This low power circuit for the FM band was found in a spy book from the 90s. The tunnel diode is not an easy component to obtain and the power can be supplied with a single button cell.
This basic configuration can be found in a lot of articles on this site. In this case, the relay...
This high pass filter was found in the February 1996 Electronics Now magazine. The circuit...
Found in General Electric documentation from 1967, this circuit uses an unusual PUT today. The...