Waveport excitation and mode alignment

Dear All,

I have designed a dielectric filled cavity resonator BPM and want to measure its position sensitivity.

From the S-parameter anaylsis, I could observe difference in signal levels for offset positions even in monopole mode. This I believe happens as I am having 4 floating cavities. However, I see higher sensitive dipole mode peaks for offset beams. However, when I plot the S parameter for the opposite pickup I see a dip/valley at the corresponding frequency. Could some one help me in figuring out how to sort this out?. I believe this happens because the field in the opposite cavity is out of phase by 180°.

Thanks a lot for your effort.
Cheers
Sud