Jim, *** True, the point I'm trying to make to you folks is that it *ISN'T* the absolute level of the signal that you are necessarily concerned with but rather the dynamic range in that signal. Simply knowing there's a 3mV signal out there won't do you a damn bit of good unless you have enough signal range to decode the contents. In actuality you could be emitting GW's of signal and if there was only say 1uV of signal range you'd never get anything off it. *** The distinction you are trying to express is that not all signals have equal information content and that it is the levels of the high-content ones that matter. Fine, but shielding is indiscriminate so shield away. And run your Tesla coil and some Ramones music to increase the ambient noise while you're doing sensitive computing. BTW - Aren't most receivers sensitive to RF field strengths in the 1 uV/m range? With cryogenically cooled front ends the nV/m range is probably usable. Look for MIB carrying Dewars in your neighborhood! How cold do Peltier junctions get? Probably good enough for a sidewinder or a low-noise rcvr. Mike Gabba-gabba hey!