In article , Bill Turner
writes:

On 07 Jul 2004 20:39:22 GMT, (Avery Fineman) wrote:

No. 10 pFd at 1 GHz has a reactance of 15.9 Ohms. That tosses
the VSWR in the bucket if that is part of a "line section."

_________________________________________________ ________

A section of coax has both capacitance and inductance. Together, they
form the Zo of the line. You are quoting only capacitance while
ignoring the inductance.

"Ignoring?" No. Spare me a lecture. 10 puffs IS a "small
capacitor. You were not talking "inductance." 10 pFd IS a
15.9 Ohm reactance at 1 GHz. That's also a small reactive
shunt on this mythical "line section."

I grant you, a typical feedthrough capacitor is designed with large
capacitance and is used for filtering, but that does not preclude one
from being designed to "look" like a piece of coax if the designer
wishes. And that was the original question.

Sorry, no. The original question concerned testing of what
LOOKED LIKE "feedthrough capacitors." The tester was thinking
of "treating it like a small section of transmission line."

Based on the response of actual testings, such as very low
VSWR measured, the gadgets are probably just wire feed-
throughs intended for something like passing through a
hermetic seal. Those would have very little capacitance and
thus would have very little discontinuity...the VSWR would not
be raised very much.

If those "feedtrhoughs" had really been capacitors, then, in all
probability, they would have tested as capacitances on a bridge
such as a 1 MHz bridge. A handheld C or L/C meter could have
read that on a 1 nFd or 10 nFd scale.

Without further ado, based on what was written by the person
who has the devices, my opinion is that they are just wire feed-
throughs with minimum capacitance and the inductance of a very
short piece of straight wire. Those MIGHT work as "line sections"
going through a hermetic seal wall of something...but not a very
practical way to run a transmission line through that wall.

Feed-through CAPACITORS were NEVER rated by "characteristic
impedance" (Zo). Not in 1964, not in 2004, not in years between.

Wire feedthroughs for sealing-type bulkheads are seldom rated
for anything but current carrying capacity and withstanding or
flash-over voltage. Those have just conductors going through
with minimal capacity to "ground" and minimal inductance
straight through.