Well here's what I did: I treated it like a peice of coax. I made a
fixture to hold the "feedthru" with semi-rigid coax on one side and a
50 ohm chip resistor on the other. I dropped in a short peice of coax
and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
- 1.06, which works out to about 53 - 55 ohms or so.
How's that sound?

wrote:

Well here's what I did: I treated it like a peice of coax. I made a
fixture to hold the "feedthru" with semi-rigid coax on one side and a
50 ohm chip resistor on the other. I dropped in a short peice of coax
and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
- 1.06, which works out to about 53 - 55 ohms or so.
How's that sound?

I could be wrong, but it really sounds to me that what you have is a
50 Ohm coaxial connector of some type, not a feed through capacitor.
Hey! Any chance you could 'loose' one and ship it to me? I'd love to get
a look at this critter.

Best 72
Bob 'POQ

Maybe one of the network analyzer experts will jump in at this point --
I'm not one. If I were your client, I certainly wouldn't be satisfied
with your methodology. But I'm not your client, so what satisfies me
isn't important. Good luck.

Roy Lewallen, W7EL

wrote:
Well here's what I did: I treated it like a peice of coax. I made a
fixture to hold the "feedthru" with semi-rigid coax on one side and a
50 ohm chip resistor on the other. I dropped in a short peice of coax
and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
- 1.06, which works out to about 53 - 55 ohms or so.
How's that sound?

Could you suggest some way I could do it?
On Wed, 07 Jul 2004 18:29:31 -0700, Roy Lewallen
wrote:

Maybe one of the network analyzer experts will jump in at this point --
I'm not one. If I were your client, I certainly wouldn't be satisfied
with your methodology. But I'm not your client, so what satisfies me
isn't important. Good luck.

Roy Lewallen, W7EL

wrote:
Well here's what I did: I treated it like a peice of coax. I made a
fixture to hold the "feedthru" with semi-rigid coax on one side and a
50 ohm chip resistor on the other. I dropped in a short peice of coax
and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
- 1.06, which works out to about 53 - 55 ohms or so.
How's that sound?

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.

On Wed, 07 Jul 2004 18:34:15 -0400, wrote:

|Well here's what I did: I treated it like a peice of coax. I made a
|fixture to hold the "feedthru" with semi-rigid coax on one side and a
|50 ohm chip resistor on the other. I dropped in a short peice of coax
|and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
|took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
|- 1.06, which works out to about 53 - 55 ohms or so.
| How's that sound?

Like you don't know what you're doing.

Your DUT is a two-port device. If you really have a network analyzer,
why aren't you using it to measure the device as a two port?

Bill Turner wrote:
But think about this in relation to your statement: We both know of at
least one person (and his boss) who wants to know the Zo of a
feedthrough capacitor. Do you really think in forty years there has not
been ONE OTHER person who might have actually designed such a thing?

I've worked with at least ten "engineers" and dozens of bosses who,
given a similar bag of feedthroughs containing a slip of paper saying
"RPM = 2400 +- 1%", would work for a week designing a test to verify or
disprove the spec.

--
Spammers are people who are too lazy and cowardly to rob liquor stores, but
still want to make money by stealing instead of working.
-- Morely Dotes, The Open Sourceror's Apprentice

wrote:
Could you suggest some way I could do it?

I did, a couple of postings ago -- using a TDR. Guess my posting hasn't
made it to your server yet. If it doesn't show up soon, you should be
able to find it on groups.google.com.

Roy Lewallen, W7EL

Wes wrote,
Message-id:

On Wed, 07 Jul 2004 18:34:15 -0400, wrote:

|Well here's what I did: I treated it like a peice of coax. I made a
|fixture to hold the "feedthru" with semi-rigid coax on one side and a
|50 ohm chip resistor on the other. I dropped in a short peice of coax
|and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
|took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
|- 1.06, which works out to about 53 - 55 ohms or so.
| How's that sound?

Like you don't know what you're doing.

Your DUT is a two-port device. If you really have a network analyzer,
why aren't you using it to measure the device as a two port?




Is two port theory even being taught in engineering schools these days?
73,
Tom Donaly, KA6RUH

They're 50 ohm RF and DC feedthrus. Here's the company:
http://www.shp-seals.com/products_and_Services.htm
Click on bulletin 100. Most50 ohm types gave a VSWR of 1.05ish, giving
about 53 ( or 47 ) ohms. I also had a few of the DC type that gave a
VSWR of 1.4ish, leading to about 25 ohms ( or 75 ohms, could I check
the phase to see which?).