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Sci.Electronics.Basics -> Distributed capacitance effects inductor Q?
There are 4 messages in this thread.
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Author: Bill BowdenDate: 00:41 29-04-07
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Does anyone know why the distributed winding capacitance of a loop
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
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Author: Andrew HolmeDate: 06:56 29-04-07
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"Bill Bowden" <wrongaddress@att.net> wrote in message
news:1177821718.856885.82580@n76g2000hsh.googlegroups.com...
> Does anyone know why the distributed winding capacitance of a loop
> 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
>
In a low-Q capacitor, energy is lost in the dielectric e.g. plastic
insulation. I imagine your external tuning capacitor is a high-quality
air-spaced type.
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Author: Bill BowdenDate: 23:27 30-04-07
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On Apr 29, 3:56 am, "Andrew Holme" <and...@nospam.com> wrote:
> "Bill Bowden" <wrongaddr...@att.net> wrote in message
>
> news:1177821718.856885.82580@n76g2000hsh.googlegroups.com...
>
>
>
> > Does anyone know why the distributed winding capacitance of a loop
> > 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
>
> In a low-Q capacitor, energy is lost in the dielectric e.g. plastic
> insulation. I imagine your external tuning capacitor is a high-quality
> air-spaced type.
>From another group, someone suggested the losses are due to increased
circulating currents between the turns of the inductor. Sounds
reasonable, although the math to calculate the losses must be fairly
complex.
Apparently, the air capacitance between each turn of the inductor sets
up a LC circuit that causes current to flow that wouldn't normally
happen if there was no interwinding capacitance. The total of all
these little currents adds up to increased I^2R losses in the larger
LC circuit and reduces efficiency. Sort of the same idea as using
laminations to reduce circulating currents in the core of a power
transformer, except in this case the idea is to use a larger spacing
between turns to reduce capacitance and parisitic circulating currents
in the windings. This is probably why the Q of a self resonant
inductor is lower than the Q of the same inductor at a lower
frequency. Too many extraneous currents generating heat in the
windings.
-Bill
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Author: john jardineDate: 17:38 01-05-07
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"Bill Bowden" <wrongaddress@att.net> wrote in message
news:1177821718.856885.82580@n76g2000hsh.googlegroups.com...
> Does anyone know why the distributed winding capacitance of a loop
> 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
>
I've never noticed it.
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.
--
Posted via a free Usenet account from http://www.teranews.com
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