On Fri, 08 Oct 2010 11:14:13 -0400, John Ferrell
wrote:

how might measure the effects of the common mode

Take one piece of wire;
loop it into a half-dozen loose turns (1" diam);
connect a Red LED to the wire ends;
place loop around coax;
slide it along the coax for a distance of something more than a
quarterwave length of the frequency under consideration.

How bright did the LED get anywhere along that length?

73's
Richard Clark, KB7QHC