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| If you plan to use a loading coil, choose a good un, or better still, roll yer own. A DIY 38uH inductor. |
OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Electrically Short Compromise Aerials.
I decided to devote this chapter of Observations to electrically short portable aerials of the sort used by backpackers. The general consensus seems to be that if you can manage it, a resonant dipole suspended in some handy trees is the best way to go. Doesn't actually take much, just a couple of wires of the correct length, a few buttons (insulators), some nylon or rayon line and a means of hoisting it up over a handy tree as a sloper, or perhaps using one of the readily available and cheap telescopic fishing poles as a support as an inverted Vee. Other popular aerials include the EFWHA (End-Fed-half-Wave-Aerial) (1) fed from a cheap and simply constructed impedance matching unit (not tuner.... see earlier OBS!) The Fuchs (2) is another useful halfwave aerial for QRP work. Of course, with a dipole, EFHWA or Fuchs, you are tied down to the one spot.
Perhaps your thing is radio on the move, not so easy, and you WILL be using a compromise aerial unless you are on 70cms or above :-)
So, with regard to /P verticals, what's hot and what's not? Well, fortunately, somebody else has kindly done all the hard work, in 2002 in fact. The HFPack Group conducted a survey of how various aerials performed by comparing them with a reference aerial, in this case, a simple wire suspended on a 16.5 foot fishing pole with a trailing counterpoise. The other aerials in the shootout were then compared against this reference and ranked according to their performance.
The article can be found in full at http://www.hfpack.com/antennas/shootoutvertical2002.html
Suffice to say that there were no real surprises there for me with the shorter aerials being the worst performers. The report authors even tried the old trick of using a 75R resistor and a short whip for the last two!
Some readers may even remember a well-publicised commercial aerial that used the same technique many years ago. Don't laugh.... even a light-bulb connected to a bit of wire will radiate some RF!
The popular ATX walkabout was 4.38Db down on the simple fishing pole while the optimistically named 'Miracle Whip' performed no miracles at all, being 10.59Db down on the reference fishing pole. In short, you might as well use a cheap fishing-pole with a length of wire taped to it. It will outperform any of the commercially made short verticals. A simple DIY matching unit would make an excellent companion for your fishing pole. If your thing is QRP, try to avoid using matchers that use toroidal cores. They are lossy things and best avoided. Do yourself a favour and make a half decent matcher with a coil wound on a varnished 'bog-roll' centre and use a decent air-spaced variable reclaimed from an old radio. You might have to BUY the fishing pole but almost everything else you need can come from the junk-box. Some pre-cut counterpoises that can be swapped around according to the bands in use are advised. Such a system will work nicely with your QRP transceiver, yet cost next to nothing if you have good scrounging skills.
In the HFPack 'shoot-out' a 75R resistor connected to a 90 inch whip was 17.82Db down on the reference while a short 4 foot) whip and 75R resistor was 26.34Db down on the reference. I AM rather disappointed that they did not try some wet string along with the 75R resistor!!
Short whip type aerials can never be great performers but if one must go down that route, attention to coil design does pay dividends. You need to get the lowest losses possible so thick wire wins over thin, large diameter formers win over smaller ones, the dielectric should be of excellent quality and the wire should preferably be silver-plated. Oh, and don't forget that short verticals need a ground,
Some sort of counterpoise wire is essential to get decent results. The aerial itself is only 'half' the circuit, you won't push out much RF with yourself as the ground. That ground is every bit as important as the aerial because it is the electrical 'mirror-image' of your short vertical. Yes, you might look stupid trailing a counterpoise, but you need it and contrary to what I overheard a G3 station say on the air recently, ground current IS desirable and does NOT represent a loss. In fact you want LOTS of ground current :-)
The Miracle Whip is quite a clever idea and of course has an inbuilt matching unit but overall, it performed poorly. Perhaps part of the problem is the lossy matching unit. Better to use air-spaced, or semi-airspaced inductors in your matcher. Some base-loaded whips used built-in tapped matching coils. This can work quite well, but the coils have to be made from top class materials as mentioned above.
As they say on a certain children's TV program that's known for it's use of fairy-Liquid bottles and Sticky-Back Plastic (can you still get that stuff?)... here's one I made earlier..
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| If you plan to use a loading coil, choose a good un, or better still, roll yer own. A DIY 38uH inductor. |
I made this coil for portable use, specifically to use on my bicycle. It's constructed with silver plated copper wire wound on a spiral groove machined into a length of solid TEFLON rod. This is the best quality of coil I could manage to make and it IS worth making the effort to do it right. The TEFLON was threaded (8TPI) on a screw-cutting lathe and the ends of the rod were drilled and tapped (M8) to attach the coil to a metal bracket for the bike and fixing at the top end to take my short fishing-pole aerial. Such a coil will have much less losses than any matching system using toroids. I bought the silver-plated wire from a wire stockist and the TEFLON rod came from EBay.
Machining TEFLON rod is not easy, it tends to flop around a bit, but the end result was certainly worth the effort. The finished coil is not particularly light but it is robust, has low loss and does the job it was intended for. The silvered wire cost more than the TEFLON but I am rather chuffed with the end result. The inductance is adjusted by a metal clip that's slid up and down until I find the sweet spot. My fishing-pole whip (3 sections) has a length of M8 screwed rod epoxied into the bottom that just screws straight into the coil. At the bicycle end, I made a simple heavyweight aluminium bracket that attaches to my luggage carrier with two screws.
This is still a compromise design and was a trade off between weight, efficiency and practical considerations. A BIG air-cored coil would have been preferable but of course impractical for bicycle use. One possibility that IS worth trying is to use a piece of 'Air-Dux' or similar coil stock as a loading coil.
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| Air-Dux coil stock. Excellent but not cheap. |
Air-Dux coil stock is now available from a couple of UK suppliers. It's not cheap but it is of very good quality. The wire is slotted into four plastic spacers forming a high quality semi-airspaced coil with excellent performance. The piece of Air-Dux shown above was cut from a longer length and would serve very well as a base loading coil for a whip aerial. In fact I used to use it as such in my back garden in a permanent installation. This would make for a better coil Q than my DIY TEFLON coil but it is rather fragile by comparison. Horses for courses.
As regular readers know, I have a particular aversion to spending my hard-earned dosh on aerials. Why not make your own aerials and matching units. It's fun, it's educational, it's practical but most of all, it's cheap. Much if not most of the time, the end results are better than with commercially made aerials. It's not rocket science, just plain old aerials!
73 Al.
GM1SXX
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