[SOLVED] Ripple in S21 of a coaxial cable

[noori.re]

I am measuring S21 and S11 of a coaxial cable using VNA. I expected to see a S21 that decreases by frequency but has some ripply behavior. It means that in some regions S21 is improving by increasing the frequency. Does anyone know the reason for this behavior? At first, I thought it should be related to non-ideal input impedance but marker number 5 in the figure below rejects this idea.

 

[BigBoss]

I presume that the Calibration was done well.
The cable is bad so Characteristic Impedance is waving due to mechanical inconsistency or inner conductor has a crack. Other option your VNA has not been calibrated very well or it has some problems.
Have you ever checked the cable quality or characteristic impedance ?? Has it visible faults ??

 

[FvM]

Standard coax impedance spec is 50 +/- 2 ohm, reflections at both ends can worst case generate a ripple in the observed order of magnitude.

I would expect a complete specification anyway (cable type, length, instrument, calibration).

 

[noori.re]

I presume that the Calibration was done well.
The cable is bad so Characteristic Impedance is waving due to mechanical inconsistency or inner conductor has a crack. Other option your VNA has not been calibrated very well or it has some problems.
Have you ever checked the cable quality or characteristic impedance ?? Has it visible faults ??

The caliration using ideal kit is done and everythins is perfect with ideal kit through mode. The quality of cable seems good no visible mechanical problem. I supposed that the measurement is correct because Im getting almost similar result when measuring with R&S CMW500 also.