In article ,
Wes Stewart wrote:

The measuring setup is a standard signal generator feeding a tee. One branch
feeds the A input of the 8405A and the other a 20 db directional configured in
reverse, that is feeding the out port. The assumption is the reflected signal
will be read correctly on the in port. The in port is then terminated or
connected to an antenna.

It would also be nice it you had a 6 - 10 dB pad between the generator
and the directional coupler (DC); located right at the DC. You want
the source match to be set right there and the A probe to sample right
there.

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another. You really don't want
the oddities of the load connected to the B side to affect the
voltage/phase of the reference signal seen by the "A" probe - it'd
certainly wreck the measurement.

Using a power divider, and resistive pads for isolation is one way to
do this. Using a pad followed by a dual directional coupler (as in
the experiment page to which Owen posted a link - thanks!) is another.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!