Jim,
Sorry to disagree again, but the CM choke dissipation DOES also depend
on the SWR!
Say we had a perfectly balanced 50 Ohm antenna being fed 1kW - the
differential voltage at the feedpoint would be 224v rms, or 112v rms
from each leg to "ground". If we feed the antenna with coax, and include
a CM choke at the feedpoint, that 112v leg voltage appears across the
choke and CM path in series and is what is driving the CM current.
Now assume the antenna impedance is 75 Ohms instead of 50 Ohms
(SWR=1.5:1). For the same power, the differential mode voltage at the
feedpoint increases by a factor 1.225 and all other voltages scale by
the same factor, including the CM voltage across the choke.
The 1.5:1 SWR increases the dissipation in the choke by 50% just as
surely as if we'd simply increased the power into the 50 Ohm antenna
from 1kW to 1.5kW.
Even if the antenna isn't balanced, or if you change what is considered
to be the "ground" reference, a change in SWR changes the feedpoint
differential voltage and alters all voltages around the system by the
same factor - including the CM voltage across the choke.
73,
Steve G3TXQ
On 18/04/2012 17:29, Jim Brown wrote:
> But SWR has NOTHING to do with dissipation in a common mode choke.
> What matters is the common mode voltage, which is directly related to
> IMBALANCE in the system, and also to feedline length.
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