Port Z0

I am using Ansoft Ensemble simulator .
When I add a infinite ground layer in my structure.
Why is the Port Z0 not 50 ohms????

The value of the impedance depends on the cross sectional port. Generally it's can have other value than 50 ohm.

Regards.

dear sir :
could you explain this in detail ???
thanks

Ensemble is an electromagnetic software simulator.
Generally this kind of sw operates analyzing in advance the 2D problem related to calculate the port impedance before analyzing the full problem.
You have to consider that each port in the structure being analyzed can be viewed as a cross section of a transmission line. Then, it is very important check that cross-section; it determines the value of the impedance port.

Regards.