Dear LF Group,
Re: Loops and stray capacitance measurements.
All these methods will give you a rough idea of the stray
capacitance; but results will be somewhat variable, since they
assume that the loop can be modelled as an inductor with some
stray capacitance in paralell. An actual loop antenna, due to it's
construction, has distributed capacitance and inductance, so a
more realistic model would be a network of transmission lines. I
think models have been devised for single turn loops, but I am not
aware of any for a multi turn loop. My maths isn't up to it, i'm afraid!
In practice, this means the experimental value of stray C varies
depending on what the measuring frequency is - the effect gets
worse the closer you get to the self resonant frequency, and the
nearer the length of wire in the winding approaches the
wavelength. I seem to remember the apparent stray capacitance
goes down as the frequency goes up.
What this means to loop-makers is you have to take the
measurements with a large pinch of salt. Dont be suprised if the
resuts are inconsistent. In general, higher Q and therefore better
sensitivity occur with low stray capacitance and thick wire.
Cheers, Jim Moritz
73 de M0BMU
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