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how to measure antenna impedance ?
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 |
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