Hello Paul,


Okay, so I think it's overwhelmingly clear that adding or subtracting
any electrical length from this "baseline" - (or "reference plain" as I
think it's also called) introduces measurement error which must be
allowed for/accounted for in some way to maintain accuracy.


Depends on how much you are talking about. Man often only needs this
much precision ;-)


Let's say I make up a short and an open using N-type plugs as described
elsewhere on this thread, using identical, high quality plugs. Can I
then use these with my proprietory Suhner 50 Ohm precision N-type load
by calibrating out any differences between the home-made jobs and the
proprietory one?


Let me describe what the three cal devices look like that came with our
HP4191A (after all, this is the stuff that Hewlett-Packard has deemed
acceptable):

The "Short" is a flat nut with a closed end. It puts a flat plate
directly across the face of the DUT connector. No length there.

The "Open" is a longer threaded tube with a hollow inside, about an inch
or so long and closed off at the far end.

The "50 Ohm" is something I wasn't able to open w/o special tools but
will some day. It looks like the resistor is about 1/2" to 3/4" up there
from the DUT connector surface, firmly encapsulated.

All gold-plated, of course.


Further, I now know that a "through" is simply that U-shaped patch lead
that's often seen sticking out of the front of VNAs in catalogue
pictures. I also know that you can't make one of these with zero coax
length for obvious reasons. Given the fact then that it has an
unavoidable length; does it have to be any *specific* length in
relation to the open, short and 50 ohm terminations?

The HP4191A has that, too. It needs to match the length that makes the
DUT and ref path equal. The one on the HP4191A is about 1-1/2" for a
zero length on the DUT. In order for the analyzer to be useful in
settings where the DUT has to be a bit remote, like in a vacuum chamber,
you can remove a little face plate, remove the "through" and install
your own longer "through". It's made from rigid coax.

Regards, Joerg

http://www.analogconsultants.com