Transmission of electromagnetic pulse into the head

Characterstic of electromagnetic pulse transmission in homogeneous spherical models of human and animal heads are studied as a function of time and position. The incident pulse waveform is described by a triple exponential waveform which exhibits close agreement with measured pulse shapes. The transmitted pulse is shown to be proportional to the time rate of change of the incident pulse, with peak occuring at the leading surface of the spheres.     

Linear and nonlinear EMP diffusion through a ferromagnetic conducting slab

The penetration of an EMP field, such as a plane-wave incident upon and a surge-arrestor current terminated at a shielding plate, through a ferromagnetic conducting slab made of iron or steel is investigated. The diffusion of the electromagnetic field in the highly conducting slab is complicated by the presence of the nonlinear saturation of the ferromagnetic permeabilitymu, due to the large amplitude of the incident EMP. Such a saturation, compared to the no-saturation constantmucase, makes the field diffuse faster in the slab and admits a stronger penetration field inside the shielding if the slab is thin and the pulse duration is long, as expected. On the contrary, if the slab is thick and the puslewidth is short, as the practical cases are, the saturation reduces the penetrated field but maintains its time shape. In this report, first we solve analytically the one-dimension plane-wave incidence problem for a slab with a constantmu. Then we use the results to partly predict and to interpret the numerical values obtained by using a finite difference code for the case of a nonlinearmu. These comparisons reveal excellent agreement. Second, we solve the constantmu, cylindrical-wave incidence problem by an approximate but extremely useful analysis, with its validity parameters clearly established. The results, shown to bear a simple relationship to those for the one-dimension problem, enable one to make use of the one-dimension results and predict easily the penetrated fields caused by a cylindrical incident current. Then for the nonlinear-mucase, we justify and extend that relationship by which the behaviors of the cylindrically diffused fields are obtained from those numerical results of the one-dimensional problem.     

Power Deposition in a Spherical Model of Man Exposed to I-20-MHz Electromagnetic Fields

The induced fields and the associated power deposition in mail exposed to HF electromagnetic (EM) fields have been investigated theoretically using spherical models. The induced electric fields inside the model exposed to either plane wave or near fields can be described adequately by a combination of quasi-static electric and magnetic induction solutions. It is shown that for field impedances less than 1200/spl pi//spl Omega/ the magnetically induced energy absorption predominates. Therefore, H fields must be measured to obtain any estimate of the hazards due to HF exposure. For a 70-kg model of man exposed to a plane wave field, the theory indicates that the time-average power absorption per unit volume is less than 2.5x10/sup -3/ mW/g for each milliwatt per square centimeter incident at 20 MHz and below. This suggests that the thermal safe-exposure levels for the HF band are many orders of magnitude in excess of the 10-mW/cm/sup 2/ level recommended for the microwave region.     

An Iterative Extended Boundary Condition Method for Solving the Absorption Characteristics of Lossy Dielectric Objects of Large Aspect Ratios

The recently developed iterative extended boundary condition method (IEBCM) has been used to compute the internal fields induced in homogeneous, axisymmetric, lossy dielectric objects of large aspect ratios when exposed to incident planewave radiation. Calculations were made for both the E- and k-polarization cases. The computed results for a prolate spheroidal model of an average man are found to be accurate for frequencies up to 300 MHz, while the use of the popular EBCM [1] was found to be essentially restricted to frequencies less than 70 MHz for these models and exposure conditions. The applicability of the IEBCM to composite bodies has also been examined by studying the irradiation of a capped cylindrical object. This composite object was first partitioned into several overlapping spherical subregions, and, alternatively, into two spherical subregions overlapping with a central cylindrical subregion. Spherical harmonics were used to represent the internal fields in the spherical subregions, while cylindrical expansions were utilized in the cylindrical subregions. It is shown that the second partitioning scheme is more computationally efficient and thereby suggests that the basis functions used to represent the subregional fields should be compatible with the subregional geometry. The new IEBCM, therefore, is a very valuable procedure which provides the opportunity of using the mixed basis functions in the solution.     

The development of biomedical approaches and concepts in radiofrequency radiation protection

The approaches and concepts used in the development of radiofrequency radiation (RFR) protection guidelines evolved over the past quarter of a century. The values of exposure limits (EL) proposed by various groups are converging. Early guides specified ELs in incident power density. Recent ELs are based on considerations of the relationship between bioeffects and the magnitude of the whole body average specific absorption rate (WBA-SAR) and current densities induced in the body. Both these quantities may be considered as dosimetric ones. Thresholds for untoward health effects expressed in terms of these quantities were suggested, and may be considered as basic ELs. It is possible to derive a frequency-dependent relationship between incident RFR fields and WBA-SAR and/or induced current densities in the body. ELs specified for the purpose of determining compliance in terms of electric and magnetic field strengths or equivalent plane-wave power density existing at a point where a person could be present, but measured in the absence of the exposed subject, may be considered as derived working limits. The rationales offered for the recommended ELs indicate that the principal consideration in establishing limits for frequencies of 10 MHz and higher is the prevention of thermal injury, thermal being defined as relatable to heating, i.e. an increase in temperature. At lower frequencies, below 100 kHz or 30 kHz, direct effects on membranes of nerve and muscle cells may be the limiting factor. An additional consideration is the hazard of shock and burns from contact with ungrounded large metal objects that are charged by RFR fields. Recent advances in RFR dosimetry led to concerns that exposure to presently accepted derived ELs may result in large local SARs and induced current densities in certain parts of the body. The present review concludes that further refinements to the basis for RFR should be introduced. Threshold for health hazards should be investigated taking into account both direct and thermal bioeffects of RFR. The dose-thermal effects relationships should be quantified using the concepts of SAR, SA and thermal dosage. Several unresolved questions, such as the biological basis for SAR time-averaging, and the limitation of pulse peak power, are briefly discussed.     

Antiradar protection of a metal body with a dielectric coating

A metal body can be made invisible for a radar (radar-protected) through covering it with a dielectric film. The invisibility means that the scattered (diffraction) field induced when a plane wave radiated by a radar is incident on a body has a zero amplitude in the direction toward the radar and that this situation occurs for any direction of the arrival of an arbitrarily polarized incident wave. The permittivity of the film should be anisotropic, complex, and different on different sections of a body’s surface. A system of equations from which this permittivity can be found is derived. The problem of protection of a cylinder of an arbitrary section is considered in detail.     

Theory of laser self-locking in the presence of host dispersion

A theory is developed of laser mode locking due to saturable absorbers when the dispersive property of the active material is taken into account. The electric field within the cavity is expanded in terms of cavity modes, the problem is treated under the rate-equation approximation, and only third-order nonlinear polarization terms of the dye are considered. The field is assumed to be made of2N + 1oscillating modes. It is shown that, owing to dispersion, the phase φlof the modes (lrunning from-Nto+N) has a term that is proportional to l²and a term (smaller than the previous one) that is proportional to l⁴. In agreement with the experimental results, the term proportional to l²increases the pulsewidth of the total electric field over the case of perfect mode-locking and it also gives a positive linear sweep of the field frequency. The pulsewidth increase is expressed by a factor γ that is proportional to the fourth power of the oscillating bandwidth, to the square of the relaxation time T1of the dye, and to the product of the length of the active material times a quantity ε that is related to dispersion of the material.     

Plane wave coupling to multiple conductor transmission lines abovea lossy earth

The current induced on an infinite multiple conductor transmission line located above a lossy homogeneous medium due to a transient plane wave is discussed. An exact solution is formulated in the frequency domain using a spatial transform technique. The widely utilized quasi-TEM approximation is derived directly from the exact solution with emphasis on the physical consequences of the assumptions made. Both frequency domain and time domain numerical results are presented for typical transmission structures and documented electromagnetic pulse (EMP) excitations. Comparison of the quasi-TEM approximation to the exact solution is made in order to study the validity of its application in EMP coupling problems. The modeling of the EMP source as an incident plane wave is examined by comparing the induced current due to a dipole source with its steepest-descent contribution     

Probability distribution of CW induced currents on randomly oriented subresonant loops and wires

The deterministic prediction of both transient and CW field coupling to large complex electrical systems poses such a formidable problem that other approaches to predicting the coupling may be useful The continuous wave coupling to a complicated electrical system is analyzed in terms of random small dipole interactions in the low-frequency limit (wavelengthsggsystem components' sizes). Both random coupling to the incident wave and random interactions among the dipoles are considered. The variables being randomized are the incident direction and polarization, the sizes and orientations of the dipoles, the mutual coupling strengths, and the lumped load impedances. The resulting normalized current distributions are shown to be insensitive to the details of the model except at the extremely low and high percentiles. The magnetic dipole case is investigated in detail. Its resulting induced current distribution roughly resembles, but is not, a log-normal distribution with a standard deviation in the vicinity ofsim6dB. This result provides insight into some recent measurements obtained for EMP transient field coupling to large systems. An important implication of the results is that for a variety of complicated systems, essentially consisting of many small elements that the coupling is dominated by low-frequency magnetic fields, the central parts of the induced current probability distributions are similar and nearly log-normal. However, conclusions based on the extrapolation of log-normality from measured values near the median to the extreme percentiles may be susceptible to sizeable errors. The electric dipole ease is presented very briefly here, only to the extent of its basic formulations. We refer interested readers to our original report for some basic results for this case and also for the effects caused by an elliptically polarized incident wave versus a linearly polarized one.     

电磁脉冲斜入射时无限长近地细导线表面电流的计算

本文应用时域有限差分（FDTD）方法研究电磁脉冲斜入射下无限长细导线的表面感应电流,并考虑了有耗地面的影响,该问题可以转换为二维问题来计算,从而大大节约计算资源,文中给出了各种情况下的感应电流计算结果。     

Analysis of focusing of pulsed baseband signals inside a layeredtissue medium

The derivation and application of a method designed to investigate the focusing properties of pulsed baseband signals of short pulsewidth (~1 ns) in biological tissue media are reported. To this end, sources fed from TEM waveguides, concentrically placed at the periphery of a three-layer cylindrical lossy model, are assumed. A Fourier-series-based methodology, appropriate for a useful class of pulse train incident signals, is presented and utilized to study the dynamics of pulse propagation inside the tissue medium. The propagation of each spectral component of the incident field within the tissue medium is analyzed by applying an integral-equation technique and a Fourier-series representation is used in order to obtain the time dependence of the electromagnetic fields produced at any point within tissue due to the pulsed excitation of the array elements. Numerical results are computed and presented at several points in a three-layer geometry, 20 cm in diameter, irradiated by an eight-element waveguide array. Focusing at a specific point of interest within tissue is achieved by properly adjusting the time delay of the signals injected to the individual applicators of the array     

Scattering from a finite length rod with endpoint contribution

A uniform, high frequency analysis for calculating scattering from thin, finite length cylinders is presented. The cylinder diameter is assumed to be small (on the order of a wavelength or less) so that the modal solution is valid. The total field consists of the incident field, the scattered field from the specular point, and two endpoint contributions. A thin wire tip diffraction coefficient (TDC) is developed for calculating the end point contributions. The incident fields are those of a spherical source radiating throughout space. While the specularly scattered field and end point contributions are discontinuous, these discontinuities cancel each other, resulting in a uniformly continuous total field. Good agreement is obtained between measured and calculated results.     

静电场对向列相液晶的影响

用统计物理的方法讨论了外加静电场对向列相液晶的影响。结果表明，电场诱导的向列相取向序参量、双折射和向列相—各向同性相的相变温度的改变是场强的偶函数，在电场较弱时与场强平方成正比。     

Resonating properties of passive spherical optical microcavities

As an optically pumped device, the lasing characteristics of a spherical microcavity laser depend on the optical pumping processes. These characteristics can be described in term of the Q factor and the optical field distribution in a microsphere. We derived analytical expressions and carried out numerical calculation for Q factor and optical field. The Q factor is found to be oscillatory functions of the radius of a microsphere and the pumping wavelength, and the pumping efficiency for a resonating microsphere is much higher than that for an anti-resonating microsphere. Using tunable lasers as pumping sources is suggested in order to achieve a higher pumping efficiency. Numerical calculation on optical field distribution in spherical microcavities shows that a well focused Gaussian beam is a suitable incident wave for cavity quantum electrodynamics experiments in which strong confinement of optical field in the center of a microsphere is requested, but higher order spherical wave should be used instead for exciting whispering-gallery-mode (WGM) microsphere lasers, for the purpose of favoring optical energy transferring to WGM in optical microspheres.     

On the attenuation of transient fields by imperfectly conducting spherical shells

Exact formulas for the electric and magnetic fields at any arbitrary point within a cavity region completely enclosed by a conducting spherical shell of arbitrary size are derived under the assumption that the exciting electromagnetic field is a linearly polarized, monochromatic, plane wave falling on the external surface of the shell. It is shown that the polarization of the electromagnetic field at the center of the cavity is the same as the polarization of the incident wave. From a knowledge of this steady-state solution, the time history of the electromagnetic field at the center of the cavity is calculated for the case where the incident wave is a Gaussian pulse. Numerical information on the effectiveness of the aluminum and copper shields under steady-state and transient conditions is provided for several pulse durations, shield sizes, and wall thicknesses.     

A numerical method for transient analysis of coupled antennas

A numerical method for finding the transient response induced on coupled thin cylindrical antennas excited by an electromagnetic pulse (EMP) is introduced. Numerical examples are given for two coupled thin antenna that are broadside incident by an idealized EMP. Applications of the method are made to calculate the induced currents on corner reflector antennas, and numerical examples are given to illustrate how the transient responses are modified by coupling effects. Examples of applying the method to include the effects of the time-varying medium on the responses are also shown with an assumed time-varying air conductivity.     

Singularity expansion representations of fields and currents in transient scattering

A unified treatment of the natural mode representations for induced currents and scattered fields is obtained by use of fundamental concepts regarding causality and superposition. The transient scattered field response is shown to have the form of a constant coefficient complex exponential sum only in the "late-time," after the last driven reponse is received from the object. Prior to this, the "early.time" response is found to be due to direct physical optics fields as well as a sum of temporally modulated natural modes produced by the progressive illumination of the incident wavefront. Alternate representations ands-plane behaviors are considered. The implications of these results on natural resonance target identification schemes are discussed.     

Fields and currents in the organs of the human body when exposed topower lines and VLF transmitters

A study is made of the electric fields and currents induced in the organs of the human body when exposed to high-voltage 50-60-Hz transmission lines and 10-30-kHz high-power transmitters. Relevant analyses previously carried out are summarized and supplemented with detailed investigations that complete the picture. Incomplete, misleading, and incorrect statements and methods in the related literature are pointed out, completed, and corrected. The major contribution is to provide quantitatively accurate, relatively simple analytic formulas that relate the incident electric field to the induced field in the organs of the body. The formulation and solution of the underlying integral equation are carried out in an Appendix     

Transient electromagnetic field propagation through infinite sheets, into spherical shells, and into hollow cylinders

Gaussian electromagnetic field pulses of several durations are propagated through infinite sheets into the interior of hollow cylinders and into the interior of spherical shells. The plates, spheres and cylinders are made of aluminum and contain no slots. The time history of the propagated pulses is computed. Finally, the time sequence of the electric field is calculated in the interior of a cylinder of finite length when connected at its ends by wires to a generator delivering a current pulse of Gaussian shape. The dimensions of the cavities are assumed to be sufficiently small so that resonances are not excited by the highest significant frequency contained in the shortest pulse considered. The numerical study is restricted to thin-walled aluminum shields 1/32 inch, 1/16 inch, 1/8 inch and 1/4 inch thick. The half-amplitude widths of the pulses employed lie in the range14 musec to2400 musec. It is shown that the resultant Gaussian pulse electric fields defined on the surface of the plates and cylinders are propagated with little diminution in amplitude. This is understandable due to the requirement that the tangential fields are continuous across the interfaces, and to the fact that skin effect is almost nonexistent at low frequencies. The incident (as contrasted to resultant) field pulse undergoes reflection at the boundary surface. Hence, the attenuation sustained by the incident field is great, since reflection is the chief mechanism of attenuation of fields at low frequencies. Thin spherical shells form effective magnetic shields. The electric field is small in the interior of thin-walled cylinders carrying extremely large transient currents.     

Energy absorption mechanism by biological bodies in the near fieldof dipole antennas above 300 MHz

The energy absorption mechanism in the close near field of dipole antennas is studied by numerical simulations. All computations are performed and validated applying the three-dimensional multiple multipole software package. The numerical model of the plane phantom is additionally checked by accurate as possible experimental measurements. For the plane phantom, the interaction mechanism can be described well by H-field induced surface currents. The spatial peak specific absorption rate can be approximated within 3 dB by a formula given here based on the incident H-field or antenna current and on the conductivity and permittivity of the tissue. These findings can be generalized to heterogeneous tissues and larger biological bodies of arbitrary shape for frequencies above 300 MHz. The specific absorption rate is mainly proportional to the square of the incident H-field, which implies that in the close near field, the spatial peak specific absorption rate is related to the antenna current and not to the input power