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Build this By LEO SIMPSON 5-Element Yagi Antenna for better FM 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. But they’re not real easy to buy these days! So why not build your own? This 5-element Yagi antenna is designed specifically for the FM band. You’ll be amazed at how good those stations can sound when they have a strong signal. T hese days, many people are listening to DAB+ broadcasts and we can well understand why. There are more stations and provided the signal is OK, the sound will be OK too. But it will only be “OK” to the extent that the sampling rates used by virtually all DAB+ stations are barely adequate to give good sound quality. Yes, yes, we know that many people now listening to internet streaming services and they have thousands of music stations from around the world to choose from but again, the sound quality is just average. If you want really good broadcast sound quality, FM stations cannot be bettered. Of course, if you live outside the capital cities, there is no DAB+ and so it’s FM or nothing if you want to listen to music broadcasts with good sound quality. And if you are outside the major cities you are going to need a good FM antenna to get the best sound quality – nothing else will do. Of course, living in a large city, probably with hilly surrounds and tall buildings can still mean that you will get weak signals and “multipath” reception. This is the same effect as “ghosting” on TV. Multipath causes bad distortion and you can only cure it with a good anten­na. 72  Silicon Chip So what FM antenna should you buy? They are virtually unavailable. This was the situation when we originally presented this antenna design back in October 1988 and repeated in March 1998. Things have not improved since then and we still get asked for a good FM antenna. So we are presenting it again, with very slight changes. The antenna is a 5-element Yagi array. It has a folded dipole, a single reflector and three directors. It has an esti­ mated gain of between +8dB and +9dB with respect to a dipole and an improved front-to-back ratio compared to a 3-element array, the most common FM antenna in the past. By the way, if you have an old VHF TV log periodic array it could be pressed into service but it won’t perform quite as well as this antenna which is cut to suit just the FM band from 88 to 108MHz. Narrow acceptance angle As well as a good front-to-back ratio, this antenna is also quite directional – or to put it another way, it has a narrow­ acceptance angle. Both of these factors mean that reflected signals coming in from the sides and rear of the antenna will be suppressed. This is worth­while because the more suppression you can obtain for reflected signals, the less siliconchip.com.au 5-ELEMENT FM BROADCAST ANTENNA 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. siliconchip.com.au October 2015  73 This shot shows the fixing of the dipole top element to the boom and plate. This is done with a 70mm long stainless steel screw which passes through the top element, then the boom and spacers below to the Lexan plate. 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. distorted the resulting stereo sound will be. We’re talking here about “multi-path” reception, which used to plague analog TV reception and cause ghosts in the picture. With FM broadcasts, multi-path reception causes the sound to spit and sputter, especially if a low-flying plane passes overhead. 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 twin-lead dipole wire antenna that is supplied with many tuners. Furthermore, because 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 reception and it will be less susceptible to interference from equipment with switchmode power supplies. There will be less distortion, better separation between channels and less hiss in the background. nas are in a poor state. Aluminium may not “rust” but it does oxidise, particularly in seaside areas or in metropolitan areas where there is a lot of industrial fallout. Corrosion will also be a lot worse if you don’t use the right screws and nuts. We strongly recommend the use of stainless steel screws, nuts and washers throughout, whether for machine screws or self-tappers. They do cost a little more but they last indefinitely. You will find a good array of stainless steel screws available from ships’ chandlers. 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, 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. Tools you will need Most enthusiasts will have all the tools needed for this project: a hacksaw, electric drill and a vice. It would also help if you have a drill press but you can do without this. You will also find that a tube cutter (as used by plumbers) will be good for making clean cuts on aluminium tubing. Apart from an antenna clamp (U-bolt and V-block bracket), no special hardware or fittings are required. Making this antenna is quite straightforward. If you have all the materials available you can probably do it in a couple of afternoons. Fig.1 shows all the details of the 5-element antenna. It shows the dimensions of all the elements and the various hardware bits you will have to make to assemble the antenna. At the top is a plan view showing the length of all five elements and their spacing along the boom. Screws & nuts After a few years’ exposure to the elements, many anten74  Silicon Chip Starting work Before you start, make sure you have obtained all the alu­minium and hardware listed in the Bill of Materials. 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 and 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 substitute 25mm diameter tubing for the boom. In fact, you could use 25mm stain­less steel round tubing which is readily available from plumbing supply stores but it is expensive and not easy to work. 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 long left over as scrap. Don’t throw it away. It will come in handy later. (When reassembling this prototype antenna after years in storage, we could not find the 50mm long spacer so we substituted two 19mm lengths of 19mm plastic conduit instead). siliconchip.com.au Note the two short sections of PVC conduit which act as spacers between the underside of the boom and the dipole insulator plate. Machine screws hold it all together. This is an alternative square tube spacer shown in Fig.1. 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 70mm long machine screws, nuts and split washers. 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 drill­ ing. 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 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 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) under size and then ream them out to the correct diameter 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 too big. 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 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 though because the dipole should be assembled and mounted on the boom first. 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 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 or M4 screw, nut and lock-washer. Terminals for the dipole are provided with two 32mm long 3/16-inch Whitworth or M4 screws, each fitted with Making the dipole 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. The two short tubes, shown as diagram E on Fig.1, are cut and shaped using a file so that they key in with the top and bottom elements of the dipole. Further detail is siliconchip.com.au What Is A Yagi Antenna? The Yagi antenna design 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 (88-108MHz) 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 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. October 2015  75 We strongly recommend stainless steel (or at worst hotdipped galvanised) fittings, as seen in this close-up shot of the U-bolt and V-block. You can find stainless fittings at automotive suppliers and ships’ chandlers. Do you have trouble drilling round holes? You’ll do better by drilling the holes slightly undersize and then reaming them out to the exact size with a tapered reamer. Don’t have a tapered reamer? Buy one – they’re handy! a nut and lock-washer 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 32mm long 8-gauge selftapping screws which go through the spacer and into the boom. The top piece of the dipole is then secured to the boom with a 70mm long 3/16-inch Whitworth or M4 screw, nut and lock-washer 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 Perspex, you could use a piece of polycarbonate 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 attached to the mast and you will need a 300Ω-to-75Ω balun to match it to 75Ω coax cable. You could use 300Ω ribbon if you wish (and also if you can get it!) and omit the balun but to obtain the most interference-free signal, we recom­mend coax cable for your installation. Also, 300Ω ribbon deteriorates in the weather a lot faster than coax. Unfortunately, many antenna clamps are sold with a cadmium-plated 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 kilometre or so from the seaside we are very aware of just how quickly metal hardware can rust and corrode. If you can, buy U-bolts and clamps that are hot-dip galvanised or stainless steel, as used for car exhaust systems (or boat fittings), as these will last a lot longer. 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. 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 deteriorate 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. Whistling in the wind! We also suggest that the ends of all the elements and the 76  Silicon Chip 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, solar panels (photovoltaic or hot water), metal wall siding, nearby metal guttering, electric cabling, metal roofing or sarking (ie, reflective insulation such as Sisalation) can all 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 will help 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 its directors (the shorter elements) are aimed at the main FM station(s) of interest. You may have to call the station to check where their transmitter is located because they are seldom at the same location as the studios. siliconchip.com.au 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. To check that screws are stainless, use a magnet. Unfortunately, the transmitters are often widely separated around the cities so you might have to compromise – aim at the one you most want or between them if there are two or more. But if you are really keen, you could consider installing a rotator, to obtain the very best reception from all stations. You should find the stations come in loud and clear without too much fiddling around. If all you get is silence, or bad reception, check that your coax cable is connected properly and/or that it doesn’t have a short somewhere (usually at the beginning or end). Remember that coax cable is lossy so you want as short a length as you can install. And speaking of the coax, it should be firmly fixed to both the antenna boom and the mast. Otherwise it will flap around in the wind – annoying to start with but eventually leading to the coax failure. Use either black cable ties (don’t use white – they don’t last) or black self-annealing insulation tape. SC Because the impedance of the folded dipole antenna is 300Ω and the coax cable downlead is 75Ω, an inline balun (balanced-to-unbalanced transformer) must be used to connect the coax cable to the driven element. It is essential that this be made waterproof with either a matching boot (normally supplied with balun) or, failing that, some coaxial sealing tape. Note also how the coax is firmly attached to both the antenna boom and to the mast pipe to stop it flapping in the breeze. siliconchip.com.au A tube-cutting tool makes easy work of cutting the aluminium rods to length, with nice square cuts. They’re also handy if you have to cut PVC conduit. And they’re cheap, too (we bought ours at Bunnings for less than $20). Bill of Materials – 5 Element FM Antenna 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 white Perspex, Lexan, etc, 120 x 40 x 3mm 1 galvanised or stainless steel U-bolt and clamp to suit mast 4 8G x 13mm screws 2 8G x 32mm screws 3 3/16-inch Whitworth or M4 roundhead screws 70mm 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 AS316-grade stainless steel Miscellaneous Mast and wall mounts or barge-board mount (hockey stick style), 300Ω to-75Ω in-line balun (Jaycar Cat LT-3028 plus matching boot and F-connector), Appropriate length semi-air spaced coax cable (Jaycar WB-2004, WB-2006; Hills SSC32 or equivalent), Plastic cable ties, Silicone sealant. October 2015  77