[sdiy] RCA theremin antenna shape

[Grant Richter]

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.

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.

 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.

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.

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.

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.

> 
> Does anyone know whether the horizontal (volume) antenna of the original RCA
> theremin is part of a coil, rather than just stuck on top of a coil?
> Because, there is a debate on wheter the shape matters, or whether it was made
> like that so you could easily dip your hand in for a quick muting.
> I suspect that it is a coil, and that the 'flattened' nature is to reduce the
> external field, so that it desn't interact so much with the vertical pitch
> antenna.
> Any of you hams & RF guys? Harry??
> 
> paul perry (Frostwave analog fx melbourne australia)
> -- 
> Message sent by Webmail from melbpc.org.au
> 
>