Frank,

Your comment about my post is correct upon analysis it is not relevant.

I am trying to identify the resonance frequency of an antenna. When that point
is found I am trying to measure the input impedance (should be R + 0j).

Assuming 50 Ohm source and cable, the smith chart shows for lengths of cable
terminated in values other the 50 Ohms, say 25R 0j, those points will plot of a
constant SWR = 2 circle. I am using that circle below:

I am assuming that for a frequency where the antenna is resonant that the phase
read from the antenna and the phase read from non 50 Ohm 0j load will be the
same. If that is true then 'zeroing' the meter by adjusting the phase offset
will not effect the frequency of resonance. It is simply a convenience.

This method should yield a direct resonant frequency reading.

Dan

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

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.


Dan,

I am not sure how you arrived at the above conclusion. The procedure should
be as follows: Short the end of the coax at the antenna location, then
adjust the line stretcher for a 180 degrees phase shift. This is your
reference reflection coefficient of -1 (i.e. 1 180). Remove the short and
verify the reflection coefficient is 1 0. Connect a known 50 ohm load,
which should provide a return loss of 30 dB. The angle of the reflection
coefficient is irrelevant. If the return loss of the 50 ohm load is
significantly less than 30 dB, then either your load is not very good, or
you have poor coupler directivity. Attach the antenna, measure, and record,
the magnitude and angle of the reflection coefficient. Repeat the procedure
for all frequencies of interest. You will then be able to plot the result
on the Smith Chart. At the frequencies you are working, which I believe is
in the 100 to 200 MHz range, it is doubtful that the non-ideal short/load
standards parasitics will effect the results significantly.

I have done such measurements countless times, although with HP VNAs, which
makes the procedure far less tedious.

Hope this helps,

Frank