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
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
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