Tero Uusitupa

SAR variation study from 300 to 5000 MHz for 15 voxel models including different postures

An extensive study on specific absorption rate (SAR) covering 720 simulations and 15 voxel models (18-105 kg) has been performed by applying the parallel finite-difference time-domain method. High-resolution whole-body models have been irradiated with plane waves from 300 MHz to 5 GHz by applying various incoming directions and polarizations. Detailed results of whole-body SAR and peak 10 g SAR are reported, and SAR variation in the dB scale is examined. For an adult, the effect of incoming direction on whole-body SAR is larger in the GHz range than at around 300-450 MHz, and the effect is stronger with vertical polarization. For a child (height approximately 1.2 m), the effect of incoming direction is similar as for an adult, except at 300 MHz for horizontal polarization. The effect of the phantom (18-105 kg) on whole-body SAR is larger at around 2-5 GHz and at vertical 300 MHz (proximity of whole-body resonance for the child) than at around horizontal 300-900 MHz. Body posture has little effect on whole-body SAR in the GHz range, but at around 300-450 MHz, one may even expect a 2 dB rise in whole-body SAR if posture is changed from the standing position. Posture affects peak 10 g SAR much more than whole-body SAR. The polarization of the incident electric field may have an effect of several dB on whole-body SAR. Between 2 and 5 GHz for adults, whole-body SAR is higher for horizontal than for vertical polarization, if the incoming direction is in the azimuth plane. In the GHz range, horizontal polarization gives higher whole-body SAR, especially for irradiation from the lateral direction. A comparison between homogeneous and heterogeneous models was done. A homogenized model underestimates whole-body SAR, especially at approximately 2 GHz. The basic restriction of whole-body SAR, set by ICNIRP, is exceeded in the smallest models ( approximately 20 kg) at the reference level of exposure, but also some adult phantoms are close to the limit. The peak 10 g SAR limits were never exceeded in the studied cases. The present ICNIRP guidelines should be revised by lowering the reference levels, especially at around 2-5 GHz.

Performance of convolutional PML absorbing boundary conditions in finite-difference time-domain SAR calculations

The performance of perfectly matched layer (PML) absorbing boundary conditions is studied for finite-difference time-domain (FDTD) specific absorption rate (SAR) assessment, using convolutional PML (CPML) implementation of PML. This is done by investigating the variation of SAR values when the amount of free-space layers between the studied object and PML boundary is varied. Plane-wave exposures of spherical and rectangular objects and a realistic human body model are considered for testing the performance. Also, some results for dipole excitation are included. Results show that no additional free-space layers are needed between the numerical phantom and properly implemented CPML absorbing boundary, and that the numerical uncertainties due to CPML can be made negligibly small.

Usability Studies on Approximate Corrugation Models in Scattering Analysis

Often an actual scatterer geometry is replaced with a computationally more economical model. For example, if a major part of the scatterer surface is corrugated, one solution is to analyze scattering by replacing the corrugated areas with flat impedance surfaces. But then a question arises: what is actually the error in the scattered field phase and magnitude due to this approximation? In this paper, planewave scattering from partly corrugated structures is studied. The analysis is based on integral equations (IE) and method of moments (MoM). Two-dimensional geometry and fields are assumed. TMz and TEz cases are analyzed separately. It is investigated how the magnitude and phase of the scattered TMz and TEz fields depend on the used corrugation model: exact-geometry results are compared with the approximate results in a wide frequency band. Also the phases of the TMz and TEz fields must be predicted correctly, if studying the polarization state of the scattered field. The paper ends with an example where a corrugated corner reflector is investigated via an exact model and via different approximate models

The effect of finite-difference time-domain resolution and power-loss computation method on SAR values in plane-wave exposure of Zubal phantom

In this paper, the anatomically realistic body model Zubal is exposed to a plane wave. A finite-difference time-domain (FDTD) method is used to obtain field data for specific-absorption-rate (SAR) computation. It is investigated how the FDTD resolution, power-loss computation method and positioning of the material voxels in the FDTD grid affect the SAR results. The results enable one to estimate the effects due to certain fundamental choices made in the SAR simulation.

Mode transformer for hard-surface waveguides

Mode transformation effect inside a corrugated circular waveguide filled with gyrotropic material, e.g., magnetoplasma or ferrite, is considered. The corrugation is in axial direction and the depth of the corrugation is a quarter wave length forming a boundary condition equal to a hard surface (HS). It is assumed that the material is slightly anisotropic and gyrotropic. This can be achieved at certain frequency range with a proper axial magnetic field strength. The eigenwaves are elliptically polarised hybrid-mode fields, which propagate with slightly different propagation factors. This difference in propagation factors causes the mode transformation.

Fabrication of photonic crystal waveguide elements on SOI

Photonic crystal angle elements fabricated in silicon-on-insulator (SOI) are reported. These elements are modelled using three-dimensional finite difference time domain (FDTD) method. Photonic crystals have a two-dimensional trigonal lattice structure with cylindrical air columns. The period of the crystal is approximately 420 nm and the cylinder diameter is about 330 nm. Defect creation is performed by removing air columns from certain lattice sites. The SOI-layer is one micron thick and it also defines the column height. The FDTD modelling results imply that photonic crystal angle elements with lower height do not exhibit proper light transmission at the telecommunications wavelength window, 1550 nm. FDTD modelling results give higher transmission for TE-polarised light than for TM-polarisation. For better light coupling a taper element with widened waveguide end is designed.

Analysis of finite-length gyrotropic hard-surface waveguide

[1]xa0Electromagnetic fields and mode transformation effect are theoretically investigated inside a hard-surface (HS) circular waveguide. The waveguide section causing a mode transformation is filled with gyrotropic material, e.g., magnetoplasma or ferrite. In this paper, the gyrotropic material is ferrite. The effect of gyrotropy is taken into account exactly, i.e., no approximation of small gyrotropy or small anisotropy is made. There are different ways to implement HS boundary. One can use longitudinal strips at the surface of a dielectric coating or longitudinal corrugation. In principle, if the corrugation is in axial direction and the depth of the corrugation is effectively a quarter wavelength, a boundary condition equal to hard surface is obtained. Inside the gyrotropic waveguide section the eigenwaves are elliptically polarized and are propagating with different propagation factors. The difference in propagation factors causes a mode transformation. Reflection and transmission formulas are derived for a structure of type isotropic-gyrotropic-isotropic in a general case. Transformer examples are given.

On the modelling of ceramic combline RF-filter

In order to design filters effectively, it is necessary to be able to predict frequency characteristics of a certain kind of filter structure suitably fast. Considering ceramic filters, instead of 3-D field analysis program, a circuit simulator program is often a good choice for a modelling tool, especially in the early stages of new filter design.

Tunable polarisation transformers for axially corrugated waveguides

Tunable polarisation transformers for axially corrugated waveguides are proposed. The polarisation transformer consists of a uniaxially anisotropic waveguide section and a ferrite filled waveguide section. The polarisation of the field is changed when propagating through the anisotropic waveguide section. The state of the polarisation depends on the ratio of the axial field components which can be changed by external magnetic field in ferrite filled waveguide section.

Application of multiconductor transmission‐line theory to combine filter design

An efficient design method for combline filters is considered. The basic assumption is that the filter can be approximated as multiple multiconductor transmission lines (MTL) cascaded so that a circuit simulator can be used along the design process. Although numerical field computation is needed to obtain MTL parameter values, the mean simulation time is short.xa0© 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 113–118, 2000.