Re: Distributed capacitance effects inductor Q?
- From: "john jardine" <john@xxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 1 May 2007 22:38:44 +0100
"Bill Bowden" <wrongaddress@xxxxxxx> wrote in message
news:1177821718.856885.82580@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Does anyone know why the distributed winding capacitance of a loopI've never noticed it.
antenna, or any inductor, degrades the efficiency?
It would seem that a loop antenna with 100pF of winding capacitance in
parallel with a external capacitor of 200pF would resonate at the
same frequency as a antenna with no winding capacitance and a external
capacitor of 300pF, and perform just as well, but apparently that's
not the case.
The best explanation I got was that winding capacitance represents
'Low Q' and a external tuning capacitor represents 'High Q'
What is the difference between high and low Q, and why should a loop
antenna with no winding capacitance perform any better than one with
50% of the total capacitance in the windings? Where is the energy
loss?
Thanks,
-Bill
As the Q meter was on the bench I tested an AM ferrite radio coil. Value
measured beforehand on two LC meters as 480uH, (one at 1000Hz to preclude
self capacitance issues). (wire was Cotton covered, Litz style, 3 stranded).
600pF 300kHz Q=110
200pF 559kHz Q=140
100pF 800kHz Q=160
70pF 900kHz Q=200
30pF 1350kHz Q=220
10pF 2200kHz Q=200
I didn't extract out the self capacitance but would have thought the
measured Q values would degrade pro rata with reducing external
capacitances.
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