This is a simple and low cost wide band VHF field strength meter. The field strength is measured by converting the radio signal to DC and measuring it. The RF signal will be picked up by the coil and rectified by the diode D1.Even a very small DC voltage is sufficient to alter the biasing of FET and it will be reflected in the meter as an indication of the presence of a radio signal. The meter can be calibrated by adjusting the preset R2 to make meter M1 read ZERO in the absence of any radio signal. This circuit is not very sensitive, but can sense radio signals from hand held FM transmitters up to a distance of few meters( ideal for theoretical demonstrations).
Circuit diagram with Parts list.
- The circuit can be assembled on a general purpose PCB.
- Use a 9V PP3 battery for powering the circuit.
- Use a 250uA FSD current meter for M1.Using a lower FSD meter will improve sensitivity.
- The coil L1 can be made by making 6 turns of 20 SWG enameled copper wire on a ¼ inch plastic former.
- The antenna can be a telescopic whip antenna.