The discussion continues ---
Ok, it's time for some of you to get out your copy
of the Bell Labs tape "Similarities in Wave Behavior",
and watch that machine demonstration very carefully.
If you don't have the tape, or have not seen it at
a club meeting in the last year or so, they are
still available for $15 per copy from Hi-Res
Communications.
Watch what happens when the small motor drives
that transmission line, and the operator detunes the
dash-pot load such that a standing wave pattern
develops on the line. Notice that ALL the vertical
motion which the driving motor sends onto the
feed point of the line, and the vertical displacement
of the piston in the liquid load are still the same!
(Except, of course, when he clamps the end of he
line to show 100% reflection emulating a shorted
or open transmission line).
However, a standing wave of even higher amplitude
(higher than the "matched" traveling wave condition)
displacement develops at points along the line,
with nearly stationary nodes of movement
of vertical displacement along the line as well as points
of unusually high vertical displacement. The line now
has "fixed" points of excess vertical displacement
and "quiet" points of little vertical displacement caused
by the additive/subtractive superposition of the forward
wave and the now mismatched load's reflected wave.
The "stress" or problem is the transmission machines
excess vertical displacement, not a problem at the
"stiff" source, nor the load.
Yes the line now has a standing wave of mechanical
displacement on it, but the desired result from the input
to the output still occurs!!
Standing waves on a transmission line do not prevent
the load from receiving the desired input from a "stiff"
source; that is one which will turn the reflected waves
back to the line such that a standing wave pattern
will develop, rather than the source "absorbing" all
or some of the reflected energy. Very careful measurement
of the vertical displacement of the piston within the load
dashpot of the Bell tape, might show a slight loss of
total displacement, caused by the frictional loss at
the pivot points of the mechanical transmission line;
the analogy to slight resistive loss in our rf lines.
Also, the observation that the build up of excess
vertical displacement caused by the forward and
reflected wave superposition's reaches some fixed
limit of displacement, demonstrates that the reflected
energy is not stored forever in the line, but is in fact
re-reflected and consumed in the load, since the load
piston continues to maintain the desired degree of
vertical motion. Were it not consumed in the load,
the vertical displacement build up would continue
until the machine, in this case, would break!
So, all the energy that goes into the transmission
line has to have left the transmission line and gone
into the dash pot. Again the problem of the reflected
energy is one for the transmission line to deal with
because of the excess vertical displacement which
results.
In our RF transmission lines the problem is the excess
build up of voltage on the lines at high vswr. Do you
realize that when your line has a vswr of 2:1 on it,
that you have cut the potential power handling
capacity of your line in half?? Now that is usually
of no concern to us, as most of our coax cables
can easily handle over 2 or 3 kW, so we never
think about that. Also, we seldom operate after
our antenna tuners (where as you know, the vswr
caused by the line/load mismatch remains) with
powers so high that the excess build up of voltage
with high vswr approaches anywhere near our
cable's voltage breakdown rating; and this is
particularly true with twin lead transmission line.
BTW, we provide the "stiff" source to our transmission
lines by the Q or our output tank circuits, if they can
be set to the proper impedance to transfer the rigs
output circuit to whatever the line input impedance is
at the junction of rig output/transmission line input;
or we introduce an "outboard" matching system called
our antenna tuners.
Many more details of these phenomena could have
been demonstrated with the Labs simple but elegant machine,
but, unfortunately could not be included in the 27
to 30 minute total allowed to the Bell tape producers so
long ago. They demonstrate only the most basic ideas,
but isn't that tape packed with information, wow!
Wonder where that machine is today?
73, Jim, KH7M
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