Frank,

The Smith Chart shows that a termination value other then 50 Ohms is required to
calibrate the 8405a to zero phase for the frequency and cable. That said there
are two zero points, I will have to think about the effect of these on the
measurements.

Frank wrote:
"dansawyeror" wrote in message
...

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


Why are you using a 25 ohm termination? The procedure for calibration
requires a short, open, and 50 ohm load. It is also important that you know
the coupler directivity at the test frequency. There is nothing wrong with
calibrating the test fixture at the end of an unknown length of coax;
although you will experience some degradation in the dynamic range of the
return loss.

Regards,

Frank