Richard Harrison wrote:

Keith wrote:
"It does go back to "double think" unless you can explain how energy can
flow when power is zero."

Think! The only way the voltage can go to zero on a good transmission
line which is energized is by interference between an incident wave and
a reflected wave. Zero volts on the line is merely a manifestation of
VSWR.

No disagreement from me as long as you replace 'incident wave' and
'reflected wave' with the more precise 'incident voltage wave' and
'reflected voltage wave'. This aligns with the precise use of V in
VSWR.

Adopting Roy's more precise notation for power
P(t) = V(t) * I(t) (1)
The question is: Do you accept this expression as describing power?

a) If not, then there is no "double think", but a lot of electrical
engineering will have to be tossed as well.

b) If you do accept (1), then in a case where V(t) is zero for all time,
power must be zero as well. V(t) is zero for all time at a current
maximum in a shorted transmission line, so the power (energy flowing)
must be zero as well.

c) If you accept (1), but also claim that there is energy flowing when
V(t) is zero, then "double think" is an appropriate description.

I think these are the only 3 options.

If you choose b), then I think we are in agreement.
If you choose c), then ... I'm not sure what the 'then' is.
If you choose a), we can explore all the difficulties that will arise
when (1) is not true and with some effort you may arrive at b).

Forward (incident) power and reverse (reflected) power are both on the
line. A zero voltage on the line requires a complete load reflection so
that the reflection volts are as strong as the forward volts.

The sentence with 'volts' is correct. The sentence with 'power' leads
to a great deal of difficulty as described above.

A directional coupler, at the very spot where a slotted line probe would
sense zero volts, would show you have full power (with its volts and
amps) coming and going.

Many people use directional couplers as a reason to stay out of
camp b), but this necessarily means they are in a) or c). The first
step to enlightenment is to briefly set aside directional couplers
and 'Bird watt' meters, and realize that in a choice between a), b)
or c), b) is the only place it makes sense to be. Then go back and
figure out how directional couplers are not inconsistent with b).

In reality there is not zero volts in the incident wave or in the
reflected wave. There`s full voltage coming and going. The volts just
happen to be out-of-phase at this point.

Yes, indeed. But there is no power.

....Keith

wrote:
In reality there is not zero volts in the incident wave or in the
reflected wave. There`s full voltage coming and going. The volts just
happen to be out-of-phase at this point.

Yes, indeed. But there is no power.

Power is the same as irradiance in optics. When total V=0, it is simply
the result of destructive interference. Perhaps this quote from _Optics_,
by Hecht, will enlighten you. "The principle of conservation of energy
makes it clear that if there is constructive interference at one point,
the "extra" energy at that location must have come from elsewhere. There
must therefore be destructive interference somewhere else."

The voltage goes to zero because two voltage waves are engaged in destructive
interference. The current goes to maximum because two current waves are engaged
in constructive interference. The momentum in the voltage waves simply transfers
to the current waves and they just keep on rolling along. There is no mechanism
of physics existing at that point to change the momentum of the waves. Believing
that no energy crosses a superposed V=0 boundary is just a wet dream. The current
is at an absolute maximum point so plenty of charge carriers are crossing that
boundary.
--
73, Cecil http://www.qsl.net/w5dxp
"One thing I have learned in a long life: that all our science, measured
against reality, is primitive and childlike ..." Albert Einstein



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W5DXP wrote:

wrote:
In reality there is not zero volts in the incident wave or in the
reflected wave. There`s full voltage coming and going. The volts just
happen to be out-of-phase at this point.

Yes, indeed. But there is no power.

Power is the same as irradiance in optics. When total V=0, it is simply
the result of destructive interference. Perhaps this quote from _Optics_,
by Hecht, will enlighten you. "The principle of conservation of energy
makes it clear that if there is constructive interference at one point,
the "extra" energy at that location must have come from elsewhere. There
must therefore be destructive interference somewhere else."

My knowledge of optics is insufficient to comment on any analogies you
choose to draw. Fortunately, a knowledge of optics is unnecessary to
understand circuits and transmission lines.

The voltage goes to zero because two voltage waves are engaged in destructive
interference. The current goes to maximum because two current waves are engaged
in constructive interference. The momentum in the voltage waves simply transfers
to the current waves and they just keep on rolling along. There is no mechanism
of physics existing at that point to change the momentum of the waves. Believing
that no energy crosses a superposed V=0 boundary is just a wet dream.

This puts you in
group a) P(t) is not always equal to V(t) * I(t); or
group c) "double think".

Care to think about which and comment?

The current
is at an absolute maximum point so plenty of charge carriers are crossing that
boundary.

Yes indeed, but current by itself is not energy. Remember
P(t) = V(t) * I(t) [unless you choose option a)]
Both volts and amps are simultaneously necessary for power.

....Keith

wrote:
My knowledge of optics is insufficient to comment on any analogies you
choose to draw. Fortunately, a knowledge of optics is unnecessary to
understand ... transmission lines.

Equally unfortunately, that's just a delusion of yours.

Care to think about which and comment?

I have no idea what you are talking about.

Yes indeed, but current by itself is not energy.

Hmmmmm, I^2*Z0 is not power? (Somebody get the net).
--
73, Cecil http://www.qsl.net/w5dxp



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