EDIT: There is now a full DIY RF Condenser mic project here -- www.amx.jp137.com -- which is based around the 300+ posts now in this thread (as at Dec 2019)....I'm afraid this one comes up at regular intervals on different mic forums.....
The most detail I have ever read regarding the coils in the Beis circuit was a response to someone on the Yahoo micbuilders group who emailed Uwe Beis about the inductor details a couple of yeas ago. The reply read:
"I cannot encourage to build this circuit. It becomes too complex, it is not just winding the inductors and reproducing the circuit. And it's not a final design, it is just the intermediate result of experiments. Anyway, I want to answer your questions as far as I can. It is a very long time ago that I made it
:
Can you provide winding details for L1 and L2?...
I'm afraid that I didn't write it down. L1 primary must fit to the oscillator frequency, secondary are very few windings (1, 1½, 2 or so as far as I remember). L2 is similar. The core material is crucial: I used Siferrit (later Epcos) K1, which has been since a long time no longer produced but possibly still available somewhere. Also important is the wire: I got best results with wire wrap wire: Rather thin with a thick PTFE insulation.
Are there any special requirements for L3 and L4?...
Not at all.
What values did you use for C3 and C4?...
I guess C3 << C2 and C4 >> C3, C1 possibly not used. It all depends on experiments!
Have you made any significant changes to the circuit since you built it?...
Not at all.
And finally, though I think it’s not critical, what microphone capsule did you use, or at least, what was its nominal capacitance? ...
As far as I remember 15 to 100 pF or so, but I tried two completely different capsules, one small diaphragm from a Sennheiser RF microphone and the home made large diaphragm one on the photo.
Could one build a multi-capsule microphone using a single, common oscillator?..
I can't see anything that argues against this idea. But an advantage might be that interferences (beatings) between different oscillator frequencies would be avoided by design".
So as you see, no precise construction details - sadly!
I did try some experiments using 74HC4046 PLL, but it's not much good for this task ---the oscillator was far too noisy...
I'm guessing things like a Rasperry pi might have similar limitations?
This is not an easy DIY project, as far as I have been able to establish .....
The Sennheiser schematics probably offer the best ideas on the concept, but again sadly - but understandably - there are no inductor details available...
EDIT: Looking again at my notes, I see the replies above I copied from the 'micbuilders' forum were addressed to someone also called 'Gerard' --- probably you, I'm guessing?.......If so, sorry to repeat stuff you already knew! ...
Still, posting it in a new location might attract some new interest?.....