That is going to have the same problems as any of the other loops suggested: it works at a distance
when you are dealing with essentially a point source, but totally falls apart when you get close and
the RFI is radiating from two points on the same powerline a half wavelength apart (because when
the null points at one source you still hear the other.)
Another problem with loops, especially on the lower bands, is that they will couple to local wiring.
If you stand under a power line, you may get a null pointing down the wire regardless of where the
source actually is. This is because the loop is picking up most of its signal by coupling to the (much
longer and higher) power line. When the loop is perpendicular to the wire, coupling is minimum and
you get a null.
Unfortunately, because HF beams are otherwise impractical for most such hunting, the best you can
do when you get close is just listen for the strongest signal and ignore the (nonexistant) directional
pattern when the noise is radiating from a power line. Similar problems exist inside a building, where the
loop will couple to the AC wiring. This is true of any sort of DF receiver, professional or otherwise.
For VHF/UHF work, VK3YNG has a range of "sniffers" that are actually full-featured DF receivers:
http://www.foxhunt.com.au/Otherwise, most of the equipment designed for hunting ELTs on 121.5 MHz will work for power line noise
when you are close enough to hear it on VHF. Here are a couple that I've used:
http://www.ltronics.com/http://www.trackersecurity.com/FTV-Receiver.html
