Gerald,
To a large extent the desirable high resistive component is inherent in
the choice of core material, but the stray capacitance can also give
useful benefit.
Here's an example: A single turn on an FT240-31 core will have an
impedance of about 20+j17 Ohms at 5MHz - the high resistive component
follows from the complex permeability characteristics. Now increase the
number of turns to 17 and we benefit from the "N-squared effect"; so the
impedance becomes 5780+j4910 Ohms, ignoring stray capacitance. Now add
in stray shunt capacitance of, say, 3pF (a reactance of -j10,600 Ohms at
5MHz) and the impedance is transformed to 9870-j880 Ohms.
Steve G3TXQ
On 20/05/2014 16:30, TexasRF--- via TowerTalk wrote:
Jim, trying to understand the source of the high resistance that is
frequently mentioned for choke design.
I can see one scenario where a series to parallel conversion might explain
it. Say 400 ohms xl in series with 20 ohms R would transform to 8020 R in
parallel with 401 ohms xl. Added shunt C would increase the effective xl up
to the point of resonance.
Is this a possible explanation of the fundamentals?
Thanks/73,
Gerald K5GW
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