you should set boundary properties (lower and upper Z distance).

If I set lower and upper z distance but x is very small. Will that work?

If this is a rectangular waveguide port, it should touch all the boundaries. If you have a picture of the feed it would really help.

If there are metal boundaries near, then the port has the same problems as the real circuit, and you can make the port small, so it touches the metal boundaries, as Mohamed K. Emara says. If you have microstrip, and two microstrips are too close together to have a big enough port on either of them, then perhaps you should add some length and use it to get the tracks far enough apart so that the ports can be big enough. In this case the modes will be correct, but the way the tracks approach each other will modify the fields on each. You could also use lumped ports instead. Perhaps try it with simple microstrip to see how a lumped port compares with a waveguide port and whether it is close enough for your purposes, or better than the effect of the extra line length and bends necessary to use full-size ports.

Thank you so much everyone

Hello,

First, form one more structure around your given structure with vacuum medium. Then select age and then take waveguide port on that selected ages.

Thanks,