Sourcing Antenna Coils for EM/EW Theremin (Moog)

Posted: 6/20/2017 1:35:07 AM
top955

Joined: 6/20/2017

I've found a lot of info on this forum regarding substitutes for the Variable TOKO inductors, but less info regarding the antenna coils. The EM theremin schematic (https://www.cs.nmsu.edu/~rth/EMTheremin.pdf) calls for

(4) 10mH RIF chokes

(2) 2.5 mH RIF chokes

(2) 5 mH RF chokes

Newark electronics still has the 10 mH Miller coil in stock (http://www.newark.com/bourns-jw-miller/6306-rc/inductor-10mh-100ma-axial-leaded/dp/63K3910), and Digikey has a Hammond 2.5 mH coil in stock (https://www.digikey.com/product-detail/en/1535B/HM1516-ND/454737), but I cannot find a 5 mH coil anywhere from either manufacturer.

 

Should I just string together a bunch of 2.5 mH inductors in place of the 5 mH ones? 

Posted: 6/20/2017 6:25:59 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Hello Top,

I do not want to fill up TW with electronic gibberish.

Christopher

Posted: 6/20/2017 9:02:31 PM
top955

Joined: 6/20/2017

Hi Christopher,

Thanks for your reply! I'm actually more interested in this from a learning perspective than obtaining the best-sounding theremin, but I appreciate your suggestion.

As for raising the frequency... Doesn't the configuration in the EM schematic reflect the optimal choice for the oscillator frequency? If Moog could've saved money by scrapping inductors without detriment, I would assume he would have done it.

Posted: 6/21/2017 3:37:41 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"As for raising the frequency... Doesn't the configuration in the EM schematic reflect the optimal choice for the oscillator frequency? If Moog could've saved money by scrapping inductors without detriment, I would assume he would have done it."  - top955

You should take pretty much everything in the Theremin field with a grain of salt.  I'm not saying everyone should go digital, and I'm not saying there weren't and aren't still brilliant people working on the analog variants, but the reasons for why things were done the way they were even in the recent past may have had more to do with the technology available, and in that sense "optimal" for the time.  Designers, like everyone, tend to go for proven, practical solutions, and this can keep one in a rut.  Maybe the rut is optimal, maybe it isn't, but it's not like there has ever been a Manhattan Project for Theremins, where cost is no object, time be damned, just hire all the brightest minds and give them anything they need.

Theremins can operate very similarly at many different frequencies, much of the coil physics scales with it, and I'd think one might be better off at 1 MHz or above just to get the inductors down to an easy to wind, reasonable size.  But I don't build analog Theremins so take that with a huge grain of salt.  

You can learn a lot by studying the designs of the masters, but you might reach a point in your education where it makes sense to deviate from what they were doing.  And at that point you will probably get hollered at for not being traditional enough. :-)

Anyway, my practical advice to you is to buy some of those RF chokes and play with them in your home lab.  Measure the Q, measure the temperature dependence, things like that.  String some together, see how they resonate with a typical antenna.  A single layer air core coil will generally beat a ferrite, but if you are aiming for a traditionally lower frequency design, then ferrite is hard to avoid.  The only real trouble you might get into with ferrite chokes is in trying to make too few of them resonate too low.  Stringing several smaller chokes in series lowers the self-capacitance.  And if you go to a higher operating point with ferrites you might see more drift due to the heterodyning gain.

I could be wrong here, but weren't there several variants of the EW over time, with changes to the values of these chokes?

Posted: 8/13/2017 10:04:43 AM
mathewgail

Joined: 8/13/2017

Well i saw what you are looking for here to find antenna coils. I am sure there would be no problem... a quick go through of mine and i managed to see items of 5mh , so direclty now you can use no need to stick together 2.5 mh ones.

 

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