Hi Dan

Normally you would calibrate your test gear against a known resistive
load first. If you coupler creates a phase shift that can be compensated
for either in the test equipment or by varying the feedline length.
(ouch!) All the network analysers I have used allow you to calibrate
50r, open or short.

You can further test you setup by measuring known lengths of coax
"stubs" that would present a reactive load.

I imagine a directional coupler would introduce a phase shift as it has
an electrical length that must be allowed for.

I saw a real impressive antenna impedence measuring device that used
coaxial cable as the tuned reference elements. It was of course
frequency dependent. It was made for 2M but I guess the design would be
easy to replicate for other frequencies given. It has about 10% usable
bandwidth. I was going to make one for HF with BNC terminated coax
lengths for each band, but never did!

Go to http://www.vhfdx.oz-hams.org/and Measurements

or

http://www.vhfdx.oz-hams.org/docs/ZMeterVK2ZAB.pdf

Apologies for not answering your exact questions.


Cheers Bob W5/VK2YQA


dansawyeror wrote:


All,

I am trying to measure antenna impedance. For this I intend to us a
directional coupler to isolate reflected signal. After using the coupler
for a while I believe that it introduces a phase shift, that shift seems
to be related to frequency. This creates a bit of a catch 22. Antenna
resonance is defined as the frequency where there is no reflected
complex component. If the tool to measure this is also frequency
dependent how can this be accomplished? Is this even the best method?

Do bi-directional couplers automatically compensate for frequency shift?

Thanks - Dan kb0qil