Field strength meter circuit is a very useful device for measuring the RF output of any transmitter or experimenting different antennas with the transmitter. The work of the field strength meter (FSM) is simple it converts RF energy in to DC voltage and this DC voltage can be easily readable with digital
multimeter meter (DMM). The process of converting RF energy to DC voltage is performed by OA91 Germanium Diodes. On internet there are many field strength meter schematics but most of them are using batteries but the good thing of this circuit is that it is battery less and it is also enough sensitive. Make sure that the digital multimeter should be set on 200mV when checking any transmitter or antenna. But you can also increase it to 1000mV or more if the RF ouput of the transmitter is strong. The frequency bandwidth of this circuit is vast but I have only tried it on VHF band.