On 4/21/2012 4:26 PM, Gary Schafer wrote:
> Yes Ian, I agree with you that "common mode current" on a coax outer
> shield is a complete misnomer. It doesn't describe what it is
> "common to". If it were really common mode current it would have the
> same current on the inner conductors as well as on the outer
> conductor and be in phase.
While common mode current may be a bit of a misnomer, it is accurate
in that the effect of "outer shield current" is identical to that of
common mode current in a balanced system because of the "pin 1 problem"
that K9YC has written so extensively about. Since the signal return
in most amateur equipment is not tied to a complete and effective
shielded enclosure, the outer shield current is *indistinguishable*
from a true common mode current as it passes through the unbalanced
input/output circuits of modern equipment.
Again, if this "outer shield current" is sufficient it can appear in
in microphone inputs, be fed back into audio amplifiers or even be
impressed across control and power supply circuits. The results can
be anything from mildly annoying to fatal to the equipment depending
on the level of the common mode signal and the circuit involved.
73,
... Joe, W4TV
On 4/21/2012 4:26 PM, Gary Schafer wrote:
> Yes Ian, I agree with you that "common mode current" on a coax outer shield
> is a complete misnomer. It doesn't describe what it is "common to".
> If it were really common mode current it would have the same current on the
> inner conductors as well as on the outer conductor and be in phase.
>
> A better name for what is commonly called "common mode current" on the coax
> outer shield would probably be "outer shield current".
>
> 73
> Gary K4FMX
>
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