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Magnetic loop HF antenna

In the last few days I have been experimenting with an HF magnetic loop antenna for the first time. These antennas are well known and work well, but for some reason I've never used one apart from on 136 and 500kHz where I used an 80m square magnetic loop made from standard PVC covered wire.

For HF I used a 2m length of 6mm diameter copper pipe formed into a near circle as the low loss inductor. To tune the antenna I tried (a) a short length of coax as a capacitor, trimming it to resonance on the 10m WSPR frequency, (b) a short length of mains cable, again as a fixed tuned capacitor, (c) a tunable 365pF air spaced capacitor, and (d) a small Jackson C804 airspaced variable capacitor with a small 3-35pF trimmer in parallel. Using 5W or less, one can get away with easier solutions for the capacitor: with higher power there can be kV across it and breakdown can be a problem. Not so with QRP.

With the 365pF variable, and bringing it to sharp resonance of 14MHz, I managed to get WSPR reports from Japan at around 9500km when using 2.5W out. This was with the antenna just taped to the back of the wooden chair in the shack in the upstairs bedroom!  On 28MHz the best report so far has been from Israel. at around 3500km using 2.5W WSPR.

The antenna is matched to the rig using a small coupling loop about 1/5th the size of the main loop.

Conclusion

The fix tuned magnetic loop antenna will make a fine antenna for 10m WSPR. It can be placed in the loft and just left connected to my 10m WSPR kit and a spare PC. Ideal for the quieter years ahead where having a continuous TX and RX on the band will help to spot fleeting openings that may occur.


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