FDTD computation of temperature rise in the human head for portabletelephones

Temperature rises in the human head for portable telephones were computed with an anatomically based head model at 900 MHz and 1.5 GHz. The specific absorption rate (SAR) in the human head was determined using the finite-difference time-domain (FDTD) method, while a bioheat equation was numerically solved also using the FDTD method. The portable telephone was modeled by a quarter-wavelength monopole antenna on a dielectric covered metal box. The source geometries considered were the telephone barely touching the ear and the telephone pressing the ear, both having a vertical alignment at the side of the head. The antenna output power was set to be consistent with the portable telephones of today: 0.6 W at 900 MHz and 0.27 W at 1.5 GHz. Computed results show that a phone time of 6-7 min yields a temperature rise of approximately 90% of the steady-state value. Application of the ANSZ/IEEE safety guidelines restricting the 1-g-averaged spatial peak SAR to 1.6 W/kg results in the maximum temperature rise in the brain of 0.06°C, and application of the ICNIRP/Japan safety guidelines restricting the 10-g-averaged spatial peak SAR to 2 W/kg results in the maximum temperature rise in the brain of 0.11°C, both at 900 MHz and 1.5 GHz     

The dependence of electromagnetic energy absorption upon human-headmodeling at 1800 MHz

The authors of a previously published paper on the dependence of electromagnetic (EM) energy absorption concluded that homogeneous modeling of the human head is suited for assessing the spatial-peak absorption for transmitters operating at 900 MHz or below. Additional studies became necessary for the frequency bands utilized by new mobile communications systems (i.e., 1.5 and 2.5 GHz) since some peripheral tissue layers have a thickness of the range of λ/4-λ/2. The results of the simulations combined with worst-case considerations confirmed the anticipated and more complex relationship between absorption and anatomical details at these higher frequencies. Nevertheless, a homogeneous representation of the head is suited for assessing the maximum specific absorption rate (SAR) in the head of the user of mobile telecommunication equipment (MTE) if the appropriate dielectric parameters are chosen     

人体受到电磁辐射的仿真研究

基于可变网络的时域有限差分法,仿真了暴露于900MHz移动通信基站天线远区场中的人体和移动电话天线近区场中的人体头部电磁模型中产生的比吸收率分布,分析了基站天线和手机天线辐射对人体的影响。以理想点源天线作为基站天线,在人体正前方入射频率为900MHz的正弦平面波,仿真结果显示,人体在基站天线照射下的平均SAR值符合国际卫生部标准;以900MHz单频PIAF天线作为手机天线置于高仿真人体头部1cm处,仿真结果与环保标准比较,人体头部受到的照射剂量远低于安全标准。     

Energy Deposition in a Model of Man: Frequency Effects

A computer-controlled scanning system and implantable, nonperturbing electric field probes were used to measure spatial distributions of the electric field in a full scale homogeneous model of a human body. The measurements were performed at three frequencies (160, 350, and 915 MHz) in the far-field and in the near-field of resonant dipoles. The specific absorption rate (SAR) distributions and the averages for body parts and the whole body are analyzed as functions of frequency. In the far-field, the SAR decreases exponentially in the direction of wave propagation in the torso at all frequencies, and large gradients of the SAR are observed along the body main axis, particularly for the E polarization. At 160 and 350 MHz high local SAR's are produced in the neck. It appears that for plane wave exposures the ratio of the peak SAR to the whole-body average SAR does not exceed 20. In the near-field, large SAR gradients are also produced, and the ratios of the peak spatial SAR to the whole-body average SAR vary from about 30 to 250 depending on the frequency and polarization. It is suggested that for near-field exposures the whole-body average SAR is not a proper dosimetric measure, and the SAR averaged over any 0.1 of the tissue volume is recommended instead.     

Absorption of microwave energy by muscle models and by birds of differing mass and geometry

Spheres composed of phantom muscle of radius 1.5, 2, 3, 4, 6 and 8 cm, as well as birds (parakeets, quail, pigeons, chickens, turkeys) were exposed to far-field plane waves at power densities of incident radiation between 182 and 560 mW/cm2 and at frequencies of 775, 915 and 2450 MHz. Specific absorption rate (SAR) patterns were determined by thermographic techniques for both spheres and birds. The measured SAR patterns in spheres were comparable to those from theoretical predictions. The SAR patterns in birds, however, varied markedly from those obtained from spheres of comparable mass. The results indicate that the geometrically complex animal is not represented by simple geometric models for making absorption studies. Thermograms of birds exposed in the flying position indicated that the SAR is high in the wings. The behavioral response of the birds to the exposure was variable. Threshold power densities for biological or behavioral reactions were determined for each bird at all three frequencies. The lowest power density associated with reactivity by the chicken was 5.8 mW/cm2 (corresponding to SARs of 3.1 W/kg in the head and 3.9 W/kg in the neck) at 775 MHz.     

Human Head Size and SAR Characteristics for Handset Exposure

Using scaled models for an anatomical head model and a simple head model, we investigated the effects of head size on specific absorption rate characteristics for two mobile phones operating at 835 MHz and 1765 MHz. Our results showed that a larger head produced a higher localized SAR at 835 MHz. However, at 1765 MHz, the differences among the head models were insignificant since the superficial absorption was dominant over the effects of head shape and size. A larger head produced a lower whole‐head averaged SAR at both frequencies.     

Evaluation of the SAR distribution in the human head for cellularphones used in a partially closed environment

The purpose of this paper is to calculate the specific absorption rate (SAR) distribution in a human head exposed to the electromagnetic field emitted from a handheld cellular phone operating in the 900 MHz range in a partially closed environment. The environment could be, for example, the interior of a car, a condition of exposure which is largely diffused nowadays. The presence of reflecting surfaces near the phone modifies the current distribution on, and the emitting properties of, the phone antenna. Therefore, the distribution of the absorbed power inside the head is different from that absorbed in the free space exposure condition. The finite-difference time-domain (FDTD) method has been used to evaluate the SAR in a realistic anatomically based model of the human head for different antenna-handset configurations and for different antenna-head distances. The environmental effects have been simulated through partially or totally reflecting walls located in various positions with reference to the phone. It is found that the presence of a horizontal reflecting wall over the head decreases the SAR values in the part of the head directly exposed to the phone antenna, while it increases the SAR values in the part not directly exposed. On the contrary, the presence of a vertical wall, located in proximity of the phone and parallel to it, raises the SAR values everywhere into the head     

Parametric dependence of SAR on permittivity values in a man model

The development and widespread use of advanced three-dimensional digital anatomical models to calculate specific absorption rate (SAR) values in biological material has resulted in the need to understand how model parameters (e.g., permittivity value) affect the predicted whole-body and localized SAR values. The application of the man dosimetry model requires that permittivity values (dielectric value and conductivity) be allocated to the various tissues at all the frequencies to which the model will be exposed. In the 3-mm-resolution man model, the permittivity values for all 39 tissue-types were altered simultaneously for each orientation and applied frequency. In addition, permittivity values for muscle, fat, skin, and bone marrow were manipulated independently. The finite-difference time-domain code was used to predict localized and whole-body normalized SAR values. The model was processed in the far-field conditions at the resonant frequency (70 MHz) and above (200, 400, 918, and 2060 MHz) for E orientation. In addition, other orientations (K, H) of the model to the incident fields were used where no substantial resonant frequency exists. Variability in permittivity values did not substantially influence whole-body SAR values, while localized SAR values for individual tissues were substantially affected by these changes. Changes in permittivity had greatest effect on localized SAR values when they were low compare to the whole-body SAR value or when errors involved tissues that represent a substantial proportion of the body mass (i.e., muscle). Furthermore, we establish the partial derivative of whole-body and localized SAR values with respect to the dielectric value and conductivity for muscle independently. It was shown that uncertainties in dielectric value or conductivity do not substantially influence normalized whole-body SAR. Detailed investigation on localized SAR ratios showed that conductivity presents a more substantial factor in absorption of energy in tissues than dielectric value for almost all applied exposure conditions.     

Modeling of normal-mode helical antennas at 900 MHz and 1.8 GHz formobile communications handsets using the FDTD technique

A simple technique is proposed for modeling short normal-mode helical antennas using a commercial finite-difference time-domain (FDTD) code with a rectangular grid and a nominal extension of the wire. The approach allows affects on the input impedance and radiation performance of the helix to be examined and importantly does not require modification of the excitation subroutines. Normal-mode helical antennas for mobile communications use at 900 and 1800 MHz were designed using the proposed method and good agreement with measurements of impedance and near-magnetic field strength was found. The radiating performance of the helix was compared to that of a λ/4 monopole and generally found to be inferior at 900 MHz due to only 19.2 % efficiency in the presence of the head. At 1.8 GHz the two antenna types showed similar characteristics except in regard to bandwidth, 36.1 % for the monopole and 7.8 % for the helix, in the presence of the head. The modeled helix antennas produce spatial peak specific absorption rate (SAR) figures that are up to 27 % greater at 900 MHz and up to 49 % greater at 1.8 GHz than the corresponding monopole values due to the shorter antenna     

Channel model for wireless communication around human body

A channel model for a wireless body area network at 400 MHz, 900 MHz and 2.4 GHz is derived. The electromagnetic wave propagation around the body is simulated with a finite-difference time-domain simulator. Creeping waves were identified as the propagation path around the body. Its impact on the delay spread in an indoor environment is discussed.     

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.     

An Extended-AMATA Indoor Propagation Model for GSM 900/1800 MHz and Wi-Fi 2.4 GHz Frequencies

In a previous article, we reported on a novel indoor propagation model; we called the AMATA model, which we applied at 900 MHz and 2.4 GHz frequencies. The model could be applied at both GSM and wireless LAN frequencies. The developed formula merits, on its own, as a novel fourth power effective attenuation equation, which relies on the number of wall separations within the floor. This paper reports an extended-AMATA indoor propagation model that generally describes university and office type buildings. A sample of four different multi-floor building structures that have a stone block type outer wall was chosen. Those flat roofed, stone built, multi floor buildings are very common, not only in Palestine, but probably in vast areas in the Middle East region. The new model benefits over the previous one, applied at 900 MHz, in that it can be extended to cellular base-stations, transmitting at 1800 MHz frequency and outdoor Wi-Fi basestations, as opposed to indoor access points, transmitting at 2.4 GHz. The work is of paramount importance to cellular and Wi-Fi network operators, transmitting at 900/1800 MHz and 2.4 GHz frequency bands. Our new model can be applied with a high confidence level to buildings, similar to the sample of buildings, we measured.     

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.      

Equatorial scintillations: A review

With the advent of satellite communications systems at frequencies varying fromsim140MHz to 1600 MHz as well as navigation and ranging systems in the 1200-1600 MHz portions of the spectrum, the effect of equatorial irregularities on fading signals has become of importance. Recent observations of the signal statistics of scintillations at frequencies ranging from 136 MHz to 6 GHz reveals a power law fall off of irregularity sizes. Power spectra are now available for a variety of conditions and for frequencies from VHF to microwaves. During periods of intense equatorial activity, at frequencies to 360 MHz, Rayleigh scattering is frequently experienced. The latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus12deg. A correlation of in-situ measurements of irregularities from satellites had revealed the great variations in longitudinal patterns during any season. It has also allowed theDelta Nobtained from in-situ measurements to be utilized to predict scintillation excursions. New facets of scintillation activity in the equatorial region recently reported include weak daytime scintillation and the patchy nature of irregularities (small irregularities embedded in large structures) particularly noted during periods of low solar flux. Future studies will assist in the delineation of the extent of the equatorial region at frequencies from 1.5 to 6 GHz, and in the UHF range. From the viewpoint of the communicator the morphology of scintillations at microwaves has still not been reported from any long term program of measurements.     

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.     

Development of a 2.45-GHz Local Exposure System for In Vivo Study on Ocular Effects

We developed a new exposure system to irradiate microwaves locally on a rabbit eye using a small coaxial-to-waveguide adapter filled with low-loss dielectric material as an antenna. A numerical rabbit model was also developed using X-ray computer tomography images, and the specific absorption rates (SARs) in the rabbit, especially in the eye, were analyzed with the finite-difference time-domain method. The temperature elevation in the exposed eye was also evaluated by solving a bioheat equation. Our exposure system can generate incident power density of 15 mW/cm² at the surface of a rabbit eye with input power of 1 W. When the incident power density on the rabbit eye is 300 mW/cm² , average SAR over the exposed eye and the whole body were approximately 108 and 1.8 W/kg, respectively. The exposure system can realize localized exposure to the eye with the ratio of exposed-eye averaged SAR to the whole-body averaged SAR was 60. The developed exposure system can achieve high-intensity exposure such as the threshold of cataracts, i.e., the eye-averaged SAR over 100 W/kg or the lens temperature over 41 degC with the incident power density of 300mW/cm² without significant whole-body thermal stresses     

Abnormal cardiovascular responses induced by localized high powermicrowave exposure

A hypothesis of microwave-induced circulatory under perfusion was tested in ketamine anesthetized rats whose heart rate, mean arterial pressure, pulse pressure, respiration rate, and body temperatures were monitored continuously. Fifty-eight ventral head and neck exposures in a waveguide consisted of sham-exposure and exposure to continuous wave (CW) and pulsed 1.25 GHz microwaves for 5 min. The 0.5 Hz (10 microseconds, 2 W average) and 16 Hz (1 microsecond, 6.4 W average) pulse-modulated microwaves were delivered at 400 kW peak power. The CW microwaves were 2 and 6.4 W. The average specific absorption rate was 4.75 W/kg per watt transmitted in the brain and 17.15 W/kg per watt transmitted in the neck. Respiration rate and mean arterial pressure were not altered. Changes in heart rate and pulse pressure were observed in rats exposed to higher power (16 Hz pulses and 6.4 W CW) but not to the lower average power microwaves (0.5 Hz pulses and 2 W CW). Depression of pulse pressure, an indication of a decrease in stroke volume, and increased (tachycardia) or decreased (bradycardia) heart rate were noted in presence of whole-body hyperthermia. The cardiac output of those animals exposed to higher average power microwaves was considered to be below normal as hypothesized. Decreased cardiac output and normal mean arterial pressure resulted in an increase in the total peripheral resistance which was contrary to the anticipated thermal response of animals.     

A novel low SAR water-based mobile handset antenna

The effect of mobile phones on human health is becoming a serious concern in the last decade. This paper suggests a novel water-based cellular phone antenna for reducing the electromagnetic wave radiation toward human head. Two antennas are considered: a single band PIFA operating at 1.8 GHz, and a dual band PIFA operating at 900 MHz and 1.8 GHz. The specific absorption rate (SAR) is decreased up to 0.6 W/kg by limiting the propagation of near electromagnetic fields toward the human head and therefore reducing the current density distribution. The reduction of SAR is carried out by introducing an U-edge wall made of an absorbing water material at each corner of the ground plane.     

Microwave-Induced Pressure Waves in Mammalian Brains

This paper presents direct measurements of acoustic pressure waves in brains of rats, cats, and guinea pigs irradiated with pulsed 2.450 and 5.655 GHz microwaves. A smal disk hydrophone transducer was surgically implanted in brains of anesthetized animals. Rectangular pulses (3 kW peak, 2.5 and 5.5 , us wide at 2.450 GHz and 200 kW peak, 0.5 ?s wide at 5.655 GHz) were applied through horns, waveguides, and direct contact antennas. The results clearly indicate that pulsed microwaves induce acoustic pressure waves in the brain, confirming earlier theoretical predictions. Furthermore, hydrophone output waveforms and on-line analyzed spectra show that fundamental and second harmonics were nearly identical to those predicted by the thermoelastic theory. However, the hydrophone records show complex sequences of higher order vibrational modes which deviate from predictions based on a homogeneous spherical model of the head.     

Electromagnetic Absorption in Multilayered Cylindrical Models of Man

The absorption characteristics of multilayered cylindrical models of man irradiated by a normally incident electromagnetic plane wave are investigated. Numerical calculations for a specific skin-fat-muscle cylindrical model of man predict a layering resonance at 1.2 GHz with an average specific absorption rate (SAR) about double that calculated for the corresponding homogeneous model. The layering resonance frequency is found to be the same for incident waves polarized parallel and perpendicular to the cylinder axis. The effects of layers on whole-body absorption by man are determined by averaging the effects obtained for many combinations of skin and fat thicknesses. Absorption effects due to clothing are also investigated.