At certain frequencies, when the human head becomes a resonant structure, the power absorbed by the head and neck, when the body is exposed to a vertically polarized plane wave propagating from front to back, becomes significantly larger than would ordinarily be expected from its shadow cross section. This has possible implications in the study of the biological effects of electromagnetic fields. Additionally the frequencies at which these resonances occur are not readily predicted by simple approximations of the head in isolation. In order to determine these resonant conditions an anatomically based model of the whole human body has been used, with the finite-difference time-domain (FDTD) algorithm to accurately determine field propagation, specific absorption rate (SAR) distributions and power absorption in both the whole body and the head region (head and neck). This paper shows that resonant frequencies can be determined using two methods. The first is by use of the accurate anatomically based model (with heterogeneous tissue properties) and secondly using a model built from parallelepiped sections (for the torso and legs), an ellipsoid for the head and a cylinder for the neck. This approximation to the human body is built from homogeneous tissue the equivalent of two-thirds the conductivity and dielectric constant of that of muscle. An IBM SP-2 supercomputer together with a parallel FDTD code has been used to accommodate the large problem size. We find resonant frequencies for the head and neck at 207 MHz and 193 MHz for the isolated and grounded conditions, with absorption cross sections that are respectively 3.27 and 2.62 times the shadow cross section.

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A method for importing data from computer-aided design (CAD) files for a mobile telephone into finite-difference time-domain (FDTD) simulation software is described. Although the FDTD method is well suited for the bio-electromagnetic simulations and has become the method of choice for most researchers in this area, there may be some limitations to its use. Limitations include, the description of the source (e.g., the mobile telephone) and the fact that the FDTD method requires large amounts of memory and computational power. The size of the computational space is dependent upon both the physical size of the model and its resolution. Higher frequencies of operation require higher resolutions. This could place the solution of some problems outside the capabilities of the technique. Often the telephone has to be represented by a plastic covered metal box, which approximates the shape of the actual device. The paper addresses these problems. Wires and circuit boards inside the telephone can act as resonant elements if they are not shielded. This potential problem is also considered. The large problem size associated with high-resolution FDTD simulations is accommodated by the use of a parallel implementation of the FDTD method (run on an IBM SP-2). The techniques developed here are used for two anatomically based head models that have been developed from magnetic resonance imaging (MRI) of two human subjects. The usefulness of the techniques developed and comparisons of the specific absorption rates (SAR's) in the two models are discussed.

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The development of an antenna capable of radiating a band-limited pulse with minimal distortion, negligible loss, and significant directivity is reported. A design was derived by modification of the conventional logperiodic dipole array to permit independent feeding of each dipole. This was modeled with a time-domain integral equation program and iterated to find a design that minimized phase dispersion across the operating band. The optimal design was realized in hardware, using a printed structure to feed the dipoles independently; this was located in the normal plane to prevent distortion of the radiated fields. When tested, the antenna was found to give a good quality radiated pulse in the main beam direction, with weaker and dispersed waveforms in other directions, indicating significant directive gain. KeywordsBroad-band antenna, electromagnetic susceptibility, log-periodic dipole array, pulsed radiation, timedomain Disciplines Computer Engineering | Computer SciencesComments ©1999 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder. This Abstract-The development of an antenna capable of radiating a band-limited pulse with minimal distortion, negligible loss, and significant directivity is reported. A design was derived by modification of the conventional log-periodic dipole array to permit independent feeding of each dipole. This was modeled with a time-domain integral equation program and iterated to find a design that minimized phase dispersion across the operating band. The optimal design was realized in hardware, using a printed structure to feed the dipoles independently; this was located in the normal plane to prevent distortion of the radiated fields. When tested, the antenna was found to give a good quality radiated pulse in the main beam direction, with weaker and dispersed waveforms in other directions, indicating significant directive gain.Index Terms-Broad-band antenna, electromagnetic susceptibility testing, log-periodic dipole array, pulsed radiation, timedomain integral equation method.