On Tue, 28 Jun 2005 14:36:23 -0400, Scott in Baltimore
wrote:

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)
*****

And I have the biggest laugh because most CBers as well as Hams have a
peanuts view of what a transmission line is or how signals act on and
in them.

First off, while the coax can be inside the field of radiation, the
signal from the transmitter to the antenna travels solely inside the
transmission line. That is between the center conductor and the
shield. The energy transmitted travels in the dielectric and it is the
dielectric that slows the wave down and casue loses. Even the worst
coax, RG-58 has sufficient shield as to not cause leakage through the
shield at 27 MHz. Maybe a 10 GHz. but not 27 MHz.

Common mode currents occur on the shield and are just that currents.
They can come from poor ground connection at the antenna feed point or
can be induced currents due to the coax being within the fear feild
energy of the antenna. Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


james