On Wed, 29 Jun 2005 20:53:42 GMT, james wrote
in :

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

****

You may be right there. Don't have all my library restored here at my
new place and having to remember.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.
********

1) The impeadance of the coax never changes unless its dielectrtic, or
ratio of outer diameter to inner diamter changes.

2) the reflection coefficient of the load does not change unless there
is a physical change in the load.

3) currents on the shield from other sources will not change the
impedance of the load reflected towards the source.

4) currents on the shield may alter what the source sees as a load
impedance. It may well be possible that the source could see the
antenna and some other load in paralell and then vectorally add the
two impedances.


And #4 is exactly why #1 is incorrect: the 'characteristic' impedance
of a coax is constant, but it's 'input' impedance varies according to
load mismatch at the other end. If it wasn't for this fact, a tuner at
the radio end would be useless. But the point here is that if the SWR
meter is left floating with the coax shield (both of which should be
RF grounded) then the measurement can be darn near anything.






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