Characterization of the Interactions Between a 60-GHz ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 12, DECEMBER 2012

Characterization of the Interactions Between a 60-GHz Antenna and the Human Body in an Off-Body Scenario Nacer Chahat, Student Member, IEEE, Maxim Zhadobov, Member, IEEE, Laurent Le Coq, Stanislav I. Alekseev, Member, IEEE, and Ronan Sauleau, Senior Member, IEEE

Abstract—Interactions between a 60-GHz microstrip patch antenna array designed for off-body communications and the human body are investigated numerically and experimentally. First, the array is characterized in free space and on a homogeneous skin-equivalent phantom in terms of reflection coefficient, radiation pattern, and antenna efficiency. Second, a multiphysics dosimetry technique is proposed and implemented to determine the specific absorption rate (SAR) and incident power density (IPD) from the heating dynamics measured on an experimental phantom using a high-resolution infrared (IR) camera. The SAR and IPD are found by fitting the analytical solution of the bio-heat transfer equation to the measured heating dynamics. The experimental and numerical results are in a very good agreement. They demonstrate that for the considered scenario the impact of the body on the antenna characteristics is almost negligible, and even relatively high radiated powers (up to 550 mW) result in exposure levels that are below international exposure limits. Index Terms—Millimeter waves, body area networks (BAN), body-centric wireless communications, high-data-rate communications, skin-equivalent phantoms, specific absorption rate (SAR).

I. INTRODUCTION

B

ODY-AREA NETWORKS (BAN) provide wireless networking between wearable antennas and other communicating devices placed on, off, or implanted directly in the body [1]. They are highly attractive for a number of applications including sport, military, personal healthcare, entertainment, smart home, etc. Today there is a growing interest in investigating the performance of BAN in the 60-GHz band, since they offer significant advantages compared to those operating at lower frequencies [2]. First, the large available bandwidth (7 GHz worldwide, unlicensed) allows reaching very high data rates (up to 5 Gb/s) [3]. Second, due to the presence of oxygen molecules in the Manuscript received January 11, 2012; revised May 22, 2012; accepted July 02, 2012. Date of publication August 02, 2012; date of current version November 29, 2012. This work was supported by “Agence Nationale de la Recherche” (ANR), France under Grants ANR-09-RPDOC-003-01 (Bio-CEM project) and ANR-09-VERS-003 (METAVEST project), and by “Centre National de la Recherche Scientifique (CNRS)”, France. N. Chahat, M. Zhadobov, L. Le Coq, and R. Sauleau are with the Institute of Electronics and Telecommunications of Rennes (IETR), UMR CNRS 6164, University of Rennes 1, 35042 Rennes, France. (e-mail: [email protected]). S. I. Alekseev is with the Institute of Cell Biophysics, Russian Academy of Sciences, 142292 Pushchino, Russia. Digital Object Identifier 10.1109/TAP.2012.2211326

atmosphere, the strong attenuation of 60-GHz signals [4] results in high level of security and low interference with adjacent BAN [5]. Finally, the size of the on-body equipments is strongly reduced compared to similar systems already available at microwaves. Only a few studies on 60-GHz BAN have been presented in the literature so far, and very encouraging results have been already reported. First, it has been demonstrated that 60-GHz BAN mitigate the cochannel interference, thus allowing a greater number of BAN users to be colocated within a certain area [5]. Besides, using ray tracing methods, propagation studies performed for interconnecting various subsystems worn by soldiers have shown that it is possible to establish a BAN with reliable radio link and coverage [6]. Furthermore, two 60-GHz antennas for on-body communications have been recently designed [7] and the electromagnetic and thermal interactions of millimeter waves with the human body have been reviewed in [8]. In parallel, a tissue-equivalent phantom emulating the dielectric properties of the human body in the 55–65 GHz range has been recently proposed [9]. Such phantoms are very attractive to evaluate the antenna characteristics in body-centric environments (reflection coefficient, radiation pattern, efficiency, etc.) and to quantify the power absorbed within the body. In contrast to real measurements performed on human bodies and for which the results are expected to fluctuate due to interindividual differences [10] and body movements, the use of phantoms allows obtaining reproducible results under well-controlled experimental conditions. At microwaves, it is widely accepted that antennas placed in close proximity to a lossy medium experience strong power absorption, radiation pattern distortion, shift in resonance frequency, and changes in the input impedance, e.g., [11]–[14]. Therefore, when placed close to the human body, wearable antennas need to be designed to operate in a robust way so that the influence of the body on the antenna performance is minimized. At microwave frequencies, patch antennas have been identified as one of the best solutions for off-body communications [1]. These are simple and low-cost structures, and their radiation at broadside allows improving the power budget link and reducing radiation towards the body direction. However, at millimeter waves, the electromagnetic coupling between antennas and the human body, and the possible perturbations of antenna characteristics due to the body remain unexplored. In addition, in this frequency range, a particular attention

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CHAHAT et al.: HARACTERIZATION OF THE INTERACTIONS BETWEEN A 60-GHZ ANTENNA AND THE HUMAN BODY

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must be paid to the power absorbed in the body since this absorption is only superficial (the penetration depth in tissues is limited to a few tenths of a millimeter [8]). Therefore, to better understand the interactions between a wearable antenna and the human body, a representative fourpatch antenna array for off-body communications has been designed and characterized numerically and experimentally, both in free space and on a skin-equivalent phantom. Besides, the specific absorption rate (SAR) and incident power density (IPD) distributions on the phantom have been measured using a multiphysics dosimetry technique specifically implemented for this study. This paper is organized as follows. The antenna array is briefly described in Section II-A. Its main characteristics when placed in free space or on a skin-equivalent phantom are discussed in Sections II-B and C, respectively. The electromagnetic and thermal dosimetry results (numerical and experimental data) are given in Section III. Finally, conclusions are drawn in Sections IV. II. ANTENNA DESIGN AND CHARACTERISTICS A. Antenna Model An on-body antenna needs to be as compact as possible to be integrated inside a transceiver. It has to be efficient with minimal power absorption inside the human body that behaves as a highly lossy dispersive dielectric material at millimeter waves. The antenna also has to be light weight and, in some particular cases, conformable to fit to the human body shape. Because of the high atmospheric attenuation at 60 GHz and limitations on the radiated power, medium-gain antennas ( dBi) are often required [15]. Indeed, in controlled environments, line-of-sight (LOS) channels can be efficiently exploited using medium-gain passive antennas, whereas directive beam steering antennas are desirable for nonline-of-sight (NLOS) channels so as to comply with the power link budgets [2], [16], [17], [21]. In this paper, we only consider LOS scenarios and thus restrict our consideration to passive medium-gain antennas. To satisfy the aforementioned criteria, a microstrip-fed fourpatch single-layer antenna array has been chosen. It is printed on a thin RT Duroid 5880 substrate ( m, ). The layout is represented in Fig. 1(a). A single rectangular patch antenna typically provides a 7 dBi gain; a 2 2 antenna array is chosen here to reach a gain of 12 dBi with about the same beamwidth in E- and H-planes. The interelement spacing is selected to achieve a good trade-off between high gain and low side lobes. Similar 2 2 antenna arrays have already been reported in a multilayer configuration [19] or fed by a coaxial probe [18], [20], which would make them difficult to fabricate on flexible or textile substrates. Hence, here all patches are fed using a single-layer corporate feed network. The antenna is linearly-polarized along -direction, and for measurement purposes it is mounted on a 3 mm-thick ground plane [see Fig. 1(b)] to avoid significant substrate bending and to achieve an accurate and stable placement of a V-connector. For the up-coming BAN applications, this kind of antennas will be directly integrated into the clothing or wearable devices.

Fig. 1. 2 2-patch single-layer antenna array at 60 GHz. (a) Schematic representation of the antenna model and dimensions. (b) Manufactured antenna with a V-connector.

Fig. 2. Measured reflection coefficient of the antenna. In free space. On the skin-equivalent phantom.

B. Antenna Performance in Free Space is represented in The measured reflection coefficient Fig. 2 in solid line. It remains below dB from 59 to 65 GHz. In addition, the radiation patterns in - and -planes are plotted in Fig. 3 at 60 GHz. The gain was measured by the comparison method with a 20-dBi standard horn, and the directivity is found from an intensive 3-D radiation pattern measurement. At this frequency, the measured and computed gains equal 11.8 dBi ( dB), which demonstrates a very good agreement between numerical and experimental results. From the measured 3-D radiation pattern at 60 GHz, the measured directivity was assessed to be equal to 13.9 dBi ( dB). This is in perfect agreement with the computed directivity (13.9 dBi). Comparison of the measured and simulated directivities with both measured and simulated gains leads to antenna

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Fig. 3. Measured and simulated normalized radiation patterns at 60 GHz in - and -planes. Measurement in free space. Measurement on the phantom. Simulation in free space. Simulation on the skin-equivalent phantom. (a) -plane (front radiation), (b) -plane (front radiation), (c) -plane (back radiation: copol only), (d) -plane (back radiation: copol only).

efficiencies of 62% in practice and in theory. This efficiency value is typical in V-band for this kind of antennas and could be further improved, for instance using a fused quartz substrate [19] instead of RT Duroid 5880. The measured copolarization components are in good agreement with the computed ones (see Fig. 3). The cross-polarization level is below dB at broadside in - and -planes. It is higher in measurements than in simulations; this is attributed to spurious radiation of the coaxial probe and scattering on the V-connector and metallic support. The cross-polarization discrimination could be further improved using multilayer antenna designs [19], [22]. C. Antenna Performance on the Body At microwaves, it is well known that the characteristics of on-body antennas are affected by the body depending on the antenna type (patch, dipole, PIFA, etc.), the distance from the human body, and the operating frequency [1], [11]–[14]. Here we investigate the interactions between the antenna array presented above and the human body using a numerical and experimental millimeter-wave semisolid phantom emulating the dielectric properties of the human skin [9]. This phantom is also used for SAR and IPD measurements, as explained in Section III.

For the numerical modeling, Debye model has been used to express the complex permittivity of the phantom [25] (1) where is the angular frequency, is electrical conductivity, is the static permittivity, is the optical permittivity, and is the relaxation time. The best fit of this theoretical model to the target values was obtained for s, , and . The good agreement obtained between the theoretical model and target values demonstrates that the choice of the Debye model is appropriate (see Fig. 4). The composition of the experimental phantom has been defined to coincide with the reference values of the human skin permittivity in the 55–65 GHz range [9]. The complex permittivity of the phantom was measured using an open-ended slim coaxial probe designed by Agilent Technologies (Santa Clara, CA) to determine the complex permittivity of lossy liquids and semisolids up to 67 GHz [23]. Our results are represented in Fig. 4. The phantom relative permittivity and conductivity respectively equal and S/m at 60 GHz (Table I); it is in good agreement with the reference data available in the literature ( and S/m) [24]. In the 55–65 GHz range, the maximum deviation between the reference values and the measured ones is less than 10%.


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Fig. 4. Measured and computed characteristics of the phantom. Skin refCalculated skin properties (Debye model).——Measured erence values. % of the reference values. skin-equivalent phantom. Error bars represent

TABLE I PROPERTIES OF THE SKIN-EQUIVALENT PHANTOM AND HUMAN SKIN AT 60 GHZ

It is worthwhile to note that the phantoms available at lower frequencies exhibit similar deviations [26], [27]. Besides, the small deviation of the phantom complex permittivity compared to the skin one induces SAR deviation of only 3.2%. Therefore such variations are acceptable for bioelectromagnetic studies. Moreover, at millimeter waves, a 10% deviation has a negligible impact on the antenna characteristics and SAR levels since it has a very small influence on the power reflection coefficient and penetration depth (see Table I). A cylindrical phantom with a thickness of 1 cm and a diameter of 14 cm has been manufactured to measure the reflection coefficient (see Fig. 2) of the antenna array placed on it [see Fig. 5(a) and (b)]. Comparison with measurements in free space shows that the phantom does not affect at all the in the 55–65 GHz range (see Fig. 2). Moreover, we have studied numerically and experimentally the variations of for different antenna/phantom separations [see Fig. 5(c)]. Our results show that the is not affected (deviations in the order of 0.1 dB compared to free space data), even when the radiating elements are placed at the same level as the phantom surface ( mm; in practice the phantom is drilled, and the thick ground plane is immersed into the phantom). The measured directivity of the antenna mounted on the phantom ( mm) equals to 14.1 dBi ( dB). This is in a good agreement with the computed directivity (14.3 dBi). The simulated and measured gain on the phantom equals 12.3 and 11.9 dBi ( dB), respectively. Hence the computed and measured efficiency equal 63% and 60%, respectively. The antenna gain (11.9 dBi) and efficiency (60%) remain close to the one measured in free space (11.8 dBi and 59%).

Fig. 5. Antenna mounted on a cylindrical skin-equivalent phantom. (a) Schematic representation. (b) Photograph. (c) Side view and dimensions.

The radiation pattern has been numerically and experimentally studied in free space and on a phantom. It can be seen in Fig. 3(a) and (b) that front radiations remain very slightly affected. Besides, the backward radiation was measured separately in both configurations. Whereas the measured level on the phantom is mainly reduced in the -plane [see Fig. 3(c)] due to the absorption and reflection, it remains very slightly affected in the -plane [see Fig. 3(d)]. Whereas the antenna efficiency at microwaves can be strongly affected by the body presence even for patch antennas [14], it is found here that it remains stable in V-band. III. POWER ABSORPTION IN THE BODY The antenna studied in Section II is used here to quantify the power density (PD), IPD, and SAR from the measured temperature rise of the skin-equivalent phantom. These parameters are determined using a specific multiphysics dosimetry technique based on the high-resolution infrared (IR) thermometry as described hereafter. It is worthwhile to note that the dosimetric quantity used in the International Commission of nonionizing radiation protection (ICNIRP) guidelines for frequencies above 10 GHz is the incident power density [34]. The ICNIRP recommends limiting the IPD to 20 mW/cm for the local exposure (averaged over 1 cm ) for the general public. However, as near-field exposures at millimeter waves have had a limited practical interest so far, ICNIRP guidelines do not really provide a dosimetric quantity/methodology for this type of exposures. Therefore we used here the four main dosimetric quantities, namely PD, IPD, SAR, and temperature. A. Experimental Dosimetry Setup The experimental set-up is represented in Fig. 6. A continuous-wave signal is generated by a Gunn oscillator at 60 GHz.

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Fig. 6. Multiphysics dosimetry set-up. The main parts of the millimeter-wave generator (QuinStar Technology Inc.) are the following: (1) 60-GHz tunable Gunn oscillator; (2) V-band tunable attenuator; (3) V-band power amplifiers (25 dB gain); and (4) Isolator (20 dB isolation).

After amplification, the signal is transmitted to the antenna through a set of WR-15 rectangular waveguides. The measured input power of the antenna equals to 322 mW. It has been measured using an Agilent E4418B power meter (Agilent Technologies, Santa Clara, CA, USA). A FLIR SC5000 IR camera (FLIR Systems, Wilsonville, OR, USA) operating in the 2.5–5.1 m range is used to record the heating pattern and its dynamics on the surface of the phantom. The camera is located below the phantom, on the same side as the antenna, at a distance of 34 cm (see Fig. 6). The thermal sensitivity of the camera is 0.02 C (data provided by FLIR Systems). Its spatial resolution equals 640 512 pixels. The sequence of thermal images is recorded with a sampling rate of 25 frames per second. B. Experimental Methodology The experimental protocol implemented to determine the PD, IPD, and SAR consists of three successive stages: 1) experimental determination of the peak values of the PD, IPD, and SAR; 2) measurement of the relative spatial distributions of SAR and IPD; and 3) the absolute SAR and IPD distributions are found by normalizing the relative spatial distributions with respect to their peak values. First, we determine the peak PD on the phantom surface. To this end, the temperature dynamics, recorded using the IR camera as illustrated in Fig. 6, is fitted to the theoretical model issued from the one-dimensional bio-heat transfer equation. The solution of this equation is provided in [28] for semi-infinite tissue-equivalent media. Thermal conductivity and specific heat used in the model are equal for the considered phantom to 0.5 W/(m C) and 3.5 kJ/(kg C), respectively [26]. The fitting procedure is performed by minimizing the standard deviation value varying the PD (the only unknown parameter) [29]. Once the peak PD value corresponding to the minimal standard deviation has been determined, the peak IPD and SAR can be found

Fig. 7. Temperature dynamics obtained for: (a) 30 s of exposure for the antenna mW, placed on the skin-equivalent phantom and on the filter paper ( mm, peak SAR location); (b) the antenna placed on the skin-equivalent mW, mm, peak SAR location). phantom (

as follows:

and [31], where is the mass density of the phantom equal 880 kg/m . The values of and are provided in Table I. Second, we determine the PD, IPD, and SAR spatial distributions over the phantom surface. For this purpose, the skin-equivalent phantom described previously cannot be used directly. Indeed, for some regions of the phantom, the recorded IR signal is too low to be detected reliably. However, it was demonstrated that when decreasing the thickness of the phantom, the PD value increases while its distribution does not change [32]. Thus we used a thin 0.2 mm-thick filter paper saturated with distilled water ( % of free water [32]). This allows increasing substantially the PD level and therefore signal-to-noise ratio of the IR signal without modifying the relative distributions of the PD, IPD, and SAR. This is clearly demonstrated in Fig. 7(a). A temperature increase of 0.57 C is recorded with the filter paper whereas an increment of only 0.20 C is obtained with the skin-equivalent phantom. In our experiments, both sides of the filter paper are covered by a 5 m-thick water resistant and IR-transparent plastic film. This allows avoiding water evaporation during temperature measurement. It is important to emphasize that this technique only provides information on relative distributions of PD, IPD, and SAR, but not on their absolute values [33]. The latter are found by normalizing the relative spatial distributions to their peak values.


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C. Numerical and Experimental Results The IPD and SAR distributions have been computed with CST Microwave Studio. The numerical model takes into account the feed connector and the thick ground plane, as shown in Fig. 5(c). These distributions have been measured using the methodology described above. The measured temperature dynamic and the optimized theoretical thermal model are shown in Fig. 7(b) for the peak SAR location and for an antenna/phantom separation of 5.6 mm. At the peak SAR location the temperature increase remains lower than 0.13 C for exposure duration of 10 s. The IPD and SAR spatial distributions are represented in Fig. 8. We can notice a very good agreement between simulations and measurements. Ten successive measurements have been carried out without moving the antenna and phantom to study the reproducibility of our results. The corresponding values of peak PD, IPD, and SAR are provided in Table II with the IPD averaged over 1 cm [the experimental data are presented as mean standard deviation (SD)]. A peak IPD of mW/cm , a peak SAR of W/kg, and an averaged IPD of 3.8 mW/cm have been found experimentally. The error between computed and experimental values is only 1.4% and 3.7% for the peak IPD and SAR, respectively. Besides, the SD for the PD, IPD, and SAR remains very small, which confirms the reproducibility of results. The influence of the exposure duration has been assessed. The main results are summarized in Table III for 10 s . The maximum deviation compared to the averaged measured SAR remains lower than 6%. Finally, it is worthwhile to note that, at 60 GHz, the penetration depth in the skin is very shallow (roughly 0.5 mm [8]). This leads to a localized energy absorption (essentially in the upper skin layer) and therefore to higher SAR values compared to those measured at microwaves. For the experimental determination of the PD, IPD, and SAR, we used literature data for the thermal conductivity and specific heat [26]. However, for the thermal modeling of the skin-equivalent phantom, these data may not be as accurate as for other conventional materials. In this case, it is important to know the contribution of the deviation of these parameters from their actual values to the resulting SAR. When both parameters are varied in % range, the changes in SAR are below %. This suggests that the proposed methodology implemented to determine the PD, IPD, and SAR is not very sensitive to the accuracy of the thermal parameters. The measured SAR results presented above have been obtained for an antenna/body separation of 5.6 mm because of the connector size. However, in real off-body scenarios, the distance between the antenna and the body could be very different. The variations of the peak SAR and IPD versus are represented in Fig. 9 when ranges from 1 mm to 10 mm. Here the numerical model does not take into account the feed connector and thick ground plane. For 1 mm mm, the peak SAR decreases very rapidly from 492 to 118 W/kg. Beyond 5 mm, the peak SAR fluctuates between 129 and 62 W/kg.

Fig. 8. SAR and IPD distributions at 60 GHz. (a) Numerical results for the antenna on the skin. (b) Measurements on the skin-equivalent phantom.

TABLE II MEASURED AND SIMULATED PEAK PD, IPD, SAR, AND IPD AVERAGED OVER 1 CM AT THE PHANTOM SURFACE FOR AN INPUT POWER OF 322 MW AT 60 GHZ

TABLE III INFLUENCE OF THE CONSIDERED EXPOSURE DURATION

The maximum SAR obtained for mm corresponds to an IPD of 21.4 mW/cm . For mm, the maximum IPD averaged over 1 cm , equals to 11.7 mW/cm . Therefore, for the considered antenna and for mm, input powers up to

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Fig. 9. Computed peak SAR and IPD on skin for different antenna/body separation for an input power of 322 mW at 60 GHz.

550 mW would comply with the international exposure limits fixed by the ICNIRP. However, as an IPD of 1 mW/cm results in a 0.1 C heating of the skin [8], the input power should be practically limited to 15 mW to avoid any significant heating of the skin. IV. CONCLUSION The interactions between a 60-GHz antenna array and the human body have been studied numerically and experimentally. The antenna reflection coefficient and radiation patterns have been computed and compared with measurements using a skin-equivalent phantom. It has been shown that, in spite of the small distance between the antenna and the body, the impedance matching and antenna gain are almost not affected by the presence of the body. A multiphysics dosimetric system has been implemented to investigate the temperature dynamics and power absorption in the phantom. It has been shown that the experimental and numerical results are in very good agreement. An IPD of 7.3 mW/cm and a peak SAR of 169.1 W/kg have been found experimentally for an input power of 322 mW and for an antenna/body spacing mm. Moreover, we have shown that, for the considered antenna, the exposure level of the body is below the exposure limits established by the ICNIRP provided the input power is lower than 550 mW for mm. In practice, in future wearable millimeter-wave devices, the total radiated power is expected to be restricted to several mW to reduce the power consumption. For these power levels, the off-body antennas will automatically comply with the international exposure limits. REFERENCES [1] P. S. Hall and Y. Hao, Antennas and Propagation for Body Centric Communications Systems. Norwood, MA, USA: Artech House, 2006, ISBN-10: 1-58053-493-7. [2] S. L. Cotton, W. G. Scanlon, and B. K. Madahar, “Millimeter-wave soldier-to-soldier communications for covert battlefield operations,” IEEE Commun. Mag., vol. 47, no. 10, pp. 72–81, Oct. 2009. [3] T. Baykas, C. S. Sum, Z. Lan, J. Wang, M. A. Rahman, and H. Harada, “IEEE 802.15.3c: The first IEEE wireless standard for data rates over 1 Gb/s,” IEEE Commun. Mag., vol. 49, no. 7, pp. 114–121, Jul. 2011. [4] R. C. Daniels, J. N. Murdock, T. S. Rappaport, and R. W. Heath, “60 GHz wireless: Up close and personal,” IEEE Microw. Mag., vol. 11, no. 7, pp. 44–50, Dec. 2010.

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[26] K. Ito, K. Furuya, Y. Okano, and L. Hamada, “Development and characteristics of a biological tissue-equivalent phantom for microwaves,” Electron. Commun. Japan, vol. 84, no. 4, pp. 67–77, Apr. 2001. [27] T. Takimoto, T. Onishi, K. Saito, M. Takahashi, S. Uebayashi, and K. Ito, “Characteristics of biological tissue equivalent phantoms applied to UWB communications,” Elec. Commun. Japan (Part I: Commun.), vol. 90, no. 5, pp. 48–55, May 2007. [28] K. R. Foster, H. N. Kritikos, and H. P. Schwan, “Effect of surface cooling and blood flow on the microwave heating of tissue,” IEEE Trans. Biomed. Eng., vol. 25, no. 3, pp. 313–316, May 1978. [29] S. I. Alekseev and M. C. Ziskin, “Local heating of human skin by millimeter waves: A kinetics study,” Bioelectromagnetics, vol. 24, no. 8, pp. 571–581, Oct. 2003. [30] M. Zhadobov, R. Sauleau, Y. Le Drean, S. I. Alekseev, and M. C. Ziskin, “Numerical and experimental millimeter-wave dosimetry for in vitro experiments,” IEEE Trans. Microw. Theory Tech., vol. 56, no. 12, pp. 2998–3007, Dec. 2008. [31] O. P. Gandhi and A. Riazi, “Absorption of millimeter waves by human beings and its biological implications,” IEEE Trans. Microw. Theory Tech., vol. 34, no. 2, pp. 228–235, Feb. 1986. [32] S. I. Alekseev and M. C. Ziskin, “Reflection and absorption of millimeter waves by thin absorbing films,” Bioelectromagnetics, vol. 21, no. 4, pp. 264–271, May 2000. [33] E. P. Khizhnyak and M. C. Ziskin, “Heating patterns in biological tissue phantoms caused by millimeter wave electromagnetic irradiation,” IEEE Trans. Biomed. Eng., vol. 41, no. 9, pp. 865–873, Sep. 1994. [34] “Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz),” Health Phys., vol. 74, no. 4, pp. 494–522, 1998.

Nacer Chahat (S’09) was born in Angers, France, in 1986. He received the B.Sc. degree in electrical engineering and radio communications (valedictorian and summa cum laude) from the Ecole Supérieur d’ingénieurs de Rennes (ESIR) Rennes, France, in 2009. He received the Ph.D. degree in signal processing and telecommunications at the Institute of Electronics and Telecommunications of Rennes (IETR), University of Rennes 1, Rennes, France, in 2012. His current research fields are electrically small antennas, millimeter-wave antennas, the evaluation of the interaction between the electromagnetic field and human body, and terahertz antennas. He accomplished a six-month master’s training period as a Special Research Student, in 2009, at the Graduate School of Engineering, Chiba University, Chiba, Japan. Mr. Chahat was the Recipient of the 2011 Best Paper Award from Bioelectromegnetics Society. He also received the 2011 CST University Publication Award. In 2012, he was the Recipient of the IEEE Antenna and Propagation Society Doctoral Research Award.

Maxim Zhadobov (S’05–M’07) received the M.S. degree in radiophysics from Nizhni Novgorod State University, Nizhni Novgorod, Russia, in 2003, and the Ph.D. degree in bioelectromagnetics from the Institute of Electronics and Telecommunications of Rennes (IETR), University of Rennes 1, Rennes, France, in 2006. He accomplished postdoctoral training at the Center for Biomedical Physics, Temple University, Philadelphia, PA, in 2008, and then rejoined IETR as an Associate Scientist CNRS (Centre National de la Recherche Scientifique). He has authored or coauthored more than 80 scientific contributions. His main research interests are in the field of biocompatibility of electromagnetic radiations, including interactions of microwaves, millimeter waves and pulsed radiations at the cellular and subcellular levels, health risks and environmental safety of emerging wireless communication systems, biocompatibility of wireless noninvasive biomedical techniques, therapeutic applications of nonionizing radiations, and bioelectromagnetic optimization of body-centric wireless systems, experimental, and numerical electromagnetic dosimetry. Dr. Zhadobov was the Recipient of the 2005 Best Poster Presentation Award from the International School of Bioelectromagnetics, 2006 Best Scientific

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Paper Award from the Bioelectromegnetics Society, and Brittany’s Young Scientist Award 2010.

Laurent Le Coq received the B.Sc. degree in electronic engineering and radiocommunications, and the french DEA degree (M.Sc.) in electronics in 1995, and the Ph.D. degree in 1999, all from the National Institute of Applied Science (INSA), Rennes, France. In 1999, he joined the nstitute of Electronics and Telecommunications of Rennes), University of Rennes 1, as a Research Laboratory Engineer, where he is responsible for measurement technical facilities up to 110 GHz. His activities in antenna measurements and development of related procedures involved him in more than twenty research contracts of national or European interest. He is author and coauthor of 21 journal papers and 30 papers in conference proceedings.

Stanislav I. Alekseev (M’10) received the M.S. degree in physics from Kazan State University, Kazan, Russia, in 1971, and the Ph.D. degree in biophysics from the Institute of Biophysics, Pushchino, Russia, in 1977. He is currently a Senior Researcher at the Institute of Cell Biophysics, Russian Academy of Sciences, Moscow, Russia. He is involved in research of neural effects of millimeter (mm)-wave irradiation. His research interests include theoretical and experimental study of microwave and mm-wave interaction with the skin, dosimetry and thermodynamics, mechanisms of microwave and mm-wave effects on model, and neuronal membranes, on electrical activity of neurons, and cutaneous sensory afferents.

Ronan Sauleau (M’04–SM’06) received the B.Sc. degree in electrical engineering and radio communications from the Institut National des Sciences Appliquées, Rennes, France, in 1995. He received the Agrégation degree from the Ecole Normale Supérieure de Cachan, Cachan, France, in 1996, and the Ph.D. degree in signal processing and telecommunications and the “Habilitation à Diriger des Recherche” degree from the University of Rennes 1, Rennes, France, in 1999 and 2005, respectively. He was an Assistant Professor and Associate Professor at the University of Rennes 1, Rennes, France between September 2000 and November 2005, and between December 2005 and October 2009, respectively. He has been appointed as a Full Professor in the same University since November 2009. His current research fields are numerical modeling (mainly FDTD), millimeter-wave printed and reconfigurable (MEMS) antennas, substrate integrated waveguide antennas, lens-based focusing devices, periodic and nonperiodic structures (electromagnetic bandgap materials, metamaterials, reflectarrays, and transmitarrays), and biological effects of millimeter waves. He has been involved in more than 30 research projects at the national and European levels and has cosupervised 14 Postdoctoral Fellows, 18 Ph.D. students, and 40 Master students. He has received eight patents and is the author or coauthor of 125 journal papers and more than 270 publications in international conferences and workshops. He has shared the responsibility of the research activities on antennas at IETR in 2010 and 2011. He is now coresponsible for the research Department “Antenna and Microwave Devices” at IETR and is deputy director of IETR. Prof. Sauleau received the 2004 ISAP Conference Young Researcher Scientist Fellowship (Japan) and the first Young Researcher Prize in Brittany, France, in 2001, for his research work on gain-enhanced Fabry-Perot antennas. In September 2007, he was elevated to Junior member of the “Institut Universitaire de France”. He was awarded the Bronze medal by CNRS in 2008. He was the corecipient of several international conference awards (Int. Sch. of BioEM 2005, BEMS’2006, MRRS’2008, E-MRS’2011, BEMS’2011, IMS’2012, Antem’2012). His is currently a Guest Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPOGATION Special Issue on “Antennas and Propagation at mm and sub mm waves.”