On Tue, 28 Jun 2005 19:36:53 GMT, james wrote
in :

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.


You can say -that- again.....


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.


The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.


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.


One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.


Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


Hogwash. Harmonics don't just appear because of common-mode currents.
They must come from a source -- i.e, the transmitter. And conductors
of common-mode currents don't have any magical properties that let
them conduct or radiate harmonics any better than the fundamental
frequency. That's RF voodoo.







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