HP has some good reading for thinking about and measuring Q for
circuit elements.
www.hparchive.com/Boonton/BRC-The-Notebook-04.pdf
www.hparchive.com/Boonton/Boonton-Manual-260A.pdf
www.hparchive.com/Boonton/BRC-The-Notebook-13.pdf
The Q-meter (ie: Boonton 160, 260, TS-617, Heathkit QM-1) is a
seldom used, but very useful instrument to the ham's workshop.
They are plentiful and usually inexpensive.
http://www.prc68.com/I/Qmeters.html
73 Bill N4LG
At 02:12 PM 3/1/2018, Richard (Rick) Karlquist wrote:
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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