5.2.1    Transmission line port termination

Transmission line port (template port) will serve as a source or load depending whether Source or Load option is chosen. Transmission line load is equivalent to transmission line source with excitation equal to zero. For the transmission line load, zero exciting amplitude is assumed.

In case of transmission line load the mode template calculation is also required similarly to the case of transmission line source. There are six mode types available for transmission line port loads, similarly as for the transmission line port sources: TEM, multiTEM, planeTEM, low-order rectangular waveguide modes, low-order circular waveguide modes, low- and high-order analytical rectangular and circular waveguide modes, Arbitrary mode.

Transmission line port loads may be used for S-parameter extraction. To allow application of the differential method, they must be accompanied by reference planes. The reference plane is the plane at which the data for S-parameter calculation is collected. If the port is not accompanied by reference plane, it will serve as a load but it will be ignored from the point of view of S-matrix analysis. As default, the transmission line port load is accompanied by reference plane.

The user should place the reference plane in the proper position taking into account the following criteria:

- The port must be defined on a segment of a transmission line such that its geometry and material filling between the port plane and the reference plane, and three cells beyond the reference plane, do not change.

- The reference plane must not be closer than two cells from the port plane but for better accuracy it should rather be moved at least 4-5 cells towards the circuit.

- The S-parameters are calculated using the vector product of the simulated fields and the mode templates to filter out the influence of the modes other than the considered one. Nevertheless the presence of other modes may have some influence on the accuracy, especially when calculating wide-band characteristics of inhomogeneously filled lines. That is why it is recommended that the reference plane be kept at a safe distance from the discontinuities, which produce a high, content of unwanted modes.

- The position of the reference plane rectangle in the plane perpendicular to the direction of propagation should be exactly the same as of the corresponding port. If you move the reference plane graphically on the screen please observe that it is moved only in the direction perpendicular to the port plane.

Note that a reference plane defined in the Input Interface remains at its user-defined position only if this position coincides with FDTD cell edges (i.e., a plane where tangential E-fields are available in QW-Simulator). If not, the reference plane is shifted by the software to the nearest E-field plane for ports oriented along the Z-axis;  or to the nearest E-field plane in the positive X- or Y-direction for ports oriented along the X- or Y-axis, respectively.

Let us also note that it is possible to define in the same place two different ports operating on two different transmission line modes (like for example vertical and horizontal polarisation in a square waveguide). In this case we define in the same place two transmission line ports and assign to each of them different mode template by a proper choice of excitation parameters for template generation. Such a process has been exemplified in User Guide 3D: Septum polariser, User Guide 3D: First insight template generation - low-order waveguide modes, and User Guide 3D: Septum polariser as a six-port with higher-order waveguide modes.

Transmission line ports are matched to the appropriate transmission lines by means of impedance template. The matching is realised by introducing superabsorbing boundary conditions in the plane right behind the port. To assure high level of absorption, appropriate effective permittivity for absorbing boundary conditions needs to be assumed. QuickWave allows the user to manually define the permittivity effective for absorption or take advantage from Auto option, which assigns the value of effective permittivity calculated during the template mode generation process.