Variability in EMF permittivity values: implications for SAR calculations

Digital anatomical models of man and animals are available for use in numerical calculations to predict electromagnetic field (EMF)-induced specific absorption rate (SAR) values. To use these models, permittivity values are assigned to the various tissues for the EMF frequencies of interest. There is, as yet, no consensus on what are the best permittivity data. This study analyzed the variability in published permittivity data and investigated the effects of permittivity values that are proportional on SAR calculations. Whole-sphere averaged and localized SAR values along the diameter of a 4-cm sphere are calculated for EMF exposures in the radio frequency range of 1 MHz to 1 GHz. When the dimensions of a sphere are small compared to the wavelength (i.e., wavelength inside the material is greater than ten times the dimensions of the object), the whole-sphere averaged SAR is inversely proportional to the permittivity of the material composing the sphere. However, the localized SAR values generally do not have the same relation and, as a matter of fact, vary greatly depending on the location within the sphere. These results indicate that care must be taken in choosing the permittivity values used in calculating SAR values and some estimate of the dependence of the calculated SAR values on variability in permittivity should be determined.     

SAR distributions for 915 MHz interstitial microwave antennas usedin hyperthermia for cancer therapy

Theoretical and experimental specific absorption rate (SAR) distributions are presented for single insulated antennas operating at 915 MHz in muscle phantom; the SAR is deduced from measured temperature rise. Results show that dipoles with lengths much shorter than their resonant length have a characteristically large reactive input impedance component and a substantially smaller absolute SAR magnitude than resonant dipoles. All cases investigated demonstrate that the maximum SAR occurs near the junction, regardless of insertion depth. Experimental results show that an antenna with the tip section equal to a quarter-wavelength and the insertion depth equal to a half-wavelength achieves a substantial increase in the longitudinal power distribution compared to other antenna designs that were evaluated     

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.     

低谐振频率电磁超材料单元设计与仿真

针对目前人工电磁超材料的谐振频率(吉赫兹(GHz)或太赫兹(THz))较大的问题,该文提出了一种新型尺寸较大的双面螺旋结构单元模型。通过HFSS仿真软件,建立了电磁超材料单元模型,分析了结构单元的S参数。采用Smith提取算法,得出了等效介电常数和等效磁导率的数学表达式。仿真结果表明,在2.0～2.1 MHz时,等效介电常数恒正,等效磁导率实部达到负极值,而等效磁导率的虚部也达到了最大值,即该频段为材料板的谐振频段,呈磁单负材料属性。     

Measurement of Low-Loss Dielectric Materials Using Dielectric Rod Resonator

To obtain the complex permittivity of low-loss dielectric materials at 60 GHz, a measurement method is developed. Using a dielectric rod resonator excited by a dielectric waveguide, effective conductivity of conducting plates for short circuiting the resonator is determined. The complex permittivity of the dielectric rod is determined by the resonant frequency and unloaded quality factor of the TE_0m1-mode resonator. Moreover, the complex permittivity of single crystal sapphire, polycrystalline ceramics, and cordierite has been investigated in virtue of numerical simulation. For all the measured specimens in this study, the proposed method is seen to provide much better accuracy for values.     

Effectiveness analysis of lossy dielectric shields for athree-layered human model

In this paper, we discuss the shielding effects of lossy dielectric materials located in front of a human model. Using the method of moments, we investigated the shielding effects by calculating the “whole average specific absorption rate (SAR)” and the “local SAR” for a three-layered elliptical model of the human body, which simulates the skin, fat, and muscle tissues. According to the results, in the low-frequency range of 200-800 MHz, the multiple reflection between the shield and human model gives rise to an increase in the whole average SAR when a low-loss material shield is placed in front of the human model. On the other hand, the local SAR increased not only at the skin layer, but also at the muscle layer. At higher frequencies, the SAR became a continuously decreasing function of frequency     

Dielectric permittivity and electrical conductivity of fluid saturated bone

The dielectric permittivity and electrical conductivity of freshly excised and formalin fixed samples of rat femoral bone were determined over a frequency range of 10 Hz-100 MHz. Impedance measurements were performed in the frequency domain using a vector impedance meter and an impedance analyzer. The results of these measurements show that the conductivity of fixed and fresh bone is nearly independent of frequency below 100 kHz, with the conductivity of fresh bone being two to three times greater than that of the fixed sample. At higher frequencies, the conductivity increases as a power function of frequency. The permittivity of bone reaches very high values at low frequencies, but decreases rapidly with increasing frequencies and approaches a limiting value of about ten. This high-frequency limit is consistent with the water content of the tissue, and with the permittivity of the anhydrous matrix. It is suggested that the olarizability observed at audio and radiowave frequencies is in part sssociated with the collagen phase, although other interfacial polarization effects can also be present.     

Dielectric properties of tissue-equivalent liquids and their effects on specific absorption rate

The radio frequency safety of mobile phones has been evaluated in terms of specific absorption rate (SAR). Standard methods for measurement of the SAR, including recipes for tissue-equivalent dielectric liquids, have recently been the subject of discussion among international standards organizations. Standards currently recommend glycol-type liquids as tissue-equivalent liquids for frequencies above 1 GHz. Although the ingredients are specified in the recipes provided, some fundamental information, such as the stability of dielectric properties, remains unclear. We measured the change of dielectric properties with time and with temperature of tissue-equivalent liquids recommended in the standard documents, and evaluated their effects on SAR. The conductivity decreased with increasing temperature in all glycol-type specimens. The permittivity, on the other hand, was almost constant. With the evaporation of water, the permittivity decreased, although the conductivities remained constant. Experimental results proved that dielectric properties are affected by environmental conditions, and that it is inevitably necessary to adjust the dielectric properties regularly, through the addition of ingredients, in order to follow the standards. The SAR values, however, were not affected significantly by the change in dielectric properties; thus, a larger tolerance of the dielectric properties may be acceptable in practical SAR measurements.     

Dielectric properties of fluid-saturated bone--the effect of variation in conductivity of immersion fluid

The dielectric permittivity and electrical conductivity of fluid-saturated rat femoral bone were determined as a function of the conductivity of the immersion fluid over a frequency range of 10?100 Hz to 8 MHz. The specimens were equilibrated in various solutions of Hank's balanced salt solution modified to contain various sodium chloride concentrations, with conductivities ranging from 0.13 to 3.55 s/m. The results of these measurements show that the dc conductivity of the tissue is about 1 percent of the conductivity of the immersion solution. Similar conductivity values were predicted from a simple model of the tissue, in which it is assumed that the only significant contribution to the conductivity arises from the capillaries that pass through the tissue. The permittivity of the tissue exhibits a dispersion whose mean relaxation frequency is proportional to the conductivity of the immersion fluid. The permittivity ranges from about 1000 at low frequencies to 10-20 in the high frequency limit. The dielectric data could be fitted to a Cole-Cole function having a distribution parameter of about 0.5-0.6, which suggests the presence of a diffusion-controlled ionic polarization mechanism, although other relaxation processes could also be present.     

Effects of Dielectric Values of Human Body on Specific Absorption Rate Following 430, 800, and 1200 MHz RF Exposure to Ingestible Wireless Device

In order to assess the compliance of ingestible wireless device (IWD) within safety guidelines, the SAR, and near fields of IWD in two realistic human body models, whose dielectric values are increased from the original by $pm$10% and $pm$ 20% are studied using the finite-difference time-domain method. The radiation characteristics of the IWD in the human body models with changed and unchanged dielectric values are compared. Simulations are carried out at 13 scenarios where the IWD is placed at center positions of abdomens in the two models at the operation frequency of 430, 800, and 1200 MHz, respectively. Results show that variation of radiation intensity near the surface of abdomen is around 2.5, 2.6, and 3.5 dB within 20% variation of dielectric values corresponding to the frequency of 430, 800, and 1200 MHz, respectively. Electric fields in the anterior of the human body models are higher than those in the posterior for all scenarios. SAR values increase with the increase of conductivities of human body tissues, and usually decrease with the increase of relative permittivities of human body tissues. The effect of the dielectric values of human body on SAR is orientation-, human-body-, and frequency-dependent. A variation up to 20% in conductivities and relative permittivities alone or simultaneously always causes a SAR variation less than 10%, 20%, and 30% at the frequency of 430, 800, and 1200 MHz, respectively. As far as the compliance of safety was concerned, the IWD was safe to be used at the input power less than 12.6, 9.3, and 8.4 mW, according to the IEEE safety standards at the frequency of 430, 800, and 1200 MHz, respectively.      

Approximation of aging effect on dielectric tissue properties for SAR assessment of mobile telephones

In electromagnetic dosimetry of children heads for mobile telephones, the dielectric properties of biological tissues for adults are so far being used due to the lack of the ones of children. In this paper, we derived an empirical formula according to Lichtenecker's exponential law for the complex permittivity of various tissues as a function of the hydrated rate or the total body water (TBW). We first examined its validity using the data measured by Peyman et al. for rats, and then applied the formula to the dielectric properties of 7-year-old and 3-year-old child head models by means of the relationship between the TBW and the age. With the dielectric properties for children derived in such an approach, we analyzed numerically the spatial peak specific absorption rate (SAR) for a 900-MHz mobile telephone in adult and child head models. As a result, we found that the dielectric properties for children do not affect significantly the 1- or 10- g averaged spatial peak SAR as well as the penetration depth. The finding could be qualitatively explained as cancellation of the increased conductivity and decreased electric field penetrating into the tissue because of the same degree of increase between the conductivity and permittivity in children compared to adults. Even in an extreme case, the age effect on the spatial peak SAR of dielectric properties is still within 10%.     

Agricultural applications of dielectric spectroscopy.

A brief account of interest in dielectric properties of agricultural materials is presented, and some examples of dielectric spectroscopy applied to agricultural problems are discussed. Included are wide frequency range (250 Hz to 12 GHz) permittivity, or dielectric properties, measurements on adult rice weevils and hard red winter wheat, for the purpose of assessing selective dielectric heating of the insects, and broadband (200 MHz to 20 GHz) permittivity measurements on tissues of fresh fruits and vegetables. Similar measurements are shown for tree-ripened peaches, which were obtained to assess possibilities for a permittivity-based maturity index. Broadband (10 MHz to 1.8 GHz) permittivity measurements are shown for several fruits and vegetables as a function of temperature from 5 to 95 degrees C. Measurements over the same frequency range and similar temperature ranges are presented for two other food products, whey protein gel and apple juice. A few comments are offered on likely future dielectric spectroscopy applications in agriculture.     

RF Energy Deposition in a Heterogeneous Model of Man: Far-Field Exposures

Distributions of the specific absorption rate (SAR) were measured in a full-scale heterogeneous model of man. The model contained a skeleton, brain, lungs, and muscle. All these tissues had dielectric properties close to those of the respective in vivo properties of actual tissues at the test frequencies. SAR's were measured for exposures in the far field at 160, 350, and 915 MHz for the E and H polarizations. A computer-controlled scanning system and an implantable, minimally perturbing electric field probe were used. The results are also compared with the SAR distributions previously measured in a homogeneous model.     

Perturbation of dielectric resonator for material measurement

A novel method for rapid measurement of the permittivity of a small object has been developed. Based on the perturbation of a semi-open resonant structure with a dielectric resonator excited in the TE₀₁₁ mode, the apparatus allows samples to be introduced freely and exhibits substantial mode stability. Results of permittivity measurement on low-loss dielectric samples in the form of thin plates and disks were in good agreement with values given by alternative methods employing much larger samples. The method can be extended to account for dielectric loss and permeability. In addition to being applied to material characterisation in the laboratory, the resonant structure can be developed into a transportable dielectrometer for small objects     

Effects of Dielectric Parameters of Human Body on Radiation Characteristics of Ingestible Wireless Device at Operating Frequency of 430 MHz

In order to assess the sensitivities of the radiation characteristics and the compliance of ingestible wireless device (IWD) in human body due to the uncertainty and intersubject variability of dielectric properties of human body tissues, the specific absorption rate (SAR) and radiation characteristics of the IWD in two realistic human body models with changed and unchanged dielectric values are quantitatively compared using the finite-difference time-domain method. Simulations are carried out in 13 scenarios where the IWD is placed in the center positions of abdomens in the two body models at the operating frequency of 430 MHz with three orientations. Results show that the variation of radiation intensity near the surface of abdomen is around 2.5 dB within 20% variation of dielectric values. The maximum SAR values increase with the increase in conductivities of human body tissues and decrease with the increase in relative permittivities of human body tissues. A variation of up to 20% in conductivities and relative permittivities, alone or simultaneously, always causes a variation of SAR to be less than 10%. As far as the compliance of safety is concerned, the maxima of 1-g-averaged and 10-g-averaged SARs can reach 3.16 and 0.89 W/kg at the input power of 25 mW.     

An Algorithm for Predicting the Change in SAR in a Human Phantom Due to Deviations in Its Complex Permittivity

The aim of this study is to determine a robust prediction algorithm that can be used to correct the measured specific absorption rate (SAR) in a homogeneous phantom when its complex permittivity deviates from standardized reference values. Results are analyzed over a frequency range of 30–6000 MHz. Both measurements and numerical simulations are presented. Several antenna sizes and distances to the phantom are investigated so as to study a large range of SAR distributions. It is demonstrated that the prediction algorithm, while developed using dipole antennas, also works well for mobile telephone models. Employing the prediction algorithm reduces the SAR measurement uncertainty, thereby improving the reproducibility of SAR compliance assessment between laboratories. Another benefit of the algorithm is that it enables the use of broadband tissue-equivalent liquids, whose dielectric parameters are not currently within the tight tolerances of existing standards. The use of broadband liquids reduces the cost of SAR measurement. The method presented in this paper is of benefit to the IEEE 1528 and IEC 62209 measurement standards.      

Numerical simulation of annular-phased arrays of dipoles forhyperthermia of deep-seated tumors

We have used the finite-difference time domain (FDTD) method to calculate the SAR distributions from an annular-phased array of eight dipole antennas coupled through water "boluses" in anatomically based three-dimensional models of the human body. We evaluated the effect of tapered bolus chambers, frequency (100-120 MHz), dipole length (17-30 cm), and phase and amplitude of power to the various dipoles on the ability to focus energy in the region of deep-seated tumors in the prostate and the liver. Assuming tumor conductivity and permittivity to be similar or slightly higher than surrounding normal tissues, calculations indicate that adjustment of the noted parameters should result in considerable improvement in focusing of SAR distributions in tumor-bearing regions. If such calculations can be shown to correctly predict empirical measurements from complex inhomogeneous (although not necessarily anatomically correct) phantoms, they may be useful for hyperthermia treatment planning based on patient-specific anatomic models.     

Resonance suppression of a spherical electromagnetic shieldingenclosure by using conductive dielectrics

The resonance suppression for the electromagnetic shielding enclosure is theoretically investigated. A simple model of a double-layered spherical shell with a plane-wave illumination is assumed. When a spherical shell made of conductive dielectrics is covered with a thin metal layer, the conductivity of the dielectrics has an optimum value which minimizes the Q-factor at the fundamental resonant frequency. The optimum conductivity is shown to be a function of the resonant frequency and the thickness of the dielectric layer. The improvement of the shielding effectiveness by introducing the optimum conductivity is shown     

A numerical study of a dielectric disk antenna above groundeddielectric substrate

An antenna made of a dielectric disk with a high permittivity mounted on top of a grounded dielectric substrate of low permittivity is analyzed. A numerical procedure based on surface integral equations, derived from the equivalence principle, is used to compute the natural resonant frequencies for the HEM₁₁ mode from which the radiation Q factor of the antenna is obtained. Then the radiation pattern of the antenna, operating at the resonant frequency evaluated previously, is computed with an electric dipole excitation located within the dielectric substrate under the dielectric disk. The effect of various parameters on the radiation characteristics of the antenna is studied, and presented in the form of diagrams. The low values of the radiation Q, combined with the high values of the dielectric Q and conductor Q, indicate that this antenna promises to be more efficient then the microstrip antenna     

Dielectric,Impedance and Conduction Behavior of Double Perovskite Pr<Subscript>2</Subscript>CuTiO<Subscript>6</Subscript> Ceramics

Polycrystalline Pr₂CuTiO₆ (PCT) ceramics exhibits dielectric, impedance and modulus characteristics as a possible material for microelectronic devices. PCT was synthesized through the standard solid-state reaction method. The dielectric permittivity, impedance and electric modulus of PCT have been studied in a wide frequency (100 Hz–1 MHz) and temperature (303–593 K) range. Structural analysis of the compound revealed a monoclinic phase at room temperature. Complex impedance Cole–Cole plots are used to interpret the relaxation mechanism, and grain boundary contributions towards conductivity have been estimated. From electrical modulus formalism polarization and conductivity relaxation behavior in PCT have been discussed. Normalization of the imaginary part of impedance (Z″) and the normalized imaginary part of modulus (M″) indicates contributions from both long-range and localized relaxation effects. The grain boundary resistance along with their relaxation frequencies are plotted in the form of an Arrhenius plot with activation energy 0.45 eV and 0.46 eV, respectively. The ac conductivity mechanism has been discussed.