I’ve recently been researching multi band antennas. My first thought was to remove the balun from my 40m dipole, feed with ladder line from an external auto-ATU and treat as a doublet. However, in several auto ATU manuals one antenna kept being mentioned: the loop (often a delta loop).
Many documents refer to dimension calculations for a single band of operation. Others state dimensions but are vague about how these calculation are made whilst also referring to use over multiple bands. I couldn’t find much regarding feeding of random loops.
In a spare 30 minutes I thought I’d string up the largest loop I could fit in the garden. What’s the worse that could happen?
The resultant loop was approximately 70m in length and strung up in a horizontal sloping rectangle, loop ends entering the upstairs shack window and fed directly from an old KW ATU using balanced outputs. The wire used was very thin, solid core ‘bell wire’; far from ideal for an antenna.
The ATU tuned most bands but on TX I noticed a brief arc on a speech peak. I had a 4:1 balun I’d previously made for another project so placed this between the ATU and loop. The loop then presented a much easier load and tuned without issue on every band I tried (80m to 10m).
Later in the evening I performed some WSPR tests using 20W output on both 80 and 20m. The results are show above (East was predominantly 80m, West 20m)
On RX, comparing the loop and 40m resonant dipole, a 2 S point (~6dB) difference was observed in favour of the dipole.
However, in summary, surprisingly good performance for a loop made of unsuitable wire and thrown up with no thought for dimensions. More investigation and experimentation definitely required!