Unable to measure the correct reflected power using ADL5920 EVAL board

Hi,

I have just purchased and setup my ADL5920 EVAL board and trying to measure the S11 of some simple loads such as 50 ohm matched load and T-resonator etc.

I am able to setup the board and connect it with quikeval.

I am also able to calibrate the board using the GUI and the measured input power given by GUI is almost correct (with some offset but I am not too concerned about it at the moment).

I have signal source at J1 (with 0dBm power typical) and I have connected load at J2.

My problem is that the power measured at J1 (incident power) is correct but the reflected power at J2 is not true representation of the load.

For example, when I connect a 50 ohm matched load at J2, the reflected power measured at J2 should be very small (in the range of -20,-30 dBm) but the GUI measures the reflected power of almost -3 dBm. The same amount of reflected power (-3 dBm) is measured even if J2 port is left opened. So, I suspect that the reflected power is not being measured correctly.

Can anyone suggest how to debug the problem?

It should be noted here that the incident power measured by GUI follows the exact amount of input power I put using signal source. 

My EVAL board