The RF Sniffer described here is a field strength meter and has many uses besides finding secret transmitters. It has been found invaluable both inside and outside the shack for checking various functions on transmitters, antennas and ATUs. No doubt there are also occasions where it could help out with the odd EMC problem, too.
The circuit diagram is shown below.
RF is received on the short wire antenna and detected (rectified) by the diode, D1. The relatively small voltage is then amplified by the FET to give an indication on the meter.
The RF sniffer is constructed on plain perforated board with wires from the components bent underneath, thus avoiding the need for a PCB (see photograph). The antenna is about 30cm of stiff wire - bending the end into a small loop helps avoid the possibility of eye damage! The meter can be any surplus type of less than 1mA full scale deflection - not so common as they used to be but often available at rallies. We used an audio level meter which was quite small. The antenna is soldered to a solder tag bolted to the board with a small nut and bolt.
Construction is very simple. When complete, just check the connections carefully. We do not use an on/off switch but we just pull off the battery connector when the RF sniffer is not in use. Connect the battery and adjust VR1 until the meter needle is just coming off the zero stop (with no RF present). Testing is best carried out with a low power transmitter which should give a reliable indication on the meter. The circuit has been tested for operation between 1.5MHz and 175MHz but it should also work at higher frequencies.