On 11/8/2011 3:58 PM, Owen Duffy wrote:
John wrote in :

On 11/8/2011 2:37 PM, Jeffrey Angus wrote:
...
The problem is that these measurements are designed to be made
as close to the instrument as possible. Adding a piece of
transmission line adds loss and phase shift to the measurement.

The problem is that whilst putting the unknown right at the instrument
might solve some problems, it creates others, particularly when the
unknown is an antenna system.


So unless you know how to work backwards from the measure you get to
what you're really measuring at the far end, you won't really have a
valid answer.

Well, you can work backwards as I explained, though again at the expense
of some uncertainty (error).

...
Obviously, the antenna can't be sitting in front of the test equipment
so I'm trying to find a way to minimize proximity effects.

Don't overlook that the reference plane need not necessarily at the end
of the directional coupler, it could be anywhere that you can
conveniently or inconveniently apply the calibration s/c (eg at the
antenna connector for flange as appropriate).

AHA! I think this is the answer I was needing. So, if I short the end of
the coax and calibrate my vector voltmeter I can then believe the
voltmeter when it says my load is a certain impedance?

I don't know were I got the idea that I had to have the coax
characteristics to be used to modify the readings for accuracy.

Try this on the bench with a substantial length of coax to the reference
plane, and measure a 25 ohm load (pair of 50 ohm terms on a Tee). The
reference plane is determined by where you apply the calibration s/c.

Okay. I'll do that as soon as I can find the instruments. They are
somewhere around here.

It was common practice using slotted lines to make such measurements
with the slotted line a long way from the reference point, which might
be quite close to the antenna, possibly a s/c applied at the antenna
flange, or on a W/G switch near the antenna.

BTW, you have read the AN, and noted no doubt the issue with error due
to the probes loading the test circuit. I would not obsess too much over
perfection unless you are prepared to go to great lengths to try and
allocate the errors and obtain a better result.

Owen

I don't obsess. I'm usually happy with errors of less than 1.5 to 1
(depending on circumstances, of course).

Thanks, Owen.

John