A wobulator was used in some old microwave signal generators to create what amounted to frequency modulation.

Ionospheric delay of a microwave signal depends on its frequency.

Also, microwave signal can penetrate, to a certain extent, the vegetation canopy and retrieve information from ground surface.

Trees and forests attenuate the microwave signal, and hills make it difficult to establish line-of-sight propagation.

A remote receiver could then have been used to decode the modulated microwave signal and permit sounds picked up by the microphone to be overheard.