RCA theremin technical notes written by Reid Welch.
The RCA Pitch Resonant Coil
Consists of more than a thousand turns of approximately 38 ga. ( B&S gauge) SSC enameled wire wound on a 12″ tall by a 2 15/16″ diameter, 1/16″-wall phenolic impregnated fiber tube.
|Winding length-||10 3/4″|
|Z at 1kHz||531 ohms|
In series with the big coil is the so-called “concentrated coil”. It is fitted planar fashion inside the bottom of the large coil. An air-core, low self-capacity, RF choke. 1″ diameter lattice wound, 7/16″ thick, wound in fine, enameled wire on a 1/2″ diameter wooden spool. The concentrated coil contributes the remaining inductance needed for a 175kHz-resonant system. There wasn’t enough room on the big coil form for all that wire.
Between the two coils there is a small blocking cap (.0022??) presumably to prevent DC voltage ever getting to the antenna and posing a shock hazard. At any rate the cap doesn’t appear sized for tuning/peaking purposes.
The RCA Volume Resonant Coil
The RCA Volume Resonant Coil employs the same 38 ga. SSC wire and the same size tubular form as the big pitch coil. The winding length is only 4 1/4″. Inductance measures 11.9mH.
The volume pick-up coil consists of eight turns of ~ 20 ga. wire placed over the volume resonant coil, right at the lower (hot) end of the big coil’s winding length.
[Copyright 1997 by Reid Welch]