Steve wrote:
How effective are these for tracking down sources of RFI?

http://www.zapchecker.com

I have one. I've found cables underground, wires in the walls, etc.
I've used it to get a rough idea where RF is leaking. For what you pay,
it is a good device. It is not a test instrument.

I have a rather interesting application for the zapchecker. At a
certain military base that doesn't exist, I can detect when they fire
up their radar. The peak detector on the zapchecker (the LED, not the
meter) will flash about every 13 seconds. This is from about 10 miles
away.

I should point out that this device was sold for year at local flea
markets at a substantially lower price than what you pay for it online.
The varients of the basic model are really just hacks the designers saw
done with the original zap checker.

Note also that if you crank up the sensitivity, the zapchecker will
detect reradiated RF. That is, metal objects in the presence of a RF
field will reradiate the RF just like a passive radiator, and the
zapchecker will think it is alive.

You can use the zap checker to get relative readings. In RF shielding,
you take an initial reading, note the gain on the dial and the reading
on the meter, then shield a compent and see if the RF level drops.