From The Dungeon. 19 Nov 2006                                                                                                     Allan Copland    GM1SXX

Compromise aerials.

Insects have 'antennae', radios have 'aerials'. Seems that the term 'antenna' was obsolete as far back as the 1940's. So now you know.

Lets face it, unless one is stinking rich or lucky enough to live on a farm,  the majority of us have to put up with compromise aerials for HF. The average home in the UK has a garden the size of a large postage stamp but of course a simple dipole for 80M is around 132feet long, hardly suitable for the modern home and therefore usually out of the question unless one has very understanding neighbours who welcome wire and poles attached to their roofs!

What to do to get a signal out? Well, a compromise antenna is usually the answer. Some sort of loaded vertical or the like, usually with radials, either elevated or buried might be used. Another less common solution is the so-called 'mag-loop', short for 'magnetic loop', a small single turn loop with a high voltage  capacitor to bring it to resonance. Such compromise aerials are poor performers on the lower bands but can be quite efficient on 20M and above as you'll see below.

I've noticed that many radio amateurs and CB'ers continually fret about SWR but the truth is that they are usually trying to get a great match on something that often radiates as well as a wet rag. I think they should look at the bigger picture. In almost all situations, they will sacrifice a great deal of power in losses to make use of their compromise aerial. Why worry if the SWR is 2:1 when the aerial efficiency is only a few percent? This point has always amazed me.

With loaded vertical aerials and small loops, this is often the sort of efficiency achieved when operating on 80M.
Moving to 40M, things look a bit more rosy and these aerials can perform quite well on 40M and the higher bands.

For this episode, I'll look at magnetic loops. These can be bought commercially or built at home from readily available materials such as copper pipe and 'Yorkshire' AKA solder-ring, fittings. The downside of mag-loops are the high voltages present across the tuning capacitors even at QRP power levels.  Commercial ones are expensive so this is one area where you can roll your own and save money.  Commercial mag-loops are usually quite small and are circular in shape. Other shapes are also quite suitable and squares, octagons and rectangles are quite popular.  The price, availability and ease of use make copper water-pipe quite a popular material for DIY loops.  Octaconal loops are easily made using 45 degree 'elbows' and pieces of straight pipe. Likewise, rectangular loops are also very simple to make. Whichever shape you choose, please build your loop on a flat surface .. concrete or paving slabs are ideal, so your loop is nice and straight. Few things look worse than a twisted loop!  generally, a gas blowtorch and a junior hacksaw or pipe cutting tool are all that you need. 

Mag-loops may be fed either via a coupling loop or by a transformer arrangement. It's normal practice to use a motorised tuning capacitor to tune the loop and small geared model motors may be employed for this purpose. It's normal practice to place the tuning capacitor at the top of the loop in a sealed enclosure and feed the loop at the bottom (at the current maximum). The figures below are for a loop with it's lowest part mounted 3M above ground level.


I'll look at a loop made from 15MM copper water pipe with 90degree 'elbows' of the solder-ring type. These parts are cheap and readily available at any plumber's merchant. It's normal to use a wide-spaced tuning capacitor that has no end-stops... one that can be rotated more than the usual 180 degrees. An even better alternative is to employ a vacuum capacitor but note the voltages involved... they are considerable. If you increase the pipe diameter from 15mm to 22mm, the efficiency will rise by about 10% (not on 80M) but the cost more then doubles. Your call!


A loop with vertical arms of 2M and horizontal arms of 1M is explored here. Built ftrom 15MM copper water-pipe with 2M Vertical by 1M horizontal arms as a vertically oriented rectangle. This is a reasonable compromise between size and performance.The bottom of the loop is 3M above the ground.

 The numbers stack up as follows.
 

Note the low efficiency on 80M. This is actually not too bad and is comparable with many expensive commercially made trapped vertical aerials. So much for worrying about your SWR!

If you raise the drive power to 100W, the voltage across the capacitor plates rises to 6560, 8843, 6813 & 2374 volts respectively. Now you know why a good high-voltage tuning capacitor is so essential. It's also reassuring that the high-voltage nodes on the radiating element are high enough to avoid being touched by anyone in view of the voltages present. Ordinary variable capacitors are not ideal for mag-loops because of the high circulating currents. Large, wide-spaced 'Butterfly' types are to be preferred since they don't rely on wiping contacts.  Serious DIY loop builders often substitute a high quality vacuum capacitor. These can usually handle the high voltages generated at the 100W level and are amenable to being motor driven. They also cost plenty. 

You can see from the above that small loops can be very acceptable performers for 7Mhz and above. What happened to 28 and 29.5Mhz? Well if you want those with a loop measuring 6M in total length you need to swap the capacitance for inductance OR reduce the size of the loop somewhat to make it resonant. Do your own modelling with the software listed below.

Mag-loops are usually fairly small, this explains their poor performance on 80M but against this must be set their relatively small visual impact. If you go for a loop made with copper pipe, it's easy to paint it in a colour that will blend in with your surroundings. For the Caribbean, sky blue or sand coloured might be nice while for Scotland, a dirty slate grey mixed with black might be more appropriate. Your tuning system must be protected from the weather and a sealed plastic box is one option so long as you use some sort of sealing gland where the pipes and cables enter/exit. some people like to drill a very small diameter hole in the base of the enclosure to let it 'breathe'. By small I mean a miniscule hole 'drilled' with a heated needle or similar. It's exceedingly difficult to keep water out of sealed enclosures.  It's generally easier to vent the enclosure to allow water to drip out.  The best plastic enclosures have a rubber gasket around the lid with the fixing screws on the OUTSIDE of the gasket.

The tuning capacitor will be the most expensive item for such an aerial. Don't be tempted to scrimp on this, the voltages listed above are real and most tuning capacitors won't handle them. Even at 5W PEP, running on pen-cell batteries, there's usually around 2KV present!

For anyone wishing to model their own DIY loops, you can download modelling software at http://www.observations.biz/Reg_Edwards_G4FGQ/rjeloop2.exe  and http://www.observations.biz/Reg_Edwards_G4FGQ/rjeloop1.exe   These programs were written by Reg Edwards G4FGQ.

Mag loops are frequently fed by a loop around one fifth of the diameter or the main loop. This can be squashed and positioned for the best match. Copper microbore pipe is a suitable material for such a loop and some types come with PVC cladding already in place.  An other alternative is to place a large toroid of adequate power rating over the lower part of the loop and feed it via a tapped primary. The loop acts as a single turn on this transformer into which the RF is coupled. Loop or transformer, it's up to you.  In an ideal world, the size of the feed loop (or number of turns on the transformer primary) would vary with the band chosen to give the best match to 52R.  In practice, a compromise is used. With a transformer, it is certainly possible to use a tapped primary but most users wont bother.  The connections to the capacitor carry considerable circulating power (tens of amps)  so should be copper straps or heavy copper braid. Don't even think of using stranded wire.

Do be aware that magnetic loop aerials generate strong local fields.  I'd only recommend using them outdoors, away from the shack for safety reasons.

I notice a recent trend back to the simple vertical antenna with no radials and some sort of unspecified 'broadband matching device' at their base. They are advertised in the radio magazines at silly inflated prices. What sort of efficiency might you think such an aerial would have? I suspect that a fair number of them consist of nothing more than a large carbon resistor ... the 'matching device' and a short whip. You might get out just as well with an old mains lamp and a whip connected to the co-ax centre conductor.

Take my advice. If unlike me you have the space, sling up a DIY wire dipole. I'll perform a LOT better at a miniscule fraction of the cost.  If you do have a postage stamp garden, consider a DIY magloop. If you really must have a vertical, remember those radials. They make ALL the difference.  Don't believe the hype... sorry lies!

73 Al.

GM1SXX