RF Condenser Microphone C2-OD4

The Stephens Tru-Sonic company was founded in Los Angeles in the 1940s by Robert Stephens, who had worked with Lansing, Shearer, and Hilliard on the academy award winning MGM loudspeaker system for motion pictures. Like Lansing, he absorbed the manufacturing philosophies and techniques that had been pioneered by Western Electric, and his company was regarded during the 1950s in the same professional class as Altec and JBL.

In the early 1950s Stephens introduced the first RF microphone intended for recording and reinforcement activities, the model C2-OD4. The C2 capsule assembly was connected to the OD4 oscillator-demodulator through a coaxial cable of any length, as long as the overall cable length (transmission line) was a multiple of 37.5 inches, a requirement for efficient signal transfer from the capsule to the demodulator. The capsule portion contained only the half-inch (12 mm) diaphragm closely coupled to an oscillating tank circuit. The system operated on the FM principle in the range of 9 MHz, and signal demodulation took place in the associated circuitry.

-from "Eargle's Microphone Book: Mono to Stereo to Surround: A Guide to Design and Application", pg. 437-438 (Ray A. Rayburn)

Here you find 12 models, 12 with images and 0 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
All listed radios etc. from Stephens Manufacturing Co., Culver City, CA