I'm writing to ask a theory question about an aspect of RFI management
that I have always been unclear on.
Consider a device connected to L and N from a 120V outlet, and having a
0/+12 cable to some other small device. I understand that common mode
current on either the DC cable or the AC wiring will radiate, and that
choking impedance is the right answer.
Thinking about building a transmitter, it seems tricky for common-mode
current to be efficient if it isn't sort of balanced on the two wires.
Otherwise, it's sort of an end-fed antenna. Therefore, having an
effective choke on the DC cable, even with no choking impedance on the
AC side, should greatly reduce the current on the AC wiring. This is
pretty much all you can do if the device has a plug and not a power
cable, unless you use a choked extension cord.
My questions
- Is the above analysis iscorrect or confused?
- If confused, does choking the DC cable side mostly address the RFI
issue, by some other mechanism?
- What is the equivalent circuit that explains generation of
common-mode current?
A related question is that given a physically small device that plugs
in, how are these able to generate common-mode current, given the lack
of a counterpoise for an endfed (which is of course not quite endfed,
but very off center)?
73 de n1dam
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