rickman wrote:

On 10/4/2015 6:01 PM, Ian Jackson wrote:
In message , rickman writes
On 10/4/2015 3:00 PM, Ian Jackson wrote:
In message , rickman
writes
On 10/4/2015 5:51 AM, Jeff wrote:

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.


If the ATU is adjusted so that the Tx sees a 1:1 match then no
power is
reflected to the transmitter. A 1:1 match means that there is no power
reflected to the Tx by definition (and can be proved by measurement).

Fine, but not related to my question if that is what you were
responding to.


By conservation of energy then all of the power must be radiated or
lost
as heat, (mostly in the coax).

Uh, I'm not sure of that since there are other paths and I'm pretty
sure power can be sent back into the driver of the transmitter.


When the ATU gives a conjugate match, (ie the Tx sees a 1:1 vswr) the
phase of the re-reflected wave at the antenna is 0, ie it is in-phase
with the original forward wave and so any power that is not
re-re-reflected again adds to the power supplied to the antenna.

You seem to be mixing two things here. We have been talking about an
ATU at the transmitter. The feed line cable is between the ATU and
the antenna. You then talk about reflections both at the TX to ATU
interface and the cable to antenna interface. You also make
assumptions about the reflected power phase when it returns to the
antenna. If you want to talk about the phase at that point you have
to take into account the electrical length of the cable and *three*
reflections.


If the coax were lossless, and there were no losses in-the ATU,
then ALL
of the power that was supplied by the tx would be radiated
regardless of
the mismatch at coax to antenna interface.
However, in reality even small coax losses add up to a significant
loss
when the mismatch at the coax to antenna interface is high due to the
number of times that the power bounces up & down the coax suffering
loss
on each trip..

One poster has tried to say the reflections are all such that the
phase of the reflected power ends up in phase with the original signal
delivered to the antenna. But no one has done a power phase analysis.
You seem to be saying the same thing.

Power is reflected from the complex impedance of the antenna. Is that
power not out of phase from the incident signal by some amount
determined by the complex impedance? On traveling down the cable
there is a phase shift by an amount related to the ratio of the
electrical length and the wavelength. The ATU reflects some of the
signal back up the cable and some is passed directly to the TX (since
there is no or a very short cable between them) and is partially
reflected by the TX, again with some phase shift determined by the two
impedances (TX output and the ATU input). Another phase shift by the
cable and the reflected wave arrives at the antenna.

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

I don't think you need to think about phase. The forward-going signal
has no need to know about the reflected signal, so just think of them as
being independent. One simply passes through the other.

When they are reaching the antenna you damn well *do* need to know
about phase. If the incident wave and the reflected wave returning to
the antenna are 180 degrees out of phase the power subtracts!


I understand Jeff correctly, if the ATU input presents a perfect 1-to-1
SWR to the transmitter output, it follows that none of power in the
multiple reflections in the coax is getting back into the TX.

I think you do *not* understand Jeff. The TX to ATU match has nothing
to do with the energy reaching the TX, just the opposite, it means all
power from the antenna passing through the ATU is absorbed by the TX.
But I have not seen anyone say the ATU matches the TX impedance, only
that it is a conjugate to the antenna.

Should be not be a conjugate match to the impedance as seen looking into
the coax at the TX end?

There are three interfaces, TX/ATU, ATU/cable, cable/antenna. Each one
can have reflections and each of the four parts of the system can
absorb (or radiate in the case of the antenna) power. This is not so
easy to analyze fully. At least until some of the fundamentals are
revealed that we have not talked about.


Therefore,
if we ignore the losses in the ATU, all the TX output power must
eventually, after countless ever-diminishing reflections between the
antenna and the ATU output, be either reaching the antenna, or being
lost in heating up the coax.

A given reflection may "reach" the antenna, but will it contribute
positively to the radiated field or counter it?

I think we are looking at the scenario in two different ways. You are
trying to see a 'snapshot' of the vectorial additions of all the forward
and reverse signals, and I am simply looking at them as being
independent entities, rattling up and down the coax, and progressively
either escaping via the antenna feedpoint and being radiated, or heating
up the coax.

You seem to want to ignore the possibility that the reflected waves can
interfere with your incident wave. This is entirely possible. Even if
reflected energy is radiated it can cancel the incident wave and reduce
your signal strength.

Conservation of energy suggests that it can only reduce it in one
direction by increasing it in another. If feeder reflections can have
this effect it should be possible to demonstrate it in small model
aerials at high frequencies.


--
Roger Hayter