Built one this morning for 10 Mhz (99 feet). Took about 15 minutes. It
works pretty well only 10' up. I'm going to put a bigger one on the
roof. Very low noise. Costs nothing. (You can drive the feedpoint with a
CATV balun, I'm using a 4:1 LDG balun).

VSWR is about 3:1 from 7 Mhz to 15 Mhz with a little dip at 9.9 MHz
(resonance). As this is a receive only antenna I didn't bother trimming
for absolute minimem VSWR. I didn't test lower with the bridge, but I
can get KFI (0.64 MHz) on it. I hear very little QRM on the loop.

http://www.radioworks.com/nloop.html