Near-Field Absorption in Prolate Spheroidal Models of Humans Exposed to a Small Loop Antenna of Arbitrary Orientation

The power absorption characteristics of the prolate spheroidal model of an average man have been studied when the model is exposed to the near fields of an arbitrarily located small loop antenna. An integral equation is formulated and the fields radiated by the loop are expanded in terms of the vector spherical harmonics. This equation is then solved using the extended boundary condition method (EBCM,). For three different loop-spheroid configurations, the power distribution and the average SAR have been calculated as a function of the frequency and the separation distance. It is shown that the results obtained for separation distances larger than lambda /2 agree well with those obtained from the plane wave exposure case. Furthermore, the average SAR value calculated as a function of separation distance for the case where the magnetic dipole moment is aligned parallel to the major axis of the spheroid are found to oscillate around the constant value obtained from the H-polarized plane wave exposure case. On the other hand, the average SAR values for the E-polarization case (magnetic dipole is parallel to the spheroidal minor axis) are found to increase monotonically with the decrease in separation distance. It is also shown that despite the complicated nature of the near fields, the absorption characteristics can still be explained in terms of the variations of the incident radiation. These loop results, together with those obtained from other simple soures, can be used as building blocks in arriving at a qualitative understanding of the near-field absorption characteristics for more general exposure cases.     

Near-Field Absorption Characteristics of Biological Models in the Resonance Frequency Range (Short Paper)

In this paper, we utilized the new iterative extended boundary condition method (IEBCM) to calculate the near-field absorption characteristics of a spheroidal model of man in the resonance frequency range. These calculations complement our previous near-field results in the preresonance frequency range. Calculations were made for simple sources such as an electric dipole and a small current loop antenna. The near fields of these sources are known exactly and hence helped in explaining the absorption results in terms of the field components of the incident radiation. Numerical results for the normalized average SAR values are presented as a function of frequency for different near-field separation distances from the sources. It is generally observed that while the far-field results converge to the plane wave values, different near-field absorption characteristics occur for the two different sources. It was possible in both cases to explain the differences in the near-field average SAR values in terms of the incident near fields from the sources.     

Absorption Characteristics of Prolate Spheroidal Models Exposed to the Near Fields of Electrically Small Apertures

Irradiation of prolate spheroidal models of biological models by the near fields of electrically small apertures is analyzed. The solution procedure involves the replacement of the aperture source by an equivalent configuration of electric and magnetic dipoles. The specific absorption rate (SAR) induced in the irradiated object is then calculated using the extended boundary condition method (EBCM). Numerical results are presented for the exposure case where the incident electric field at the spheroid location is parallel to the major axis of the model. This polarization is associated with the maximum low-frequency absorption in biological models and, hence, is the most important polarization case in microwave dosimetry.     

Comparison of the absorption characteristics of block and prolate spheroidal models of man exposed to near fields of short electric dipole

It is shown that the average specific absorption rates (SAR's) in a prolate spheroidal model of man are essentially the same as those for a block model of man irradiated by a short electric dipole at 27.12 MHz, even in the near fields. For purposes of average SAR, this allows use of the simpler and less expensive prolate spheroidal calculations.     

Irradiation of Prolate Spheroidal Models of Humans in the Near Field of a Short Electric Dipole

Analysis of the near-field irradiation of prolate spheroidal models of humans and animals by a short electrical dipole is described. The method of solution involves an integral equation formulation of the problem in terms of the transverse dyadic Green's function and expanding the fields irradiated by a short dipole in terms of the vector spherical harmonics. The extended boundary condition method (EBCM) is employed to solve the integral equations. The power distribution and the average specific absorption rate (SAR) are calculated and plotted as a function of the separation distance. It is shown that for a dipole placed along the major axis of the spheroidal (k-polarization), and for a very short separation distance, d= 0.15 lambda, the relative power values at both ends of the spheroid are about 40 compared with the ratio of 15 in the planewave exposure case. Furthermore, the calculated average SAR values as a function of the separation distance were found to oscillate around the constant value obtained from the planewave irradiation case. Differences between the near-and far-field exposure cases occurred only at separation distances shorter than 0.5 lambda where the magnitudes of the electric and magnetic energy densities are higher than the time-average radiation power density.     

Limitations of the Cubical Block Model of Man in Calculating SAR Distributions

Block models of man which consist of a limited number of cubical cells are commonly used to predict the internal electromagnetic (EM) fields and specific absorption rate (SAR) distributions inside the human body. Numerical results, for these models, are obtained based on moment-method solutions of the electric-field integral equation (EFIE) with a pulse function being used as the basis for expanding the unknown internal field. In this paper, we first examine the adequacy of the moment-method procedure, with pulse basis functions, to determine SAR distributions in homogeneous models. Calculated results for the SAR distributions in some block models are presented, and the stability of the solutions is discussed. It is shown that, while the moment-method, using pulse basis functions, gives good values for whole-body average SAR, the convergence of the solutions for SAR distributions is questionable. A new technique for improving the spatial resolution of SAR distribution calculations using a different EFIE and Galerkin's method with linear basis functions and polyhedral mathematical cells is also described.     

Whole-body SAR in spheroidal adult and child phantoms in realistic exposure environment

In the past, the absorption of electromagnetic fields in a human body has been studied for a homogeneous worst-case single plane wave exposure or for a deterministic heterogeneous exposure, i.e. a measured incident electric field distribution. The work presented in this report, investigated the absorption in homogeneous spheroidal human body phantoms in a realistic exposure environment for frequencies ranging from 150 to 950 MHz using a statistical multipath exposure tool. The absorption has been investigated for five different sizes of the spheroid model. It is reported that the highest absorption occurs in the smallest phantom and that the ICNIRP reference levels do not satisfy the absorption limits for a realistic exposure scenario. Therefore, ICNIRP reference levels should be adapted to provide compliance to the basic restrictions for realistic exposure scenarios.     

Theoretical and Experimental Evaluation of Power Absorption in Elongated Biological Objects at and Beyond Resonance

The recently developed iterative extended boundary condition method (IEBCM) is used to calculate the average specific absorption rates (SAR's) in biological models of an average man and a sitting rhesus monkey at and beyond their resonance frequencies. The average SAR's for these objects are also experimentally determined in scaled saline phantoms. Close agreement thus found between the calculated and the measured SAR values validates the postresonance absorption calculations made using the novel IEBCM. Also, in order to check the adequacy of using the mixed basis functions in the IEBCM procedure, the SAR in a capped cylindrical model of an average man is calculated and the result verified experimentally. It is, therefore, concluded that to achieve an improved computational efficiency in the IEBCM, the choice of the basis functions for a particular lar subregion should be based on the geometry of that subregion.     

Dependence of the Occupational Exposure to Mobile Phone Base Stations on the Properties of the Antenna and the Human Body

This study assesses human exposure in the close vicinity of mobile phone base station antennas by finite-difference time-domain simulations. The peak spatial average specific absorption rate (SAR) and the whole-body average SAR are analyzed in three different anatomical models (55–101 kg) with respect to the basic restrictions for occupational exposure. The models are at distances between 0.5 and 4 m from various antenna types operating at frequencies ranging from 450  to 2140 MHz. The validity of the simulations is confirmed by an analysis of the impact of the mesh resolution on local and whole-body average SAR and by experimental validation of the numerical models. The results demonstrate that the whole-body absorption generally determines the maximum permissible antenna output power for collinear array antennas. Local exposure depends on various effects that fluctuate strongly among individuals. In particular for short antennas, the peak spatial average SAR can be more restrictive than the whole-body absorption because they may only expose a fraction of the body. Therefore, compliance must be demonstrated for both quantities.      

Average SAR and SAR Distributions in Man Exposed to 450-MHz Radiofrequency Radiation

Fifth-scale phantom models were exposed to 2450-MHz electromagnetic fields to obtain the average specific absorption rate (SAR) and SAR distribution in man exposed to 1 mW/cm/sup 2/ 450-MHz radiofrequency radiation for various polarizations and body positions. The average SAR was measured calorimetrically and SAR distribution was determined thermographically using an interactive computer system, The mean SAR, as averaged over the body, remained relatively constant at 0.050 W/kg, with a standard deviation of +-0.007 W/kg for all polarizations and body postures considered in the study. Peak SAR values were as high as 0.650 W/kg, occuring typicaly in the wrist.     

Preliminary studies: far-field microwave dosimetric measurements of a full-scale model of man.

Measurements of microwave heating were made in a full-size, upright human model. The 75-Kg model, composed of electrically simulated muscle, was placed in the far-zone of a standard-gain horn inside an absorber-lined chamber. Pulsed energy at 1.29 GHz was obtained from a military radar transmitter (AN/TPS-1G) and produced radiation at 6-14 mW/cm2 average power density at the location of the model. Microwave heating at the front surface was measured at nine locations on the phantom. Measurements at several depths within the phantom were also made at a central location to gain information on the depth-of-penetration of the microwave energy. Results of the frontal surface measurements and of the penetration study permitted a calculation of the approximate whole-body average specific absorption rate (SAR) when the model's long axis was parallel to the E-field vector. For a normalized power density of 1 mW/cm2 at a frequency of 1.29 GHz, the whole-body average SAR approximated 0.03 W/Kg. This result agrees well with theoretical predictions based on absorption in prolate spheroidal models of man.     

Effects of ear-connection modeling on the electromagnetic-energyabsorption in a human-head phantom exposed to a dipole antenna field at835 MHz

Specific absorption rate (SAR) compliance measurements for wireless personal devices are usually performed in anatomically correct phantoms. The phantoms have a lossless spacer to model the external ear (pinna). The use of a lossless spacer has been questioned. The purpose of this paper was to study the effects of the lossy pinna by E-field and numerical assessments validated with thermal measurements. The measurements were performed in a box with a rectangular well simulating a pinna compressed during phone usage. Various openings were created in the septum separating the box and the well to simulate the connection between the head and pinna. A balanced half-wave dipole was used as the RF source. The results of this study lead to the conclusion that for SAR values averaged over 1 gram, within our current probe resolution, complicated lossy pinna structures are not necessary     

Inverse Source Problem in the Spheroidal Geometry: Vector Formulation

A formulation based on Lagrangian optimization and spheroidal vector wave functions is presented for the vector electromagnetic inverse source problem of deducing a time-harmonic current distribution that is confined within a spheroidal volume, that generates a prescribed radiation field, and that is subject to given constraints on the source functional energy, which characterizes antenna current level, and the source's reactive power, which models antenna resonance matching. The paper includes computer simulation results illustrating the derived inverse theory.     

Long-wavelength analysis of near-field irradiation of prolate spheroidal models of man and animals

Expressions are derived for the induced internal electric fields and average specific absorption rate (s.a.r.) in prolate spheroidal models of man and experimental animals irradiated by electromagnetic (e.m.) fields having arbitrary orientation and arbitrary impedance. These expressions are of value in estimating the s.a.r. in man and test animals exposed to e.m. near fields when the wavelength is long compared to the dimensions of the body.     

Numerical optimization of 3-D SAR distributions in cylindricalmodels for electromagnetic hyperthermia

Numerically optimized SAR (specific absorption rate) distributions in a source free 3-D multilayered concentric cylindrical (MCC) model are presented. The fields were expanded in the modes of the MCC. Cost functions which specify mathematically the relative weight assigned to differences between an SAR distribution and a desired SAR distribution were defined. The coefficients of the modes, which minimize the cost function, were obtained using gradient search optimization methods. The optimized SAR distributions shown were computed using three different cost functions and two different radial locations for the center of the region where the desired SAR is largest. A five-layered model, including the outer water layer for cooling and improved matching with the source, was used. The frequency was 70 MHz. The current and charge distributions computed on a perfectly conducting cylindrical surface just outside the model are also shown. The surface current and charge distributions depend strongly on the relative importance of the cost for acute heat and systemic heat. A technique is developed for generating a new set of basis functions for reducing the number of unknowns to be optimized. We suggest that the approach shown could be useful in designing hyperthermia applicators.     

An Empirical Formula for Broad-Band SAR Calculations of Prolate Spheroidal Models of Humans and Animals

An empirical relation for calculating approximate values of the average specific absorption rate (SAR) over a broad-frequency range for any prolate spheroidal model is derived for E-polarized incident plane waves. This formula provides a simple and inexpensive method for calculating the SAR for human and animal models, which otherwise requires complicated and expensive methods of calculation. The formula satisfies the f/sup 2/ SAR behavior at lower frequencies, the resonance characteristic at intermediate frequencies, the 1/f behavior past resonance, and the dependence on the dielectric constant at the geometrical optics limits. An expression for the resonance. frequency f/sub 0/ in terms of the dimensions of the model is also derived. The unknowm expansion coefficients were determined by curve-fitting all the data available in the second edition of the Radiofrequency Radiation Dosimetry Handbook. Numerical results obtained from the empirical relations are generally in good agreement with those calculated by other methods. Limitations of the formula and suggestions for its improvement are also discussed.     

Method for accurate measurement of complex permittivity of tissueequivalent liquids

A microwave method for determining the electrical properties of lossy liquids is presented. The method is optimised for measuring the complex permittivity of human tissue equivalent liquids typically used together with human head phantoms in SAR (specific absorption rate) measurements. The method presented has a considerably better performance than conventional methods     

Four Element Wideband Rectangular Dielectric Resonator Antenna Terminated in a Bio-medium

In this paper, a new four element rectangular dielectric resonator antenna (RDRA) for 5.0 GHz WLAN/WiMAX band is proposed. The simulation results for its radiation characteristics and the specific absorption rate (SAR) distribution in a homogenous bio-medium (muscle layer) for different antenna-to-muscle layer spacings are presented at different frequencies in 4.9–5.9 GHz band. The input characteristics of the proposed antenna and a single element RDRA of same size as the element are compared. The effect of changing antenna input power on maximum SAR value is also analyzed. The simulation study has been carried out using CST Microwave Studio software.     

Radiation Characteristics of Ingestible Wireless Devices in Human Intestine Following Radio Frequency Exposure at 430, 800, 1200, and 2400 MHz

In order to assess the compliance of ingested wireless device (IWD) with related international safety guidelines, the studies on the biological effects and the signal intensity of an IWD in two realistic human body models using the finite-difference time-domain (FDTD) method are reported in this paper. Simulation studies are carried out in 21 scenarios where the IWD is placed at seven positions with three orientations, for each of the two human models. Specific absorption rate (SAR), temperature rise, near fields and far fields are analyzed in the 21 scenarios at four selected operation frequencies of 430 and 800 MHz, and 1.2 and 2.4 GHz, respectively. Our study indicates that the radiation intensity outside of the human body decreases with the increase of operation frequency. Furthermore, the radiation characteristics of the IWD are orientation and position dependent. The polarization of electric field outside of the human body is similar to that of the Radio Frequency (RF) source. As far as the compliance of safety is concerned, among all the simulated scenarios the maxima of the averaged-1g and averaged-10g SARs can reach 3.71 W/kg and 1.37 W/kg at the input power of 25 mW, respectively.      

Various Optimization Problems of Electromagnetic Power Absorption in Homogeneous and Heterogeneous Phantoms

Over the last decade, problems related to electromagnetic power absorption by human bodies or phantoms have been motivating a large scientific and technical effort. In many situations, the ultimate goal is to characterize and get as close as possible to the minimum or maximum value of a fraction of the total dissipated power (TDP) in a given volume. As is well known in mathematics, to find an optimum of a function can be a very tough question, whose complexity strongly depends on the constraints imposed, and the number of variables. Considering this point, the following paper gives a brief review of some optimization problems involving radiating sources and phantoms, and tries to highlight their complexity. The particular situation of a two-dimensional (2-D) source, radiating close to homogeneous or two-layer cylindrical phantoms, is analyzed. Thanks to an equivalent junction model derived from a modal-based approach, the sources minimizing and maximizing the TDP in these phantoms are obtained, as well as the sources giving the maximal or minimal difference between the TDP in two chosen phantoms. Finally, the obtained results are discussed, and shortly extrapolated to the problem of specific absorption rate (SAR) due to wireless devices