[TowerTalk] Fwd: Inductor Design and Voltage Breakdown

[Richard (Rick) Karlquist]

On 7/31/2015 11:35 AM, Steve Hunt wrote:
> Jim,
>
> This one may be of interest:
> http://hamwaves.com/antennas/inductance.html
>
> It's the most accurate I've found. To quote the web page:
>
> Steve G3TXQ/
>

I have done a lot of calculations with this app and
the results are almost always within a few percent of
Wheeler's formula for inductance.  This is not saying
anything bad about the web site; simply that Wheeler's
formula isn't broken and doesn't really need fixing.
The calculations for Q seem somewhat questionable,
despite the high falutin' talk on the web site.
It seems to say that Medhurst's formula for Q is too
pessimistic.  I've never seen a coil with a Q
exceeding Medhurst's formula.

Medhurst also has a formula for self resonant frequency.
Again, the hamwaves calculator basically validates
this formula.  The SRF depends on whether one end of
the coil is grounded or not, so it is application
dependent.

BTW, there is a graph in Reference Data For
Radio Engineers showing predicted Q of coils.  This
graph follows Medhurst's formula, without any attribution
as to where it came from.  EXCEPT, that there is an
error on the graph.  Where it says "diameter in inches"
you should instead use "radius in centimeters".
Apparently even the publishers are not aware of this
error since it occurs in many editions of the handbook.

If you need a "calculator" for inductors, you can
program Wheeler's and Medhurst's formulas into Excel
in 5 minutes.  Medhurst's Q and SRF formulas are most readily
accessible in the Radiotron Designer's handbook.

Rick N6RK