[sdiy] RCA theremin antenna shape

[Magnus Danielson]

From: Grant Richter <grichter at asapnet.net>
Subject: Re: [sdiy] RCA theremin antenna shape
Date: Sun, 27 Jan 2002 09:06:23 -0600

> You can build a functional Theremin out of two CD4046 chips. Set one to a
> fixed frequency of around 200KHz with a tuning knob that goes from 190Khz to
> 210 Khz (just a pot in series with the frequency set resistor). Make another
> 4046 with the same frequency and attach an antenna to one leg of the
> frequency setting capacitor. Combine the two frequencies with the EXOR gate
> inside one of them, and listen to the output. You can power it with a 9 volt
> battery. Theremin for under a $1.

Oh my... yes, the 4046 is a versatile chip ;O)

> Since this is so cheap, I goofed around with the circuit for a couple years.
> Since the output is a pulse train, you can use another CD4046 to convert the
> frequency into a voltage. Easy proximity controller for synthesizer.

Indeed. See above comment. I love the 4046s for bringing simlicity in
for many cases. BTW, They have their limits and I've run over them too.

>  With no inductors in series with the antenna going to the cap lead, you get
> a range of about 10Khz over a distance of 6" inches. If you put an 80
> milliHenry choke in series with the antenna on the cap lead, it (I think)
> acts as an antenna loading coil or impedance matcher? Anyway this will
> spread the response out to a 2 Khz change over about 24" of hand travel. It
> will then behave very much like an Big Briar Etherwave, which is where I got
> the 80 milliHenry value from. Thanks again to Dr. Bob Moog, patron Saint of
> all things Theremin.

Indeed.

> A theremin with both antennas in parallel does not work as well as at 90
> degree angle. The two circuit interact even with 100Khz difference in local
> oscillator frequencies.

Ouch!

> So based on experimental evidence, I would say that the horizontal antenna
> is NOT an inductor, the orientation is to prevent interaction between the
> two antennas. Since the horizontal antenna is now a cantilever, it was
> probably doubled it into a loop for mechanical strength, not electrical
> properties.

Most interesting! Thanks!

> Also the schematic for the RCA Theremin is published on the inside of the
> Clara Rockmore CD. I do not remember seeing the volume antenna as part of
> the tank circuit.
> 
> It is only because of Bob Moog that the Theremin is even remembered today. I
> am fairly certain the Theremin is Bobs real passion, the synthesizer is more
> business related. I hope over time, Dr. Moog gets the credit he deserves for
> single handedly carrying this marvelous instrument into the 21st century.

Right. You can also pick up some on him, Leon Theremin, by reading
Albert Glinsky's book "Theremin - Ether Music and Espionage".

I just haven't had the time to read mine.

Cheers,
Magnus