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The subject of this example is a single layer patch antenna. The feed details for the patch are not considered in detail in this example and the feed stripline is excited using a simplified feed structure proposed by Luebbers[7]. The model is based on original work by Sheen[8] who constructed several microstrip circuit elements for comparison of measurement with FDTD model (the next two examples are taken from this work also). The dimensions of the patch are shown in Figure 1. The patch is mounted on a Duroid substrate with relative permittivity of 2.2.
Figure 1 shows the feed point on the stripline connected to the patch. The feed is implemented by exciting a line of electric field nodes between the perfectly conducting ground plane and the stripline. The drive circuit attached to the cells beneath the stripline looks like a Thevenin network equivalent. The internal source resistance of the feed is useful in these calculations to dissipate the energy within the domain and this leads to a much shorter time period for the analysis.
The excitation function in this example is a Gaussian which is broadband in nature, thus the input characteristics over the bandwidth DC to 20GHz can be calculated from a single computation.
Strictly speaking the lower frequency limit is imposed by the behaviour of the boundary conditions and no calculation is able to reach DC realistically. In this class of calculation the location of the boundary is important as a result of the evanescent wave modes generated locally around the patch. Transmissive boundaries within computational methods are designed to terminate waves with an impedance of free space. Near field and evanescent modes are not of this nature and so radiation boundary conditions have difficulty terminating these waves. At lower frequencies the near field and evanescent modes extend further from the patch and so the efficiency of the RBC falls as a result. The PML RBC performs well under these circumstances (Gedney[?]).
The computational domain illustrating the patch geometry is shown in Figure 2.
Figure 3 shows a carpet plot of the sub-surface electric field on the patch during the decay of the transient excitation. The AVI illustrates the time dependent variation of the field on the patch (a higher quality download is provided of size 2Mb).
Figure 4 shows the input impedance of the patch compared to experiment.
Figure 4 -