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how to measure antenna impedance ?
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. |
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