How to find Fresnel's transmission coefficient using CST MWS?

Although CST is not for such a simple thing but I am interested to find the Fresnel transmission and reflection coefficients using CST to see that the results are matching with the textbook equation for non-dispersive materials. I have taken a dielectric material of epsilon=4 and Miu=1 placed along z direction. open boundary is considered in x and y direction. open(add space ) BC at Zmin and open BC at Zmax. Two waveguide ports are placed at Zmin and Zmax in such a way that port_1 is in the back groun medium and port_2 is at the dielectric end. Wavelength range is considered as 1.45 to 1.65 um.
As the dielectric is a non-dispersive material the amplitude transmission coefficient is expected to be 0.666 for the entire range of wavelength. But the simulated results is varying widely between 0 and 1. Can not understand the reason. Please help.

Could you show the results obtained from CST simulation?