[TowerTalk] need advice on calculating coil Q

[Bill Cotter]

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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