----- Original Message -----
From: <wng@daimlerchrysler.com>
> A common mode filter passes ALL of the CURRENT-CARRYING conductors through
> the series element, but NOT the non current-carrying conductors (i.e.:
> safety or shield ground). There must be NO SERIES IMPEDANCE in these
> conductors; it defeats their purpose and may create unsafe conditions.
I respectfully disagree with your statement, Bill. When
I was trying to solve the RFI problems with my Honda
EU2000i, I tried using a B&W AC line filter that I had
lying around the shack. It did absolutely nothing to
decrease the common-mode RFI from the generator.
When I took the filter apart it was almost immediately
obvious why the filter did no good. Although the AC hot
and neutral lines each had two large inductors in series,
the AC safety ground passed straight through the filter
via the metal chassis that enclosed the filter. Also each
of the two current carrying legs were bypassed to safety
ground with ceramic bypass capacitors at both ends
(and the center nodes) of the filter. The common-mode
RF impedance of this filter was very low making it
essentially worthless for the problem I was trying to
solve. Only when I added significant RF impedance in
series with all three lines by passing them all through the
same toroid was I able to quell the noise.
I think the last thing you want to do is go around the filter
with the AC safety ground. AC safety ground generally
connects to the exciter/amplifier chassis which in turn
connects directly to the antenna coax shield. Any RF
noise on the AC safety ground can easily end up at the
receiver input if the antenna has poor common-mode
rejection (verticals, for instance, have notoriously poor
common-mode rejection).
In any case, I don't see how passing the AC safety
ground wire through the toroid creates much of a
safety issue. If for instance, N turns of the safety wire
passing through the toroid produces 1000 ohms of
series impedance at 1.8 MHz, this only amounts to
about 33 millohms of series impedance at 60 Hz.
And even so, in a fault situation the AC safety ground
will most likely be returning current from hot phase
wire in the same circuit so the net flux in the toroid
will be zero.
What am I missing?
73 de Mike, W4EF.......................................
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