> >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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