Detecting voltage of RF signal

Hi,

just wondering how I would detect the voltage of a rf signal passively. I would like to be able to turn on a receiver on a hand held device but only when its a certain distance away from the source as to save batteries. ANy input would be greatly appreciated

Amateur Radio july 2002 has a "A low cost
microwatt meter" article. I dont remember site, you can try by google.

But actually level detection is done in receiver itself :
for FM it is about to monitor high freq audio noise detector (through that is for limiting values), for AM based - level of AGC in RF stages.