what are the problems of my directinal coupler_help

I designed a simple single-step strip-line directional coupler.
the sumulation has coupling degree of-40dB, directivity of 30dB from 350MHz-600MHz,
but the test results of directivity is almost 0. I do not know where the problems lie in.
feeling there is no theory problem, should be the wrong choice of substrate ? (I am using Rogers4350).
and my stripline coupler is connected with microstrip line. will the discontinuities of strip line to microstrip line cause these Deteriorations?
This is the first time I designed directional coupler. I wonder if there any recommendations?

Hello HouJun,

Your model looks fine.
You said there is a change from stripline to mictrostrip line, but I can't see it in model.

The problem could be when trying to match the coupler from striplinr to microstrip.
In general, the striplines are between two conducted ground and microstrip is a conductor above ground.

How did you bypassed the ground planes for the microstrip transmission line?

I think you grounded the main signal transmission by this.

Please check it.

IH

Thank you, itzikhaim. I got it.
Exactly, the problem is the transition from stripline to microstrip, the top ground of the stripline is bad grounded.
I use some copper foils to wrap the edges of the coupler. It's fine now.
Thank you again.