This is the time to fetch the magnetic loops, so that I can keep going on the lower HF bands whilst the winds flatten everything outside.
Whilst magnetic loops are available from a limited number of makers, their price is very high. One of the latest entrants is Inac of Spain, who seem to have a very nice line-up. But at 1000 Euro for the rather small 80m upwards loop of the 3-30MHz version, I'm happy to make my own and simply tune manually.
Last year, I had a nice G3JKF three-loop array to play with. It was successful and very easy to match, but it did take up a lot of room in the kitchen! So, this year, I'm back to a single loop of 1.2m per side, made of wide, 28mm copper tube. Any wider than this, and you start to cry at the hardware shop's checkout!
Wider tube increases efficiency somewhat at the lower bands, but it is not really necessary to use much above 15mm; according to Sutherland, gain calculations for loops are vastly out of kilter with reality (up to 30dB!), owing to questionable use and misinterpretations of the Chu-Wheeler criterion, first published in the late 1940s. Look these up online for further understanding.
Loops can be difficult to tune owing to their very high Q. So, this is how I tame my loop:
(1) Use a 4:1 balun near the feedpoint.
(2) Connect one side of the balun output to the centre of the bottom tube of the loop. Soldering is best.
(3) Connect a longish, stiff wire like hard-drawn copper to the other balun's pole. Run this about 1cm or less from the bottom tube, parallel with it. Run it up maybe 60-70% of one upright side and connect (tap) with a crocodile clip or hose clamp/Jubilee clip.
(4) Adjust the position of the tap such that you get the best matching/lowest SWR.
(5) Turn your capacitor until you hear the loudest signal. This should be close to perfect matching.
(6) If you can't get below 1.5:1 or so, adjust the position of the tap, make sure there are no metal objects around your loop which are detuning it, and adjust the shape/positioning of the gamma match wire. I have previously found, under advice from G3JKF, that running a second wire directly from the gamma match pole of the balun directly to the tap point, in addition to the parallel wire, gives a good, broad matching that makes achieving low SWR much easier. In other words, this looks like a triangular loop, connected at the upper apex to the normal tap point up one side of the loop. It's a kind of parallel gamma match.
(7) To work out what kind of capacitor you need for your particular diameter pipe and loop, use this site, which has proven tobe spot-on in its predictions for me (though I suspect the efficiency output is based on old formulas, and hence underestimates efficiency.) Vacuum capacitors, if you can find (and afford) them, are better than air-spaced ones, but both can be used to good effect.
So there's the theory. Does it actually work?
Well, have a look at this 11/11/15 results on 60m WSPR. I'm one of only four stations being heard by VK7BO from anywhere; the map shows all stations active on the band. And no, this wasn't a 'ten minute' wonder where the spots simply reflect transient conditions - this situation persisted for some time. Not bad for a kitchen-based loop!
Happy building!
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