Ian White GM3SEK wrote:
Cecil Moore wrote:
"... when two waves of equal amplitude and wavelength that
are 180-degrees ... out of phase with each other meet, they
are not actually annihilated, ... All of the photon energy
present in these waves must somehow be recovered or redistributed
in a new direction, according to the law of energy conservation ...
Instead, upon meeting, the photons are redistributed to regions
that permit constructive interference, so the effect should be
considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."

The killer is that word "somehow"... "all of the photon energy must
somehow be redistributed".

That's not a killer, Ian, that's a challenge to people
like me to figure out how. If there is indeed a "somehow",
then there has to be a "how". Please don't try to dampen
my curiosity like the church priests tried to dampen Galileo's
curiosity.

Well of course it must! Nobody denies that conservation of energy will
hold, in a system with properly defined boundaries. But the weakness of
a photon model is that it cannot provide a detailed nuts-and-bolts
explanation of the mechanism by which that energy becomes redistributed
in time and space.

I'm sure a QED explanation exists but we might have trouble
understanding it. I would like for you and others to follow
me through an energy analysis to see if you can find anything
technically wrong with it besides your revulsion to the approach.

A wave model will provide all of that detail - and in transmission-line
problems we can use it. If we trace what happens to forward and
reflected waves of voltage (and/or current) we can predict the
magnitudes and phases of those quantities at any location, at any
instant. That gives us a complete time-dependent map of the voltage and
current across the entire system.

From that, we can also find out where the energy is - the inputs,
outputs, losses and stored energy. Sure enough, we will find that energy
is conserved within the system boundaries... but that is no big deal, we
always knew it would. In a wave model, conservation of energy is
something you should check for, but only as an overall confirmation that
you've done the sums correctly. All the useful detail came from the
analysis of the voltage and/or current waves.

I agree with everything except your last sentence.
There is lots of useful information to be had from
tracking the energy through the system including
how and why the energy in the reflected wave changes
direction and momentum. If you think that information
doesn't matter or is not useful, then that's your
opinion. But please don't condemn the individuals who
find that information useful and go for an explanation.
And please don't say that explanation is wrong if you
cannot prove it to be invalid.

In the process of tracing forward and reflected waves,
we must remember that they obey the laws of physics
including their energy contents. The average forward energy
per unit time in a forward voltage of Vf RMS volts is
Vf^2/Z0 joules/sec, an assumption upon which the S-Parameter
analysis system is based. The average reflected energy per
unit time in a reflected RMS voltage is Vr^2/Z0 joules/sec.

In an S-Parameter analysis, if you square any of the
normalized voltage terms, you get joules/sec. Someone
said that at microwave frequencies, the powers are
often easier to measure than the voltages and currents.
The powers can be measured and the voltages and currents
calculated from the power measurements.

In optics, physicists don't have the luxury of dealing
with voltages and currents. They must necessarily deal
with energy and power. That field of physics is older
(and wiser) than RF engineering and they deal with power
reflection coefficients, not voltage reflection coefficients.
--
73, Cecil http://www.w5dxp.com