[TowerTalk] Balun Recommendation

[Jim Brown]

On 4/19/2012 9:25 AM, Steve Hunt wrote:
> I was contacted privately on this issue by someone whose engineering
> ability I well respect. He confirmed what I have been saying, but
> suggested that the link between choke dissipation and SWR is "best not
> mentioned" for fear some folk would interpret it to mean that SWR
> *causes*  CM current.

I'm very short of time, because I'm entertaining house guests, about to 
take them out to dinner, and leaving in the morning for the Visalia 
DX/Contesting convention.

The thing that is WRONG about Steve's statement is that it is not SWR 
that changes the voltage on the choke, it is the common mode voltage, 
which is the result of imbalance in the system. If there is no 
imbalance, there is no common mode voltage, and thus no dissipation, no 
matter what the SWR. Consider, for example, a direct short at the 
antenna, or an open circuit at the antenna, but no imbalance in the 
SYSTEM.  No common mode voltage on the choke, no stress.

Steve has cited an example where the 50 ohm coax is terminated by 75 
ohms. Yes, there is a mismatch, but the thing that can change the 
voltage across the choke is the change in the LOAD impedance, not the 
fact that it's a mismatch. And that change doesn't matter if the system 
is BALANCED.  Changing to 75 ohm coax and a 75 ohm load would preserve 
the match, but the higher voltage required for the same power level 
would also increase the choke's sensitivity to imbalance.

Bottom line -- dissipation in a common mode choke on a COAX line is 
directly, and primarily related to the IMBALANCE in the SYSTEM, the TX 
power, and the system impedance. A common mode choke on a parallel wire 
line has the additional dissipation due to leakage flux from 
differential current.   In my experience, the latter effect is small at 
(legal) ham power levels.

73, Jim K9YC