On 9/28/2015 2:40 AM, Jeff wrote:

I agree that we are not all on the same page.

Then some of us much prefer to argue rather than discuss. Is there
anything about Waynes post you like? Are the facts more clear now at
least?


The situation is that with the ATU at the Tx end there are 2 mismatches
to consider.

1, the mismatch at the ATU output ( which hopefully has been adjusted by
the ATU to be small to keep the Tx happy.)

2. the mismatch between the antenna and the characteristic impedance of
the feeder.

Mismatch 2 causes the part of the signal to be reflected back down the
feeder towards the ATU which then re-reflects it back to the antenna,
which of course then re-reflects part of it back towards the ATU and so
on ad infinitum.

The ad infinitum can be expressed with a simple ratio. The reflections
constitute an infinite series that approaches a limit easily calculable.
Regardless, any incident energy reaching the antenna will be reflected
in the ratio determined by the impedance mismatch and measured by the
VSWR or the return loss ratio. It doesn't matter where the energy comes
from so talking bout infinite reflections serves no point when
discussing return loss ratio.


Of course each time the signal passes along the feeder it suffers the
loss due to the feeder.

As the amount of signal that is reflected at the antenna depends on the
SWR (or return loss) it is obvious that as the mismatch between the
feeder and antenna increases more of the power is subjected to multiple
trips along the feeder and so encounters more loss.

If you had lossless coax there would not be a problem, but even quite
low loss in the coax will cause significant loss when the mismatch
between the feeder and antenna gets a little high.

So getting the feeder impedance closer to that of the antenna can help.

Why can't the ATU also match the impedance of the cable eliminating
reflections there?

--

Rick