The strength of a radiated signal from a transmitter is measured by this circuit. It is commonly used by model airplane builders to check whether their transmitter are really transmitting the radio signal. One can quickly determine whether the problem lies on the transmitter or the receiver module using this FS meter.

Field strength meter circuit schematic

The only active element in this circuit is a single transistor T1. The transistor is designed as a “controlled resistor” forming one part of a balanced bridge circuit. The base of T1 is connected to the receiving antenna directly. The transistor conducts and disturbs the balance of the bridge circuit with increasing radio frequency voltage at the transistor base. Currents flows to the meter through R3, the meter and the emitter-collector junction of the transistor with the bridge out of balance. How to use: by adjusting P1 before you turn on the transmitter, the meter is set to zero.