This is an instrument which is very useful for radio frequency hobbyist and enthusiast. Especially when one has to adjust the final RF transmission. One of the example is setting up an antenna impedance matcher. Following is the schematic diagram of field strength meter :
As the active component, this circuit uses only one transistor, the MPSA18. 1 mA full scale analog meter is used as a signal strength meter display. If we don’t calibrate the scale using a standard instrument, this RF field strength meter has no absolute scale. However, the most important thing for this uncalibrated RF meter is that it is able to display the changes of an RF field when we calibrate the antenna or the matching network for the RF transmitter-antenna loading. Just observe the analog meter to get the maximum deflection if we are tuning the antenna size or the transmitter-antenna matcher filter.
If we pay attention to the circuit diagram, the base of the transistor is biased by the voltage from 1 Mohm resistor and connected to 1N914 diode to ground. The voltage at this connector is around 0.6-0.7 Volts. The voltage is the delivered to the base of the transistor through the Radio Frequency Choke inductor. This voltage bias has the function to make the transistor ready to conduct. The signal received from the antenna does not have to reach 0.6 volt base-emitter barrier to make it works. The circuit will not draw any current from battery if there’s no received RF signal. It is because the bias voltage is set by the diode just below the point at where the transistor begin conducting. Therefore, it doesn’t need a power switch to cut the power consumption of the supply (the battery). You need to make sure to move it away from strong RF transmitter when this device is not being used. [techlib.com]