jawod wrote:
Please help a newbie. Can I get a good definition of "common mode"?
I've been reading ARRL pubs but can't seem to get my head around this
concept.

First understand differential currents as occur in
transmission lines. The two differential currents
in the two wires are equal in magnitude and 180
degrees out of phase. This is the ideal case for
a transmission line and differential currents
average out to zero, i.e. equal currents flowing
in opposite directions.

Common-mode currents are the currents left over
when the average of the total currents is not zero.
Icm = (I+ + I-)/2

From the IEEE Dictionary: "common mode signal -
The average value of the signals at the positive
and negative inputs of a differential input waveform
recorder. Vcm = (V+ + V-)/2"
--
73, Cecil http://www.w5dxp.com