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Cecil Moore · August 26th 06, 05:55 PM posted to rec.radio.amateur.antenna

wrote:
When I, and others, write "The
electric field is the superposition of a forward and reverse traveling
wave" maybe it would be better to say "The electric field has two
terms, one that appears to be a forward traveling wave and one that
appears to be a reverse traveling wave." or something like that.
There's one electric field vector and one Poynting vector. Or there
are two. The structure of the electric field and the structure of the
real part of the Poynting vector both admit BOTH explanations of what's
happening.

I know and accept both explanations. The problem is the other side
refuses to acknowledge the validity of the wave reflection model.
If you have gotten the idea that I reject the superposed wave model,
you are mistaken. I fully accept both models. The problem is that
others have rejected the non-superposed component wave model.

I contend that one gets the same results using the components of
superposition, i.e. the forward wave and the reflected wave, that
one obtains after the superposition of those two waves. Others say
that is an invalid treatment because superposition causes the
reflected wave to cease to exist and the energy just "sloshes"
around inside the transmission line. (Never mind that RF energy
must necessarily travel at the speed of light and only reverses
direction at an impedance discontinuity.)

Rather, there's a 100W net forward power flux and THAT will give
you the energy contained in the part of the field that's actually
moving from source to load. The energy contained in the reactive part
has an integral that's going to cyclically vary with the length of the
line, and sometimes goes through zero (kL or kL - phi equal to an
integer multiple of Pi... or any integer multiple of a half wavelength,
which happens to be the length of an impedance repeating line, eh?)

That's one model. The other model is, assuming a purely resistive Z0,
the forward voltage is in phase with the forward current and therefore
there are no reactive vars in the forward wave. The reflected voltage
is in phase with the reflected current and therefore there are no
reactive vars in the reflected wave. This model works just as well as
the one above, sometimes better because of simplicity. It has the
advantage of being easily able to track the real energy because there
is no "unreal" energy in the model. :-)

If the forward wave component is analyzed separately, there are no vars
because the forward voltage is in zero phase with the forward current
(assuming a perfectly resistive Z0). The same is true for the reflected
wave. So we are easily able to calculate how much energy is contained
in those two waves devoid of any calculation of vars.

Assume that we have a one megahertz signal into a transmission line
that is electrically 360 degrees long, near lossless, the forward
power is 200W, and the reflected power is 100W. I am willing to bet
the energy contained in the feedline during steady-state is very
close to 300 microjoules no matter how complicated the math used to
get the answer that I just came up with off the top of my head.

Note that the transmission line is one millionth of a second long
and therefore contains one millionth of the energy of a one
second long line.
--
73, Cecil http://www.qsl.net/w5dxp