[TowerTalk] Balun Recommendation

[Jim Brown]

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 
re-analyze it.

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 
that failure.

73, Jim K9YC