Well, After many trials and much learning I believe the 8405a is currently
working and actually making measurements. The breakthrough was a web article
showing Tau ranging from -1 to +1. This finally made sense of the comments
posted here.

Now to the setup and the measurements. I have built a loaded vertical for 2
meters and placed it over a 3 foot by 5 foot ground screen. The exact frequency
of the antenna is unknown. This is assembly moved into another room and
connected via an unknown length of 50 Ohm cable. The technique is labor
intensive, but in the end produces consistent results.

The 8405a is connected to a dual directional coupler. The connector lengths are
tuned to produce zero phase. The coupler is driven by a signal generator and
connected to the antenna assembly.

Now the tuning process: It involves tuning the antenna for a zero phase return
and then temporally replacing the antenna with a 25 Ohm terminator. The 8405a is
set to zero phase adjusted to he terminator. The antenna is then re-substituted
for the terminator and the frequency is adjusted to produce the next zero phase.
The cycle is the frequency is adjusted to the antenna and the meter zeroed phase
adjusted to the terminator. This is repeated.

The first attempt did not resolve. That is it the values did not trend toward a
common frequency and phase. However the second attempt, which was just a few MHz
away did. The result was a measurement that was consistent and repeatable. The
meter was set to the 6 degree scale and the measurements were sensitive to 5 kc
differences in frequency. The results were consistent to 1/5 a degree.

The Smith Chart shows for a purely resistive load that the return phase from a
fixed cable and frequency is independent of R. That is the return phase is
constant for a frequency, fixed cable, and pure resistive load. That would imply
that when the phase return is the same for a resistive load and an unknown then
the unknown is a pure resistive load. This is a way to normalize the effects of
an unknown cable to determine resonance.

I think this actually works. Is there a better way to do this?

Thanks - Dan