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By LEO SIMPSON & BOB FLYNN Build this 5-element FM antenna for better reception What’s your FM reception like? Pretty poor? Does the music sound distorted or are there lots of sibilants on voice? If so, you need a proper FM antenna, not a bit of wire hanging out the back of your tuner. Why not build this 5-element Yagi antenna which is designed specially for the FM band? You’ll be amazed at how good those stations can sound. 34  Silicon Chip It’s amazing isn’t it. People spend thousands of dollars on their hifi equipment and then just hang a bit of wire out the back of the gear to pick up the FM stations. Well, the music might sound first class from CDs but it will definitely not be up to par from the FM stations. This doesn’t make sense really. Most people know that they need a good antenna for TV reception; the same applies to FM. The same recep­ tion problems that plague TV also Fig.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. March 1998  35 Bill of Materials Aluminium 2.3 metres of 19mm square aluminium tubing with 1.8mm wall thickness 8.5 metres of 10mm diameter aluminium tubing with 1mm wall thick­ness Hardware 1 piece of thick Perspex, 120 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 or M4 roundhead screws 70mm long 1 3/16-inch Whitworth or M4 roundhead screw 60mm long 2 3/16-inch Whitworth or M4 roundhead screws 32mm long 2 3/16-inch Whitworth or M4 roundhead screws 19mm long 7 3/16-inch or 4mm ID split or lockwashers 7 3/16-inch Whitworth or M4 nuts 2 3/16-inch Whitworth or M4 wing nuts 2 3/16-inch or 4mm ID flat washers Note: all screws, washers and nuts should be stainless steel Miscellaneous Mast and wall mounts or bargeboard mount (hockey stick style), 300Ω-to-75Ω in-line balun (for outdoor use), semi-air spaced coax cable (Hills SSC32 or equivalent), plastic cable ties, silicone sealant. affect FM stations. You wouldn’t put up with just a bit of wire hanging out of your TV so why compromise with FM signals? If you do, you are bound to get weak signals and multipath reception which is the same effect as “ghosting” on TV. Multipath causes bad distor­ tion and you can only cure it with 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 better results. And if you changed over to a UHF TV antenna or pay TV, you 36  Silicon Chip 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 University in Japan in 1926. In the VHF (very high frequency) bands, of which the FM band (88108MHz) forms a small part, most antennas depend on electrically resonant elements; ie, elements which are a half-wavelength 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 the don’t have that option. So what FM antenna to buy? There are only a few available and of those that are, few are suitable for fringe areas. To be specific, the most common FM antenna available is a 3-element Yagi which gives, at best, about 6dB gain with respect to a simple dipole. These are OK in strong signal areas but if you want more than just a couple of stations at reasonable signal strength, you need more gain; ie, you need a “fringe-area” anten­na. You also need more directivity to cope with poor reception 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 single reflector and three directors. It has an esti­mated gain of between +8dB and +9dB with respect to a dipole and an improved front-to-back ratio compared to a 3-element array. Narrow acceptance angle As well as an improved front-toback ratio, this antenna is also more directional. To put it another way, it has a narrow­er acceptance angle. This dipole. Part of the electromagnetic energy 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 cer­ tain number of parasitic elements, no useful increase in gain is obtainable. There is a definite trade-off between the practical size of a Yagi and the amount of gain it provides. means that reflected signals coming in from the side of the antenna will be suppressed. This is worth­ while 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 prob­ lems with distorted sound from FM stations (due to reflected signals or “multipath”), you should get a notice­ able improvement with this antenna. Apart from reducing multipath problems, the big reason to build this antenna is to obtain lots more signal than you would get from a random piece of wire or the common twinlead dipole wire antenna that is supplied with many tuners. Furthermore, be­ cause it will be installed outside your home, the signal pickup will be even better. In fact, our observations show that with a good FM antenna such as this, it is possible to pick up stations (in stereo) which may be more than 160km away. Finally, by feeding more signal to your tuner, even from your strong local stations, you will improve the re­ ception. There will be less distortion, better separation between channels and less hiss in the background. In fact, with a good FM tuner combined with a good antenna, it can be difficult to pick the difference between a CD player and the same piece of music “off air”. Does that sound outra­geous? Well, it’s not, as far as average CDs are concerned even though CD players have far superior noise and distortion compared to signals broadcast on FM. Tools you will need 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 this. Apart from an antenna clamp (U-bolt and V-block bracket), no spe­ cial hardware or fittings are required. Making this antenna is very straight­ forward. If you have all the materials available you can probably do it in a single afternoon. Fig.1 shows all the details of the 5-element antenna. It shows the di­ mensions 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. The dipole insulator plate has wing nut terminals to connect 300Ω ribbon or a 300Ω-to-75Ω balun. The plate is made from Per­spex, Lexan or other acrylic material. The square boom makes mounting easy. Screws & nuts 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 screws and nuts. We recommend the use of stain­ less steel screws, nuts and washers throughout, whether for machine screws or self-tappers. They do cost 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 recom­ mend galvanised, bright zinc or cadmium plated steel screws. In seaside areas these can be visibly cor­ roded 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 Materi­ 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. als. After all, you will be frustrated if you get half-way through and find you can’t progress further 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 makes drilling and assembly easier. If you are experienced in metalwork and have access to a set of V-blocks and a drill press, you could substi­ tute 25mm diameter tubing for the boom. In fact, you could use 25mm stain­ less steel round tubing which is readily available from plumbing supply stores. While you’re at it, cut the folded dipole spacer which also uses the 19mm square tubing. It is 50mm long. March 1998  37 The ends of the folded dipole are fabricated 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. will have the elements skew-whiff. A few words of advice on drilling is appropriate here. Drilling in thin wall aluminium tubing can be a problem and many people 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. 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. Each director element and the reflector is held in the boom with a self-tapping screw, as shown in dia­ gram 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 though because the dipole should be assem­ bled and mounted on the boom first. Making the dipole The mast clamp and V-block assembly can be purchased from elec­tronic parts or automobile accessory retailers. We strongly recommend hot-dip galvanised types if possible. Avoid cadmium plated or zinc plated clamps which can rust quite quickly, par­ticularly in seaside areas. You should have a piece of tubing about 120mm long 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 (ie, 1270mm). Next, centre-punch the boom for all holes prior to drill­ing. Note that the 38  Silicon Chip 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 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 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 ele­ ments of the dipole. Further detail is shown in the accompanying photos. The top and bottom pieces of the dipole are held at each end with a 70mm long 3/16-inch Whitworth or M4 screw, together with a nut and lock washer. At the centre, the lower halves of the dipole are terminated on an insulating plate (shown in dia­ gram 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 or M4 screw, nut and lockwasher. Terminals for the dipole are pro­ vided with two 32mm long 3/16-inch Whitworth or M4 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 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. long self-tapping screws which go through the spacer and into the boom. The top piece of the dipole is then se­ cured to the boom with a 60mm long 3/16-inch Whitworth or M4 screw, nut and lockwasher. The details of the dipole insulating plate and fixing to the boom can be seen in the accompanying photos. Note that while we used white Per­ spex, you could use a piece of clear material if that is what you have on hand. However, note our remarks on painting, later in this article. By this time the antenna looks just about complete. You need to add the antenna clamp, to enable it to be at­ tached to the mast and you will need a 300Ω-to-75Ω balun to match it to 75Ω coax cable. You can use 300Ω ribbon if you wish and omit the balun but to obtain the most interference-free signal, we recom­mend coax cable for your installation. Unfortunately, many antenna clamps are sold with a cadmium plat­ ed and passivated finish. These have a “gold” finish. This is barely adequate for inland areas but rusts quickly 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 metal hardware can rust and corrode. If you can, buy antenna clamps that are hot dip galvanised. These last a The reflector and director elements are attached to the boom using self-tapping screws. Ideally, all screws, nuts and washers should be stainless steel to avoid corrosion. 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 hot dip 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 stopped up with 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. It is also a good idea to paint your antenna, if you live in an area where corrosion is a problem. If nothing else, the dipole insulating plate should be painted as acrylic material does dete­ riorate in sunlight (ie, UV). 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 an­ tenna 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 insu­ lation such as Sisalation) can have a bad effect on antenna performance. And 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 gutter­ ing. 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 an­ tenna. 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 consider installing a rotator, to obtain the very best reception from SC all stations. March 1998  39