SAR and temperature increase in the human eye induced by obliquely incident plane waves

Specific absorption rates (SARs) and temperature increases in the human eye are calculated for exposure to obliquely incident plane waves in the frequency range of 600 MHz and 6.0 GHz. The average SARs and the temperature increases in the lens are found to take maximum values only in the hot-spot frequency range for oblique incidence (30/spl deg/-50/spl deg/).     

THE RELATIONSHIP BETWEEN THE EM ENERGY ABSORPTION OF HUMAN BODY AND THE INCIDENT DIRECTION AND POLARIZATION OF PLANE WAVE

The problems of EM energy absorption of human body irradiated by plane wave arediscussed by the Finite-Difference Time-Domain(FD-TD)method.The local Specific AbsorptionRates(SARs),the whole-body average SARs and the layer average SARs for the inhomogeneousblock model of human body with different incident direction and different polarization of theincident waves are calculated.The results show that the appearance of maximum EM energyabsorption is not always at the situation of the front incidence and the local SARs are moreimportant for the interaction of the EM fields with human body.All results provide more infor-mation about the electromagnetic dosimetry for human body.     

Temperature increase in human eyes due to near-field and far-field exposures at 900 MHz, 1.5 GHz, and 1.9 GHz

This work investigates the effect of frequency, polarization, and angle of incidence of an electromagnetic (EM) wave on the specific absorption rate (SAR) and maximum temperature increase in the human eye at 900 MHz, 1.5 GHz, and 1.9 GHz. In particular, the temperature increase in the eye is compared for near-field and far-field exposures. The difference of a maximum temperature increase in the lens is also discussed between the head models of an adult and children. Throughout the investigations, our attention is paid to a maximum temperature increase in the lens for SAR values prescribed in safety standards. For the results of our investigation, the SAR and temperature increase in the eye are found to be largely dependent on the separation between the eye and a source, and the frequency, polarization, and angle of incidence of the EM wave. The maximum temperature increase (0.303/spl deg/C-0.349/spl deg/C) in the lens of the adult for the SAR value of 2.0 W/kg for the eye tissue (about 10 g) is marginally affected by the above-mentioned factors. No clear difference of a maximum temperature increase in the lens at the SAR limit is observed between the adult and children models.     

Temperature increase in the human head due to a dipole antenna at microwave frequencies

The temperature increases in a human head due to electromagnetic (EM) wave exposure from a dipole antenna are investigated in the frequency range of 900 MHz to 2.45 GHz. The maximum temperature increases in the head and brain are compared with the values of 10/spl deg/C and 3.5/spl deg/C (found in literature pertaining to microwave-induced physiological damage). In particular, the estimation scheme for maximum temperature increases of the head and brain tissues is discussed in terms of a peak average specific absorption rate (SAR) as prescribed in safety standards. The rationale for this attempt is that maximum temperature increases and peak average SARs have not been well correlated yet. For this purpose, the SAR in the head model is initially calculated by the finite-difference time-domain method. The temperature increase in the model is then calculated by substituting the SAR into the bioheat equation. Numerical results demonstrate that the temperature increase distribution in the head is largely dependent on the frequency of EM waves. This is mainly because of the frequency dependency of the SAR distribution. Similarly, maximum temperature increases in the head and brain are significantly affected by the frequency and polarization of the EM wave. The maximum temperature increases in the head (excluding auricles) and brain are determined through linear extrapolation of the peak average SAR in these regions. According to this scheme, it is found that the peak SAR averaged over 1 g of tissue in the head should be approximately 65 W/kg to achieve the maximum temperature increase of 10/spl deg/C induced in the head excluding auricles. This corresponds to a factor of about 40 compared to the FCC standard. On the other hand, the peak SAR for 10 g of tissue should be around 40 W/kg, which implies a factor of about 20 compared to the ICNIRP standard.     

Fully polarimetric bistatic radar scattering behavior of forestedhills

The bistatic radar scattering measurements of forested hills were performed at grazing incidence and at azimuth scattering angles from 28° to 66° from the forward scatter plane. Using pulse-to-pulse switching between orthogonal transmitted polarizations, the radar simultaneously measures two orthogonally polarized components of the scattered wave to obtain full polarimetric information about the scattering process. These are the first fully polarimetric terrain clutter measurements to be conducted at large bistatic angles. The complete Stokes matrix, computed by averaging successive realizations of the polarization scattering matrix, is used to examine the polarization sensitivity of the bistatic clutter. It is found that the polarization state of the EM wave scattered out of the plane of incidence strongly depends on the polarization orientation of the incident electric field. Unlike the monostatic case, these two incident wave polarization states are found to produce substantially different scattered wave behavior when trees are viewed at large bistatic angles. Scattered fields resulting from vertically oriented incident fields are found to be highly polarized and to produce bistatic clutter power levels that are strongly dependent on the polarization of the receiving antenna. In contrast, horizontally oriented incident fields are found to produce weakly polarized scattered waves with bistatic clutter power levels that are insensitive to the polarization of the receiving antenna     

基于超表面的高效宽带双极化转换器设计

超材料和超表面因表现出天然材料所不具备的特 殊性能而广泛应用于电磁波调控。针对宽频带、高效率和小型化极化 转换器应用需求,本文设计一款宽带高效反射型超表面双极化转换器。基于电磁波极化调控 原理,在两角切方形贴片的基础上, 设计一款圆角切方形贴片周期性阵列超表面,实现单一宽频带内高效率线极化和圆极化转换 。研究表明,该极化转换器在6.850—11.050 GHz频段内极化转换效率(po larization conversion ratio,PCR)达90%,且性能可维持在入射 角 ≤45°范围内,覆盖整 个X 波段,实现高效率宽入射 角线极化转换;在6.225—6.402 GHz和12.562—13.472 GHz频率范围内轴比(axial ratio, AR)低于3 dB,实现高效率圆极化转换。此外,该超表面具有结构简单、宽入射角、加工方 便等优点,可满足微波通信、显微成像等领域的应用需求。     

Hot Spots Generated in Conducting Spheres by Electromagnetic Waves and Biological Implications

The distribution of the heating potential generated by an incident electromagnetic plane wave on a conducting sphere simulating the human head was investigated. It was found that for a sphere of 10-cm radius having the same electrical characteristics as those of biological tissues, no hot spots are generated inside. While at lower frequencies the heating is relatively uniform with some polarization effects, for frequencies above 1000 MHz only skin heating takes place. For a sphere of the same size but of conductivity ?= 10 mmho/cm (which for f>1000 is lower than that of biological tissues) hot spots occur inside for f>1000 MHz. Intense hot spots also occur inside spheres of radius 5 cm having the same electrical characteristics as those of biological tissues in the frequency region of 250 MHz相似文献   

Temperature rise for the human head for cellular telephones and forpeak SARs prescribed in safety guidelines

The bioheat equation is solved for an anatomically based model of the human head with a resolution of 3 × 3 × 3 mm to study the thermal implications of exposure to electromagnetic (EM) fields typical of cellular telephones both at 835 and 1900 MHz. It is shown that similar to the measured data, up to 4.5°C temperature elevation may be caused for locations of the pinna by a cellular telephone warmed by electronic circuitry to temperatures as high as 39°C with temperature increases for the internal tissues such as the brain and eye that are no more than 0.1°C-0.2°C higher than the basal values. Similar to previous studies by other authors, additional temperature increases due to EM fields of cellular telephones are fairly small and typically less than 0.1°C. Another objective was to study the thermal implications of the SAR limits for the occupational exposures of 8 W/kg for any 1 g, or 10 W/kg for any 10 g of tissue suggested in the commonly used safety guidelines. Such specific absorption rates (SARs) would lead to temperature elevations for the electromagnetically exposed parts of the brain up to 0.5°C with 10 W/kg for any 10 g of tissue resulting in somewhat higher temperatures and for larger volumes. Similar temperature increases are also calculated by increasing the arterial blood temperature, except that the temperature increases due to the SAR are for the more limited volume rather than the entire brain     

Heating patterns in biological tissue phantoms caused by millimeterwave electromagnetic irradiation

Distribution of millimeter wavelength electromagnetic energy absorption in surface layers of biological tissue models was studied using methods of Infrared Thermography. 0.1 mm thin-layer phantoms were irradiated in the near field using different types of horn antennas in the 37-78 GHz frequency range. Heating patterns were recorded during microwave irradiation, and surface SAR distributions mere calculated. The temperature resolution was better than 0.05 K. It was found that horn antennas produced nonuniform heating patterns in irradiated objects. These nonuniform patterns were due to a geometrical resonance resulting from a secondary wave-mode interaction between an irradiated object and the corresponding critical cross-section of the horn antenna. Local SAR values in hot spots exceeded the spatially averaged values by over 10 times, and the widths of these hot spots at 5 times the average SAR were often 1 mm or less. The location, quantity, number and size of the local field absorption maxima of irradiated objects strongly depended on the frequency of electromagnetic irradiation, with equivalent Q-factors of 500 or more. These findings provide an explanation for a number of frequency-dependent effects of millimeter wave electromagnetic irradiation     

Compton散射对三元磁化等离子体光子晶体的影响

应用多光子非线性Compton散射模型和传输矩阵法,研究了Compton散射对一维三元磁化等离子体光子晶体中TE波禁带特性的影响。结果表明,与散射前相比,随等离子体频率增大,散射使左旋极化和右旋极化波的禁带展宽和透射峰值分别减小0.1 GHz和0.21 GHz,禁带中心频率向高频区移动位置增大0.5 GHz。随等离子体碰撞频率增大,散射对左旋极化和右旋极化波的禁带宽度有一定的影响。随等离子体回旋频率、填充率、光入射角和介质相对介电常数的增大,散射对左旋极化波和右旋极化波的禁带有明显的调谐作用。     

基于超构表面的高效宽带交叉极化转换器设计

基于近年来利用超构表面在调控电磁波极化状态方面的热度,应用超构表面设计了一种适用于微波频段的新型极化转换器,可以在比较宽的带宽内,将入射的线极化波高效地转换为其交叉极化的反射波,在四个频点附近转换效率甚至达到100%。在整个工作频带内,仿真和测试结果基本吻合,仿真得到极化转换效率高于90%,频带宽度为7. 88-20. 70 GHz,相对带宽达到了89. 7%。理论上给出了极化转换器的设计原理,仿真确定了转换器谐振点的位置,分析了极化转换器工作的物理原理,实验验证了极化转换器的效果。所设计的极化转换超构表面在极化选择控制方面有重要应用。     

Compton散射对1维3元未磁化等离子体光子晶体禁带影响

为了研究Compton散射对1维3元未磁化等离子体光子晶体中TE波禁带影响,采用Compton散射模型和传输矩阵法,进行了理论分析和实验验证,取得了一些重要数据。结果表明,随着等离子体频率增大,左旋和右旋极化波禁带展宽比散射前减小0.09GHz,禁带主频率向高频区域移动增大0.48GHz。随着等离子体碰撞频率增大,两种极化波禁带宽度发生一定变化。随着等离子体回旋频率、填充率、光入射角和介质相对介电常数增大,左旋和右旋极化波禁带明显调谐效应。这一结果对等离子体光子晶体应用是有帮助的。     

电磁波在等离子体中的热致非线性衰减

研究了电磁波对等离子体进行加热所产生的非线性效应,分析了电磁波在等离子体中传播的热致非线性衰减,通过与线性衰减的计算结果进行比较,指出当电磁波频率较低且传输距离较远时线性计算产生较大误差,必须考虑电子温度升高引起的非线性衰减,并且分析了非线性衰减随频率、入射电波场强、传输距离等的变化关系,指出在一定条件下,电波对等离子体的加热可使电波的非线性衰减小于线性衰减。     

人体吸收电磁能量与入射方向和极化的关系

本文讨论了用时域有限差分法计算在平面电磁波照射下人体对电磁能量的吸收问题。计算了各种入射方向和极化条件下非均匀块状人体模型中的局部比吸收率(SAR)分布。计算结果表明,人体并不总是在正面入射时吸收能量最多;同时还表明,局部比吸收率比平均比吸收率更值得重视,因为局部吸收率的最大值往往比全身平均值大几倍到十几倍。本文的计算结果丰富了人们在人体电磁剂量学方面的知识。     

不均匀磁等离子体的隐身机理——圆极化电磁波的碰撞吸收

研究了圆极化电磁波在不均匀磁等离子体的传播和吸收。讨论了不均匀磁化等离子体片对平行于磁场方向传播的左、右旋圆极化电磁波的碰撞吸收，计算了不同条件下衰减率。计算表明，当电磁波的频率接近电子碰撞频率时，磁等离子体对电磁波的吸收达到最大值。当入射电磁波的频率很低时，不均匀磁化等离子体中磁撞对雷达波的吸收非常小。当入射电磁波频率较高时，等离子体的碰撞对入射电磁波的衰减很有效。     

FDTD Simulation of Electromagnetic Reflection of Conductive Plane Covered with Imhomogeneous Time-Varying Plasma

A finite-difference time-domain (FDTD) algorithm is applied to study the electromagnetic reflection of conduction plane covered with inhomogeneous, collision, warm, time-varying plasma. The collision frequency of plasma is a function of electron density and plasma temperature. Under the one-dimensional case, transient electromagnetic propagation through various plasmas have been obtained, and the reflection coefficient of EM wave through inhomogeneous time-varying plasma (ITVP), homogeneous time-varying plasma (HTVP) and inhomogeneous plasma (IP) are calculated under different conditions. The results illustrate that a plasma cloaking system can successfully absorb the incident EM wave.     

The influence of fiber orientation on electromagnetic shielding in liquid-crystal polymers

The influence of conductive carbon-fiber orientation and weight percentage on the electromagnetic (EM) shielding effectiveness (SE) in liquid-crystal polymer (LCP) composites was investigated experimentally and theoretically. The experimental results show that the SE of LCP composites with longitudinal fiber orientation is higher than random fiber orientation under the same weight percentage of carbon fibers filled. This is because longitudinal fiber orientation is parallel to the electric field of the incident EM wave, and most of the energy of the incident wave is reflected by the longitudinal fiber. In comparison with nylon66 composites, the SEs of LCP composites with longitudinal fiber orientation are also higher than nylon66 composites with the same content of carbon fibers. Furthermore, the SE of 20% conductive carbon-fiber-filled LCP composites was measured to be 50 dB at a frequency of 0.3 GHz and 53 dB at 1 GHz, which is at least 10 dB higher than that of nylon66 composites. The SE predicted by theoretical models and measured by experiments was in good agreement for carbon-fiber-filled LCP composites of longitudinal and random fiber orientations.     

Analysis and design of a two-octave polarization rotator formicrowave frequency

The analysis and design of a 45° polarization rotator are presented. A two-octave frequency band is achieved with only three grids of strips, each rotated with respect to the previous one. Over this band the transmission loss is less than 0.5 dB, the circular polarization ratio is less than 2.0 dB, and the major axis of the polarization ellipse forms 45°±3° with respect to the linear polarization direction of the incident plane wave. The analysis method is an original scheme based upon the conjugate-gradient fast Fourier transform (CG-FFT) method to analyze periodic structures and the generalized scattering matrix method to study their connection. A comparison between numerical and measured data is presented     

Design and analysis of an ultrathin triple-band polarization independent metamaterial absorber

In this paper, an ultrathin triple-band polarization insensitive metamaterial (MTM) absorber has been presented for C-band applications. The proposed structure is explained as a combination of two absorbers, named as Absorber-I and Absorber-II. It exhibit 91%, 98.9% and 99.5% absorptivity at 4.2 GHz, 7 GHz and 7.4 GHz frequencies respectively. Absorber-I and Absorber-II are individually responsible for the origin of two separate frequency bands at 4.2 GHz and 7 GHz respectively. Further, mutual coupling between Absorber-I and Absorber-II is responsible to generate one more absorption band at the frequency of 7.4 GHz. The fourfold symmetric nature of the proposed unit cell provides polarization insensitivity phenomena. The absorbing mechanism of the designed absorber has been explained by two methods. One is by plotting input impedance of the absorber. Another one is by analyzing the EM wave propagation inside the structure. The proposed absorber has been fabricated and simulation results are experimentally verified.     

Potential Temperature Rise Induced by Electromagnetic Field in Brain Tissues

The differential temperature rise in a spherical region stimulating a potential hot spot in the central region of a human head has been determined, takdng into account heat conduction as well as heat convection due to blood flow. The results apply in general to ali cases where the dimensions of the model and the frequency of the incident electromagnetic wave is such that a hot spot is formed near the central region. For a heat deposition rate of 10 mW/g in a hot spot of 1 cm radius, a differential temperature rise over the ambient tissues and blood is approximately 0.5°C. This corresponds to the case of a differential absorption cross section of 1 cm-1 for a 5 cm radius sphere exposed to an incident power density of 10 mW/cm2. For larger models of man's head the differential temperature rise is smaller.