In 2012 I made a simple magnetic loop antenna from RG213 coax using only the outer sheath. The loop was about 1.2-1.5M across and was tuned by an air variable capacitor connected to a small servo. The performance far exceeded expectations.
The first test was on the beach at Dunster in Somerset one summer evening when the tide was in, the water literally lapping round my feet. I was using a YouKits HB-1B Mk2 with 4-5W output. I put out a call on 20m and was answered by a station in Eastern Europe who gave me 599, described his mega rig, linear and tower and asked what I was using. I explained and passed transmission back to him. Silence. He eventually came back with a hurried TNX and 73.
Next day I worked into the US with the loop just outside the door, see pic. It was not a brilliant contact but good for QRP and a cheap homebrew loop. My intention was to perfect the loop using better materials and a few tricks I had learned but life took me in another direction. Now I am back and making a similar loop for 30m, 20m and 17m. It will work on 40m, but the efficiency will be low, and the bandwidth will make it a pig to tune.
What surprises me is the number of companies selling loops for hundreds of £s when they are not that difficult to make. The trick is to keep the losses as low as possible. To begin, get a copy of ‘loopcalc’, it is free!
Put some characteristics into loopcalc and play with different loop diameters. Then ask how anybody manages to get a loop that covers 80m to 10m. If I wanted to operate on 80m and 40m then I would make a loop for those bands, another for 30m-17m and another for higher HF frequencies.
Some important information: I use only CW and QRP. QRO loops are possible but an order of magnitude more difficult mechanically and more dangerous. Yes, dangerous for two reasons; the voltage on the loop will be several KV so you would not want to touch it. RF burns hurt and get deep into the skin. Secondly, the intense magnetic field close to a QRO loop will drastically exceed safe levels. Laugh if you want but exposure causes damage to people and if somebody gets hurt they will likely sue you for everything they can get!
Some figures from assuming a circumference of 4m, a diameter of 20mm and an operating power of 5W.