Antennas & antenna coils for EM theremin

Hi everyone! Sorry if I'm repeting previous topics, but I searched forum and didn't found whole description of my problem.

I'm now building the EM theremin according to the 1996 article (http://www.emusician.com/gear/0769/build-the-em-theremin/146698). The onle problem are inductours [what a surprise! :)], because in my country it's absolutely impossible go get specified ones in a retail and at reasonable prices. I managed to get some replacement for tunable coils but stuck with antenna coils. So here are my questions:

1. What properties of antenna coils are essential exept for the inductance itself?

2. If I deside to use antennas different from what was advised by the article, how should I change coil's parameters? For example if I'm going to use telescopic antennas from home radio instead of copper tubing.

Thanks in advance! Best regards, @//!

1. What properties of antenna coils are essential exept for the inductance itself?

Low capacitance (High SRF .. SRF = Self Resonating Frequency)

see

http://www.thereminworld.com/Forums/T/28568/coils-in-series-question

2. If I deside to use antennas different from what was advised by the article, how should I change coil's parameters? For example if I'm going to use telescopic antennas from home radio instead of copper tubing.

Dont change the coils parameters - doing this will severely affect linearity - the oscillator is designed to work with the loading provided by the tuned antenna circuit - unless you really know what you are doing, and have the test equipment required, leave the antenna circuit as specified!

Telescopic antennas are too crude - see http://www.thereminworld.com/Forums/T/28592/coil-tuning?Page=1

for an option which allows antenna length adjustment - with this idea you use a different diameter antenna if you wish, and ajust the length so as to get the same antenna capacitance as required for optimum operation.

Once you have set the antenna, oscillators (VFO and REF) for optimum operation, adjustment of the antenna length (rather than adjusting the REF oscillator with the tuning control) will keep the theremin at its optimum operating point.

If I was building a EM, I would make the tuning potentiometer a preset (or eliminate the tuning circuit altogether, and adjust the reference frequency once using its tunable inductor) and only ever tune the theremin by adjusting the antenna length.

Fred.

1. Low capacitance (High SRF .. SRF = Self Resonating Frequency)

see http://www.thereminworld.com/Forums/T/28568/coils-in-series-question

 My mistake. Have seen this before, was not very attentive (:

2. Telescopic antennas are too crude - see http://www.thereminworld.com/Forums/T/28592/coil-tuning?Page=1

Now that's what I certainly should consider in my design. Thanks a lot for the idea (way simple but so neat!).

 3. for an option which allows antenna length adjustment - with this idea you use a different diameter antenna if you wish, and ajust the length so as to get the same antenna capacitance as required for optimum operation.

Is it then possible to somehow estimate the capacitance provided by this or another antenna?

Use copper or brass tube for the antenna.   - will perform better and telescopic antenna just looks lame  -will make your all your hard work look like  Rabbit-ears/ TV antenna.

 You could always just hand-wind large antenna coils.  

1200 turns of #32 enameled magnet wire on a 3"( x12.5" cardboard tube ) will make the pitch coil~= 32mH. 

Volume - 600 turns of #32 on 3" x 10" tube= 12.5-ish mH

You'll just need to find a small 2.5mH inductor for the other part of the volume antenna circuitry.

I built my first EM this way. Actually performed quite well. The lower inductance of the pitch coil compensates for it's larger capacitance.   (BTW- these are pretty much the same coils for the KEP tube theremin.)

When I first built the EM i couldnt find any of the inductors, so I had to get resourceful.    I had good results with winding the variable inductors on surplus slug-tuned coil forms.  Id recommend having a inductance meter for that tho'.

  Chobbs, thanks for the answer! 

What concerns telescopics - it was more an example, I've already decided to make "authentic" copper antennas. 

As for hand-wind coils - they are pretty big according to you. How did you mount it on the t-vox?

By the way, the article says to cut antennas longer then necessary and then trim them when first tuning assembled EM. How would I understand that the lenght is right, when trimming? 

You'll need about 1meter of tube for a "regulation" size loop- check the RCA drawings in the tech section for loop dimensions. bend it, then cut it down.

rod is about 16-18"

copper is very soft so you may want to consider filling the tube with epoxy (as per the EM article) after bending to prevent it from getting messed up.

 - sorry i think I misunderstood your question.

the engineers may roll their eyes, but I'd just make it and then mess with the tuning/antenna tweaking later.   Start with either the em antenna instructions or the RCA drawings, then tweak the circuit later for best results (after its all working)  The circuit is pretty forgiving and even if things arent perfectly tuned, as long as long as the components are assembled correctly, itll work.

Yeah-  big coils will not fit it in a small etherwave or tvox style cabinet.  I had to make a junior-sized rca style to house everything.

"Is it then possible to somehow estimate the capacitance provided by this or another antenna?" - snazzy

Well yes, it is - and its not difficult... Dewster produced a spreadsheet recently, Theremin_simulation_v3_2012-04-28.xls , but I cannot find this..

 But the essential formula he gave was:

From:

"Physics of the Theremin" by Kenneth D. Skeldon, Lindsay M. Reid, Viviene McInally, Brendan Dougan, and Craig Fulton, 1998 (PDF)

Antenna capacitance above a ground plane:

Cant = 2*pi*epsilon0*L / (ln(2*L/D)-k)   <- Corrected

where:
L = length of antenna in meters
D = diameter of antenna in meters
epsilon0 = permittivity of free space = 8.85E-12 Fm^-1
k is a constant depending on how far above the ground it is mounted
k = 0.4 for almost at ground level.

Extra capacitance for the above antenna due to a hand in proximity:

Chand = pi*epsilon0*L / (10*ln(4*x/D))

where:
L = length of antenna in meters
D = diameter of antenna in meters
x = distance from hand to antenna in meters
epsilon0 = permittivity of free space = 8.85E-12 Fm^-1

To get total capacitance for antenna with hand at any given distance, add antenna and hand capacitances.

I think that to compute the antenna capacitance (without taking the hand capacitance) all you need is:

2*PI()*e*L / (ln(2*L / D) -k))*10^12

L = Length of antenna in metres

D = Diameter of antenna in metres

e = 8.85E-12

k = 0.4

The result should be antenna capacitance in pF.

If you calculate the antenna capacitance for the EM using the above, you should be able to mess with length and diameter values for whatever antenna you want to substitute, so that you get the same capacitance.

Fred.

 Dewster - Please correct me if Im wrong about the above - I am not sure about the 2*PI() in the formula.. This formula was lifted from your spreadsheet but doesnt fit the formula in the text.

Thanks you a lot guys for such detailed answers. I couldn't have hoped to get more info! Rushing back to the workshop!

"Dewster - Please correct me if Im wrong about the above - I am not sure about the 2*PI() in the formula.. This formula was lifted from your spreadsheet but doesnt fit the formula in the text." -FredM

I got the paper indirectly from you Fred (over at element14) titled "Theremin_Am_J_Phys_excellent.pdf".

My spreadsheet notes are wrong, but the spreadsheet calcs seem OK (compared to the paper). The antenna capacitance calcs should have 2pi, the hand calcs just have pi in the paper.

Note to self: never try to slip one by Fred!

My archive of Theremin stuff (including the paper, updated spreadsheet, and digital Theremin verilog code):

http://www.mediafire.com/?w36b3brqyg2g3

"My spreadsheet notes are wrong, but the spreadsheet calcs seem OK (compared to the paper). The antenna capacitance calcs should have 2pi, the hand calcs just have pi in the paper." - Dewster

Thanks for that confirmation, Dewster -

I used the 2pi version - after I ran the formula in the text, and the results looked wrong, I examined what was actually 'coded' in the spreadsheet, used 2pi and things looked a lot more 'right'.. But I was a little worried about the discrepancy.

Fred.