wrote:
In the shorted or open transmission line (from Magid, above), the
analogue is two clumps of charge rushing towards each other and meeting
at a voltage maximum (current zero). No charge crosses this point
(obvious because the current is zero), but the charge coming from each
direction builds to a voltage maximum and bounces away again (since
like charge repels).

Unfortunately, the analogy is not a good one. In a transmission line,
there must exist a discontinuity to cause a reversal of momentum of
the waves. No such discontinuity exists so there is nothing to reverse
the momentum of the forward and reflected waves.

Not possible since NO energy crosses the zero voltage and zero current
points (unless you want to reject Pinst = Vist x Iinst).

Ramo, Whinnery, and Van Duzer disagree. They say that the power
reflection coefficient is equal to the reflected Poynting vector
divided by the forward Poynting vector which in this case would
be unity.

The difficulty I encountered, while trying to understand, is that
simultaneously holding the views that:
1) ghosting is caused by reflected energy flowing back along the line;
and 2) Pinst = Vinst x Iinst required too much double think.

Life is tough all over. :-) What I am saying doesn't require double
think. :-)

Although it seems unrelated, it is worthwhile to consider how to send
information along a line without sending energy in the same direction.

Information transfer doesn't require energy? Methinks you are confusing
the carriers of the wave with the waves themselves. Zero carriers may
indeed cross the zero power point, but that does not prevent energy
from crossing the boundary. It just means that the energy crossing the
boundary must be equal in both directions.

One point. The power is not Vavg * Iavg. The power is Vavg*Iave*cos(theta)
All up and down a shorted lossless transmission line, theta is equal to
90 degrees. Around a voltage zero point, on one side the voltage is equal
to 0.00000001 volts and is 90 degrees out of phase with the current, i.e.
zero power. On the other side of the voltage zero point, the voltage is
equal to -0.00000001 volts and is 90 degrees out of phase with the current.
There are an infinite number of points where the voltage is 90 degrees out
of phase with the current. That's why the average power is always zero, not
because the average voltage is zero, but because the voltage is *ALWAYS* 90
degrees out of phase with the current in a lossless shorted feedline.

When the source is not matched, it takes several round trips for the
line to settle and ghosting is what you see.

Put an amplifier between the TV set and the feedline. Let the amplifier
have as high an impedance as possible (insulated gate FET). That will
cause reflections. Those reflections will still be there during steady-
state and observing the ghosting will prove that, during steady-state,
the ghosts have made one or more round trips back to the source where
they are re-reflected (assuming a mismatched source or a Z0-match
provided by a network).
--
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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