rickman wrote:

On 10/4/2015 4:48 AM, Jeff wrote:

What is the reflection coefficient at the ATU/feedline interface?


Assuming a 'perfect' ATU and a prefect conjugate match, giving 1:1 at
the TX, then it is 1. ie all of the reflected power that reaches the ATU
is re-reflected back up towards the antenna.

Is that good? If the cable length is 1/4 wavelength (as it is in the
info provided for this case) the reflected power is nearly 180 degrees
out of phase with the initial power at the antenna. I think reducing
this through cable losses would not be so bad, or better to dump it in
the ATU?

It is good in as much as some of the re-reflected power is radiated (and
some re-re-reflected) since the ATU causes the phase of the re-reflected
wave to be 'in-phase' at the antenna.

It is bad in as much as the reflected power suffers 2 times the cable
loss, and dissipates that in heat, on each return trip, up and down the
coax.

With a high VSWR at the antenna there will be many return trips before
the re-reflected power drops to a negligible level.

No power is 'dumped' in the ATU; although there will be losses, but that
is another story.

If not the ATU, then the transmitter. I'm sure not all of the power is
reflected back from the ATU. Exactly what is the phase of the reflected
power from the ATU? I haven't seen an actual circuit for the ATU in
question. For that matter, what is the phase of the power reflected
from the antenna? I'm pretty confident we are not looking at the return
of the reflected wave in phase with the incident wave.

Having specified the transmitter power output, by definiton, no power
can be "lost" in the amplifier, because transmitter power output is
*defined* as the net power it actually manages to get out of its output
socket net of any reflections. The mismatch may make it harder for the
transmitter to achieve that; but, by definition, whatever it does
achieve is its power output. You can't draw a valid defiinition of a
difference between power it never produced at all and power sent back to
it.

--
Roger Hayter