The penetration of EM waves through loaded apertures

In many cases, the effectiveness of an electromagnetic shield is determined by apertures that exist in the shield. To minimize the penetration of EM fields through a large aperture, the aperture is sometimes loaded with conductive material. The solution of the loaded aperture problem can be reduced to the calculation of equivalent magnetic surface currents, M&oarr;_s, that exist over the surface of the aperture. In the paper, the relevant integro-differential equations have been solved using the method of moments to determine M&oarr;_s for a small, square aperture loaded with a number of impedance sheets of practical interest. These values of M&oarr;_s have been used to calculate the magnetic and electric insertion losses of these impedance sheets. The numerical results are compared with shielding measurements that have been made on carbon composite materials and wire meshes and grids     

Analytic determination of the pinhole collimator's point-spread function and RMS resolution with penetration

Pinhole collimators are widely used to image small organs and animals. The pinhole response function (PRF) of knife-edge pinhole collimators has been estimated previously using geometric constructions without considering penetration and using "roll-off" models that employ an exponential model for the flux. An analytic expression for the PRF on the imaging plane that includes the effect of aperture penetration is derived in this paper by calculating the flux for photons passing through the aperture and those passing through the attenuating material. The PRF is then used to approximate the angular-dependent root-mean-square resolution in the directions parallel and perpendicular to the tilt of the point source. The corresponding aspect ratio is then obtained. The formulas are then compared with experimental data.     

Electromagnetic transmission through inhomogeneously filled slotsin a thick conducting plane-arbitrary incidence

The penetration of an arbitrarily incident electromagnetic wave through a slot filled with inhomogeneous material in a thick conducting plane is analyzed. The solution is obtained via a combined finite-element method/method of moments algorithm based on the generalized network formulation. The discretization of the generalized network formulation is performed via the method of moments. The finite-element method is then used to compute the fields within the inhomogeneous interior cavity region, leading to the construction of the interior aperture admittance matrix. It is shown that with the use of entire domain basis functions, the construction of the aperture admittance matrices is computationally efficient. Furthermore, this method is attractive since it preserves the sparsity of the finite-element method matrix, reducing computational memory requirements. Some examples of the penetration of inhomogeneously filled slots of various cross sections are presented     

Evanescent-mode applicators (EMA) for superficial and subcutaneoushyperthermia

Evanescent-mode waveguide aperture applicators are proposed for hyperthermic treatments of superficial and subcutaneous tissues. They consist of air-filled waveguide segments that work below the cutoff frequency and therefore support only evanescent transverse modes. These are excited by radiators of suitable symmetry and configuration to produce modal heating fields of selected cross-sections. This field emerges from the waveguide active aperture and enters the tissue to be heated through an air gap. These devices work in a very large range of frequencies and are extremely simple to manufacture, even with a variety of cross-section sizes and shapes, because of their air-filled feature. This enables good heating field flexibility with improved penetration to be obtained. Their operation is safe and practical also on irregular and curved tissue surfaces     

三维口面衍射场的复射线展开

本文利用Ｇａｂｏｒ原理把三维任意口面场表示成离散结构－波数相位空间中三维高斯函数的叠加将二维高斯基函数与相位空间谱双重正交的函数推广到三维，从而得到口有开函数。     

Equivalence of surface current and aperture field integrations for reflector antennas

The "extinction theorem" is used to prove that the fields of reflector antennas determined by integration of the current on the illuminated surface of the reflector are identical to the fields determined by aperture field integration with the Kottler-Franz formulas over any surfaceS_{a}that caps the reflector. As a corollary to this equivalence theorem, the fields predicted by integration of the physical optics (PO) surface currents and the Kottler-Franz integration of the geometrical optics (GO) aperture fields onS_{a}agree to within the locally plane-wave approximation inherent in PO and GO. Moreover, within the region of accuracy of the fields predicted by PO current or GO aperture field integration, the far fields predicted by the Kottler-Franz aperture integration are closely approximated by the far fields obtained from aperture integration of the tangential electric or magnetic field alone. In particular, discrepancies in symmetry between the far fields of offset reflector antennas obtained from PO current and GO aperture field integrations disappear when the aperture of integration is chosen to cap (or nearly cap) the reflector.     

基于电磁计算理论的雷达成像仿真研究

逆合成孔径雷达在战场监测、导弹突防等方面有着广泛的应用。文中采用电磁计算理论研究雷达成像仿真技术。首先,采用物理光学法得到目标表面电流,计算目标大带宽、多角度范围内散射特性。针对计算量大、耗时长的缺陷,文中采用最佳一致逼近理论拟合出目标在大频带、多角度范围内的散射特性,通过计算验证,在不影响精度的前提下大大提高了计算效率;接着,利用计算得到的散射数据,构建宽带目标回波信号,利用转台理论,进行小角度内的距离-多普勒成像;最后,以锥台为例,计算了从不同角度入射时的目标雷达图像,可以验证该方法的正确性和高效性。     

Application de la méthode de résolution par différences finies en régime transitoire à la détermination des champs électromagnétiques pénétrant dans une cavité via une ouverture

Study of the penetration of an electromagnetic wave through a large aperture is made using a finite difference time domain method. Considering the great storage capacity needed by a classical technique and thein stability of internal fields, a modification of the code has been done. In the first part of this article, we propose a method which avoids these problems, it consists in using Huygens sources at the location of an aperture. Two applications are presented : a model of plane, opening in an aircraft carrier.     

PolInSAR analysis of X-band data over vegetated and urban areas

This paper investigates the polarimetric and polarimetric interferometric synthetic aperture radar (PolInSAR) information contained in the high-resolution X-band data acquired by the RAMSES airborne SAR system over an area around Avignon, France containing bare surfaces, vegetation, and urban areas. The interferometric coherences are computed over natural and urban areas for all possible baseline copolar polarizations. In the complex plane, the obtained regions of coherence corresponding to most vegetation areas display small angular extents, meaning that if penetration occurs in the foliage, it is shallower than the system height accuracy. To quantify the PolInSAR information, an analysis of the interferometric height accuracy is first performed, and the results are compared with those associated with a theoretical and an empirical model. Concerning vegetation, a 6-m height difference is measured between the different polarimetric phase centers over a sparse pine forest, probably due to the presence of holes in the canopy. Crop study reveals also that wheat-type fields present oriented media properties at X-band due to their vertical structure. Over urban areas, in most cases, building height can be accurately obtained by using Pauli polarimetric phase center information.     

Characteristics of circular patch on thick substrate andsuperstrate

Accurate formulation for the wall admittance of a circular patch on a thick substrate/superstrate is presented. The principle of equivalence is invoked at the physical aperture of the patch, which yields equivalent electric and magnetic surface currents for the outside fields. The unknown electric current, in terms of the aperture electric field, is obtained from the field continuity condition on the aperture surface. Numerical results reveal that for a small substrate thickness, the equivalent electric current may be ignored. It should, however, be included in the analysis for larger substrate thicknesses. Numerical results for the wall conductance, wall susceptance, and radiation efficiency of a patch with various substrates and superstrates are presented     

Analytical form of EM fields radiated by circular aperture antennas of various current distributions

~~Analytical form of EM fields radiated by circular aperture antennas of various current distributions[1] Schelkunoff S. A., Advanced Antenna Theory, John Wiley &Sons, 1952. [2] E|liott R.S., Antenna Theory and Design, Prentice-Hall, Engie-wood Cliffs, N J, 1981. [3] R.W.P. King and G.S. Smith, Antennas in Matter: fundamen-tals, Theory and Applications, MIT Press, Cambirdge, MA,1981. [4] W.L. Stutzman and G.A. Thiele, Antenna Theory and Desigrn,John Wiley & S…     

Transmission of an EM Wave Through the Aperture of a Cylindrical Cavity

The penetration of an electromagnetic wave through an aperture in a cylindrical structure is investigated. By using the moment method, the electrical and magnetic fields in a cavity behind the aperture are determined as a function of frequency of the incident field. The accuracy of the numerical solution is established through tests of satisfaction of the boundary condition, edge condition, and convergence with respect to the number of modes in the aperture. Depending on the frequency and location inside the cavity, the stored electromagnetic energy density varies very rapidly. Its peak value can be two orders of magnitude greater than the incident energy density. The frequencies where the peaks occur can be identified approximately as the resonance frequencies of the cavity when the aperture is closed.     

通过导电壁上窗口透入的电磁波对人体作用的研究

本文用时域有限差分法和人体非均匀电磁模型研究了通过无限大导电壁上窗口透入的电磁场与人体的相互作用。首先对平面电磁波通过导电壁上窗口的透入特性进行了计算,并与解析解进行比较,证明了计算的可靠性。在此基础上对不同极化情况的入射平面波的透入场与靠近窗口的坐姿人体模型的作用进行了研究。计算了人体模型中的电磁场分布和所吸收的电磁能量。     

Slot apertures having depth and losses described by local transmission line theory

The problem of electromagnetic penetration of a narrow slot aperture in a thick conducting plane is considered when the conductivity of the plane (and slot walls) is large but not necessarily infinite and when somewhat lossy gaskets are placed in the slot. The problem is partitioned into a local region, where transmission line theory is used to describe the antenna modes along the slot, and a nonlocal region, where a filament-type integral representation is used. The result is a modified Hallen-type integro-differential equation for the slot voltage (or magnetic current). The local transmission line theory allows the effects of wall loss and gaskets to be included in a simple and intuitive manner. Examples involving a rectangular slot aperture are given. The finite conductivity of the conducting plane, even for good conductors, reduces the penetration when realistic slot dimensions are used. Gaskets, even with relatively small loss tangents, also reduce the penetration when realistic slot dimensions are used.<>     

Feed gap aperture field and input admittance of an infinitely longinsulated antenna

Expressions are derived for the input admittance and aperture electromagnetic fields in the feed gap of a solid insulated antenna of infinite extent. This is done by solving the boundary value problem where the fields are represented by Fourier series built up by superposition of basic sets of φ-independent cylindrical waves. Such waves are obtained from the solution of the Helmholtz equation governing the z-directed electric Hertzian potential. The axial aperture electric field in the gap is in the form of a Fourier-Bessel series and is used to find the input admittance. Results are plotted for the aperture fields and tabulated for the input admittances at different radii, insulating shell dielectric constants, and gap widths     

Improved Single and Multiaperature Waveguide Coupling Theory, Including Explanation of Mutual Interactions

Bethe's small aperture coupling theory, modified by Cohn for large coupling apertures, is improved by including correction terms obtained by averaging the fields over the large aperture. Additionally, inclusion of nonempirical thickness correction factors derived previously by McDonald give coupling formulas which result in theoretical predictions for multiaperture couplers substantially in exact agreement with experiment (correcting small discrepancies previously noted by the author in a 1968 paper). This agreement is now so close that it becomes possible both to identify and explain the mutual interaction effects between closely spaced apretures in multiaperture couplers. It is shown that the mutual interaction is due to contradirectional (or backward) waves in the secondary arm, so that multiaperture interactions are manifested as elimination of the self-interactions of the individual apertures (since the high directivity of typical multiaperture couplers implies negligible backward wave amplitude).     

Shielding Theory of Enclosures with Apertures

This paper develops the theory of EM radiation from metallic enclosures with apertures, excited by an internal source at frequencies below the fundamental resonance of the enclosure. The enclosure with an aperture is analyzed from two different points of view: as a cavity with a small aperture in a wall, and as a waveguide section short-circuited at one end and open at the other. Rectangular geometries are used throughout, since these are by far the most commonly encountered in practical enclosures and cabinets. Using the corresponding dyadic Green's functions, the fields generated inside the enclosure by some simple sources are determined. The fields radiated through small apertures in a cavity are determined using Bethe's theory of diffraction by small holes. The radiation from an open waveguide is calculated with the help of field-equivalence theorems, with assumptions applicable to the case of evanescent waves. The final step is to derive expressions for the " insertion loss" of the shield, defined as the ratio of the field strength at a point external to the shield, before and after the insertion of the enclosure. To do so, the effect of the shield upon the input impedance of the antenna is analyzed, and expressions obtained for the applicable cases. The resulting insertion-loss expressions are numerically evaluated for some representative cases, and graphically compared with a series of measurements performed to obtain experimental confirmation. Very good agreement is obtained in all cases, establishing the validity of the analysis.     

Gaussian mode analysis of Cassegrain antenna efficiency

Gaussian mode analysis is a convenient way to characterize long focal length systems, such as the Cassegrain antenna at the secondary focus. We use multimode Gaussian optics to derive several interesting results concerning the aperture efficiencies of Cassegrain antennas fed by corrugated conical horns. The highest efficiency is obtained when the antenna is illuminated by a wide-band, wide-angle horn which has its phase center at the secondary focus, or equivalently by a narrow-band diffraction limited horn with its phase-center located at a confocal tertiary focus. In both cases this corresponds to placing the horn aperture at a position where the incoming fields are frequency-independent and have limited spatial extent. We generalize these results to show that a true image of the aperture distribution can always be formed on the spherical cap in the aperture of any arbitrary scalar horn, by using a single refocusing tertiary lens or mirror. Since both the incoming fields and the horn aperture fields on this surface are frequency independent, it follows that the aperture efficiency is also independent of frequency with either of these arrangements. We suggest that devices for shaping the aperture illumination, such as shaped lenses or dual shaped subreflector systems, should, in general, be installed near an image of the telescope aperture distribution.     

Low-Frequency Electromagnetic Penetration of Loaded Apertures

Low-frequency (quasi-static) electromagnetic penetration of an aperture can be reduced by loading the aperture with a conductive film or a bonded-junction wire mesh. A quantitative analysis of this phenomenon is carried out in this paper for a loaded circular aperture in a perfectly conducting ground plane of infinite transverse extent. Contact resistance between the aperture loading and the rim is taken into account. The quasi-static magnetic-field problem and the electrostatic field problem from which the aperture polarizabilities and penetrant fluxes are determined are shown to reduce to the problem of solving a single Fredholm integral equation. Exact (numerical) and approximate (variational) solutions to this integral equation are obtained, and the latter are used to represent the polarizabilities and penetrant fluxes by simple analytical formulas and equivalent circuits. These representations are found to be quite accurate when the contact resistance is not too large     

Radiation from a flanged rectangular waveguide

The radiation from a rectangular waveguide with a perfectly conducting infinite flange is rigorously studied by using the method of the Kobayashi potential (KP). The fields in the waveguide and half-space are expanded in terms of the waveguide modes and the Weber-Schafheitlin discontinuous integrals, respectively. Continuity of the tangential aperture fields yields matrix equations for the expansion coefficients and the matrix elements consist of double infinite integrals and double infinite series of Bessel functions, which are calculated efficiently by applying the asymptotic approximation of the Bessel function. Numerical results are presented for various physical quantities, such as the aperture admittance, reflection coefficient of the incident wave, and magnitudes of higher-order mode waves, as well as the far-radiation pattern and aperture fields. To verify the validity of our method, the results are compared with other methods and excellent agreement is obtained.