The selected dielectric material is FR4, the dielectric constant is 4.4, and the plate thickness is 1mm. First, roughly calculate the line width of the 50 ohm transmission line:
The line width of the 50 ohm transmission line is about 1.92mm, and then the accurate simulation is carried out with HFSS:
HFSS version 13.0:
File--new, create a new project:
save as named microstrip.hfss
Project--Insert HFSS Design
Click on Draw box 
Right-click CreateBox, select properties, and set the parameters:
Right-click Box1, select properties, and set the parameters:
Get the following image:
Click on Draw rectangle 
Right-click rectangle1, properties, set:
to get the line:
Click on the Draw box again
Get the baseplate model:
Click on the Draw box again
Get the model diagram:
Click View--active view visibility to hide the vacuum layer:
Revert to:
Change the Draw plane to YZ:
Click on Draw rectangle
get the model:
Select port1, point 
Click OK, right-click line--Assign Boundary--Perfect E, click OK.
Right-click port1--Assign Excitation--Wave Port, click OK continuously, complete
Right-click port1_1--Assign Excitation--Wave Port, click OK continuously, complete
右键boundary--Assign Boundary--Radiation，OK
Right-click analysis in the Project manager, add solution setup, OK
Right-click setup1 and select add frequency sweep:
Click the button 
Click the button 
After completion, right-click Results--Create Modal Solution Data Report--Regular plot in the project manager,
Choose Zo:
Click on New Report:
While characteristic impedance is a fairly stable property, it decreases slightly with increasing frequency:
Due to the skin effect (when there is alternating current or alternating electromagnetic field in the conductor, the current distribution inside the conductor is not uniform, and the current is concentrated in the "skin" part of the conductor,
That is to say, the current is concentrated in the thin layer on the outer surface of the conductor. The closer it is to the surface of the conductor, the greater the current density, and the actual current inside the wire is smaller)
When the frequency increases, the current concentrates on the surface of the wire, the current density in the wire decreases, and the internal inductance of the wire decreases.
(The essence of inductance is the number of turns of the magnetic field around the current)
However, the external inductance remains unchanged, so the total wire inductance decreases, resulting in a weakly smaller characteristic impedance.