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by Ross Tester Collinear Antennas for ADS-B (or anywhere else!) In the August issue, Jim Rowe told us that he found the little “toy” whip antennas that come with USB DVB-T Dongles work about as well as anything else for ADSB, especially when cut down to a quarter-wave at ADSB operating frequency (1090MHz). Here are a couple of antennas which will deliver more signal. You can use the same principles for just about any frequency. A fair amount of research has backed up what Jim said – you don’t need a you-beaut antenna to receive ADSB signals. It was suggested by one source that the reason for this is that the signals, emanating from aircraft and their straightline, unobstructed paths, are not likely to suffer as much degradation as ground-based signals. That’s as good an explanation as we can come up with, too! However, it was also suggested that there is one antenna type which does offer better performance than a simple 42  Silicon Chip quarter-wave whip – and that antenna is the collinear. The big advantage of the collinear is that it costs peanuts to make, is quite easy to build and should give a useful improvement in gain. What is a collinear antenna? These antennas have been around for the best part of a century, having been first described in the jounal of the Institute of Radio Engineers by PS Carter in 1932 and further by CW Harrison in 1945. They have become very popular siliconchip.com.au I’m holding BOTH antennas in this shot – in my right hand is the little wire antenna, with the much larger coax collinear in my left. There’s about 3dB difference between them. wave phasing stub between each section. And yet another method is to include an inductor or coil between each section which achieves the same result. There are many other phasing methods as well but we won’t get bogged down on the technicalities here. We just want to make an antenna! Collinear antennas are also very much suited to a limited frequency range – ideal for single-frequency ADSB – and they also have the feature of being very easy to increase the antenna gain, within reason, simply by adding more elements. The collinears we are describing here are ‘end fed’ – that is, the feed to the receiver comes from the bottom end of the antenna. This is a very convenient way to feed the antenna, particularly when it is vertically polarised, as it must be for the vertically-polarised ADSB signals. A properly-designed antenna should be suitable for both transmitting and receiving, so if you want to use the information later in this article to change dimensions and make (say) an antenna suitable for UHF CB radio (476-477MHz) you can easily do so. Our simplest collinear both in amateur and professional ranks over the years. In a nutshell, a collinear is a vertical antenna whose resonant elements are connected along a common line (ie, co-linear) so that each element is opposite in phase to its neighbour. If you’re not into antennas, that mouthful is, fortunately, very easy to achieve. In some collinears (and the second one we will be making here) this phase transition is achieved simply by reverseconnecting each element. Another approach (especially used in larger, high-frequency collinears) is to use a 1/4siliconchip.com.au As we said earlier, collinears have been around for quite a while and come in all shapes and sizes. Therefore anything we describe here has almost certainly been described elsewhere before. And so it is with this one – in fact, we acknowledge that the whole inspiration came from one we saw on the ’net (http://martybugs.net/wireless/collinear. cgi). That was for a 6dBi collinear for the WiFi band (2.45GHz); the dimensions simply scale up for the longer-wavelength ADSB frequency. The beauty of this antenna is that it is made from bits and pieces you may have lying around – the most important one being a length of 2.5mm2 copper wire. Hmm, where do you get that from? How about some mains building cable? You’ll need the single-strand variety – not quite as common as multi-strand these days – but it doesn’t matter if it’s old and tarnished. For an 8-element collinear, you’ll need a length about 500mm; to add more elements, you’ll need more length! Even if you have to buy a length of this cable, it should set you back not much more than a dollar or so for a metre. The other hardware you’ll need is a length of 20mm or 25mm plastic conduit (again, used in electrical installations – short lengths are regularly discarded from building sites), an end cap to suit (a few cents from a hardware store) and some plastic saddle clamps to mount it (ditto from hardware store). The easiest way to connect to your antenna is to use the mini base that was supplied with your USB dongle. Admittedly, this only gives you about 1.2m of cable, so if you want to use this over more than that length (outside, for example), you’re going to need to make some form of base with low-loss coax to connect to your receiver. The USB Dongle is likely to have a very small “MCX” connector; so unless you get really lucky and find an MCX plug which can fit on your coax, some form of adaptor is likely to be required between the coax cable and the dongle. But we’d think twice about using this simple antenna and a long length of coax – this one is quick and easy to make but the second antenna should be a better performer. September 2013  43 Before we start 90% of 1/4 (62mm) The frequency we want to receive is 1090MHz. This has a wavelength () of 275mm, derived from the formula: ~420mm OF 25-30mm CONDUIT WITH TOP CAP  = C/f, where C = the speed of light (near enough to 300,000,000m/s) and f = the frequency in Hz. ONE-TURN COIL AS LARGE AS WILL FIT INSIDE CONDUIT There are three lengths we need to know, derived from the full wavelength: a quarter wave (¼) = 69mm a half wave (½) = 138mm a three quarter wave (¾)= 206mm Remember these – you’ll need them! ENSURE TOP AND BOTTOM OF COIL DO NOT TOUCH IF BARE WIRE Making the antenna 3/4 (206mm) ALL DIMENSIONS SUIT ADSB (1090MHz) ONE-TURN COIL AS LARGE AS WILL FIT INSIDE CONDUIT 1/2 (138mm) ENSURE TOP AND BOTTOM OF COIL DO NOT TOUCH IF BARE WIRE “CRANK” WIRE TO ALIGN BASE WITH MIDDLE OF COILS SUITABLE MOUNT/ CONNECTOR – EG 3mm THREADED STANDOFF 44  Silicon Chip The simple wire antenna is made from a ~500mm length of 2.5mm copper wire. For such a simple antenna, it gives a surprisingly good result. Above is shown the completed antenna mounted on the mini base which comes with the USB dongle. It’s a little misleading as both coils need to be at right angles to the elements, not as the camera has distorted here. And be careful not to bend the wire – it should be as straight as possible. As Mrs Beeton’s cookbook almost says, “first catch your wire!” If you happen to have a length of stiff copper wire, great. Otherwise, you’ll need to strip it from a scrap of single-conductor T&E 2.5mm building cable. You don’t want the plastic insulation on it, so remove that as well. We worked with a 500mm length. You need to first make the wire as straight as you can – and one of the easiest ways to do this is to firmly grip one end of the wire in a vice, just as firmly grip the opposite end with a large pair of pliers, and pull firmly. You’ll feel a little “give” as the wire stretches slightly and presto! A straight length of wire. Once you’ve straightened it out, try not to bend it – this will reduce its performance. Carefully remove the wire from the vice and cut off any damaged wire (eg, from the vice or pliers) at the end and place it on a flat surface, ready to measure out. As our diagram shows, the wire collinear is in three sections or elements: from the bottom, a ½-wave length, a ¾-wave length and a not-quite-¼-wave length. These lengths are as shown on the diagram. Between each of the elements there is a single-turn phasing coil, wound from the same wire but at 90° to the elements. You might be wondering why the top element is less than a ¼-wave length. All antennas exhibit either capacitance or inductance At left is a close-up of one of the two “coils” – note that its start and finish do not touch. Again, this coil is at right angles to the vertical wire elements. At right is the bottom of the antenna, soldered into a 3mm threaded stand-off so it can be used with the base which comes with the TV USB dongle. Note the crank at the base which aligns the base to the middle of the coils above. siliconchip.com.au off a millimetre could easily make the antenna not perform properly (by the same token, it could do the opposite. But you have no way of knowing). So all you can really do is compare this antenna to the ADSB antenna you made by clipping the whip supplied with the dongle down to ¼ wave (69mm). We’d be surprised if it didn’t do somewhat better – that is, receive ADSB signals from further-away planes. Finishing off Taken from a Gratten spectrum analyser, this shows a 1090MHz signal received by the bare wire Collinear antenna. As you can see, the signal is well above the background noise and this would be further improved by the coax version of the Collinear. or both. In this case, it is the capacitance that affects the length, so it is made 10% less than you would normally expect to reduce the capacitance effect and so make the length “seem” like a ¼ wave. Start at the top of the antenna and measure down, say, 70mm. Mark the wire with a felt-tip pen. Using the photo as a guide, carefully bend the wire straight out at 90° and wind a single-loop coil around a former, as large as will fit into your electrical conduit (20mm conduit is about 16mm ID; 25mm conduit is about 21mm ID). Note that the start and finish of the coil must not touch each other, particularly if you’re using bare copper wire. Also make sure that the start and finish are directly over each other and the coil is as round as you can make it. Now you can carefully measure back up the wire 63mm (69mm - 10%) from the coil and snip off the remainder. The middle length, down to the next coil, is the ¾ wave length or 206mm. Measure this, mark the wire and bend the second coil the same way as the first, so that the coils are directly under one another. Finally, the bottom section of the antenna is the 1/2-wave section, 138mm. There is a “crank” in the bottom of this wire so that the bottom of the antenna is in line with the centre of the coils. There’s a second wrinkle here: the 138mm must be from the end of whatever you use to mount the antenna. We used the same base that comes with the USB dongle antenna – it has a 3mm threaded end which makes it convenient to use a 3mm (internal) threaded standoff soldered to the wire. Just make sure that you don’t push the wire all the way through (leave enough to screw onto the base) and don’t fill it up with solder. Adjustments Without some rather specialised equipment, it is not possible to adjust this simple antenna. At 1090MHz, snipping The wire antenna is a little prone to damage so it’s best housed in some form of protective “case”. A short length of electrical conduit is ideal – the whole antenna can be slid inside it with conduit caps to seal it. On the top, the cap slides straight on, whereas the bottom cap will need a hole drilled through it to allow the coax to pass through. Conduit caps don’t tend to fit tight like other PVC pipes, so once everything is finished to your satisfaction (and tested!) we would glue the caps on with either PVC pipe cement (very permanent!) or even a dab of super glue (easier to prise off), just in case you want later access to the antenna. The web version used a male and female “N” connector but we think this is a bit of overkill – they’re not cheap – so why not simply make the coax captive (ie, glue it in) and save the possible losses from the connectors? The coax collinear This is the antenna which has been reported as giving excellent results on ADSB – one report we read said that the user could pick up signals from planes as much as 250 nautical miles away (>460km). That’s no mean feat – we’d be interested to know if any readers have the same experience. The coax collinear one is made up of short (approximately ½ wavelength) lengths of coaxial cable, secured together so that the inner conductor of one length connects to the braid of the next and vice versa. This gives the necessary phase reversal of each element. The reason we said ~1/2 wavelength lengths of coax is that there is a slight complication factor here. All coaxial cables exhibit what is known as velocity factor, which is the speed an electromagnetic wave travels along the cable compared to the speed in a vacuum (which approximates the speed of light). The velocity factor in a vacuum is 1.0; all cables are less than that because they are less than perfect! The dielectric in the cable (the insulation which separates the inner conductor from the braid) effectively slows the signal down. Velocity factor, therefore, varies from cable to cable depending on the type of dielectric – some, such as polyethylene and solid PTFE are quite low (0.695) while others such as foam polyethylene can be higher – 0.79 to 0.88. What this means to the constructor is that the length of the elements in the collinear need to be adjusted to take the velocity factor into account, simply by multiplying the theoretical half wavelength (in this case <at> 1090MHz = CUT OFF FLUSH INNER WIRE 10mm RG-6 COAX LENGTH = 0.5 x Vf (for RG-6 FOAM COAX: 0.5 x 275 x 0.85 = 117mm) 10mm Here’s one element of our coaxial collinear, shown exactly same size so you could even use this as a template. Shown at left is a typical “quad core” coax cable, with the layers cut to reveal its construction. siliconchip.com.au September 2013  45 We found it easiest to mark off the elements by using a rule. The length of centre conductor (copper wire) emerging from each end is not at all critical – just long enough to work with – but the length of the element itself is! As seen here, we cut each element to 117mm. You need to end up with a clean cut as shown at right – make absolutely certain there are no wisps of wire shorting between the centre conductor and braid. If necessary, check for shorts with a multimeter. 138mm) by the velocity factor. For a foam dielectric collinear, this would be 117mm (138 x 0.85). If you have a coax with clear identification, there are many references on the net which will tell you its velocity factor. If you can’t identify it, look at the dielectric: if it is foam, use the 0.85 figure. If it is solid, use the 0.695 figure. Because the receiver input is 50 impedance, you should ideally use 50 coax. But we’ve made ours from RG6 coax (because we had some) which is 75, so if you happen to have a spare length of 75, give it a go. Sure, it’s not quite according to Hoyle – but you won’t break anything! How many elements? This is entirely up to you! While there would be little point in making a one-element collinear, it can be done. But theoretically, the more elements there are the more gain your antenna will have (doubling the number of elements should give you a 3dB increase in gain). However, there is a law of diminishing returns as there are losses (in the coax) which start to become significant fairly quickly. 8-12 elements appears to be about optimal both from a performance viewpoint and also ease of construction and stability. A collinear with 12 elements at 1090MHz will have a gain of about 6dB and be just over 1.4m long, which is probably a good compromise between gain and size. If you’re stuck for space, 8 elements should still give a reasonable performance and be less than 1m long. You can cut the coax with a very sharp hobby knife (be warned, the blade will be blunted) but one of these rotary strippers makes the job so much easier. and straightened for some time. Pulling it tight will help straighten it. Even then, it will have a tendency to curl back up again. It’s a lot easier to work with small sections of coax so cut as many lengths as elements you want, perhaps 150mm in length – that gives you the 117mm element length required plus about 15mm or so of inner conductor to join to the next element. Carefully remove the outer insulation, braid and inner insulation (dielectric) from each end, leaving the inner conductor poking out, so that you are left with lengths measuring 117mm from insulation end to insulation end. A sharp hobby knife can be used to cut coax and/or remove insulation and braid but a rotary coax cutter makes this job a lot easier and repeatable - but just be careful that you get those lengths right. And before you move on to the next sections, check the Construction There are several options available here – we’ll look at just two of them. The first is arguably the more “permanent” arrangement, and that requires soldering the elements together. The downside of this is that it is quite easy to damage or distort the dielectric, especially if it is foam, which can degrade performance. It is essential that new, unweathered coax cable be used for this method because you need the solder to take to both the braid and centre conductor very easily and quickly. The second method doesn’t need soldering but relies on a “friction fit” connection between centre conductor and braid, held in place by the coax cable’s outer insulation. While this works well for a time, we’d be inclined to think that eventually corrosion or weathering will make the connection between the sections at best problematic. Still, it’s a quick and easy way to make an antenna and has many supporters on the ‘net. Cutting the coax You’ll need some nice, straight coax so if it has been wound on a drum or coiled, it will need to be unwound 46  Silicon Chip Here’s what you want to end up with: 12 (or 8, 6, etc) identical lengths of coax cut to size and ready to be assembled. siliconchip.com.au cut ends with a multimeter and/or a magnifying glass/ loupe. It’s far too easy to leave strands of wire which might short between the braid and centre conductor, rendering your antenna useless. You should end up with absolutely identical lengths of coax as shown in our pictures. The next step depends on which of the two methods above you’re going to use. (a) Soldered collinear This is not our preferred model, as soldering to co-ax braid is not as easy as you might imagine. This is particularly so if (a) the braid is at all weathered or oxidised or (b) if the outer braid is actually woven aluminium – that’s very hard to solder to without special fluxes and solder. But it can be done! Remove another 10mm of outer insulation (only) from each end of each of the prepared lengths, being careful not to cut the braid underneath. Cut all the centre conductors to 10mm. It will pay you to pre-tin all centre conductors and a ‘strip’ along the braids, making sure the tinning is on the same side as each other. It’s also easiest to make a jig to solder the sections of coax together because you need to ensure they go together (a) in a ‘stepped’ straight line (see photo and diagram); (b) with their ends actually touching each other, as long as the braids and conductors aren’t shorting and (c) so that they soldered elements are as mechanically rigid as possible. Our photos should help explain this. Repeat for all the elements (coax sections) but for the top-most element, simply clip off the centre conductor so it cannot short to the braid. The bottom element connects directly to the coax lead-in (to your receiver) in the same way as the rest of the elements connect to each other. Because you now have a number of exposed solder joints, cover with some self-sealing adhesive tape to minimise oxidation and corrosion. This antenna needs to be housed a plastic conduit, just like we did the wire collinear above. Simply follow those details to weatherproof your coaxial collinear. Slide a length of insulation tape over one of the centre conductors. The tape is to prevent the two braids shorting out when the elements are brought together. Then pass the other centre conductor through the tape as you bring the two elements together. Slide the centre conductor from one between the outer insulation and braid of the other . . . and vice versa. (b) “Friction fit” collinear. This is our preffered antenna because very little soldering is involved. Prepare your elements in the same way as you did for the soldered model but don’t remove any extra outer insulation – that is, the insulation, braid and inner insulation should all be cut off cleanly, leaving the inner conductor exposed. Shorten the centre conductors to about 10mm. Cut a 75mm length of insulation tape and push one conductor through the centre of the tape, close to one end. Take the second element and push its centre conductor through the tape from the opposite side about 3mm away from the conductor already pushed through. Again, see the photos to view this. Now you have to carefully slide both centre conductors between the outer insulation and the braid of the opposite element. It may pay you to warm the outer insulation first – say with a hair drier – if you have problems doing this. Push the two elements together as far as they will go then secure them in position using the insulation tape. As well as holding the elements together, the tape prevents shorts siliconchip.com.au Continue pushing the two elements together until they touch. You can see that they are slightly offset one to the other and the tape forms an electrical barrier between them. Finally, wind the excess tape around the join to hold the two elements together. You can relax – just as soon as you’ve finished another 11 elements . . . September 2013  47 CONDUIT CAP SNIP OFF LAST WIRE ELEMENT N 20mm CONDUIT (LENGTH TO SUIT NUMBER OF ELEMENTS) ELEMENT 3 Drill out a conduit end cap to accept a BNC connector and solder its centre pin to the bottom element conductor. Force a short length of wire between the insulation and braid, and solder that to the BNC connector earth lug. These caps (top as well) will need glueing to the conduit as they are invariably a loose fit. INSULATION TAPE BETWEEN ELEMENTS – WRAP AROUND WHEN JOINED ELEMENT 2 FORCE INNER WIRES UNDER OUTER COAX INSULATION (REPEAT FOR EACH ELEMENT) ELEMENT 1 SOLDER BNC SOCKET FITTED INTO CONDUIT CAP between the braids of the two cable sections. Repeat these steps for as many elements as you have, as well as the coax lead-in to your receiver. Once again, snip off the centre conductor from the top wire. Mount the antenna in a suitable length of electrical conduit as per the wire collinear above. We mounted ours via a pair of worm-drive hose clamps on a length of water pipe so its base ends up about 2-3m above ground level. It would appear that the higher above ground the antenna is erected, the better the performance. However, that means a longer coax lead in with its own losses at 1090MHz, so you might need a little experimenting to find the “sweet spot” of height vs loss. And that’s it: two different collinears which will offer better performance for ADSB reception than a simple whip. Now if you want to make a collinear for a different frequency, read on . . . Our “friction fit” collinear – we’ve only shown four elements but we actually made it from twelve. Any more than this will not give appreciably better results. At right is the antenna, inside its conduit housing, secured to a pipe with a couple of hose clamps. Lash the coax to your receiver to the pipe for security against wind damage. Making a collinear for other frequencies The above steps can be used to make coax collinears for any frequency or band you want to listen to. We’ve seen them made for UHF CB, for 2.4GHz WiFi, for VHF amateur frequencies (2m, 6m, 70cm, etc) . . . we’ve even seen one monster made for the 20m amateur band, hanging from a very tall tree! Naturally, physical constraints come into play with lower frequencies – a 12-element coax collinear for the 80m amateur band might be just a bit of a stretch (It would be a bit over 500m high!). Simply remember that formula: wavelength = 300,000,000/frequency (Hz) [in metres] So for that 20m (14MHz) amateur band collinear, the wavelength would be 300,000,000/14,000,000 or 21.42m and each element (½ wave) would therefore be 10.71m long. From memory, it had four elements so was over 43m high. Other common wavelengths: WiFi band (2.45GHz) UHF CB band (476MHz) 70cm amateur band (say 430MHz*) 2m amateur band (say 146MHz*) Aircraft band (say 125MHz*) FM broadcast band (say 100MHz*) 6m amateur band (say 53MHz*) 48  Silicon Chip = = = = = = = 122mm 630mm 680mm 2050mm 2400mm 3000mm 5660mm For other frequencies you might want, simply use the formula shown earlier. Remember, these are full wavelengths – multiply by 0.5 to get 1/2 wave element length, by 0.25 to get a 1/4 wavelength and of course by 0.75 to get a 3/4 wavelength. * We have nominated frequencies which are either in the middle of the band or where much of the action is located. (EG, aircraft band is 108-136MHz but voice communication is mostly towards the upper end of the band). SC siliconchip.com.au