On 11/8/2011 7:29 AM, Owen Duffy wrote:
John wrote in :

I have about 94 inches of RG-142B/U. I am using a Fluke 6061A signal
generator, an HP 8405A Vector Voltmeter, and a Narda dual directional
coupler. I have tried to measure the line characteristics at 434 MHz
but I am not satisfied that the results are accurate. It is very
difficult to get good short and open circuits at this frequency and I
also wonder if the 8405A accuracy suffers since a short is well away
from the nominal system impedance of 50 ohms.

What if I simply calibrate the 8405 with a short on the end of the
line (the measurement plane) then attach my antenna and accept the
readings? Will they be very far from the real value?

I am a little confused about your objective. The subject line seems
inconsistent with your discussion.

If you are trying to measure Z at the reference plane in the simplest
manner, then what you propose in your last par should give you the
magnitude and phase of the reflection relative to a s/c (where
Gamma=-1).

This simple paired measurement of the reflected wave from a s/c and
unknown load depends on the forward wave being constant. That is true if
the Thevenin source impedance of the source at the coupler is equal to
the nominal characteristic impedance of the coupler and the coupler
terminations, cables etc that you use. That would usually be met by a
standard signal generator etc, but some SSGs depart from ideal on their
highest output settings (check the specs). Measuring the forward wave
under significantly different loads will provide an indication as to
whether you can assume that it remains constant with different loads.

I give an explanation of why Vf is constant when Zs=Zo at
http://vk1od.net/transmissionline/VSWR/Zs50.htm .

You will recall that there is an ongoing argument that a ham transmitter
is well represented as a Thevenin source with Zs=50+j0 ohms, as some
accident of design. The article describes a simple test using an
accurate directional wattmeter to demonstrate that under different drive
level and different frequencies, that Vf is often not necessarily
independent of load impedance and that calculations that depend on
constant Vf (such a Mismatch Loss) are in error.

Owen

I think I am the confused one. Do I even need to know the transmission
line characteristics if I am going to short the load end and set the
vector voltmeter for a phase reference of 180 degrees?

I am following the HP app note AN-77 and they do not mention a
transmission line. They say to short the load end of the coupler. I need
to get my antenna away from the test setup, so I add the transmission line.

Has this made any sense?

John