On Jan 2, 7:47*pm, "AI4QJ" wrote:
"Roy Lewallen" wrote in message

...

Jim Kelley wrote:

I have a piece of coax around here somewhere that I once burned up. *I
recall telling you about it. *The insulation is bubbled and melted at
half wavelength intervals. *Please explain what particular aspect of a
traveling wave might have caused that to happen.

It's the consequence of having *two* traveling waves, which occurs any
time the line isn't terminated with its characteristic impedance. Betcha
yours wasn't.

As you have stated before, the standing wave is the ENVELOPE of maxima
resulting from forward and reflected waves. The ENVELOPE maxima and minima
will occur at 1/2WL distances in the cable. If the line is lossless, no
power is consumed in the unterminated line.

If *I were to see a cable with melted insultion at 1/2 wavelengths and the
coax was open, the first thing I would suspect is that the voltage on the
inner conductor greatly exceed the dilectric strength of the insulation
between inner conductor and shield at the original standing wave maxima and
minima. Two 3-500Z's in series could produce 6KV or even more. Standard
Radio Shack grade RG58 might not be able to handle that. When the insulation
breaks down it turns into a carbon resistor and we no longer have a lossless
situation; real power is dissipated. Whther or ot the standing wave is still
present, power will dissipate where the dielctric material has turned into
carbon (even with DC). Less likely would be current heating the inner
conductor, again a resistive power loss. But these discussions have been
assuming a lossless line of infinite dielectric strength where real power
disspiation cannot occur unless we terminate the line and only then, in the
terminating component since the line is supposedly 'lossless'. Adding the
real life burnt coax example when the discussion assumes ideal components is
comparimg apples and oranges; all we are doing is exposing the weakness of
the assumption when using ideal lossless components. BTW, in any case in
which insulation melting occurs and we have deviated from the ideal
transmission line, at least some of the reactive power that was contained in
the standing wave has now been changed into a travelling wave with I/V in
phase thru the 'resistors' between inner conductor and shield.

This is the discussion I was hoping Cecil would take up. I can assure
you that the coax I was using was not lossless; RG58 is quite real in
that respect. The source was probably good for a kilowatt at 2.4 GHz,
though it's likely only a small fraction of that was actually coupled
from the cavity by Langmuir probe into a 50 ohm termination of some
sort. (This was 20 years ago, so some of the details have faded
now.) The cable got hotter than a firecracker in a very short period
of time over centimeter sized regions spaced some number of
centimeters apart. The coax appeared to have melted due to excessive
I^2R heating by the conductors. Skin effect apparently played no
small role. I believe the voltage in question is not the voltage
across the dielectric, but rather along the conductor. At my
suggestion the experiment was not repeated. :-)

73, ac6xg