Jerdvisanop Chakarothai

Dosimetry of a Reverberation Chamber for Whole-Body Exposure of Small Animals

In order to determine the whole-body exposure level in small animals inside a reverberation chamber (RC), an ideal electromagnetic (EM) environment has often been assumed inside an RC for numerical dosimetry analyses. In an RC where a strong EM coupling between antennas and animals exists, however, such an EM environment is difficult to be realized, so that it remains unknown whether or not the RC could provide a target dosimetry. In this study, we adopt the Poggio-Miller-Chang-Harrington-Wu-Tsai formulation of the method of moments to directly determine the specific absorption rate (SAR) of a rectangular dielectric phantom placed in an RC, and we demonstrated its validity via comparing the numerical temperature rise with those obtained from experiments. We then demonstrated a nonideal EM environment inside an RC using a prolate spheroid as an exposure target. Furthermore, we performed dosimetry evaluation for multiple rat-shaped tissue-equivalent phantoms inside the RC and determined their whole-body average SARs for many orientations and arrangements of the phantoms. Finally, based on our numerical results, we offered a design rule when using an RC as a whole-body exposure system for small animals.

Estimation of current distribution by near-field measurement

Approaches to estimate the current distribution on EM source such as antennas and printed circuit board (PCB) by measuring the near-field distribution are demonstrated. These approaches include the matrix inversion method and sampled pattern matching (SPM) method. All of these approaches require accurate near-field measurement and field-current inverse algorithm. Experimental results are presented and compared with the numerical results, confirming the accuracy and performance of these methods.

A Hybrid MoM/FDTD Method for Dosimetry of Small Animal in Reverberation Chamber

A hybrid approach combining the method of moment (MoM) and the finite-difference time-domain (FDTD) method is applied to a problem of evaluating specific absorption rates (SARs) in a small rat inside a reverberation chamber (RC). The hybrid method is an alternative technique used to overcome the problem of poor convergence for a solution of the FDTD method in analyzing the RC. First, the RC with a dipole or a helical antenna is designed numerically to function properly at 2 GHz. Then, the MoM/FDTD method is used to calculate the whole-body average SAR (WBA-SAR) for a small animal inside an open cavity. The results are compared with those obtained with the FDTD method in order to verify accuracy. The SAR distributions when stirrers are rotating are also determined. Finally, it is numerically found that the ratio of the WBA-SAR of the 118 g rat to the mean squared electric field is 0.149 (mW/kg)/(V/m) 2 and an input antenna power of 2.2 W is required to produce a WBA-SAR of 4 W/kg for our designed chamber.

SAR assessment of a human body exposed to electromagnetic fields from a wireless power transfer system in 10 MHz band

Wireless power transfer (W PT) technology has attracted great attention for its high transfer efficiency with a long transfer range. Concerning biological hazards caused by strong electromagnetic fields in proximity to the WPT system, the dosimetry of the WPT system needs to be investigated in detail. In this paper, exposure assessment of the WPT system with a homogeneous cylinder in various operating situations possible was performed to characterize the dosimetry. It was found that the maximum allowable input power of the WPT system is restricted based on the whole-body average specific absorption rate (SAR) rather than the peak 10g-average SAR for some exposure conditions. Finally, a hybrid MoM/FDTD method is us ed to calculate the induced electric field inside the realistic human body and maximum allowable power into the WPT system.

Convergence of a Single-Frequency FDTD Solution in Numerical Dosimetry

In this paper, we propose a new approach for determining convergence in finite-difference time-domain (FDTD) method for numerical dosimetry at a single frequency. Both amplitude and phase of the FDTD calculation are simultaneously determined and traced in the time domain in order to evaluate the convergence of the solution and terminate the FDTD computation. Validity of our approach has been demonstrated via dosimetry of a homogeneous dielectric sphere in comparison with Mies analytical results at frequencies from 100 kHz to 100 MHz. Induced SARs calculated by the proposed approach are in good agreement with Mies results, having differences in the whole-body average SAR for less than 9%. It is also found that convergence of the FDTD solutions is related only to permittivity, conductivity, and time-step interval. Finally, the proposed approach has been applied to analysis of an inhomogeneous realistic human body of a Japanese adult exposed to an electromagnetic (EM) plane wave and to localized EM fields radiated from a wireless device at intermediate frequencies around 10 MHz.

Evaluation of EM absorption characteristics in realistic adult and child models in vicinity of wireless power transfer systems

There are public concerns about biological hazards caused by strong electromagnetic near-fields produced by WPT systems. In particular, absorbed power averaged over mass and the specific absorption rates (SARs) in a child model tend to show a higher value than those in adults. In this paper, we investigate numerical exposures of adult and child models in proximity of a WPT system in 7 MHz bands. We then compare the dosimetry results with the levels prescribed in the ICNIRP guidelines to find the maximum allowable input power of the system. Finally, we found that the exposures of human models in vicinity of the WPT system is highly localized, where the peak 10g average SAR is higher than the whole-body average SAR in all exposure scenarios in this study.

Exposure Evaluation of an Actual Wireless Power Transfer System for an Electric Vehicle With Near-Field Measurement

In this paper, we propose an experimental approach for determining the internal electric field for exposure evaluation of wireless power transfer (WPT) systems by using measured magnetic near-field data. Two WPT systems are fabricated and used in the measurements: one without ferrite tiles, and the other with ferrite tiles and a metal plate. The amplitude and phase of the magnetic near field in the vicinity of the WPT systems are then measured by using in-house magnetic-field probes and a near-field measurement system. Numerical dosimetry of human exposure is performed using the measured near field as an incident field in the impedance method to derive the internal electric field strength inside numerical human models. Validation of the proposed approach has been demonstrated by comparing measurement results with those obtained from numerical simulations. Additionally, the coupling factor, which represents the relationship between the incident magnetic field and the induced electric field in the human body, at different distances is derived for realistic exposure scenarios.

Numerical Techniques for SAR Assessment of Small Animals in Reverberation Chamber

Reverberation chamber (RC) has been recently developed and used in bio-electromagnetic (EM) field research for investigation of possible adverse health effect of EM waves to human body. Concerning the use of an RC as an exposure device, accurate dosimetry or quantification of EM energy absorbed in exposed animals inside an RC is actually of importance. However, the dosimetry of animals inside an RC is one of challenging problems due to its size and complex behavior of EM fields inside the chamber. This paper is dedicated to the demonstration of three different numerical techniques developed for dosimetry of small animals exposed to EM fields inside an RC at microwave frequencies. First we briefly review procedures of each numerical method and clarify its advantages, disadvantages, and range of applications. Then we demonstrated their validity by either experiments or cross-verification. Finally we discuss the results obtained from each numerical technique.

Hybrid approach of SPM and matrix-inversion to estimate current distribution of electromagnetic radiation source

A hybrid approach using the sampled pattern matching (SPM) method and matrix inversion is utilized to estimate current distribution of electromagnetic radiation sources with multi-mode current distribution. The current distribution of a loop antenna is detected as an example. The capability of the hybrid method in handling practical problems is demonstrated by estimating the amplitude and phase of electric current sources.

Derivation of some expansions of near-field gain of antennas in lossy medium

To estimate SAR (specific absorption rate) of the wireless devices operated in HF band, the behavior of the near-field gain in the near-field region of the reference antennas operated in the tissue-equivalent liquid is required. Some asymptotic expansions of the near-field gain of the reference antennas are derived by using an integral form of the transmission coefficient between the antennas and expansion theorem.