On 4/9/2016 12:29 AM, Jim Brown wrote:
Dissipation in a choke is the SUM of the differential mode power and the
common mode power. N6BV wrote an excellent piece that ran in QST a
couple years ago on the issue of differential mode dissipation in chokes
with severely mismatched antennas (he predicted disaster), and ran it
past me first. I told him that his analysis of differential mode was
right on, but that he had failed to consider common mode, which makes
matters even worse!
Note that differential mode dissipation is strictly due to
conductor losses in the windings, and doesn't depend on the core.
It's mainly an issue because the windings concentrate the
heat in a small area. Also, it's not just the mismatch that
matters, but whether the choke is at a high current or low
current location, assuming low loss dielectric is used.
For example, a balun used to feed a 1 wavelength long dipole
will have low differential mode dissipation because the
dipole impedance is high (low current). Of course it will
have high core loss.
Rick N6RK
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