Suggestion for antennas

Posted: 8/12/2019 12:45:32 PM

Joined: 10/19/2017

Hi everyone!
I'm making a theremin based on the Robert Moog's article. He used 10mm diameter copper pipes for both antennas, but I have only 8mm and 12mm diameter copper pipes.
Wich one is the best replacement? (8mm or 12mm)
And in consequence of this choice should I modify the value of the inductors related to the antennas?
Thank you all.

Posted: 8/12/2019 2:37:47 PM

From: Colmar, France

Joined: 12/31/2007

The combination of all four, oscillator inductance, oscillator capacitance, antenna series inductance, and static antenna capacitance (it's here where antenna dimensions come into play) is a delicately balanced and optimized thing to obtain an optimum of pitch range and linearity. Thus, you can't only modify one or two of these four parameters without the risk of reducing the pitch range or degrading the intended linearity and response.

I've never seen a hardware store where they did not carry 10mm (or 3/8") copper tubes for plumbing. Don't make yourself unhappy by trying to do the impossible if you haven't a well equipped RF lab and several decades of EE experience. Go and buy 10mm tubing. If you can't get copper, then nickel or brass will do it, too. The correct physical dimensions are ways more important than slight differences in conductivity.

Posted: 8/12/2019 7:30:19 PM

From: Theremin Motherland

Joined: 11/13/2005

The effect of antenna dimensions is greatly exaggerated.

It’s actually enough to keep the static capacitance of antenna like the Etherwave's one.

(which is 6.4pF at 10mm diameter / 50cm length).

Just to be within the limits of tuning abilities of L5.

So antennae 10mm/50cm, 8mm/53.6cm, 12mm/46.6cm will be equivalent (6.4pF).
Plus, linearity of these three antennas will be almost the same:

Posted: 8/12/2019 7:48:17 PM

From: Berlin Germany

Joined: 4/27/2016

Following the advices of Thierry is surely the right way I recommand too. I agree also with ILYA. 
You can keep the oscillator frequency unchanged if antenna capacitance of your antenna rod remains just the same. The behavior should show this formula. 

Example: original radius=5 mm and lenght=480 mm. Using r=6 mm leads to a shorter rod with l=452 mm. For r=4 mm you will need 505 mm. 

The difference to the original is the hand capacitance. Note that these also vary with the dimensions of a person's hand and body. Between the thicker antenna rod and your hand is a bit more capacitance. So you would have a little more or less pitch range which can be compensated with the offset knob. The material of the rod does not play any role as long it is metal. 

Posted: 8/13/2019 7:22:35 PM

From: earth

Joined: 5/8/2017

The plate antenna is best, most forgiving and easiest to hold a note.

but, when I played the Ehterwave, I found that a copper wire coil worked very well and could easily be adjusted to compensate for environmental issues. Since it is a spring it adds its own vibrato. In over to keep the vibrato in check, I inserted some plastic foam within the void of the coil. If you ever have problems with your pitch field, either pull the antenna out to make it longer, or squish it back down to make it shorter.

The coils, being wider than the toilet inlet pipe, is a bit more forgiving when it comes to lateral extraneous hand movements,

But toilet inlet pipes do have their charm, and the chrome plate is spiffy.

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