The chances of a Reactive choke decreasing the feedline CM path
impedance may be much higher than many folk might think, particularly on
a multiband antenna. Trying to assess the CM path impedance on various
bands and constructing a Reactive choke which "tunes" the impedance to
create "multiple quarter waves" on all of those bands is well beyond the
capability of most Hams, if not impossible altogether. The safe option
for them is a choke with high Resistive impedance.
I just added a section to the CM chokes page of my web site which
illustrates the point:
http://www.karinya.net/g3txq/chokes/index.html#inductive_chokes
Notice in the example given, where the coax is short of a
half-wavelength and the CM path impedance is 28-j200, adding any choke
with a reactance between 0 and +j400 will *increase* the common-mode
current. Now imagine trying to construct a choke which avoids the
"problematic" reactance ranges on every band in a multiband situation.
73,
Steve G3TXQ
Dr. Gerald N. Johnson wrote:
One has to take care that the added reactive impedance doesn't become
series resonant with the available wire length. You wish to shift to a
wire plus reactance that makes an odd multiple of a quarter wave to
minimize the current in that wire. If you slip up and the combination
makes any multiple of a half wave, you have maximized the current in
that wire.
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