# Coil Winding To 9mH

Posted: 8/17/2011 5:51:20 PM

From: UK

Joined: 8/17/2011

Hi,
Just building an SC/Jaycar Theremin into a full size case and I need to know how to calculate and wind an air cored linearisation coil for the pitch antenna.

From what I can work out... I should need about 9mH to 10mH air core coil. To work with my Pitch Antenna, which is 3/8" diameter and 18" Long.

I have some winding wire on hand that is 0.25mm so to save on the costs I plan to use this.

I have no idea however, how large a diameter the core should be or how many turns of this diameter wire I will need to achieve 9mH or 10mH..... Can anybody advise me how to work that out please??

Best Regards,
Julian (Fozzy The Bear)
Posted: 8/19/2011 10:37:45 AM

From: Colmar, France

Joined: 12/31/2007

[i]If you want to make a single layer air coil with this 0.25mm wire, you could take a 10cm diameter and about 70cm long plastic tube (European sanitary accessory...) and wind first 2304turns, make a tap for 9mH and wind other 253 turns for 10mH...[/i]

Edit: These results are atrociously false... I made a big mistake when converting the wire diameter from mm into inch before feeding all that in a software tool, please accept my apologies!

Here the correct values:

1000 windings for 10mH, tap at 900 windings for 9mH. Coil length 250mm, diameter 50mm. (There are also 50mm PVC tubes available which in general serve as bathtub drain pipe...)
Posted: 8/19/2011 2:27:34 PM

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Thierry, congratulations on being promoted, I think?

Your numbers show why I like operating a pitch oscillator at a higher frequency. If I grasp my centimeters correctly…that is going to be a large spool of wire (+80 m). The coil should look nice, especially if it is mounted vertical with the antenna on top.

Edit: Thierry I wasn’t going to question your math on your day of celebration! That coil reminded me of my very first theremin build many years ago which ended up more like a Tesla coil throwing tiny sparks. LOL

Posted: 8/19/2011 3:17:43 PM

From: UK

Joined: 8/17/2011

Thank you very much for your help Thierry. I will try those exact sizes and winds and see how it works out.

Also congratulations on becoming "Technical Editor" here. Very much justified.

Best Regards,
Julian (Fozzy The Bear)
Posted: 1/15/2013 8:34:20 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Here the correct values:

1000 windings for 10mH, tap at 900 windings for 9mH. Coil length 250mm, diameter 50mm. (There are also 50mm PVC tubes available which in general serve as bathtub drain pipe...)" - Thierry

For a 50mm tube and 0.25mm wire I calculate 1084 turns to get 10mH.  The 9mH tap would be at 983 turns.

Many on-line calculators use the old Wheeler formulas that can be rather inexact when it comes to different coil form factors (particularly when the coil length is short compared to the coil diameter).  Here is an excellent paper on the subject:

"An introduction to the art of Solenoid Inductance Calculation With emphasis on radio-frequency applications" By David W. Knight, Version 0.18, provisional, 24th October 2012

I'm using method W82-7W in my spreadsheet (page 43, error bounded within +/-20ppm).

Posted: 1/15/2013 9:42:04 PM

From: santiago, chile

Joined: 3/17/2006

"Here the correct values:

1000 windings for 10mH, tap at 900 windings for 9mH. Coil length 250mm, diameter 50mm. (There are also 50mm PVC tubes available which in general serve as bathtub drain pipe...)" - Thierry

For a 50mm tube and 0.25mm wire I calculate 1084 turns to get 10mH.  The 9mH tap would be at 983 turns.

Many on-line calculators use the old Wheeler formulas that can be rather inexact when it comes to different coil form factors (particularly when the coil length is short compared to the coil diameter).  Here is an excellent paper on the subject:

"An introduction to the art of Solenoid Inductance Calculation With emphasis on radio-frequency applications" By David W. Knight, Version 0.18, provisional, 24th October 2012

I'm using method W82-7W in my spreadsheet (page 43, error bounded within +/-20ppm).

Posted: 1/15/2013 9:45:05 PM

From: santiago, chile

Joined: 3/17/2006

I used a Calcoil.exe program and i got other results, can someone explain me what did i did wrong?

[URL=http://imageshack.us/photo/my-images/687/10mhcoil.jpg/][IMG]http://img687.imageshack.us/img687/1970/10mhcoil.jpg[/IMG][/URL]

thank you

Posted: 1/15/2013 10:57:04 PM

From: Northern NJ, USA

Joined: 2/17/2012

I'd stay away from any programs that don't have good papers associated with them.  Lots of misinformation out there in terms of SRF, even from official seeming sources.

You might also want to buy an inexpensive LC meter.  I got this one off of eBay for pretty cheap and it seems to work well.  It works off of resonance so the test frequency depends on the value of the component being tested.  Measures really small values of L & C.

Posted: 1/15/2013 11:57:17 PM

From: Northern NJ, USA

Joined: 2/17/2012

Thanks RS!

He says to no one because RS deleted his post...