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

What you are measuring with a directional coupler is the complex reflection
coeficiant. If the measurement is for low frequencies (i.e. 30 MHz), and
the load is at the input of the directional coupler, then you will probably
obtain a realistic figure for complex "Gamma". Ideally you need a short
circuit, open circuit, and 50 ohm load to determine if the system is
calibrated.

Short circuit Gamma = 1 180
Open circuit Gamma = 1 0
50 Ohm Gamma = 0

If the load is at the end of a length of coaxial cable you have to
comphensate for the phase shift error at every measurement frequency.

Since you are dealing with complex numbers it is tedious to determine the
actual load impedance. The following app. note should help:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/742

HP's app. note at http://www.sss-mag.com/pdf/hpan95-1.pdf is also very
helpful.

Regards,

Frank