This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
For the 7 to 10 MHz band L1 is formed by 3 turns of 22 to 28 AWG wire in a ferrite core 15 to 20...
This circuit show the performance of a GraetzBridge or Diode Bridge usedin full-wave rectifers....
This adjustable source was found in a 1979 documentation, but all components are still common...