Walter Maxwell wrote in
:

....
It appears to me that even with all the successive posts on the
subject of power in the standing wave, you all seem to be missing the
ingredient that proves why there is no useable power in the standing
wave. It is because the current and voltage in the standing wave are
90° out of phase. Multiplying E x I under this condition results in
zero power.

Walt,

I am trying to make sense of this and the first issue is what you mean by
the term "standing wave".

The only meaning that seems possible is that it is the magnitude of the
time alternating voltage or current at some displacement along the
transmission line.

If that is the meaning, then the situation you describe of 90° phase
difference between E and I is rather specific, it can only occur with a
distortionless line AND a load that is (s/c OR o/c OR purely reactive).

Is that the case?

If so, should you have stated the assumptions and how does the case you
discuss help in explanation of general principles?


In addition to another comment above that implies that reflected power
is reactive power, this is not true--reflected power is as real as
forward power. The only differences are that they are traversing in
opposite directions, and that while the voltage and current travel in
phase in the forward direction, they are traveling 180° out of phase
in the rearward direction. Multiplying voltage and current while 180°
different in phase results in the same power as when they are in
phase.

You seem to be inferring that it is legitimate (in a general sense) to
calculate the power of forward and reflected waves as voltage times
current (eg Vf*If).

Isn't the instantaneous power at a point a function of time, and it is p
(t)=v(t)*i(t) and the expansion of that equals Vf*If-Vr*Ir ONLY when the
other two terms of the expansion cancel, and that is the special case of
a distortionless line.

Are you illustrating general principles with a special case without
stating the underlying assumptions.

Why is it that so many attempts to explain transmission line behaviour,
particularly regarding real and imaginary components of power at a point,
aren't consistent with basic AC circuit theory?

Owen