INTERACTION BETWEEN SMALL THIN-WIRE ANTENNAS AND THE HUMAN HEAD STUDIED WITH THE ADI-FDTD/MoMTD HYBRID METHOD

The design of electrically small antennas is a topic of current interest connected with the growing development of mobile-communication devices that require their components to be even smaller and lighter. From the designers' point of view, apart of seeking antennas with the appropriate radiation characteristics, it is critical both to know how the presence of the head affects the performance of antennas, as well as how much energy is absorbed by the head in front of the antenna in order to meet the maximum Specific Absorption Rate (SAR) directives. In this work this mutual interaction of small thin-wire antennas in front of the human head has been studied. The thin-wire small antennas considered were genetically optimized (GA) monopoles with either prefractal or Euclidean (zigzag or meander) geometries (M. F. Pantoja et al., "GA design of wire pre-fractal antennas and comparison with other euclidean geometries", IEEE Antennas and Wireless Propagation Letters, 2, 2003. pp. 238-241). In the first part of this work we study how the presence of the head modifies the resonance frequency and the input impedance of these antennas depending on the specific geometry, showing that the resonance frequency has greater variation in the prefractal antennas than in their non-fractal counterparts. In the second part, we focus our attention on the calculation of the SAR of a human head in front of those antennas, and show that the SAR maps are almost identical for all of them, since the main radiation actually comes from the excitation point. In order to meet the recent IEEE recommendation on SAR calculation (Std. C95.1-1999, Std. C95.3-2002), we have developed a new algorithm managing the average volume to be exactly a 1gram cube. A biologically realistic model of a human head, obtained from a Magnetic Resonance Image (MRI) has been employed. The head is divided into 122x133x155 voxels 2 mm side, specifying for each one the constitutive parameters (permittivity, permeability and conductivity) at 900 MHz from a Debye model. The novel Alternating Direction Implicit Finite Difference Time Domain (ADI-FDTD) method has been used to simulate the head and the free-space around the antenna, and the Method of Moments in Time Domain (MoMTD) to simulate the thin-wire antenna. Making use of Huygens' principle, the hybridization of ADI-FDTD with MoMTD (R. G. Martín et al. Time-domain hybrid methods, in Time Domain Techniques in Computational Electromagnetics. Pp. 133-172. WIT press) yields a tool which permits to simulate the full problem of the antenna in front of the head, thus overcoming the well-known inability of FDTD techniques to accurately handle arbitrarily-oriented thin wire structures. The use of this hybrid tool was crucial to get accurate results due to the intricate shapes of the antenna geometries considered. The employment of ADI-FDTD, instead of the classical Yee-FDTD, allows us to overcome the conditional stability limit of the latter while keeping its ability to treat general inhomogeneous bodies. Thus, the time increment can be increased over this limit, independently of the space increment, to reduce the computation time. Furthermore the hybrid code ADI-FDTD/MoMTD has proven to reduce the apparition of the late-time instabilities which, under some circumstances, arise in the hybrid Yee-FDTD/MoMTD code.