Nicolas Chavannes

Effects of Hand Phantom on Mobile Phone Antenna Performance

With the continuous trend to smaller mobile devices with more functionality and more wireless bands, the possible insufficient over-the-air (OTA) becomes a growing concern for service providers. In addition to reduced radiation efficiency by smaller antennas in smaller phones, the head and in particular the hand may further impair radiation performance. The aim of this paper is to investigate the influence of the various parameters of the hand upon the OTA performance criteria to support the standardization process of the hand phantom and to provide the tools for optimization of handheld transmitters. The results reveal the following order of significance for small variations in hand parameters: 1) position of index finger; 2) distance between phone case and palm; 3) position of the other fingers; 4) size of the hand; 5) dielectric parameters; 6) wrist length and tilt. Some configurations may lead to a slight increase of the peak spatial SAR in the head. OTA assessment without the hand does not provide a good measure of phone performance in real life. However, good reproducibility of evaluations requires a precise definition of the hand, in particular its geometry and the phone position within the hand.

Suitability of FDTD-based TCAD tools RF design of mobile phones

This paper discusses the general suitability and possible limitations of an enhanced finite-difference time-domain (FDTD) simulation environment for straightforward and efficient RF design of complex transmitters. The study was conducted using a current commercially available multi-band mobile phone. Simulations were conducted in free space and with various dielectric loads, whereby different parameters were evaluated such as impedance, efficiency, far-field as well as near-field distributions of e-fields and h-fields, and the specific absorption rate (SAR). The results were compared to measurements. In addition, mechanical-design issues that showed a significant influence on the electromagnetic (EM) field behavior could be predicted by simulations and were experimentally reproduced. The accurate prediction of all essential performance parameters obtained by straightforward simulations suggests that appropriately enhanced software packages are suitable for device design in industrial research and development environments with few limitations, provided flexible graphical user interfaces (GUIs) and graded meshes combined with local grid-refinement schemes are available.

A Patch Antenna Design for Application in a Phased-Array Head and Neck Hyperthermia Applicator

In this paper, we describe a specifically designed patch antenna that can be used as the basis antenna element of a clinical phased-array head and neck hyperthermia applicator. Using electromagnetic simulations we optimized the dimensions of a probe-fed patch antenna design for operation at 433 MHz. By several optimization steps we could converge to a theoretical reflection of -38 dB and a bandwidth (-15 dB) of 20 MHz (4.6%). Theoretically, the electrical performance of the antenna was satisfactory over a temperature range of 15degC-35degC, and stable for patient-antenna distances to as low as 4 cm. In an experimental cylindrical setup using six elements of the final patch design, we measured the impedance characteristics of the antenna 1) to establish its performance in the applicator and 2) to validate the simulations. For this experimental setup we simulated and measured comparable values: -21 dB reflection at 433 MHz and a bandwidth of 18.5 MHz. On the basis of this study, we anticipate good central interference of the fields of multiple antennas and conclude that this patch antenna design is very suitable for the clinical antenna array. In future research we will verify the electrical performance in a prototype applicator.

A new 3-D conformal PEC FDTD scheme with user-defined geometric precision and derived stability criterion

A new conformal finite-difference time-domain (CFDTD) updating scheme for metallic surfaces nonaligned in the grid is presented in this paper. In contrast to existing conformal models, the new model can be formulated with the original Yee FDTD update equation. Therefore, the proposed scheme can be easily added in standard FDTD codes even if the codes are already parallelized or hardware-accelerated. In addition, based on the commonly used conventional stability criterion, a derivation of the stability is presented and based on the conformal geometric information, a time step reduction formula is presented. The time step reduction is used as a user-defined parameter to tradeoff speed versus accuracy. The achievable geometric precision is optimized to a given time step. Therefore, even with the conventional time step (no reduction) the presented scheme profits from the conformal discretization. To show the performance and the robustness of the proposed scheme canonical validations and two real world applications were investigated. A broadband low profile (circular) antenna was successfully simulated showing the benefit of the conformal FDTD method compared to the conventional scheme. Furthermore, a CAD based mobile phone was conformally discretized and successfully simulated showing that the proposed scheme is highly suited for the simulation of advanced engineering problems.

Human Exposure to Close-Range Resonant Wireless Power Transfer Systems as a Function of Design Parameters

In this study, human exposure to close-range wireless power transfer (WPT) systems operating in the frequency range 0.1-10 MHz with coil diameters up to 150 mm are investigated. Approximation formulae, which include scaling factors derived from numerical simulations that take variations of complex human anatomies into consideration, are proposed to conservatively estimate human exposure with respect to the most authoritative exposure guidelines. The approximation has been verified for two precommercial prototype WPT systems, the first of which, a 5-W system operating at 100 kHz, has been evaluated in this study; the second system been verified was reported in a separate study and operates at 6.78 MHz with a nominal current of 5.4 A rms. Based on the results obtained, the optimal operational frequency range for WPT with respect to compliance with exposure safety guidelines is revealed to be ca. 1-2.5 MHz. In summary, this study provides novel and insightful information for the design of an exposure-compliant close-range magnetic resonant WPT system.

Novel conformal technique to reduce staircasing artifacts at material boundaries for FDTD modeling of the bioheat equation

The modeling of thermal effects, often based on the Pennes Bioheat Equation, is becoming increasingly popular. The FDTD technique commonly used in this context suffers considerably from staircasing errors at boundaries. A new conformal technique is proposed that can easily be integrated into existing implementations without requiring a special update scheme. It scales fluxes at interfaces with factors derived from the local surface normal. The new scheme is validated using an analytical solution, and an error analysis is performed to understand its behavior. The new scheme behaves considerably better than the standard scheme. Furthermore, in contrast to the standard scheme, it is possible to obtain with it more accurate solutions by increasing the grid resolution.

Benefits of superficial hyperthermia treatment planning: Five case studies

Purpose: To demonstrate the benefits of treatment planning in superficial hyperthermia. Materials and methods: Five patient cases are presented, in which treatment planning was applied to troubleshoot treatment-limiting hotspots, to select the optimum applicator type and orientation, to assess the risk associated with metallic implants, to assess the feasibility of heating a deeper seated tumour, and to analyse the effective SAR coverage resulting from arrays of multiple incoherent applicators. FDTD simulation tools were used to investigate treatment options, either based on segmented or simplified anatomies. Results: The background, approach and model implementation are presented per case. SAR cross-sections, profiles and isosurfaces are visualized to predict the effective SAR coverage of the target and the location of the maximum power absorption. In addition, the followed treatment strategy and the implications for the clinical treatment are given: for example, higher temperatures, relief of treatment limiting hot-spots or increased power input. Conclusions: Treatment planning in superficial hyperthermia can be applied to improve clinical routine. Its application supports the selection of the optimum technique in non-standard cases, leading to direct benefits for the patient. In addition, treatment planning has shown to be an excellent tool for education and training for hyperthermia technicians and physicians.

Quantitative validation of the 3D SAR profile of hyperthermia applicators using the gamma method.

For quality assurance of hyperthermia treatment planning systems, quantitative validation of the electromagnetic model of an applicator is essential. The objective of this study was to validate a finite-difference time-domain (FDTD) model implementation of the Lucite cone applicator (LCA) for superficial hyperthermia. The validation involved (i) the assessment of the match between the predicted and measured 3D specific absorption rate (SAR) distribution, and (ii) the assessment of the ratio between model power and real-world power. The 3D SAR distribution of seven LCAs was scanned in a phantom bath using the DASY4 dosimetric measurement system. The same set-up was modelled in SEMCAD X. The match between the predicted and the measured SAR distribution was quantified with the gamma method, which combines distance-to-agreement and dose difference criteria. Good quantitative agreement was observed: more than 95% of the measurement points met the acceptance criteria 2 mm/2% for all applicators. The ratio between measured and predicted power absorption ranged from 0.75 to 0.92 (mean 0.85). This study shows that quantitative validation of hyperthermia applicator models is feasible and is worth considering as a part of hyperthermia quality assurance procedures.

Influence of the Hand on the Specific Absorption Rate in the Head

The influence of the users hand holding a mobile phone to the ear on the peak spatial-average Specific Absorption Rate (psSAR) averaged over any 1 g and 10 g of tissue in the head is investigated. This study is motivated by recent reports that found substantial increases in psSAR by the presence of the hand in some cases. Current measurement standards prescribe the measurement of SAR in a head phantom without a hand present. The mechanisms of interaction between the hand and mobile phone models are studied. Simulations and measurements at 900 and 1800 MHz have been conducted to complement the understanding of the hand grip parameters leading to higher SAR in the head. Numerical simulations were conducted on four mobile phone models, and parameters such as the palm-phone distance and hand position were varied. Measurements of 46 commercial mobile phones were made, and the maximum psSAR with different hand positions and palm-phone distances was recorded. Both simulations and measurements have found increases in the psSAR in the head of at least 2.5 dB due to the presence of the hand. Furthermore, the psSAR is sensitive to the hand grip, i.e., the variations can exceed 3 dB.

Reliable prediction of mobile phone performance for realistic in-use conditions using the FDTD method

The objective of this study was to analyze whether advanced simulation platforms provide the effectiveness, accuracy, reliability, and efficiency to predict impairment of mobile-phone RF performance under various usage patterns. The investigation was based on the mechanical CAD data of a commercial phone with two alternative antennas. Three significant hand positions were modeled and evaluated with the device against the SAM head. The results demonstrated high reliability and suitability for providing decision rationale for the design of complex high-end multi-band mobile phones.