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Issues with HP8753D, while calculating the tuning frequency of an antenna

Question asked by saviour2009 on Jun 10, 2013
Latest reply on Jul 3, 2013 by drkirkby
I am using a monopole antenna and trying to verify its resonant frequency using an HP8753D VNA. The process I am using is as explained below:

1. Calibrate the VNA using S11-1 port calibration method.
2. Connect the antenna using a co-axial cable to the VNA and check the S11 parameter graph over the frequency range. 
3. For a length of 7.6 cms, I should have resonant frequency around 1 GHz, however I am also getting larger dips at higher frequencies (at around 3.5 GHz). As per the logic I should only have a dip at the resonant frequency. 

Please advise what could be the possible reason behind the above mentioned situation.