Tom (Others)
You bring up all good points, and instructive. My intention was to
give a quick answer in a world where no quick answers are adequate. I
had in mind all-air dielectric and same-conductivity inner and outer
conductor materials, for application to *the* lowest attenuation and
*the* highest power handling (be patient for definition) rigid coaxial
line.
When one accepts these startup 'restrictions' it is indeed found that
b/a ~ 3.6 (Zo = 77) yields lowest attenuation and that b/a ~ 1.65 (Zo
= 30) yields highest power handling (be patient), based on the STP 30
kV/cm corona limit and presuming the temperature is stabilized
especially for the inner conductor of the 30 ohm line.
Introduction of solid dielectric in the 77 ohm line increases
attenuation due to dielectric hysteresis and reduces the Zo by sqrt
(epsilon r). Filling a (otherwise nice) 77 ohm air line with a (nasty
solid) dielectric of epsilon r ~~ 2.2 indeed reduces the Zo to near 50
ohms (52 ohms). Since attenuation due to dielectric hysteresis is
independent of line dimensions as you point out (attenuation depends
on the quality of the dielectric only) the 52 ohm "filled" coax may be
considered a new and different kind of "lowest attenuation" definition
coax, based on different startup restrictions (from air line).
Low average power pulsing approaching 30 kV/cm at the surface of the
inner conductor of 30 ohm line does reduce the inner conductor
temperature rise from that of the CW case but the (air line) remains
corona limited. That is to say, highest CW power handling *and*
highest pulse peak power handling occur with 30 ohm air line as
defined. Again, the introduction of (nasty solid) dielectric will
cause a new and different kind of filled "highest power handling"
definition coax, based on different startup restrictions from air
line.
It is further instructive to consider gaseous insulations other than
air, whether pressurized or not, for increasing power handling and to
consider I-squared-R reductions achieved with cryogenic cooling.
Gawrsh, we got to be careful with our words in this business.
regards
dan bathker
("never met a dielectric that I really liked")
Received on Thu Mar 15 2001 - 05:11:06 EST
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