On Sat, 03 Dec 2005 17:33:18 -0800, 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?

This depends a lot on what instrument you are connecting to this
coupler. If it's nothing more than a power sensor, then you are
making scalar measurements and phase is meaningless.

You would calibrate by placing a short on the measurement (antenna)
port and getting a 100% reflection reference (rho=1). You would
determine the magnitude of the reflection coefficient by ratioing this
to the measured value.

If you have a magnitude and phase sensitive instrument (vector
analyzer) then, as others have answered, you calibrate with additional
reference standards. In any event, the phase shift through the
coupler is compensated for by the calibration process.


Do bi-directional couplers automatically compensate for frequency shift?


No. The provide for a simultaneous sample of the forward and
reflected signals.