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Frequency dependence of the auto-balancing bridge?

Question asked by wbuller on May 24, 2011
I am attempting to measure the capacitive coupling of objects in the near-field of large antennas via impedance measurements with a hp4294a.  The measurements show strong resonances commensurate with the attached antenna, but the sensitivity of the measured impedance is much higher for higher frequencies (or higher resonant modes).  My prejudice is that the resonant standing wave has greater penetration and capacitive coupling at lower frequencies.  So, my question is, does the auto-balancing bridge perform some stability correction that is frequency dependent.  I'm not looking at frequencies below 1 MHz, so it should be using the same auto-balancing method.  Above 15 MHz the measurements are much more sensitive to capacitive coupling for the DUT.  Has anyone seen this behavior, or have a good hypothesis?
William Buller, Michigan Tech Research Institute