On 10/5/2015 9:36 AM, Ian Jackson wrote:
In message , Jeff writes

NO, as above, if the ATU is adjusted for a 1:1 match then there is no
power reflected back to the TX.

You are saying the ATU doesn't reflect power back from the TX. If the
match is 1:1 that is true. But you aren't considering the power
reflected from the antenna. The antenna reflects power back to the ATU
and there is nothing in the ATU to prevent that power from being handed
to the transmitter.

The whole function of the ATU is to provide a 1:1 match for the Tx,
when that is the case NO POWER GOES BACK TO THE TX.

Yes the antenna reflects power back towards the ATU, BUT the ATU then
reflects ALL of that power back again towards the antenna and none
into the TX (assuming that the ATU achieved a conjugate match.)


When adjusted for a 1:1 match there are no reflections between the Tx
and ATU, but there are multiple reflections between the ATU and teh
antenna. That is where a lot of power is dissipated in the antenna is a
poor match to the feeder impedance.

No one is talking about the power from the TX being reflected back to
the TX by the ATU.



The reflected wave will start off at the antenna out of phase with the
forward wave, (the actual phase depends on the complex impedance of the
mismatch), what the conjugate match that the ATU provides, when
adjusted
so that there is a 1:1 match and no power reflected power sent to the
Tx,

This doesn't cover the reflected power from the antenna, just the power
from the TX.

NO. It covers the reflected power from the antenna. There is no
reflected power between the ATU and Tx if the match is 1:1.



is a phase shift such that the re-reflected wave from the ATU
towards the antenna is in phase with the original forward wave, so when
it reaches the antenna the portion of that re-reflected wave that is
not
bounced back again down the coax by the mismatch is delivered to the
antenna.

How does that work? There is phase shift in the reflections which may
be compensated for by the ATU, but there is also phase shift in the
cable.

The ATU applies a conjugate match at the end of the cable, the
impedance that it sees, and applies the conjugate of, is the impedance
of the antenna modified by the length of cable.

It just so happens that the conditions for a 1:1 vswr, and that of a
conjugate match, are that match causes the phase at the antenna end of
the cable to be the same phase as the original forward wave. That is
the physics of a conjugate match.


NO, no power gets back to the tx if the ATU is adjusted for a 1:1
match.
after all that is the definition of a 1:1 match; no reflected power.

You keep saying that the 1:1 match between the TX and the ATU prevents
any power from being sent to the TX which is not true. You are
confusing the power from the TX which is not reflected and the power
reflected from the antenna which passes through the ATU to the TX.


YES IT IS TRUE. I am not confusing anything.


Is anyone going to tell me that all this has to add up to a reflected
signal arriving *in phase* with the incident signal?


YES.

I may have to look at the math for this. How does the ATU reflect all
the power from the antenna back to the antenna? I thought only an open
or a short can reflect all the power.


Please look it up, do some experiments yourself. Try some simulations
in Spice or similar. You will find that I am correct.

While I've being saying "Let's ignore the losses in the ATU", presumably
you can assign a loss to it, and the reflected signal will suffer this
loss each time it bounces off the ATU output. If so, the loss can be
treated in the same way as the loss in the coax. In fact as 15' of
decent coax will only have (say) 0.5dB matched loss on 14MHz, the
summation of the 'return and go' ATU losses (say 2dB each time?) could
be more significant than the 1dB each time the signal traverses the coax.

I don't mind treating the cable and ATU as "ideal". I'd like to
understand how you can turn one knob and get a match to the transmitter
*and* a perfect reflection *with* phase matching to the cable. It seems
to me there are three variables you need to tune for and if such a
circuit is possible will require more than one knob.

--

Rick