[TowerTalk] need advice on calculating coil Q

Richard (Rick) Karlquist richard at karlquist.com
Thu Mar 1 14:12:15 EST 2018


No!

Since you refer to this as an "antenna coil", I
will assume it is air core.  In that case,
generally Q scales with square root of frequency.
If it has Q=25 at 100 kHz, you can estimate that it
will have Q=209 at 7 MHz.  Not as good as actually
measuring at 7 MHz, of course.

If the coil has a magnetic core, then you
can't conclude anything about 7 MHz from
100 kHz measurements.

You can alternately calculate the Q using the formulas
developed by Medhurst et al.  See the "Radiotron
Designer's Handbook" for details.  There is also
a graph in "Reference Data for Radio Engineers"
that attempts to emulate Medhurst, but it
underestimates Q by a factor of 1.27, apparently
due to a mix up about units.

Rick N6RK

On 3/1/2018 10:46 AM, John Keating wrote:
> I'm attempting to characterize an antenna coil. I have a meter than can
> measure at 100kHz, which indicated L = 13uH and Rs = 0.322 and Q = 25. Those
> would indicate X(L) = 8.17 ohms at 100kHz. The value of L is consistent with
> other measurements I did with an antenna analyzer.
> 
>   
> 
> Is it valid to use the value of L and Rs meaured at 100kHz for calculations
> at the operating frequency of 7 MHz? I get a nonsensically high result for
> Q. Suggestions appreciated.
> 
>   
> 
> 73, John / AI6LY
> 
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