Microstrip low-pass filter analysis using 3D FDTD code with UPML

Microstrip low-pass filter analysis is performed by the 3D FDTD + UPML routine.

FDTD 3D with UPML is used to calculate scattering coefficients S_{11} and S_{21} for planar microstrip low-pass filter following by the original paper by D. Sheen, S. Ali, M. Abouzahra, J. Kong "Application of the
Three-Dimensional Finite-Difference Time-Domain Method to the Analysis of
planar Microstrip Circuits", IEEE Trans. on Microwave Theory and Techniques http://dx.doi.org/10.1109/22.55775).
Current code includes some improvements in comparison with the original calculations:
1) UPML instead of Mur ABCs;
2) using real metal (copper) as a patch conductor material instead of PEC;
3) applies matched load at the ends of filter's transmission microstrip lines to prevent physical reflections;
4) do not apply "magnetic wall" or "electric wall" conditions at the Ez source plane.