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S11>1 Error - Imperfect reference plane?

Question asked by rf_mark on Jan 7, 2009

I am using an HP4396B SA/NA to attempt to match a dual band antenna (920MHz & 1800MHz). Unfortunately, I must use a length of coax for physical attachment to PCB antenna, and it is difficult to get equal length for open and short calibration. I realize there will be errors associated with this measurement, but I am seeing an S11 magnitude greater than 1 over part of the band. Also, when I disconnect the coax (after cal), shortening the reference plane, the raster goes to the limits of the CRT screen in Smith chart mode.

Is this normal?

Thanks in advance for your response!!

- Mark -