Help installing the pitch antenna?

Posted: 6/23/2012 5:55:49 PM

Joined: 6/23/2012

Hi guys

Me and my couple other friends are working building a EM theremin (based on this

So far everything works well--the variable & fixed pitch oscillator and volume oscillator and tuning works fantastic.

However when we were trying to connect the pitch antenna to our breadboard, it seems that it is not responding. For the purpose of building a demo, we are using an alligator clip on one of the antenna and have a wire feeding into the inductors.

Any ideas?

Posted: 6/23/2012 9:42:52 PM

Joined: 6/23/2012

So after doing some research I found this same scenario from another member:

The pitch antenna (~50cm aluminium rod, not tube -> will be changed to copper tube with 10mm outer diameter when the cabinet is available) is mounted approximately 7-12cm away from the board. A grounded copper foil is mounted between the board and the antenna.
First thing I've learned is that it's really hard to tune if you do it on the workbench next to PC, oscilloscope, etc. :) This environment has given me only 2-3cm for the distance of the pitch antenna, mainly independant of the oscillator settings. Also the pitch range is poor.

So I did it again on the clean living room desk without any near active electrical device or something like that. It improved the range to approximately 15cm and even the oscillator tuning has now a better effect on the pitch range. But this is not enough. If I'm right, I can expect about 50-60cm distance on a good tuned circuit, right? Also, I assumed that the pitch range is at least three octaves, but for my unexercised ears it's somewhat of a half octave in best case (also it's hard for me to check the 3kHz mentioned in the EM article checklist with my ears, and if I measure audio output frequency with the scope the signal is very noisy so I cannot read the exact frequency).
Now, I wonder if I can expect pitch distance and range of a Etherwave on my quick test assembly.


Is this common when working the antenna near the workbenches?


Posted: 6/24/2012 2:39:47 AM

From: Northern NJ, USA

Joined: 2/17/2012

All kinds of stuff in the nearby vicinity affects a Theremin.

Try adjusting the pitch coil for maximum voltage (position a scope probe near the pitch antenna) and that may get you close to where it should be.  I don't think the EWS should be sold as a kit actually, at least not without proper instructions.

Posted: 6/24/2012 5:07:52 PM

Joined: 6/23/2012

Thanks for the reply. 

We checked and we are receving max voltage through our coil, but nothing have changed.


However, I found another forum member having trouble with his antenna because he was using a breadboard. 


The 4 x 10mH inductors may not have too much static winding capacitance themselves. That's why you must use the recommended 3-pi-wound RF inductors with a SRF > 700kHz and you can't mount them on a breadboard since its copper dots or stripes would add too much capacitance and thus lowering the SRF

This may be our solution. If anyone can confirm this, that would be great. As of now, I'll be doing more research on debugging and I'll let you guys know if we were able to progress.


Posted: 6/24/2012 9:57:37 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

". If anyone can confirm this, that would be great."

I posted the following somewhere on TW, but cannot find it - So sorry for this "double" posting - When I find the link I will insert it.

When it comes to equalizing coils, high Q and low capacitance are, IMO, the fundamental requirements.. These can be achieved with big air coils, but equally can be achieved with smaller series connected coils on ferrite formers.

For those who dont understand this, I will explain:

It is the 'trick' of connecting coils in series which is where one gets the best of all worlds - the capacitances of these coils is reduced for the total capacitance of the final composite inductor - For example, the Miller (Bourns) 6304-RC coils (which I mainly use) are 5mH 5pF each.. Connect 8 of these in series and one has 40mH (series inductances add together), but the capacitance becomes 1/((1/C)*N) = 1/((1/5pF)*8) = 1/(0.2*8) = 1/1.6 = 0.625pF ... You will have difficulty winding a coil which gives 40mH with capacitance of 0.625pF!

Using 10mH coils, each with 8pF, one gets (for 4 coils) 40mH with 2pF - this is ok, not as good as using 8 5mH coils, but ok (and 1/2 the price of using 8 inductors).

Using one 40mH coil on a single ferrite former, one gets a lousy equalising coil, because capacitance is usually much higher. 


(in fact, the Miller / Bourns 630x inductors partly achieve their low capacitance by being wound as 3 seperate coils on a common former - using the same 'trick' I have described above)

"and you can't mount them on a breadboard since its copper dots or stripes would add too much capacitance and thus lowering the SRF"

You can mount them on breadboard - but keep the leads long and raise the inductors well above the board.. Its not as good as you will get from a PCB (capacitance is likely to be a bit higher) but it is good enough.

But why mount the inductors on the board? solder them end to end and place them in a plastic tube - connect one end directly onto the antenna mounting tag with a really short connection between the antenna and inductor, and run a thin wire from the other end to the PCB.. Keep the inductor tube away from any metalwork or board etc (use plastic clips or whatever to hold it in position).

It is far better to have any connection wire on the "cold" end (the connection to the oscillator, which is not extremely sensitive to capacitance changes) than on the "hot" end (the end of the last inductor, which is highly sensitive and is the antenna) - you want as little "antenna" inside your theremin as possible.

Posted: 6/24/2012 11:07:33 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

The missing link. :-)



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