Re: Distributed capacitance effects Q?

Bill Bowden <wrongaddress@xxxxxxx> wrote in news:1177821138.653191.285430
@u30g2000hsc.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,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?

Bill,

Some thoughts about inductor loss and self capacitance:

Consider and ideal coil (ie lossless, no distributed capacitance) in
series with a small ideal resistor to represent its loss, the combination
having high Q. Connect it to a constant voltage source at some frequency
and observe that the current lags the voltage by almost 90 deg.

Now shunt that combination coil+resistor with a small lossless capacitor,
and note that the current in the capacitor will be small in magnitude,
and leading the applied voltage by 90 degrees.

The effect of the capacitor is to reduce the total current, and not
change its phase slightly. So the combination of coil & series
resistance, & shunt capacitance draws less current and at slightly lower
(lagging) phase, so it appears to be a smaller but lossier inductor.

The discussion above is about conditions below the self resonance of the
total combination.

Now, real inductors might be represented by a simple circuit as dealt
with above, but it is an approximation only. A better representation of
real inductors is more complex and highly dependent on the frequency,
geometry and materials.

An example of the influence of these factors is that a ferrite cored
inductor usually needs less turns (and less capacitance) than an air
cored inductor of the same inductance; a bifilar split transformer
winding on a toroid increases the self capacitance compared to a normal
winding, albeit with higher flux leakage; close spaced windings reduce
the number of turns needed, and resistance due to decreased wire length
however proximity effect increases the resistance per turn. Design is
about finding an optimal solution to these effects for the intended
usage.

Distributed capacitance is not of itself necessarily lossless, the
materials in which the electric field alternates might not be ideal
dielectrics, and so a further loss is contributed by dielectric losses.

Operation of coils approaching their self resonance increases the loss
due to this effect.

Owen
.

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