Antennas & antenna coils for EM theremin

"I would always recommend some sort of insulation coating on the volume loop for several protective reasons. Three immediate thoughts are protecting the metal, shielding from static discharge and the fact that some volume circuit designs react poorly to the loading affect of direct contact, your sound could come back on." - RST

Yes, so would I - I also seem to remember (this may be complete baloney) that some early EW's squeeled loudly when their volume antennas were touched - now, as I say, this may be baloney.. But you dont get a much earlier "EW" than the "EM"!

My "recollection" is that, on these theremins, touching the antenna (making galvanic contact) pushed the volume oscillator into a frequency range which interfered with the pitch oscillator, and simultaneously overloaded the volume circuit, turning the theremin VCA on.. And the fix for the problem was insulation of the volume antenna..

But it is just possible I have this wrong - could be a different theremin I am thinking about..

Fred.

If you try annealing the pipe, before bending you will then need cap off an end, then pack it full of fine sand- then seal off the other end.  This will prevent the tube from collapsing. 

I know that water can be used for this too. Need to freeze water filled tube carefully (probably with one end open for ice to expand) and then bend it before ice melts.

making a "proper" loop isnt that hard- decent mounting hardware on the other hand is a real PIA

Well, mr. Moog's advice is to use tube fittings. But I found it as difficult as to buy proper inductors to get needed fittings, so I came up with an idea. The idea is to cut a couple of inches of screw-thread on the end of a tube and then mount the tube in the hole in the wooden panel with two nuts (one on each side of panel).

http://img826.imageshack.us/img826/9391/adjustableantenna.jpg

Resembles FredM's variant (http://www.thereminworld.com/Forums/T/28592/coil-tuning?Page=1). Antenna's length also can be made adjustable (FredM, hope You forgive me for using Your picture as a source).

"(FredM, hope You forgive me for using Your picture as a source)." - Snazzy

Please feel free to use it - The real "perk" of contributing is when you see that it isnt a waste of time!

Fred.

k is a constant depending on how far above the ground it is mounted
k = 0.4 for almost at ground level.

Hey Fred, what does that k actually mean and what would it be if it wasn't mounted at almost ground level.  Is there a separate equation for k that you can point me to.

Thanks

This k=0.4 is a "wet thumb" estimation which shall compensate the difference between theory and practice. The authors of the paper where it appeared for the fist time didn't give an exact reason for using just this value...

I personally have totally stopped caring about formulae and precision computing in the theremin domain. They helped me at the beginning to understand several basic  mechanisms. And I learned that there are too many variables for exact predictions. Thus I rely rather on my RF experience and when I observe a kind of phenomenon, I just adapt slightly the virtual parametric equation system in my mind to match the new reality. 

At the end there are professional musicians who want to make music on what I created or modified. And since I started thinking more like a musician and less like an engineer, my clients are still more satisfied with the results.

Mathematics and electronic engineering are just some tools, not an end in itself.

Well the reason i ask is because some friends and i are making a theremin for our senior design project (we are engineers) so understanding what the k really means might aid in understanding the capacitance of the antenna.  Right now, using k as 0.4 with an antenna height of 18 inches and diameter of 3/8 inches i calculate the capacitance to be around 9.4 pF.  Using this value doesn't give us the right resonant frequency of 260 kHz (using 40 mH inductors).

So is there something we are missing?

??? When I apply the Thompson formula f=1/(2 x pi x Sqrt(9.4E-12 x 4E-2)) with 9.4pF and 40mH I find a resonant frequency of about 259.6kHz which is well in the acceptable (but not optimal) range for an EM-Theremin... All these formulae allow just a rough estimation to see if you are near the order of magnitude. 

Later a 1/100 picoFarad which will make the difference between musical and unmusical.

Stop calculating... build it, feel it, then adjust and optimize it by hand. It's all about making music, not about precision oscillator and antenna matching design.

"Hey Fred, what does that k actually mean and what would it be if it wasn't mounted at almost ground level.  Is there a separate equation for k that you can point me to." - Antman

The equasion was extracted by Dewster from this article and did not come from me... As in, I have absolutely no idea what k is or where it comes from, I can see its function in the equasion, but that is all.. The article is one of the few which was not put into the collection at Element-14 by me, and I certainly do not vouch for the accuracy of its contents - its from the University of Glasgow after all! ;-)

I am inclined to agree with Thierry on this matter - It is strange how it seems we have almost had a "role reversal" LOL - I am only really concentrating on theory and maths and simulations etc because I am not able to be "hands on" at the moment -

When it comes to theremins, you cannot run a simulation and expect to get your circuit to behave exactly like the simulation does! - You cannot really expect this even with non-critical circuits - but certainly dont expect it with theremins or capacitive sensors or any high impedence RF constructions! Simulations are mathematical evaluations, and the same therefore applies to any computations you make for a theremin circuit.. Fine to get you into the right "ball park" if you are lucky, but thats about it!

As for the 'k' and, in fact, the whole equasion - I have not found it particularly 'realistic' - I now just insert a realistic antenna capacitance (which you can work out by knowing the required resonance and knowing the value for the series inductor, or transposing these as required). I think a simplified capacitance computation which just computes hand capacitance as if the hand was a 'plate' and setting a reasonable capacitance for 'background' may be simpler - I will let you know if the spreadsheet I am working on right now verifies this (or looks sensible).

Fred.

"When I apply the Thompson formula f=1/(2 x pi x Sqrt(9.4E-12 x 4E-2)) with 9.4pF and 40mH I find a resonant frequency of about 259.6kHz which is well in the acceptable (but not optimal) range for an EM-Theremin"

Sorry, 9.4pF was the value I calculated from the resonance equation. I meant to say the antenna capacitance calculated from the equation FredM found doesn't give the right resonance frequency, therefore the k in the equation is doing something in reality.

"Stop calculating... build it, feel it, then adjust and optimize it by hand. It's all about making music, not about precision oscillator and antenna matching design"

Believe me, we want to do away with the calculation but since this is for a class, the professor requires a report that's why we are interested in the calculation.

for now we are just gonna build each piece and hope it works when we put them together.

In that case, find a value for k which makes the formula match the 9.4pF and tell your professor that during your research you found an optimized value on the empiric way! ;-)