On my web site I explain how a predominantly reactive common-mode choke
has the potential to increase CM current if the reactance of the CM path
is of similar magnitude and opposite sign to the choke reactance. It's a
reason I generally recommend resistive chokes, and avoid materials like
Fair-Rite's #61; however, I'm now wondering if that is an
overly-cautious approach.
I've spent some time over the past couple of days looking at the CM
complex impedance presented at the feedpoint of a dipole by varying
lengths of coax feedline. With the coax well-grounded at the "shack"
end, the CM impedance varies cyclically as we would expect, being low
when the path length is a multiple of a half-wave and high when an odd
multiple of a quarter wave. For coax lengths greater than about 0.28
wavelengths the highest reactance I saw was 1800 Ohms - when just
shorter than, and just longer than, odd multiples of a quarter-wave;
importantly the CM resistive component was also moderately high for
those lengths.
Now consider the impedance of a typical "reactive" choke that we might
insert at the feedpoint. Taking the figures in K9YC's tutorial for 14
turns on a FT240-61 toroid, the reactance is above 5000 Ohms over a
useful 2:1 frequency range from 10MHz to 20MHz. Because (for coax
lengths greater than 0.28 wavelengths) the CM path reactance never
exceeds 1800 Ohms, the "net impedance" of the CM path can never fall
below 3200 Ohms, even for the worst-case coax lengths.
The significance, of course, is that the "reactive" choke can cope with
much higher power levels. For example, at 10MHz K9YC's quoted impedance
for that choke is 200+j5000; that means, when flowing the same current,
it would dissipate just 7% of the power of a 3000 Ohm resistive choke
which delivered the same minimum "net" CM impedance.
My modelling for the CM path used lengths of #14 wire from 0.25
wavelengths up to 0.78 wavelengths; longer lengths, fatter cable more
representative of coax, or a lossy ground connection at the shack end,
all produce *lower* CM path reactances. The CM path resistance,
reactance, and net path impedance with a 200+j5000 choke in place are
shown here:
http://www.karinya.net/g3txq/temp/zcm.png
Taking only a choke's resistive component and discounting its reactive
component is certainly a sensible approach when calculating the
effectiveness of chokes in series, but it seems to me it may be an
overly-cautious approach for a single choke at the feedpoint. If I'm
right it would make "high-Q" ferrite materials, like #61, a more
attractive choice than I had thought - certainly for single-band
applications.
Steve G3TXQ
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