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Building a real Yagi Antenna for UHF CB Can you scrounge a bit of plastic electrician’s ducting and a few wire coat hangers? If so, in a couple of hours you can build yourself a really cheap, really effective antenna for your UHF CB or PRS hand-held and enjoy significantly increased range! 72  Silicon Chip 72  Silicon Chip siliconchip.com.au siliconchip.com.au cheap by Stan Swan R emember those Christmas-stocking-filler walkie-talkie radios you had as a kid? The sort that you and your mates played spies with – before their batteries gave out on Boxing Day? Operating in the 27MHz band (CB channel 14 being most common) most of these flea-power sets had just two controls: on/off and push-to-talk. Many didn’t even rate a volume control! Not only did they have mediocre “lucky to get next door” range but lack of noise-killing squelch revealed your broom cupboard hideaway. Naturally your allowance didn’t stretch to costly 9V battery replacement every few days either. Untold numbers of such sets probably now lie discarded in those very broom cupboards. Such cheap wireless toys rightly lead to most hand-held two-way radios being dismissed as kids stuff. Serious communications surely justifies professional equipment, perhaps larger 27MHz CB, VHF marine or even (another) mobile phone? Allow hundreds of dollars. Anyway, today’s kids would probably organise their spy ring by mobile phones or text messages, to their (and their parent’s!) lament when the bills arrive. PRS = Potent Radio Surprise Surprise! The walkie-talkie has grown up, and modern 2 way-radios are now the cat’s whiskers. In most countries, evolving from an attempt to tidy 27MHz CB abuse, generous slices of the radio spectrum around 450MHz were assigned in the 1990s for licence free, low power (0.5W) UHF CB voice communications. New Zealand and much of Asia calls this the Personal Radio Service (PRS) but in Australia it’s simply referred to as UHF CB. It covers 40 FM analog channels between 476.425 and 477.400MHz, with specific limits on power and so on. In Australia, conventions have been established as to which channels are simplex and which are for repeater inputs and outputs, which channels are for emergencies, which channels truckies and travellers use, and so on. Two channels are even reserved for “non voice” data communication – more of this in a later article! The US Family Radio Service (FRS) and European Personal Mobile Radio (PMR) are similar, but with slightly different frequencies (PMR = 446MHz siliconchip.com.au siliconchip.com.au February 2005  73 February 2005  73 The simplest UHF CB antenna? A quarter-wave whip! Perhaps the simplest external UHF CB antenna is a quarter-wavelength whip, which can conveniently be rustled up at this UHF frequency with an old telescopic aerial or welding rod offcut. With one 477MHz wavelength = 628mm, then 1/4 wave = 157mm. The telescopic version allows VERY convenient tweaking of course and even extension to a desirable 5/8 of wavelength (here 392mm) when a more squashed radiation pattern emerges. Verticals give an omnidirectional pattern, so won’t have the potential gain of a Yagi but lend themselves to elevated mounting if an artificial ground can be provided. Such a ground is often configured as a few sloping radials, each themselves 1/4 wavelength long but it’s not usually too critical. A 2-way radio in fact uses loose coupling to your body itself as the ground and vehicle mounted whips employ the metalwork of the car. Although cheap TV grade “F” connectors are fine at UHF, BNC are more rugged and reliable. One simple DIY design uses an emptied plastic CD spindle as the connector holder, with an aluminium “disk” inside as a simple ground plane. Thermally conductive aluminium will be a soldering nightmare, so bolt a solder tag from the coax shield to it instead. The disk ground should be larger (usually 1/2 wavelength across = 314mm) but this could be enhanced if placed on a car roof top or the like. For antenna security it’s suggested that several strong magnets be placed inside the plastic case to grip nearby iron work, although maybe the plastic container could be filled with pebbles or sand for stability when on a platform or stationary vehicle. But don’t forget to take it down before you drive off . . . Even a simple vertical antenna can greatly enhance reception, especially if signals are otherwise weakened by nearby shielding metalwork. Assorted F and BNC connectors and adaptors allow designs to suit your application. Use heat shrink or sleeving to prevent the main antenna conductor shorting. 74  Silicon Chip In Australia, conventions have been established as to which channels are simplex and which are for repeater inputs and outputs, which channels are for emergencies, which channels truckies and travellers use, and so on. Two channels are even reserved for “non voice” data communication – more of this in a later article! The US Family Radio Service (FRS) and European Personal Mobile Radio (PMR) are similar, but with slightly different frequencies (PMR = 446MHz over 8 channels) and local regulations. In all cases however, operation is totally licence free, with no ongoing running costs. There’s no privacy (so don’t broadcast your Swiss bank account details…) and, despite some businesses in the bush trying to claim “their channel” no-one has the right to use a channel over anyone else (except the emergency channel which is backed by regulation). When first offered back in the 1990s, and even as recently as the SILICON CHIP March 2001 review, UHF CB radio sets were costly and largely appealed to commercial and events users. Such is now not the case, since mass production has lead to insanely cheap global prices. In NZ and Australia, UHF sets now flood electronic retailers and discount stores at such throw-away prices that customers may not in fact take them seriously. Even electronic gurus often consider them as glorified toys until they try one out! Perhaps because of such “cheapness”, uptake seems very modest here in larger NZ cities, no doubt also reflecting the now nearautomatic tendency to reach for one’s mobile phone. But remote regions, group bush hikes or vehicle convoys may suffer cellular flakiness. If you’re passing through Snake Gully and require group broadcasts for a comfort stop ahead, then mobile phone calls may be futile and are limited to 1:1 of course, even if coverage improves. Ranges of the modern UHF CB sets are rather more line-of-sight than 27MHz CB, although reduced atmospheric noise at the higher frequencies mean receivers can be very sensitive indeed, with quality FM audio as well. City block coverage is typical (ideal shopping!) with bush conditions perhaps a kilometre or so. Unobstructed links (eg, over water) can allow clear siliconchip.com.au wonderland of microelectronics but most importantly two generous solder pads beside the clamped spring antenna. Yay! It’s the work of moments to solder (perhaps with 1mm holes drilled for header pins) a flexible coax lead to these and direct it out via the nowempty rubber ducky space. Strictly speaking, UHF grade coax and BNC connectors should be used, but for convenience even a short length of thin flexible shielded audio coax and cheap RCA sockets are tolerable, especially while you experiment. Avoid transmitting without an antenna of course, since you may damage the set with the signal that’s now got nowhere to go! Antenna basics Jaycar’s Digitech DC-1030 is typical of modern UHF CB/PRS transceivers which sport a huge range of features. Opening the case reveals a couple of nice large pads to which we can solder an external antenna. The existing antenna is the spring-like coil (shown with its cover removed, top left of opened-up transceiver). reception at even 10km or more. Signals will penetrate buildings and vegetation to a certain extent, as 477MHz propagation is not as picky as 2.4GHz WiFi, where at times it seems even a shadow will threaten to throttle coverage! Battery life is much enhanced and most units now capable of using rechargeable NiMH AA or AAA cells, further reducing ongoing costs. Docking stations and mains chargers can make a group’s wireless operations almost seamless – particularly appealing perhaps to a sports or school group short on expertise (and $$$) but needing reliable equipment. Even budget $40 models feature near-bewildering control options, such as scanning, voice operated transmission (VOX), duplex repeater and subaudible tone calling. If you intend buying a UHF CB family pack for an outing or sports event then ensure you choose a model that’s ergonomic, else granny may waste half the day trying to deactivate dual watch and CTCSS calling, or annoy the group with VOX transmissions every time she coughs! Before Christmas, Australian discount department stores were flogging siliconchip.com.au four-packs of UHF CB transceivers for less than $100 – (that’s just $25 per set!). However, we cannot make any comment as to their “mod-ability” which follows in this article using the Jaycar DC-1030. External antenna mod Most 2004-era UHF CB sets use a traditional rubber ducky antenna, involving a coiled wire “spring” radiator inside a short flexible rubber sleeve. Radio regulations, common to both NZ (RSM) and Australia (ACA), specify that external antennas are permitted, so it’s naturally tempting to replace this rubber ducky with something more effective. At the very least perhaps an antenna can be placed higher on a building for better coverage. Naturally, opening and modifying a transceiver may void your warranty but at their bargain prices this may be incidental. Courtesy of Jaycar Electronics (which fortunately stocks largely the same range both sides of the Tasman) a pair of their DC-1030 handhelds was obtained. This model readily unscrewed (joy of joys – a Philips head, not a tamperproof screw!) to reveal a not only a In spite of white-hot changes in electronics, antennas remain near timeless devices, with designs dating back decades ripe for hands-on experimentation. Considerable design and simulation software now exists, ranging from the broadcast professional’s Numerical Electromagnetics Code (NEC) to (gasp!) DOS-based programs such as Yagi Optimizer (yo.zip). Perhaps the most complete repository is that of a Canadian ham, VE3SQB, whose web site, www.qsl.net/ve3sqb/ is justifiably praised for its Visual Basic antenna simulations. Taking 477MHz as the midpoint of the UHF CB band, radio waves at this frequency will have a wavelength (L) (in metres [=1000mm]) of: L = 3 x 108 477MHz, or some 628mm, a length convenient for “plumbers delight” designs, especially classic Yagi-Uda’s. The Yagi The “Yagi” antenna (fellow inventor Uda is usually now neglected) dates from 1929. It is probably best known as a TV or FM radio antenna but lends itself to easy home-built use. Amateur radio operators love ’em! As with theoretically all antennas, the Yagi can be used to transmit or receive electromagnetic energy (in the form of radio waves). Here we’ll describe it as a receiving antenna. It consists of at least two elements (although a minimum of three is more February 2005  75 A typical antenna design and simulation program screen. Note the slight dimensional variations from the version used in this article. It’s probably best to use such software as a guide and optimise performance by field trimming. common) and theoretically there is no upper limit. All elements are (again normally) aligned in the same plane. The most important one is the driven element (DE) – the one that connects to your transmitter and/or receiver. Behind the driven element is (usually) one reflector (R), slightly longer than the driven element. As its name suggests, its job is to reflect electromagnetic energy back towards the driven element. In front of the driven element is one or more directors, each slightly shorter than the driven element. Again, as the name suggests they direct energy towards the driven element. The more directors a Yagi has, the more directional it becomes. The Yagi is most effective when the boom (on which all the elements mount) points directly at the signal source (transmitter), with the directors towards the front. It will normally work “back-tofront” (ie, with the reflector pointing towards the transmitter) but nowhere near as well. As you turn the Yagi boom away from the antenna, performance drops off, to the point where at rightangles to the transmitter, there may be little or no signal at all. These are called “nulls”. Antenna textbook theory assigns the driven element (DE) as a half wavelength across. The actual physical length reduces somewhat by a factor relating to the elements diameter and frequency in use (typically 0.94). Thus, for 477MHz it’s 628mm x 0.5 x 0.94 = 295mm wide – about 1½ handspans. 76  Silicon Chip A birds eye view of the expected radiation from a 4-element Yagi such as ours. Note the large broad frontal lobe - this could be sharpened with even more directive elements. The rear reflector (R) is usually 1.05 times this (=310mm) and front directors (D) 0.95 times (=280mm). Tradeoffs between bandwidth, gain and feed impedance govern element spacings, with L/8 to L/3 gaps (here 78–209mm) common. Practical needs to compact the design arise as well – ours in fact follows this. Confused by the maths? Try a simulation program instead. Typically these may make all sorts of assumptions and their recommendations may be just a guide to the actual best design for your application. Measurements to just the nearest millimetre will do as well, since it’s hard for the average home constructor to cut and drill better than that. Experiment – you’ll learn by doing! Cotanger and ducting Yagi You could drill holes in a broom handle to mount Yagi elements but for flexibility a mount such the one that follows may appeal! This uses a very special type of wire called cotanger – if you haven’t heard of this before, go into your bedroom, open up you wardrobe and remove a shirt from what’s holding it up. Unless you’re unlucky (or upmarket You don’t have to label which element is which – but it might help you eliminate misteaks mstakes misstakes errors! You can see the “raw materials” at right: a length of electrical ducting and a few wire coathangers. siliconchip.com.au REFLECTOR (R) = 310mm DRIVEN ELEMENT (D) = 295mm* 120mm DIRECTOR 1 (D1) = 280mm DIRECTOR 2 (D2) = 280mm 110mm 160mm 477MHz 4-ELEMENT YAGI -- (~6dB GAIN) * BREAK DE AT CENTRE AND BEND 90° INTO TERMINAL BLOCK (90° BENT SECTIONS NOT INCLUDED IN 295mm LENGTH) ALL ELEMENT LENGTHS ARE END TO END ELEMENTS #10 GAL WIRE (2.5mm DIAM) CUT FROM COATHANGERS KEEP AS SHORT AS POSSIBLE ~450mm LENGTH 2-PART (SNAP FIT) PVC ELECTRICAL DUCTING 2-WAY MAINS TERMINAL BLOCK (MOUNTED ALONG CONDUIT) COAX CABLE (TO TRANSCEIVER) 3x 2-WAY MAINS TERMINAL BLOCKS (MOUNTED ACROSS CONDUIT) This scale diagram should help you construct the 477MHz Yagi antenna. Element lengths shown here were theoretical and may benefit from slight trimming. Spacings can readily be adjusted by drilling extra holes in the ducting walls. with wood or plastic models) you have in your hand a cotanger. (Whoops, we forgot who we are talking to. if y ou don’t hang your shirts up . . . you might have to surreptitiously raid someone else’s wardrobe). OK, so it’s made of galvanised wire about 2.5mm in diameter and when cut to appropriate lengths, is just stiff enough to make an antenna from. In our antenna, the wire elements are supported by terminal blocks that neatly fit inside a length of plastic electrical 2-part ducting (the kind which has a “lid” which snaps onto the “base”). The split driven element (DE) is secured by connectors turned through 90o. Doubling the connectors allows back ups should brass screws strip their threads on the tough iron wire and the connectors easily slide to test element positions. Thicker elements are in fact more desirable than thin, since radiation resistance and bandwidth is improved. Construction The length of the ducting and the distance from its end to the first elesiliconchip.com.au ment is not important (as long as it is long enough!). We have said around 450mm would be an adequate length; you may like to make yours longer if you want to be able to either mount it on a mast or hold it in the hand. In both cases, any extra length should be at the reflector end. First of all, you’ll almost certainly have to straighten the coathangers out first. Cut the elements to the dimensions shown from your coathangers using heavy-duty plier blades or even a hacksaw (don’t use your good sidecutters – you’ll either break them or notch them!). Using the diagram and photo as a guide, mark out the position of the driven elements on the ducting and drill all the appropriate holes. All elements pass through the sides of the ducting. The reflector and two directors are continuous lengths of wire, while the driven element is divided in two and the very end (say last 10mm) is bent at 90° to go into their respective places in the terminal block. The reflector and directors pass right through the terminal block and are held captive by the screws. One side of each of those terminal blocks is unused but you need two-way blocks to be able to mount them with a suit- Parts List – 477MHz UHF CB Yagi 1 boom made from a 450mm length 2-part electric cable ducting, of a size you can scrounge (typically ~30mm wide) 1 310mm length 2.5mm galvanised steel wire (from coathangers) – “R” 2 280mm lengths 2.5mm galvanised steel wire (from coathangers) – “D1, D2” 2 ~160mm lengths 2.5mm galvanised steel wire (from coathangers) – both for “D”. 4 2-way mains terminal blocks 4 nuts, bolts and washers (to mount terminal blocks on duct) 1 length (to suit) coax cable (preferably mini 75W low-loss) 1 plug to connect to socket you fit to your transceiver February 2005  77 With a home-brew field strength meter and a good pair of binoculars, you can do some quick performance checks of your new antenna – and compare the readings to the “rubber duck” antenna that the UHF transceivers come with. able bolt, washer and nut. To find the right spot for the reflector and director terminal blocks, poke the elements through the hole just drilled in one wall of ducting, slide the block on, the poke the element though the opposite wall and slide it out until it is roughly in position. Place the terminal block in the exact middle of the ducting (either by eye or by measure). Mark the terminal block mounting hole and drill it, then secure it with its bolt, nut and screw. Very carefully find the half-way point of each of the elements and make sure it is in the exact centre of the block. Tighten up the grub screws in the terminal block and your three elements are in the right spot. The driven elements are similar except that there are two of them and they have the 90° bend in their ends. Their terminal block mounts along the duct, rather than across it, so that the 90° ends turn into the terminals. It’s pretty important to have the ends of the driven elements mounted tight up to the terminal block. Strip off a couple of centimetres of coax cable and connect it, nice and DMM ON LOW DC RANGE (eg 200mV) GERMANIUM DIODE (eg OA91) 1λ WIRE LOOP (APPROX 628mm) 78  Silicon Chip 200mV close, to the terminal block. It doesn’t matter which way around the braid and inner conductor go. The coax can run along the ducting and out the reflector end. Give your antenna a final check, then snap the ducting cover on – and its finished. Performance Performance of this 4 element design was most satisfactory – we estimate around 6–8dB of gain, which translates into range doubling and improved signals in marginal areas. Modest direction finding was also possible, allowing homing in on a remote “lost in the bush” transmitter. This could be educational “fox hunting” fun for a scout group and may even stimulate a youthful interest in electronics or ham radio. Field testing Since classic transmitter testing equipment (usually SWR meters and antenna noise bridges) may be unavailable, it’s suggested you initially use a “Poor Man’s Field Strength Meter” (FSM). Our “poor man’s field strength meter” merely consists of a loop of wire, a germanium diode and a digital multimeter. Ideally the loop should be 628mm from multimeter terminal to terminal. But it’s not a particularly exact science: near enough will usually be good enough. The multimeter leads themselves (usually about 750mm each) will be a tad too long because that will be about 1500mm. Simply connect a germanium diode (such as an OA91 or 95) in series with a length of wire, ideally one wavelength long (628mm), and plug into a DMM switched to sensitive (200mV?) DC range. The wire loop picks up the radio signal, and the diode rectifies this to DC, much in the style of a classic crystal set. If it’s a clear diode (and most modern ones are) make sure it is well covered, or it may act as a tiny solar cell and generate a few millivolts when sunshine strikes it! Such a simple FSM may respond to all manner of passing transmitters, especially if powerful UHF TV stations, mobile phone towers and so on are nearby. Since it’s the relative “far field” readings that are of interest, these are readily determined by placing your FSM on a wooden chair some 20m away while trimming or rotating your antenna – maybe supported on a kitchen Lazy Susan – and reading the result via either a second person, or a pair of good binoculars. Meter readings from a directed antenna of some 20mV were noted at 20m distance – about as far apart as even good binnies can read on a DMM with even large LCDs. The proof of the idea of course is in performance. Since virtually all radio receivers incorporate automatic gain control (AGC) to amplify weaker signals more and attenuate strong ones, audible checks at some distance may be biased, although background “hiss” may be a good measure. With a test over an unobstructed 11km link, deep nulls – that is, drops in signal – were found much as predicted near 90o as the handheld Yagi was rotated and frontal radiation improvements were very significant. The Yagi reception was almost “arm chair quality” beside an unmodified set at the same time, and it’s feasible that Yagis at both ends would have allowed line of sight ranges of 30-50km. Mmm – just the ticket for that outback farm, off shore island or mega shopping mall! Ultimately the earth’s curvature may be the limiting factor, even if both ends are elevated. SC References www.manuka.orcon.net.nz/prs.htm hosts direct web links and pointers to simulation software. siliconchip.com.au