On 20/04/2012 03:12, Jim Brown wrote:
> 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 the system is perfectly balanced there is no common-mode voltage and
you don't need a choke. Once the system is unbalanced, the common-mode
voltage depends directly on both the degree of imbalance _and the
differential mode load impedance_
> 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.
It's an interesting argument that changing differential-mode load
impedance isn't necessarily accompanied by a change in SWR because you
could change the feedline Zo to keep it the same; but I would guess most
readers _don't_ change their feedline every time the load impedance
changes, and for them changing SWR and changing load impedance are
inextricably linked. In fact when I shift frequency 70kHz on my 80m
dipole and its impedance changes from 50+j0 to 47-j35, I really struggle
to find coax with a Zo of 47-j35 that will keep the SWR at 1:1 :)
For me - and I suspect most readers - changing SWR is a reliable
indicator of changing load impedance; and a choke designer would be wise
to cater for the extra dissipation that an SWR of 2:1 (or whatever the
expected limits are) could imply.
I originally challenged this statement from Jim: "But SWR has NOTHING to
do with dissipation in a common mode choke." I still maintain that
statement is wrong.
Steve G3TXQ
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