I tried building the HF "magnetic" loop SWL antenna from plans by KR1ST. It's made of ten feet of 1/4 inch copper tubing for the main loop and 2 feet of copper wire for the coupling loop. I used a tuning cap from an old boombox like the directions say. It turned out to work pretty well, and tunes to resonance from 5,150 khz to 21,700 khz. I haven't yet built a stand for it so for now it just hangs on a door. This would have been a great apartment antenna back when I was renting. I wish I'd built it then.
I followed the directions pretty closely except for the addition of a salvaged SO 239 connector.