On Wed, 28 Dec 2005 21:37:05 -0800, dansawyeror
wrote:

Owen,

Thanks for the example. I do not have an impedance meter. I do have a vector
voltmeter that will read the phase and value of the reflected signal.

If you mean relative to the forward wave, then you can calculate Z
from that.


The core problem is: How to measure and improve the performance of a loaded
vertical. The unknowns are the value of ground and antenna resonance.

Setting the coax to a 1/4 wave multiple was a way to remove its phase altering
characteristics from the reflected signal at a frequency of interest. That seems
to be working and is predictable to measure the 1/4 wave odd, open circuit,
resonant points, etc. This simply proves consistent measurement and correct
identification of the electrical cable length.

I don't know that that is necessary or helpful.


I am pretty confident about the coax measurements at this time. Now on the to
antenna. I am not what the reflected angle is from the antenna. The coaxpair
program predicts it can vary 180 degrees for a purely resistive load.

Yes, the complex reflection coefficients for a 51+j0 ohm load and
49+j0 ohm load are both very small magnitude, and 180 deg different in
phase.

Don't worry about the reflection coefficient at the antenna, find what
it is at the instrument interface, calculate the Z, and use one of the
many calculators to work out what it is at the end of the line (they
use the input reflection coefficient and propagation constant to do
that, but they do it internally).

You know that Gamma=(Zl-Zo)/(Zl+Zo), rearrange the terms to find Zl
knowing Zo and Gamma, measure Gamma with your instrument, calculate
Zl... and the rest is easy.

Back to my worked example, if you instrument indicated Gamma was 0.195
-117 in 50ohms, you would calculate Z to be 40-j15... and go from
there.

What more do you need?

Owen

Thanks for your help - Dan



Owen Duffy wrote:
On Wed, 28 Dec 2005 08:00:51 -0800, dansawyeror
wrote:



After making an error here between the effects of odd versus even quarter waves
at the source I am getting closer to being able to measure the impedance of a
loaded vertical 'in the shack'. At the moment this is limited to a single
frequency 'close' to a frequency of interest. But even that is a triumph.


Is that as hard as it looks?

Take an example:

You have an impedance meter to measure complex impedance at the
frequency of interest, being 3.6MHz for the sake of the example.

(This technique depends on the behaviour of the tranmission lines, you
would want to be sure that the transmission lines are in good
condition and work as characterised.)

You measure the impedance 40-j15 for example looking into a cascade of
5m of RG58 and 50m of 9913 connected to the unknown load.

The Z at the load end of the RG58 is 60.42-j20.13. That is the Z
looking into the 9913.

The Z at the load end of the 9913 is 41.94+j18.03.

Is this the kind of thing you are trying to do?

Bear in mind that you cannot know the characteristics of the lines etc
to support the precision shown above. You also need to keep in mind
the sensitivity of the results to changes in parameters to form a view
of the confidence limits of your measurements.

This took more time to write about than it did to find the results.

Owen
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