As winter deepens, attention always seems to turn here to magnetic loops, an antenna that I've always enjoyed good success with. As always, the key to that success is a good environment.
A very popular portable loop is the Alex Loop, made of Heliax-type coax. At 14MHz and above, this diameter of copper conductor is perfectly acceptable, and will have an efficiency of 90% or more.
I've never tried a coax cable loop, and I have absolutely no intention of paying £300 for one! I also never use the primary and secondary loop 'Faraday' system, because it is susceptible to mechanical shape changes and, hence, matching instability. But that's the system I will use for this experiment.
I don't have any Heliax, which is more than £5 per metre. So I am building my first protoype with plain RG213. If it doesn't work, I only need 3m of conductor for a loop, so we potentially face a bill of only ~£17 for that.
As for the matching capacitor, well it's not the building of the enclosure for that which represents the biggest problem, but finding a reasonably priced one on E-bay, where competition for these is always very strong, sometimes leading to silly prices. The plastic box was £5, and the large control knob £2.75. I already had the SO239 sockets.
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| Tuning capacitor. It's a deep box, so those wires are very well separated. Range ~25-450pF. Connections are to the outer of the SO239, as we only need to use the outer of the coax. |
So, compromise again here, with a capacitor that is likely only to handle ~5W maximum before breakdown. But it will be fine for some WSPR RX and some TX as well. It has a modest reduction drive which, combined with the large diameter knob, makes fine control easier.
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| Innards of the Alex Loop, with the (many) added DC and reduction drive components. Maybe my plate spacing isn't too bad, after all! |
This (French language) site is really good, and looks somewhat more professional than my attempt; lack of time never lets me make things especially pretty! At worst expenditure case, I think this would cost no more than about £50 - a saving of £250 over the commercial version!
Next step is to get some free waste timber from the local builders' yard, and put it all together. With more wind and rain on the horizon this weekend, this will while away the time nicely. Progress reports to come...
Nice article John, I prefer the pick up loop "faraday" system as it is easier using portable. My homemade package is similair to the Alex loop. Although I use a fieldstrength meter to find a peak for tuning. I see the Alex loop is designed with a LED which contains also a small battery which will drain after a few years. I've been building the loop with RG213, LMR400 but it was not too sturdy that way. I couldn't form a nice round loop the way it should be resulting in inconcistend results. I've been working in a cable factory 25 years ago and still had some 75 Ohm scrap bamboo cable which is as sturdy as heliax. It does wel for the loop. 73, Bas
ReplyDeleteI agree with all you say, Bas. The RG213 is too floppy without a crossbar, but there's no need to form a perfect loop. I tested it out last night, and I was doing remarkably well on 60m, even with real QSOs and not just WSPR. Unfortunately, and as predicted, the cap is not widely spaced enough for reliable operation. Cap 2 ready for engineering, Sir!
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