It actually consists of a tunable field strength meter. The circuit is of an old documentation and the circuit must be tuned to the frequency to be monitored. The circuit offers the option to use a headset.
We found this circuit in an article in Electronics Now magazine from March 1996. The frequency is limited...
This circuit is designed to measure very weak currents, in the nanoampères range, using the 1 V...
This circuit is from a 1967 US publication. The configuration can serve as a basis for designs...