About 6 months ago, I visited the website of Julian, G4ILO who had detailed how to build what he has called a 'Wonder Loop' - a small indoor magnetic loop aerial. Following this visit I was inspired to build one myself.

As is my way, I've taken a little while to get around to building this but I have now completed my first build with moderate success.

I've used a 30-310pf (approx) tuning capacitor which I found on eBay for a few pounds, a loop made of RG-213 coax cable and a coupling loop made from some high current cable.

The capacitor is housed in a rather large plastic box (again an eBay purchase) using a couple of screws and a couple of cable ties to hold it in place. The terminals of the capacitor run to two terminal posts to which I can connect the main loop. The coupling loop is connected to a Phono socket, which runs directly to a BNC socket on the rear of the box. From this socket runs a patch lead to the transceiver.

I haven't been particularly precise in the construction - so imprecise in fact that when drilling holes for screws to secure the capacitor, I managed to mis-judge where on of the holes went. I ordered 2m of RG-213 for the main loop, and initially I used the entirety of this, only trimming it slightly during first tests. The coupling loop was one fifth of the size of the main loop, but since the trimming this is now slightly out of proportion.

The assembly of the aerial was completed yesterday after various trips to Maplin and B&Q to get parts, and initial tests are promising. As a receiving aerial I'm really quite happy. Sitting in the flat, it out performs my whip aerial when tuned correctly. Transmitting is a slightly different matter. Yesterday I managed to get a good match with SWR readings of around 1.5:1 or better on 40m and 20m and the PSKReporter website showed my CQ calls on PSK31 being received in Italy with 10w of power. Unfortunately today so far I can't seem to get a match on either band.

I'm experiencing an issue with the sharp tuning of the capacitor. I don't have a reduction drive so I'm having to be extremely precise with the tuning knob - possibly more precise than I am able to control. Another issue is the effect my hand has on the SWR reading. I've notice today that when tuning I can perhaps get a lowish reading but when moving my hand away, the reading worsens. Strangely I didn't notice this yesterday. I can't think of how to overcome this unless I can motorise the control. I'm not sure how possible that is.

My next steps I think are to look at the dimensions of the loop, and perhaps shorten it a bit. I will also try re-creating the coupling loop ensuring it is one fifth of the size of the main loop. I'd like to investigate the possibility of creating a reduction drive to allow for fine control but I need to source cogs and rods and figure out how to fit them into the existing design. If any readers have ideas of where to get such parts, please do leave a comment below.

Despite these initial set-backs, it's been fun to create what is my first home built aerial and to have it work albeit temporarily. Credit of course goes to G4ILO for his design, which I have pretty much copied, and write-up which was very helpful in aiding construction.

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Tags: amateur radio, magnetic loop aerial