If your physical structure is excited by a Lumped Source or a Waveguide Source or a Distributed Source, and one or more ports have been defined, the FDTD engine calculates the scattering (S) parameters, impedance (Z) parameters and admittance (Y) parameters of the selected ports. The S parameters are calculated based on the port impedances specified in the project's "Port Definition". If more than one port has been defined in the project, the FDTD engine runs an internal port sweep. Each port is excited separately with all the other ports turned off. When the ''j''th port is excited, all the S<sub>ij</sub> parameters are calculated together based on the following definition:
:<math>S_{ij} = \sqrt{\frac{Re(Z_i)}{Re(Z_j)}} \cdot \frac{V_j - Z_j^*I_j}{V_i+Z_i I_i}</math><!--[[Image:FDTD82(1).png]]-->
where V<sub>i</sub> is the voltage across Port i, I<sub>i</sub> is the current flowing into Port i and Z<sub>i</sub> is the characteristic impedance of Port i. The sweep loop then moves to the next port until all ports have been excited. After the FDTD simulation is finished, the S parameters are written into output ASCII data files. Since these data are complex, they are stored as '''.CPX''' files. Every file begins with a header starting with "#". Besides the scattering parameters, the admittance (Y) and impedance (Z) parameters are also calculated and saved in complex data files with '''.CPX''' file extensions. The following relationships are used:
:<math>\mathbf{ [Z] = [Image:FDTD83.png\sqrt{Z_0}]\cdot ([U]+[S]) \cdot ([U]-[S])^{-1} \cdot [\sqrt{Z_0}]}</math>
where :<math> \mathbf{ ['''U'''Y] is the identity matrix of order N, and = ['''vZ<sub>0Z]^{-1} } </submath>'''] is a diagonal matrix whose diagonal elements are the square roots of port characteristic impedances. The voltage standing wave ratio (VSWR) of the structure at the first port is also computed and saved to a real data '''.DAT''' file. The following definition is used<!--[[Image:FDTD83.png]]-->
where <math>\mathbf{[[Image:FDTD84.png]U]}</math> is the identity matrix of order N, and <math>\mathbf{\sqrt{Z_0}}</math> is a diagonal matrix whose diagonal elements are the square roots of port characteristic impedances. The voltage standing wave ratio (VSWR) of the structure at the first port is also computed and saved to a real data '''.DAT''' file. The following definition is used:
:<math> \text{VSWR} = \frac{|V_{max}|}{|V_{min}|} = \frac{1+|S_{11}|}{1-|S_{11}|} </math>
<!--[[Image:FDTD84.png]]-->
You can plot the port characteristics from the Navigation Tree. Right click on the '''Port Definition''' item in the '''Observables''' section of the Navigation Tree and select one of the items: '''Plot S Parameters''', '''Plot Y Parameters''', '''Plot Z Parameters''', or '''Plot VSWR'''. In the first three cases, another sub-menu gives a list of individual port parameters. Keep in mind that in multi-port structures, each individual port parameter has its own graph. You can also see a list of all the port characteristics data files in EM.Cube's data manager. To open data manager, click the '''Data Manager''' [[Image:data_manager_icon.png]] button of the '''Simulate Toolbar''' or select '''Simulate > Data Manager''' from the menu bar or right click on the '''Data Manager''' item of the Navigation Tree and select Open Data Manager... from the contextual menu or use the keyboard shortcut '''Ctrl+D'''. Select any data file by clicking and highlighting its row in the table and then click the '''Plot''' button to plot the graph in '''EM.Grid'''. By default, the S parameters are plotted as dual magnitude-phase graphs, while the Y and Z parameters are plotted as dual real-imaginary part graphs. The VSWR data are plotted on a Cartesian graph. You change the format of complex data plots. In general complex data can be plotted in three forms: