OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

A cheap and simple 'field aerial' for 80/40 & 20M.

As regular readers will know, I'm a recent convert to small loop aerials. Not the 'true' STL's (Small Transmitting Loops), but larger ones that don't quite fit the true definition of a "magnetic-loop" aerial. Small transmitting loops or mag-loops are usually around 1/10th Lambda.  I'm talking something rather bigger.  Sure, a dipole will easily out-perform a small loop, but for quick setup and operating convenience, a loop is hard to beat.  On 20M, the efficiency is quite good.

Using G4FGQ's loop modelling software, it became obvious to me that I could build a loop to cover 80,40 & 20M with an overall radiating element length of between 5 and 7 metres.  Loops can be constructed from a variety of materials such as copper pipe, heavy wire or even electrically bonded (solder jointed) copper tape over large diameter plastic pipe.  With electrically-small tuned-loop aerials, in practical terms at least, the things that affect efficiency most (apart from size) are the choice of capacitor and radiating element.

This is a compromise aerial, make no mistake about that. An ideal loop would use large diameter copper or other material with low ohmic losses along with a vacuum capacitor to tune it.  This is simply not possible (or desirable) in a portable loop so some compromises have to be made.

A normal domestic radio type two section variable capacitor of 500+500PF rating was chosen for my loop. Having identical capacitance in each section allowed me to connect the ends of the loop to each of the static vanes and allowing the capacitor frame to 'float'. This effectively halves the capacity but doubles the maximum operating voltage.  If you can't find a capacitor like this, you could try a 100-250PF single section type.

The capacitor is housed in a small plastic box with a plastic block glued to the underside, drilled to accept the top section of a short fishing pole.  A plastic knob is used for tuning. A wooden 'spreader' with forked ends holds the wire loop open and an elastic band slipped over the fishing pole stretches the bottom part of the loop.   At the bottom of the loop is a ferrite sleeve with a two turn winding to couple the RF from the transceiver into the loop.

The loop itself is nothing more than a length of figure-of-eight twin-flex with the ends bared, twisted together and soldered directly to the capacitor sections. It passes straight through the ferrite sleeve at the base of the loop. The wire is a compromise. Ideally, you'd like it to have the lowest ohmic losses possible with the largest RF 'skin' area. (He means large bore copper pipe... LA2QAA).   In reality, lighting flex will do the job, but with a reduction in efficiency.  Some people have pressed old co-ax cable into service in small loops. For a number of reasons, this is NOT ideal.  Just use the biggest gauge of insulated stranded wire you are willing to carry around with you.  Remember, this is a compromise aerial! Don't be tempted to use a cheap plastic-dielectric capacitor from a transistor radio to tune the loop. Even at QRP power levels, there are substantial RF voltages across the tuning capacitor.  Use a decent airspaced type.  For the reasons just mentioned, it's not a good idea to transmit while tuning. Even QRP power can 'bite'.  Just tune the loop for the maximum received noise and the match will be acceptable.

The table below gives starting points for a pair of loops of 7.2M and 4.88M total length arranged as a diamond shape.  The resonating capacitance values are given for common bands for the two loops. It can be seen that a loop of 7.2M total length can cover all three bands (80/40/20) with a 200pf variable capacitor.  Reducing the loop size reduces the efficiency and drives up the resonating capacitance.  Tweak the numbers to suit whatever capacitors you may have available. If you like doing your own thing, you can model various loops using rjeloop1.exe or rjeloop2.exe . These are a pair of DOS programs that can be downloaded and run on your 'Win-Doze' PC.  Just click the link, save the program on your desktop, go there and run it/them.

This simple loop will easily cover 80, 40 & 20M and the good news is that you don't need to lug along an SWR meter. Just tune the loop for maximum received noise and the SWR will be within acceptable limits.  My FT817ND is happy driving this loop.  It could hardly be any simpler.   In theory at least (and I've never tried this), it could just about be used /P... if you don't mind walking around looking like a total pillock!  Not advised for town use, but you might get away with it while out on the hills though.  You could tune the loop on the move with a fibreglass 'stick'.  I've grown to rather like small loops. The feed arrangement for this loop may seem rather unusual but it offers the advantage of being small and convenient when compared to the usual method of having a coupling loop.  Ferrite is fragile, so wrap the ferrite sleeve in several layers of ordinary electrician's PVC tape to protect it against breakage.  As for co-ax, I used a piece of RG-174. RG58 or Mini8 are just a bit too heavy for my liking.  The losses in a short piece of RG174 are not *too* bad at HF.

Small portable loops are not a new idea and I after I wrote up this article, I found another page with a similar concept at G4FON Portable Mag-Loop  In essence, this is very similar to the one I've been using for a while.  As LA2QAA is wont to say.... SSNW * .  With aerials, it's *extremely* unlikely you can do something new.  I wouldn't describe either of these loops as mag-loops, at least not on 20 & 40M where they easily exceed the accepted size for a "magnetic loop aerial". In fact I just don't like the term "mag-loop" at all .  I think some of these names are concocted just to bamboozle and mislead the gullible.   As far as I'm concerned, its just an electrically undersized loop loaded with a capacitor!   There again, everything I know about aerial design could be writ large on the rear of a postage stamp :-)

Rather than buy a commercial compromise aerial, why not just make one. It'll *still* be a compromise, but you'll have saved a load of hard-earned cash and you can always say that YOU made it.

 73 Al.
GM1SXX
 

*SSNW....Same shit, new wrapping.

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