This example demonstrates how XFDTD can be used as the total solution for evaluating the performance and field effects of a radiating device.

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An important topic of discussion today is the impact of radio frequency electromagnetic fields on human tissue. Standards for field exposure limits are under consideration by an international group of government, industry, and academic leaders involved with the IEEE Standards Coordinating Committees 28 and 34 which includes among its members Dr. Raymond J. Luebbers, President of Remcom Incorporated. A review of the activities of these committees can be found in the recent article "Safety Standards for Exposure to RF Electromagnetic FIelds," by J. M. Osepchuk and R. C. Petersen in IEEE Microwave Magazine, Volume 2, Number 2, June 2001, pages 57-69. Among the recommendations of these committees is the selection of the Finite-Difference, Time-Domain (FDTD) method as the preferred technique for computational simulations of field effects on human tissue.

This example simulation demonstrates how XFDTD can be used as the total solution for evaluating the performance and field effects of a radiating device. Here the device in question is a simplified representation of a cellular telephone. A more complex device could also be imported using the XFDTD CAD file importers.

First let's consider the handset alone to determine its characteristics and attempt to apply an appropriate match to the feed. The handset is chosen as a 12x6x2 cm perfectly conducting box with a monopole antenna meshed using the XFDTD editor into a 2mm cubical grid. Both the box and the monopole are enclosed in a dielectric material with a relative permittivity of 2. A Gaussian pulse voltage source with a 50 ohm internal resistance is applied to the base of the monopole and the box. Following the simulation, the input impedance of the feed versus frequency can be plotted as a Smith Chart (Figure 1). The desired frequency for this handset is 900MHz, and at that frequency we can see that the input impedance is about 46-j55.

The S11 plot is shown in Figure 2 and demonstrates that the handset is not well tuned for 900MHz. An appropriate reactive component can be added to the feed in the form of an inductor placed between the feed and the monopole (Figure 3). The simulation is then run again and the impedance and S11 plots indicate a much better match at the 900MHz design frequency (Figure 4 and Figure 5). A third simulation of this tuned handset is now run with a sinusoidal input at 900MHz in place of the Gaussian pulse so that we can compute a number of far-zone gain patterns for the handset. The antenna characteristics of the steady-state run shown in Figure 6 are consistent with the previous pulsed-input calculation.

The Remcom High-Fidelity head mesh is in a 2x2x2.5 millimeter grid with the top of the head oriented in the +Z direction. However our handset is in a 2mm cubical grid and the monopole of the handset is also oriented in the +Z direction. With the Remcom Remesh/Rotate module we are able to quickly re-orient and remesh the head and merge with it the handset geometry so that the phone is aligned between the ear and mouth and both meshes are in the 2mm cubical grid (Figure 7). We also are able to automatically specify the electrical parameters of the numerous head tissues with the press of a button.

Once the geometry file is ready, we can apply the 900MHz sinusoidal input to the feed of the handset with our previous matching network in place. We will also save a number of steady-state quantities including:

• the specific absorption rate (SAR) in the entire head
• the 1 and 10 gram averages of the SAR values including the peak value locations
• the whole-body average SAR throughout the head
• the electric field magnitudes in several slices of the head
• conduction currents on the outer surfaces of the head
• time-domain electric field values at numerous test points in the head and phone
• and various antenna characteristics including input impedance, S11, and efficiency

Following the simulation, the time-domain electric fields at the test points are plotted to ensure that the calculation has properly reached steady state. One example of a test point in the center of the head is shown in Figure 8. The SAR information for the head is visible in Figure 9 where the peak SAR, 1g averaged SAR, and 10g averaged SAR values and locations can be seen after the input power applied to the handset has been adjusted to 600mW. In Figure 10, Figure 11, and Figure 12 the SAR images for slices containing each peak are shown. Figure 13 shows a three-dimensional view of the head/handset geometry with the conduction currents present on the surface of the head. Figure 14 and Figure 15 show steady-state electric field magnitudes in two cross-sectional slices of the head.

The impact of the head on the antenna parameters quite dramatic. In Figure 16 we can see that the efficiency of the antenna has dropped from 100% to about 27% while the input impedance has shifted as well. The change in the gain patterns can be seen in Figure 17 and Figure 18 where the blue lines indicate the patterns of the handset alone while the red lines show the same patterns in the presence of the head. The antenna patterns in the plane of the head can also be computed by again using the Remcom Remesh/Rotate menu to generate the unrotated antenna patterns with the coordinate system adjusted so that the head is vertical and the phone is tilted (Figure 19).