On Fri, 05 Aug 2005 12:33:44 -0500, Cecil Moore
wrote:
What you say happens at a load is entirely correct. At a
load, there is only one EM wave incident upon the load.
But at an impedance discontinuity in a transmission line
with reflections, there are two EM waves incident upon
the impedance discontinuity, one from each direction.
There's a forward wave coming from the source and a
reflected wave coming from the load.
snip

There's another way of viewing the manner in which energy reflected
from a mismatch load. That is 'motor generator action'. Lest you think
I'm joking, please let me quote from my own writing in QST August
1973, 32 years ago, repeated in Chapter 3 of Reflections 1 and 2:

"... Now we'll proceed to the generation of reflections. When the
electromagnetic field reaches the end of the line, if the load
terminating the line is an open circuit, the magnetic field collapses
because the current goes to zero due to the infinite impedance of the
open-circuit. The changing magnetic field at the open circuit
produces a new electric field equal in energy to the magnetic field,
which induces a new voltage into the load circuit that is equal to,
and in phase with the voltage in the forward wave. (Keep in mind that
a voltage is induced, or generated, by mutual motion between a
magnetic field and a conductor, a phenomenon generally known as
motor-generator action. Thus, it can be said that the reflected
voltage was developed and delivered by a generator, a reflection
generator. Although in this case the field is changing while the
conductor is stationary, as in a transformer, it is motor-generator
action nonetheless.) The new electric field induced by the changing
magnetic field adds in phase to the existing electric field, and the
new induced voltage (delivered by the reflection generator) adds in
phase to the voltage in the forward wave, resulting in an increase of
voltage at the open circuit to twice the voltage of the forward wave.
At this instant, a standing wave is developing, because now there is a
current minimum and a voltage maximum at the open-circuit termination,
where an instant before, current and voltage were constant all along
the line.
The new voltage at the open-circuit termination, along with its
new electric field, starts a voltage wave traveling in the rearward
direction, as if it had been launched by a separate generator at the
open-circuit point. (It has---remember the induced voltage, generated
by the changing magnetic field?) Since no energy was absorbed by the
open-circuit load, the new rearward-traveling voltage wave has the
same magnitude as the original forward wave, which is why rho = 1,
indicating total reflection. As the new electric field starts its
rearward travel, it produces a new magnetic field, which in turn
produces a new current, launched into the line as the reflected
current wave with the same magnitude as the forward current wave, but
with opposite polarity and direction. The new electric and magnetic
fields combine to form the reflected electromagnetic field and, as in
the forward electromagnetic-field wave, the energy in the reflected
electromagnetic-field wave also divides equally between its electric
and magnetic fields."

Walt, W2DU