The XFdtd^® "thin wire" material is useful for simulating wire elements which have diameters much less than the cell size.

The XFdtd "thin wire" material is useful for simulating wire elements which have diameters much less than the cell size.In general a single cell edge PEC wire in XFdtd will have an effective radius of approximately 1/5 of the cell size.The thin wire material may be used when the radius of the wire is approximately 1/4 of the cell size or less and needs to be specified exactly.A significant amount of memory and execution time can be saved by using large XFdtd cells with the thin wire material rather than dimensioning the XFdtd mesh to the size of the wire.The important advantage of using the thin wire capability of XFdtd is that the thin wire radius can be specified independently of the XFdtd mesh cell size.

An example using thin wire material is shown in Figure 1 with a simple dipole antenna geometry. The dipole is made of 21 FDTD cells with a dimension of 5.2381 mm on a side to give a total wire length of 11 cm.Note that every cell of the dipole is entered as thin wire material, including the feed port location. The wire is entered with a radius of 0.22 mm or L/500 where L is the length of the antenna.The FDTD calculation is excited with a short Gaussian pulse and the feed port voltage and current are saved for computing the input impedance versus frequency. The results are compared with a solution from the moment method code, ESP4, and with a non-thin wire perfectly conducing (PEC) mesh edge FDTD dipole in Figure 2. The non-thin wire PEC edge dipole used cubical cells of 1.089mm (1.089mm/5 = 0.22mm).

The same 11cm dipole is simulated using different FDTD mesh cell sizes corresponding to 11, 21, 41, and 81 FDTD cells to determine the sensitivity of the cell size on the impedance calculation when using the thin wire material. As the cell size is decreased, the size of the XFdtd space increases, but the thin wire radius is held constant at L/500 and L/150 where L is the antenna length (11cm). Again, the FDTD calculation is excited with a short Gaussian pulse and compared with results from ESP4. The results are shown in Figure 3 and Figure 4. The important result is that the dipole impedance is, within numerical accuracy, independent of the FDTD mesh size but is determined by the thin wire radius.The 81 cell dipole is not used in Figure 4 since the L/150 radius is too large for the corresponding cell size.

Even with this simple example there are some clear savings using the thin wire material in XFdtd.The 21-cell dipole simulated with thin wire material required about 2.6 MB of RAM and ran in less than 4.5 minutes. The non-thin wire small cell PEC material dipole used 3.4 MB and ran in nearly 12 minutes.The ESP4 result required only 900KB but took about 6.5 minutes to run in 50MHz increments, each using 100 segments on the wire. For more complex structures with a larger number of cells, the difference in cell size will produce much more dramatic time and memory savings.All calculations were performed on a Sun Ultra 1/140.

See the paper "An Improved FDTD Model for the Feeding Gap of a Thin-Wire Antenna" by So-ichi Watanabe and Masao Taki, IEEE Microwave and Guided Wave Letters, Vol. 8, No. 4, April 1998, pg. 152-154, for more information about this example. The ESP4 computer code was produced by Dr. Edward Newman of The Ohio State University and is referenced and discussed in the book Advanced Engineering Electromagnetics by C. Balanis.