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How perfect should TRL standards be? (Seeing notches in s21 of thru)

Question asked by aminick on Nov 6, 2013
Latest reply on Nov 11, 2013 by aminick

I have fabricated TRL standards with SMAs as external connectors and suspended striplines in between. In the transmission response of the thru and line standards, I'm seeing several deep notches at different frequencies, which I suppose is a result of multiple reflection between transitions.
As a result, the outcome of the TRL calibration is not valid. For example when I re-measure the Line standard after TRL calibration I can see positive insertion loss (dB(s21)) at some frequencies.
How can I fix this problem?