This simple circuit measures the strength of a radiated signal from a transmitter. It is commonly used by model airplane builders to check whether their transmitter are really transmitting the radio signal. Using this FS meter, one can quickly determine whether the problem lies on the transmitter or the receiver module.
A single transistor T1 is the only active element in this circuit. The transistor is designed as a “controlled resistor” forming one part of a balanced bridge circuit. The base of T1 is directly connected to the receiving antenna. With increasing radio frequency voltage at the transistor base, the transistor conducts and disturbs the balance of the bridge circuit. With the bridge out of balance, currents flows to the meter through R3, the meter and the emitter-collector junction of the transistor.
How to use: Set the meter to zero by adjusting P1 before you turn on the transmitter.