Averaging all the signals over the day so far on the Wander Wand Compact Loop indoors on 10m and comparing with the external end-fed Par 10/20/40m wire antenna, I believe the loop is perhaps just 2 S-points down or less. With Es and widely varying signal levels, it is quite hard to do the comparison. Also, the loop was not rotated, and it is possible the difference is slightly less.
On lower frequencies I think the difference will be slightly greater. Overall, I am impressed with the indoor loop. Seeing how it is not positionally optimised (it just sits above the rigs and runs just over 2W), it does pretty well.
For future 10m WSPR use I think a 6mm or 10mm copper pipe (fixed tuned) loop will make a good external antenna. I suspect a thicker loop will be only be very little down on a 1/2 wave vertical or dipole at a similar height. The disadvantage of a loop, especially an efficient one, is the narrow bandwidth. This of course, helps on receive as the antenna rejects out of band interferers. Bandwidth should just about cover JT65/JT9-1, PSK and WSPR allocations 28.076-28.126kHz.
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