Improve the return loss of the transition

Hello everyone,

I am trying to simulate a stripline to microstrip transition, Here, the impedance is matched to 50 ohms.
The impedance is matched, but the return loss is very low ( -0.2 dB).
I need to acheive a return loss S11( > -20dB).
I have put a stripline, Puta waveport for stripline( 5 times the width of trace and 5 times the height of substrate).
The trace is a step transformer (i.e, 50 ohms - trans - 30 ohms).
On top of the trace, I have put a via (material - pec surrounded by a outer cylinder (teflon). The via(inner cylinder) touches the bottom ground of stripline.Below the via, there is a cylinder cut out and a cavity with top face uncovered, this is the top portion.On top of the via, I have put one more step transformer and port (S21).

When I simulate this project with solution frequency of 18GHz, the
return loss is practically zero, How do I improve the return loss ?
The frequency reange is from 15-18GHz.

Regards,
Avin