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[AMPS] Finding the Q of a Coil

To: <amps@contesting.com>
Subject: [AMPS] Finding the Q of a Coil
From: W8JI@contesting.com (Tom Rauch)
Date: Mon, 8 Oct 2001 04:52:00 -0400
> >Accurate calculation of RF resistance definitely is the tough part. 
> >There is more than one set of formulas from different sources for
> >calculating this and the results don't agree. If I were forced to
> >choose, I'd probably stick with the formulae in Terman's books.  
> >Steve T. correctly outlined the general procedure.  I too will have a
> >look at the Murphy file that Ian suggested.
> >
> I should have mentioned, it's all based on info from Terman.

This is one of the cases I have found where you had just better 
measure the component.

It seems is difficult if not virtually impossible to get close to actual 
component Q using formulas. Many things are at issue besides 
wire resistance. Every Q formula working off coil dimensions or 
program I have tried has never been close.

The work by Czerwinski in August 26 1960 Electronics comes 
closest, while some of Terman's stuff is miles from being correct.

The highest Q's I have ever measured are in the upper hundreds 
(~800) for large well designed transmitting-type inductors operated 
reasonably far below the self-resonant frequency of the inductor.

You can test or judge any formulas by looking to see if inputs 
contain:

1.) Length to diameter
2.) Conductor spacing
3.) Conductor type and construction  (unless assumed to be the 
type you are using)
4.) Frequency

What you should see is an increase in Q and a peak someplace 
below the self-resonant frequency. If you see Q's of more than 
several hundred's with any RF inductor, watch out!

The best text I have found on this is in Kuecken's "Antennas and 
Transmission Lines" in the chapter on "Reactance Elements and 
Impedance Limits", although it deals only with airwound inductors 
using bare solid round conductors.

Stray capacitance in an RF inductor greatly affects Q, and so if 
you don't see a definite upward slope of Q with frequency, limiting 
at perhaps 1000 or less, and an abrupt drop above the peak with 
much lower Q as the self-resonant frequency is approached 
something is wrong.

  
73, Tom W8JI
W8JI@contesting.com 

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