10.1 Rules of thumb and hints for FDTD simulation in QuickWave

This subsection gathers some rules of thumb and useful hints for FDTD simulation in QuickWave. Those remarks are grouped below by subject:

 

-                 The reference plane of transmission line ports should be placed at the distance of at least of 4 – 5 cells from the port towards the circuit to assure accurate S‑parameters extraction.

-                 NTF surface should be usually placed as close the object as possible provided that there is at least 3 cells distance. In case of dipoles radiating in free space, bigger distance can be required.

-                 The distance between the NTF surface and absorbing boundary must be at least 3 cells. In case of using PML ABC its thickness should be taken into account when placing the NTF surface.

 

-                 It should be remembered that the FDTD method applied with particular meshing is accurate up to particular frequency limit. As a rule-of-thumb, the FDTD cell size cannot be bigger than 0.1 of the wavelength at the limit frequency, in other words, the cell size should be set to assure at least 10 cells per wavelength. It must also be noted that severe accuracy requirements or/and complicated boundary shape can further intensify the limit in a particular application. Of course, the software is able to produce the simulation results also for the frequencies above the limit but its accuracy and applicability will need to be carefully reviewed by the user.

-                 Mesh snapping planes of electric type at the edges of metal objects enable singularity corrections at those edges. E.g. Mesh snapping planes, when applied to the edges of the strip in the planar structure, with 2-3 cells across the strip width, assure similar impedance approximation as the one obtained with 7 – 8 cells per strip width.

-                 In case of any doubts if the geometry of the project was correctly defined and correctly understood by the mesh generator, it is strongly recommended to verify the meshing results using Test Mesh option of QW-Simulator.

 

-                 The symmetry plane must coincide with mesh boundary (the meshed region cannot extend beyond the symmetry plane).

-                 If symmetry plane is placed in the port plane, at least one of the checkboxes (H or V) in the port settings dialogue, informing about the symmetry, should be checked.

-                 If symmetry plane is used in the radiation scenario, where the NTF box is used, the circuit must be situated in the positive X- and/or Y- and/or Z- direction from the symmetry plane.

-                 If symmetry plane is used in the radiation scenario, where the NTF box is used, and appropriate NTF wall (or walls) must coincide with the symmetry plane.

 

-                 PML cannot be placed in a region of boundary-conforming cells of shapes other that rectangular.

-                 PML is stable only when its sides coincide with a grid termination (extend over the entire side of the meshed area) or a metal wall.

-                 MUR can be applied in any case and its implementation takes into account the conformal cells.

-                 ABC should be placed at least half wavelength from the antenna to avoid intense interactions with antenna near fields, otherwise computational errors or even algorithm instabilities may appear.

-                 The distance between the NTF surface and absorbing boundary must be at least 3 cells. In case of using PML ABC its thickness should be taken into account when placing the NTF surface.

-                 Incorrect assumption of the effective permittivity (of the medium in which the wave propagates) for MUR increases the level of spurious reflections from ABC.

 

-                 The reference plane of transmission line ports should be placed at the distance of at least of 4 – 5 cells from the port towards the circuit to assure accurate S-parameters extraction.

-                 In the case of lossless circuits, power balance can be treated as a quantitative measure of convergence of simulation. When power balance=1, the user may assume that the FDTD simulation has converged.

-                 In the case of lossy circuits, power balance is not equal unity and indicates the efficiency of energy transmission.

-                 If power balance does not converge to unity for lossless circuits, this may indicate that some of the energy is dissipated in the source port and/or load port in modes not considered in the S-parameters extraction.

-                 The power balance below cut-off is calculated properly with the extended S‑parameters displays.

-                 Calculating the power balance with extended S-parameters is available only when the results are displayed during the simulation. The display of saved results stored on disk uses always the power balance formula for standard S-parameters extraction.

-                 The range of frequency in which we calculate the S-parameters cannot exceed the spectrum of the exciting pulse defined in the port settings dialogue. If this is the case, the S-parameters may be calculated with significantly increased numerical errors and QW-Simulator gives an appropriate warning. Formally we can choose an exciting pulse of the spectrum much wider than the spectrum needed for S-parameter analysis. This kind of choice will not change the final result to which the simulation converges. However, it may significantly prolong the computing. The reason is that a wide-band pulse may excite high-Q resonance outside the band of interest. During the simulation process the energy accumulated in the circuit at these frequencies is dissipated slowly and thus the convergence of the S‑parameter characteristics to their final shape may be much slower. Thus it is recommended as a standard approach to keep the frequency band of the exciting pulse equal to the band declared for S‑parameter extraction.

-                 It is a known advantage of the FDTD method that the number of frequency points for S-parameters calculation has little effect on the computing time. This allows for example choosing the step small enough to detect narrow (high-Q) resonances. Nevertheless we suggest that some restraint be exercised in the choice of the frequency step. Usually a reasonable number of frequency points is supposed to be between 100 and 500 and in some cases should be increased to 1000 or so. A higher number of frequency points can slow down the Results display. It also affects the memory occupied during the software runtime and needed for the storage of the results on disk.

 

-                 For lossless structures, where all ports are included in the S-Parameters post-processing, the efficiency can be treated as a quantitative measure of convergence of simulation: When efficiency=100%, the user may assume that the FDTD simulation has converged.

-                 Radiation efficiency is calculated if NTF frequencies coincide with frequency points set for S-Parameters post-processing. If this condition is not obeyed for particular NTF frequency, Ef will be 0.

-                 Radiation resistance is calculated if NTF frequencies coincide with frequency points set for FD-Probing post-processing. If this condition is not obeyed for particular NTF frequency, Rr will be 0.

-                 Power injected by source is calculated if NTF frequencies coincide with frequency points set for S-Parameters post-processing. If this condition is not obeyed for particular NTF frequency, Pi will be 0.

-                 Current injected by source is calculated if NTF frequencies coincide with frequency points set for FD-Probing post-processing. If this condition is not obeyed for particular NTF frequency, |I|^2 will be 0.

-                 NTF box must surround entirely the radiating object.

-                 NTF box must be placed entirely in the homogenous medium.

-                 Outside the NTF box, only absorbing boundaries should be placed.

-                 NTF surfacecan be relatively close to the radiating object. Three FDTD cells of separation are usually sufficient in the case of antennas fed by transmission lines. A bigger distance may be required in the case of dipoles radiating in free space since they can generate quasi-static mode around them and perturb the correct calculation of radiated power (with "NTF fields" option).

-                 The separation between NTF surface and absorbing boundary must not be smaller than three FDTD cells. Otherwise the far field calculation may be incorrect. Note that QW-Editor displays full cells in XY plane but half-cells in XZ or YZ planes. Thus when watching the QW-Editor displays in XZ or YZ we need to make sure that a distance of at least six half-cells is maintained.

-                 The absorbing boundary can be placed quite close to the antenna providing that the antenna does not generate evanescent modes sliding along the boundary. In the latter case, from the physical viewpoint, an absorbing boundary should be neutral to the wave. However, in practical numerical solutions it may generate small amounts of energy and inject them back into antenna. This may deteriorate the accuracy of the analysis of high-Q resonant antennas and sometimes even lead to instability of the FDTD simulations. Such a possibility should be considered in the analysis of planar antennas and is exemplified in User Guide 3D: Patch antenna. Here let us only note that a cure to this problem is placing the absorbing boundary at a bigger distance from antenna. The distance of half a wavelength is usually sufficient.

 

-                 The project name should be the same as the name of the UDO object, used for optimisation.

-                 The frequency range and step in the Objective configuration tab must be identical with those set in QW-Editor for the relevant post-processing.

-                 QW-Simulator MTGOMP is not recommended for parameters sweep and optimisation routines.