a: ,f;l E ita: E E E \::! . .s .s ;! ~ a: ,- w ~ iS IlliS 9 :;; w ~ a: a: 0 0 E .s ::. ... 0 ,f;l ,f;l a: iS !!i N ..,~ = z N -L BOOM 2222mm A A C A A 19 19 ... N ~ ... N ... N 50 63D 627 363 ~ 0 562 J ~694 69/ FOLDED DIPOLE INSULATOR 3mm ACRYLIC -==------------Jil -------- -----'140D -":.:....- - ------- - - -- -~...-----------~~- ~ DIPOLE TOP PIECE - 1 REQO. 642 680 DIPOLE BOTTOM PIECE - 2 REQO. I 0 19 19 50 3/16" x 70mm WHITWORTH SCREW I 0 FOLDED DIPOLE SPACER - 1 REQD. 19mm SQ. ALUMINIUM TUBING MATERIAL:- FOLDED DIPOLE, IHRECTORS ANO REFLECTOR : 10mm DIA.; 1mm WALL THICKNESS, ALUMINIUM TUBING BOOM : 19mm SQ., 1.8mm WALL THICKNESS , ALUMINIUM TUBIIG 0- FASTENERS : STAINLESS STEEL FOLDED IHPOLE BOTTOM PIECE 0 REFLECTOR ANO DIRECTORS TO BOOM CONNECTION 3/16" x 60mm WHITWORTH SCREW FOLDED DIPOLE TOP PIECE FOLDED IHPOLE ENO 0 ; 42 IHPOLE ENO PIECE 2 REQO. =,, ELEMENT BG x 13mm STAINLESS STEEL SELF-TAPPING SCREW BOOM SPLIT WASHER I 0 FM ANTENNA FOLDED DIPOLE TO BOOM CONNECTION OCT0BER1988 29 Fig.2: the dipole insulator plate has wing nut terminals to connect 300!1 ribbon or a 300!1 to 75!1 balun. The plate is made from Perspex, Lexan or other acrylic material. The square boom makes mounting easy. more directional. To put it another way, it has a narrower acceptance angle. This means that reflected signals coming in from the side of the antenna will be suppressed. This is worthwhile because the more suppression you can obtain for reflected signals, the less distorted the resulting stereo sound will be. In other words, if you have problems with distorted sound from some FM stations (due to reflected signals or "multipath" ), you should be able to obtain a worthwhile improvement using this antenna. Another reason to build this antenna is to improve on the signal available from a normal 3-element array. While a gain figure of about + 2dB with respect to a 3-element array may seem modest, it can make a substantial difference to the apparent signal-to-noise ratio when receiving weak stations. Tools you will need Fig.3: this topside view of the antenna shows how the folded dipole is attached to the square boom. Note the short section of aluminium tubing which acts as a spacer between the underside of the boom and the dipole insulator plate. Whitworth or other machine screws hold it all together. tion conditions where multipath is a real problem. After looking at what's available, we decided to design and build our own. Actually, we ended up building a number of variations before settling on the design presented here. The new antenna is a 5-element Yagi array. It has a folded dipole, a 30 SILICON CHIP single reflector and three directors. It has an estimated gain of between 8 and 9dB gain with respect to a dipole and an improved front-toback ratio compared to a 3-element array. Narrow acceptance angle As well as an improved front-toback ratio, this antenna also is Most enthusiasts will have all the tools needed for this project. You will need a hacksaw, electric drill and a vice. It would also help if you have a drill press but you can do without. Apart from an antenna clamp (U-bolt and V-block bracket), no special hardware or fittings are required. Making and assembling this antenna is very straightforward. If you have all the materials available you can probably do it in a single afternoon. The diagrams of Fig.1 show all the details of the 5-element antenna. These diagrams show the dimensions of all the elements and the various hardware bits you will have to make to assemble the antenna. At top is a plan view showing the length of all five elements and their spacing along the boom. Fasteners After a few years' exposure to the elements, many antennas are in a poor state. Aluminium may not "rust" but it does corrode, particularly in seaside areas or in metropolitan areas where there is a lot of industrial fallout. This corrosion can be a lot worse if you don't use the right screw fasteners. We recommend the use of What is a Yagi antenna? The Yagi is not a new antenna design by any means. It was developed by H. Yagi and S. Uda at Tohoku Imperial Univers ity in Japan in 1926. In the VHF (very high frequency) bands, of which the FM band (88-108MHz) forms a small part, most antennas depend on electrically resonant elements; ie, elements which are a halfwavelength at the frequency of interest. In its simplest form, the Yagi consists of a dipole element and an additional slightly longer parasitic element behind it, called the reflector. More complex designs have shorter parasitic elements in front of the dipole and these are called directors. The reflector and directors are referred to as parasitic elements because they also resonate over a frequency range similar to that of stainless steel screws throughout, whether for machine screws or self-tappers. They do cost a little more but they last indefinitely. Don't, on any account, use brass screws. When used to attach aluminium elements these will corrode away almost before your eyes. Nor do we recommend galvanised, bright zinc or cadmium plated steel screws. In seaside areas these can be visibly corroded with just a few days' exposure. In rural areas, well away from the sea or city pollution, you can probably get away with galvanised screws but the antenna will last longer if you paint it. Starting work Before you start, make sure you have obtained all the aluminium and hardware listed in the Bill of Materials. After all, you will be frustrated if you get half-way through and find you can't progress farther because you lack screws or some other item. Get 'em all before you start. Cut the boom to length first. It is 2222mm long. It is made of 19mm square aluminium tubing which the dipole. Part of the electromagnetic ene rgy they capture is re-radiated and picked up by the dipole. Hence the director and reflectors add considerably to the signal which is picked up by the dipole on its own. By suitably dimensioning the reflector and directors, it is possible to determine the overall frequency coverage of a Yagi antenna, its gain and its directional characteristics. In general, the more elements in a Yagi array, the higher will be its gain and the smaller the forward acceptance angle . There is a law of diminishing returns though. Above a certain number of parasitic elements, no useful increase in gain is obtainable . There is a definite tradeoff between the practical size of a Yagi and the amount of gain it provides . makes drilling and assembly easier. (Yes, if you are experienced in metalwork and have access to a set of V-blocks and a drill press, you could substitute 25mm diameter tubing for the boom). While you're at it, cut the folded dipole spacer which also uses the 19mm square tubing. It is 50mm long. You should have a piece of tubing about 120mm left over as scrap. Don't throw it away. It will come in handy later. Now cut the 10mm diameter tubing for the director, three reflectors and parts for the dipole. Remember the old adage about "measure twice and cut once". It's hard to lengthen elements that are too short. Note that the three directors are all the same length, 1270mm. Next, centre-punch the boom for all holes prior to drilling. Note that the boom is 2222mm long and the total of the element spacings along the boom is 2182mm - see the plan diagram on Fig, 1. Mark the hole centre position for the reflector element first, 20mm from one end of the boom, and then work your way along. If you have a drill press which Bill of Materials Alumini um 2.3 metres of 19mm square aluminium tubing with 1 .8mm wall thickness 8 .5 metres of 10mm diameter aluminium tubing with 1 mm wall thickness Hardware 1 piece of thick Perspex, 1 20 x 40 x 3mm 1 U-bolt and clamp to suit mast 4 8G x 13mm screws 2 8G x 32mm screws 2 3/16-inch Whitworth roundhead screws 70mm long 1 3/16-inch Whitworth roundhead screw 60mm long 2 3/ 16-inch Whitworth roundhead screws 32mm long 2 3/16-inch Whitworth roundhead screws 1 9mm long 7 3/16-inch ID split or lockwashers 7 3/16-inch Whitworth nuts 2 3/16-inch Whitworth wing nuts 2 3/16-inch ID flat washers Note: all screws, washers and nuts to be stainless steel Miscellaneous Mast and wa ll mounts or bargeboard mount (hockey stick style) , 3000 to 750 in-line balun (for outdoor use). semi-air spaced coax cable (Hills SSC32 or equivalent). plastic cable ties, silicone sealant. lets you drill all the element holes square through the boom you are fortunate. If not, mark the hole centre positions on both sides of the boom and drill from both sides. If you don't get the element holes lined up properly, you will have the elements skew-whiff. A few words of advice on drilling is appropriate here. Drilling in thin wall aluminium can be a problem and many people will tend to end up with holes that are more triangular than round. The way around this problem is to drill all the large holes (ie, all 10mm holes) undersize and then ream them out to the correct size using a tapered reamer. OCT0BER1988 31 though because the dipole should be assembled and mounted on the boom first. Making the dipole Fig.4: the ends of the folded dipole are fabicated using 42mm lengths of aluminium tubing shaped to mate with the upper and lower pieces. They are held together with a 70mm long machine screw, nut and split washer. Fig.5: the mast clamp and V-block assembly can be purchased from electronic parts or automobile accessory retailers. Use galvanised types if possible. Avoid cadmium plated or zinc plated clamps which tend to rust out. Be careful when reaming holes out though because it is quite easy to get carried away and then end up with holes that are oversize. Use a scrap piece of 10mm tubing to test when the holes specified at 10mm are the correct size. 32 SILICON CHIP Each director element and the reflector is held in the boom with a self-tapping screw, as shown in diagram A of Fig.1. Drill a 3mm hole at the centre point of each element but only through one side. Don't mount the elements on the boom yet The folded dipole is made from five pieces of 10mm aluminium tubing, three long and two short. The detail of its assembly can be seen from the diagram at the bottom of Fig.1. Two short tubes, shown as diagram E on Fig.1, are cut and shaped so that they key in with the top and bottom elements of the dipole. Further detail is shown in the photo of Fig.4. The top and bottom pieces of the dipole are held at each end with a 70mm long 3/16-inch Whitworth screw, together with a nut and lock washer. At the centre, the lower halves of the dipole are terminated on a insulating plate (shown in diagram D of Fig.1). This plate is made of 3mm acrylic (Perspex or Lexan). The dipole halves are each secured to the insulating plate with a 19mm long 3/16-inch Whitworth screw, nut and lockwasher. Terminals for the dipole are provided with two 32mm long 3/16-inch Whitworth screws, each fitted with a nut and lockwasher plus a wing nut and flat washer. The insulating plate is secured to and spaced off the main boom via a section of square tubing, shown as a "folded dipole spacer" in diagram F of Fig.1. The insulating plate is secured to the spacer with two 8-gauge 32mm long self tapping screws which go through the spacer and into the boom. The top piece of the dipole is then secured to the boom with a 60mm long 3/16-inch Whitworth screw, nut and lockwasher. The details of the dipole insulating plate and fixing to the boom can be seen in the photos of Fig.2 and Fig.3. Note that while we used a white Perspex, you could use a piece of clear material if that is what you have on hand. By this time the antenna looks just about complete. You need to add the antenna clamp, to enable it to be attached to the mast, and you will need a 3000 to 750 balun to match it to 750 coax cable. You can use 3000 ribbon if you wish and omit the balun, but to obtain the Fig.6: do you have trouble drilling round holes? You'll do better by drilling the holes undersize and then reaming them out to exact size with a tapered reamer. most interference-free signal, we recommend coax cable for your installation. Unfortunately, many antenna clamps are sold with a cadmium plated and passivated finish. These have a "gold" finish. This may be adequate for inland areas but they soon rust in sea air. We may seem to be paranoid about corrosion but since the SILICON CHIP editorial offices are only a few hundred metres from the seaside we are very aware of just how quickly antennas can rot away. If you can, buy antenna clamps that are heavily galvanised. These last a lot longer than the cad-plated jobs. U-bolts and clamps intended for auto exhaust systems are generally quite good in this respect. But be aware that zinc " plated" fittings are not as rust resistant as galvanised types. Zinc plated fittings have a smooth bright appearance while heavy galvanising is unmistakable - it has quite a rough appearance. If you really want to gild the lily, go to a ship's chandlers and buy stainless steel U-bolts and clamps. They'll last forever; well just about. We suggest that the ends of all the elements and the boom be Fig.7: the reflector and director elements are attached to the boom using self-tapping screws. All screws, nuts and washers should be of stainless steel to avoid corrosion. stopped up with epoxy adhesive (eg, Araldite), putty or silicone sealant. This will stop them from whistling in the wind. Better still, you can buy Delrin plugs to suit the square aluminium tubing. These look neater. You might also consider painting your antenna, if you live in an area where corrosion is a problem. We suggest you leave the antenna for a month or so to weather it and then paint it with an etch primer. Finish it with an aluminium loaded paint such as British Paints "Silvar" . Installation When you have finished your antenna you need to carefully consider its installation. There is no . point in going to a lot of trouble making it if you don't install it properly. Try to install your new antenna well away from existing TV antennas as these can have quite a serious effect on the performance. Similarly, nearby metal guttering, electric cabling, metal roofing or sarking (ie, reflective insulation such as Sisalation) can have a bad effect on antenna performance. Don't forget the effect of a hot water tank which may be lurking just beneath the roof tiles. If you live on a busy street, try to install your antenna as far away as possible from the traffic side of your house. That way you will minimise ignition noise from passing traffic. Finally, install the antenna as high as possible above the roof and guttering. If that is a problem, try to install the antenna so that it is at least a half wavelength away from the nearest metallic object such as guttering or roofing. This means a distance of about 1.5 metres away from guttering. Take care when installing the antenna. Safe working with ladders is particularly important. Take your time and don't take risks. You don't want to end up in hospital. Line up the antenna so that it is aimed at the main FM stations of interest. If you are really keen, you could also consider installing a rotator, to obtain the very best reception from all stations. If you are using 3000 ribbon, use spacers to hold it away from the mast, otherwise you'll lose signal. If using coax cable, tie it to the mast with insulation tape or cable ties, to stop it flapping in the wind. ~ OCT0BER1988 33