During our current discussion, I've been specifically been talking about
dissipation in a ferrite common mode choke due to COMMON MODE CURRENT.
Chatting at the Visalia DX Convention over the weekend, Dean, N6BV,
reminded me of our discussion some months ago on another email reflector
about the variability of dissipation in a common mode choke due to
DIFFERENTIAL current when there is significant SWR, and thus
considerable variation in the current along the line.
There is virtually NO dissipation due to differential current in a coax
ferrite choke, because the ferrite core does not "see" the differential
field, which is contained within the coax dielectric. When we wind
bifilar turns around a ferrite core, connect them as a parallel wire
transmission line, and insert them in series with any transmission line,
coax or parallel wire, there is SOME dissipation due to differential
mode current, but it is generally relatively small.
BUT -- SWR on the line causes current to vary along the line, and if the
choke is inserted at a point where the DIFFERENTIAL current is large due
to SWR, the dissipation due to the DIFFERENTIAL current WILL be greater
than if the line were matched (no peaks and dips of current). But
again, when there is considerable dissipation in a ferrite common mode
choke, it is nearly always due to the COMMON MODE voltage across the
choke, not the DIFFERENTIAL CURRENT. The exception would be a choke at
a current peak with VERY high VSWR.
Another point -- when we talk about COMMON MODE voltage and current
varying along a feedline, we are talking about the behavior of that line
as an ANTENNA, Vp is approximately 0.99, and we must use NEC or some
other computational model to learn the voltage and current
distribution. When we talk about DIFFERENTIAL MODE voltage and
current, we are talking the behavior of the feedline as an ordinary
TRANSMISSION LINE, so we must use ordinary transmission line analysis to
determine peaks and nulls, Vp will be 0.66 - 0.88, depending on the
line. If this sounds complex, it's because IT IS! And it's why Ian and
Ward have emphasized that adding a choke changes the system, so we must
But it's also why we recommend "brute force" solutions that tend to more
problem-free and have a better chance of success than others, like a
high value of resistive impedance choking.
Chatting with Dean, N6BV, at Visalia over the weekend, he observed that
Steve Hunt had been including differential dissipation in his discussion
of the effect of VSWR on dissipation in a choke. My apologies to Steve
for missing that, and the above discussion is my response to correct for
73, Jim K9YC
TowerTalk mailing list