Re: Distributed capacitance effects Q?

On 29 abr, 06:32, Bill Bowden <wrongaddr...@xxxxxxx> wrote:
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,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

Hello Bill,

I assume that you mean radiation efficiency (ratio between actual
radiated power and total electrical input power).

I think inter-winding capacitance does not decrease efficiency, it may
only change the radiation pattern when the inter-winding capacitance
is that much, that the current distribution in the coil is affected.
This is almost the case with relative large loops.

When you have a loop close to a halve wave, just the own capacitance
is sufficient to get resonance (as with, for example, a halve wave
dipole).

Radiation efficiency may be reduced by losses in the insulation. When
windings are close together, the Electric Field strength in the
insulation can be that high, that loss becomes significant. This is
mostly the case when windings are touching.

Another thing can be corona discharge (that may in the end destroy
your insulation).

Best regards,

Wim
PA3DJS



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