Showing papers on "Near and far field published in 1981"

Antenna theory and design

Abstract: Foreword to the Revised Edition. Preface to the Revised Edition. Preface. I SOURCE-FIELD RELATIONS SINGLE ANTENNA ELEMENTS. 1 The Far-Field Integrals, Reciprocity, Directivity. 1.1 Introduction. 1.2 Electrostatics and Magnetostatics in Free Space. 1.3 The Introduction of Dielectric, Magnetic, and Conductive Materials. 1.4 Time-Varying Fields. 1.5 The Retarded Potential Functions. 1.6 Poynting's Theorem. 1.7 The Stratton-Chu Solution. 1.8 Conditions at Infinity. 1.9 Field Values in the Excluded Regions. 1.10 The Retarded Potential Functions: Reprise. 1.11 The Far Field: Type I Antennas. 1.12 The Schelkunoff Equivalence Principle. 1.13 The Far Field: Type IL Antennas. 1.14 The Reciprocity Theorem. 1.15 Equivalence of the Transmitting and Receiving Patterns of an Antenna. 1.16 Directivity and Gain. 1.17 Receiving Cross Section. 1.18 Polarization of the Electric Field. 2 Radiation Patterns of Dipoles, Loops, and Helices. 2.1 Introduction. 2.2 The Center-Fed Dipole. 2.3 Images in a Ground Plane. 2.4 A Monopole Above a Ground Plane. 2.5 A Dipole in Front of a Ground Plane. 2.6 The Small Current Loop. 2.7 Traveling Wave Current on a Loop. 2.8 The End-Fire Helix. 3 Radiation Patterns of Horns, Slots and Patch Antennas. 3.1 Introduction. 3.2 The Open-Ended Waveguide. 3.3 Radiation from Horns. 3.4 Center-Fed Slot in Large Ground Plane. 3.5 Waveguide-Fed Slots. 3.6 Theory of Waveguide-Fed Slot Radiators. 3.7 Patch Antennas. II ARRAY ANALYSIS AND SYNTHESIS. 4 Linear Arrays: Analysis. 4.1 Introduction. 4.2 Pattern Formulas for Arrays with Arbitrary Element Positions. 4.3 Linear Arrays: Preliminaries. 4.4 Schelkunoff's Unit Circle Representation. 5 Linear Arrays: Synthesis. 5.1 Introduction. 5.2 Sum and Difference Patterns. 5.3 Dolph-Chebyshev Synthesis of Sum Patterns. 5.4 Sum Pattern Beamwidth of Linear Arrays. 5.5 Peak Directivity of the Sum Pattern of a Linear Array. 5.6 A Relation Between Beamwidth and Peak Directivity for Linear Arrays. 5.7 Taylor Synthesis of Sum Patterns. 5.8 Modified Taylor Patterns. 5.9 Sum Patterns with Arbitrary Side Lobe Topography. 5.10 Discretization of a Continuous Line Source Distribution. 5.11 Bayliss Synthesis of Difference Patterns. 5.12 Difference Patterns with Arbitrary Side Lobe Topography. 5.13 Discretization Applied to Difference Patterns. 5.14 Design of Linear Arrays to Produce Null-Free Patterns. 6 Planar Arrays: Analysis and Synthesis. 6.1 Introduction. 6.2 Rectangular Grid Arrays: Rectangular Boundary and Separable Distribution. 6.3 Circular Taylor Patterns. 6.4 Modified Circular Taylor Patterns: Ring Side Lobes of Individually Arbitrary Heights. 6.5 Modified Circular Taylor Patterns: Undulating Ring Side Lobes. 6.6 Sampling Generalized Taylor Distributions: Rectangular Grid Arrays. 6.7 Sampling Generalized Taylor Distributions: Circular Grid Arrays. 6.8 An Improved Discretizing Technique for Circular Grid Arrays. 6.9 Rectangular Grid Arrays with Rectangular Boundaries: Nonseparable Tseng-Cheng Distributions. 6.10 A Discretizing Technique for Rectangular Grid Arrays. 6.11 Circular Bayliss Patterns. 6.12 Modified Circular Bayliss Patterns. 6.13 The Discretizing Technique Applied to Planar Arrays Excited to Give a Difference Pattern. 6.14 Comparative Performance of Separable and Nonseparable Excitations for Planar Apertures. 6.15 Fourier Integral Representation of the Far Field. III SELF-IMPEDANCE AND MUTUAL IMPEDANCE, FEEDING STRUCTURES. 7 Self-Impedance and Mutual Impedance of Antenna Elements. 7.1 Introduction. 7.2 The Current Distribution on an Antenna: General Formulation. 7.3 The Cylindrical Dipole: Arbitrary Cross Section. 7.4 The Cylindrical Dipole: Circular Cross Section, Hallen's Formulation. 7.5 The Method of Moments. 7.6 Solution of Hallen's Integral Equation: Pulse Functions. 7.7 Solution of Halle'n's Integral Equation: Sinusoidal Basis Functions. 7.8 Self-Impedance of Center-Fed Cylindrical Dipoles: Induced EMF Method. 7.9 Self-Impedance of Center-Fed Cylindrical Dipoles: Storer's Variational Solution. 7.10 Self-Impedance of Center-Fed Cylindrical Dipoles: Zeroth and First Order Solutions to Hallen's Integral Equation. 7.11 Self-Impedance of Center-Fed Cylindrical Dipoles: King-Middleton Second-Order Solution. 7.12 Self-Impedance of Center-Fed Strip Dipoles. 7.13 The Derivation of a Formula for the Mutual Impedance Between Slender Dipoles. 7.14 The Exact Field of a Dipole: Sinusoidal Current Distribution. 7.15 Computation of the Mutual Impedance Between Slender Dipoles. 7.16 The Self-Admittance of Center-Fed Slots in a Large Ground Plane: Booker's Relation. 7.17 Arrays of Center-Fed Slots in a Large Ground Plane: Self-Admittance and Mutual Admittance. 7.18 The Self-Impedance of a Patch Antenna. 8 The Design of Feeding Structures for Antenna Elements and Arrays. 8.1 Introduction. 8.2 Design of a Coaxially Fed Monopole with Large Ground Plane. 8.3 Design of a Balun-Fed Dipole Above a Large Ground Plane. 8.4 Two-Wire-Fed Slots: Open and Cavity-Backed. 8.5 Coaxially Fed Helix Plus Ground Plane. 8.6 The Design of an Endfire Dipole Array. 8.7 Yagi-Uda Type Dipole Arrays: Two Elements. 8.8 Yagi-Uda Type Dipole Arrays: Three or More Elements. 8.9 Frequency-Independent Antennas: Log-Periodic Arrays. 8.10 Ground Plane Backed Linear Dipole Arrays. 8.11 Ground Plane Backed Planar Dipole Arrays. 8.12 The Design of a Scanning Array. 8.13 The Design of Waveguide-Fed Slot Arrays: The Concept of Active Slot Admittance (Impedance). 8.14 Arrays of Longitudinal Shunt Slots in a Broad Wall of Rectangular Waveguides: The Basic Design Equations. 8.15 The Design of Linear Waveguide-Fed Slot Arrays. 8.16 The Design of Planar Waveguide-Fed Slot Arrays. 8.17 Sum and Difference Patterns for Waveguide-Fed Slot Arrays Mutual Coupling Included. IV CONTINUOUS APERTURE ANTENNAS. 9 Traveling Wave Antennas. 9.1 Introduction. 9.2 The Long Wire Antenna. 9.3 Rhombic and Vee-Antennas. 9.4 Dielectric-Clad Planar Conductors. 9.5 Corrugated Planar Conductors. 9.6 Surface Wave Excitation. 9.7 Surface Wave Antennas. 9.8 Fast Wave Antennas. 9.9 Trough Waveguide Antennas. 9.10 Traveling Wave Arrays of Quasi-Resonant Discretely Spaced Slots [Main Beam at theta0= arccos(beta/k)]. 9.11 Traveling Wave Arrays of Quasi-Resonant Discretely Spaced Slots (Main Beam Near Broadside). 9.12 Frequency Scanned Arrays. 10 Reflectors and Lenses. 10.1 Introduction. 10.2 Geometrical Optics: The Eikonal Equation. 10.3 Simple Reflectors. 10.4 Aperture Blockage. 10.5 The Design of a Shaped Cylindrical Reflector. 10.6 The Design of a Doubly Curved Reflector. 10.7 Radiation Patterns of Reflector Antennas: The Aperture Field Method. 10.8 Radiation Patterns of Reflector Antennas: The Current Distribution Method. 10.9 Dual Shaped Reflector Systems. 10.10 Single Surface Dielectric Lenses. 10.11 Stepped Lenses. 10.12 Surface Mismatch, Frequency Sensitivity, and Dielectric Loss for Lens Antennas. 10.13 The Far Field of a Dielectric Lens Antenna. 10.14 The Design of a Shaped Cylindrical Lens. 10.15 Artificial Dielectrics: Discs and Strips. 10.16 Artificial Dielectrics: Metal Plate (Constrained) Lenses. 10.17 The Luneburg Lens. APPENDICES. A. Reduction of the Vector Green's Formula for E. B. The Wave Equations for A and D. C. Derivation of the Chebyshev Polynomials. D. A General Expansion of cosm v. E. Approximation to the Magnetic Vector Potential Function for Slender Dipoles. F. Diffraction by Plane Conducting Screens: Babinet's Principle. G. The Far-Field in Cylindrical Coordinates. H. The Utility of a Csc2 theta Pattern. Index.

Polarization dependence in electromagnetic inverse problems

Abstract: The complete description of electromagnetic scattering processes implies polarization since an electromagnetic scatterer acts like a polarization transformer. The main objective is to show that due to the vector nature of electromagnetic waves, electromagnetic remote sensing and inverse scattering techniques, if applied rigorously, require incorporation of polarization information into their formulation. By applying this approach to existing and some novel theories, remarkable improvements in quality and fidelity of the reconstructed profiles and/or images can be obtained. Thus there is ample justification for continuing efforts in developing methods and theories of vector inverse scattering for the electromagnetic polarization-dependent case or in the even more complicated seismic case of p - and s -wave interaction in elastic media as for example encountered in vertical seismic profiling.

Light emission by multipole sources in thin layers. I. Radiation patterns of electric and magnetic dipoles

Abstract: The emission of light by sources, e.g., by luminescent centers, located in a thin nonabsorbing dielectric layer 0 between two half-spaces 1 and 2 is investigated theoretically. It is assumed that the light is emitted in electric or magnetic dipole transitions. But the theory is given in such a form that it can easily be extended to electric and magnetic quadrupole and higher-order multipole transitions. The electromagnetic boundary-value problem is solved rigorously for sources in layers 0 of arbitrary thickness. The radiation patterns, i.e., the angular distributions of light emitted into the half-spaces 1 and 2, are calculated. The theory takes into account the following effects that strongly influence the radiation patterns: (1) the wide-angle interferences that are a consequence of the coherence of the plane waves emitted into different directions, (2) the multiple-beam interferences that result from the multiple reflections of the plane waves between the interfaces 0/1 and 0/2, and (3) that evanescent waves present in the near field of the source radiate into media 1 and/or 2 if these media are denser than layer 0. This emission process is influenced by evanescent-wave effects analogous to the wide-angle interferences and the multiple-beam interferences of the plane waves. The limiting case of extremely thin layers 0 with optical thickness much smaller than the wavelength is also treated. Explicit analytical expressions are presented for the dipole radiation patterns in this case. Furthermore, the theory is generalized for sources in plane-stratified-layer systems. The dipole radiation patterns are derived for the case in which any numbers of loss-free or absorbing, dielectric or metallic thin films are present between the loss-free layer 0 of arbitrary thickness containing the source and the half-spaces 1 and 2.

The phase retrieval problem

Abstract: The phase retrieval problem arises in applications of electromagnetic theory in which wave phase is apparently lost or impractical to measure and only intensity data are available. The mathematics of the problem provides unusual insights into the nature of electromagnetic fields. The theory is reviewed and illustrated. The basic issue of the phase retrieval problem, stated for a one-dimensional field, is that although a unique Fourier transform relation exists between the field F(x) in the Fraunhofer plane and the field u(x') in the object plane, the infinite fold phase ambiguity which appears as the result of the possibilities of conjugating the zeros of F(z), z = x + jy implies that additional information or processing of the object wave must be available to obtain the phase. Among the possible solutions which are described are reference beam addition, apodization and the use of multiple intensity distributions, permitting the use of iterative computational procedures in some applications.

Broadband pulse-optimised antenna

Abstract: The theoretical far-field pattern, gain, and aperture are derived for a V-shaped dipole carrying a nonuniform current wave. Particular attention is given to broadening in the far field of a sharp current pulse applied at the input. The optimum variation of characteristic TEM wave impedance is then derived with reflection coefficient and pulse broadening as constraints. Measurements on a practical design of 70 cm length show a bandwidth from 200 MHz to 1.6 GHz for an input voltage reflection coefficient less than 0.1 and a pulse broadening of 35ps. The directional diagram is±25° to half power.

Generation of a d.c. field by nonlinear electromagnetic waves in relativistic plasmas

Abstract: A finite amplitude linearly polarized electromagnetic wave propagating in a relativistic plasma, is found to generate the longitudinal d.c. as well as the oscillating electric field at the second harmonic. In a plasma consisting of only electrons and positrons, these fields cannot be generated.

Near-field control in multistripe geometry injection lasers

Abstract: Control of the intensity and position of the near field is demonstrated in an analysis of twin-stripe and triple-stripe injection lasers. Calculations of file electrooptical properties of the structures are presented, taking into account the interaction between the carrier concentration profile and the optical field. For twin-stripe devices interstripe coupling is investigated as a function of stripe separation. For stripe separation comparable with stripewidth, the properties of triple-stripe lasers become of interest. Nonlinear light-current characteristics for the twin-stripe device are obtained and lead to an interpretation of such characteristics in conventional stripe geometry lasers.

Determination of far fields signatures, for instance of seismic sources

Abstract: A method is provided of ascertaining the far field signature of an array of sound source units, each of which is small compared with the wavelength of the highest frequency of interest. The units are fired so that the interactions between them is negligible. This is achieved by firing the units sequentially so that each generates all its significant radiation before the next is fired, and/or by firing more than one unit at a time and separating the units by at least one wavelength of the lowest frequency of interest. The far field signature of each unit is measured by a pressure-sensitive detector close to the unit but in a region where the phase spectrum of the pressure field is independent of azimuth and range. The far field signature of the array is derived from the measured signatures by summation.

Energy Absorption from Small Radiating Coaxial Probes in Lossy Media

Abstract: This paper describes the calculation of energy deposition around small open-ended coaxial antenna probes in lossy media. Two theoretical methods, a small monopole approximation (I) and an equivalent magnetic current source (II), are evaluated and compared. Method I is shown to be inappropriate for determining near field energy deposition. Power contour plots determined by method II in the vicinity of the open-ended coaxial antenna are presented as well as calculations of total power absorbed as a function of distance from the antenna center for various antenna dimensions and media dielectric properties. Our calculations of absorbed power distributions near the antenna are consistent with the limited experimental data which is available for comparison. A frequency of 2.45 GHz was selected for these calculations so that the results will be of value to workers interested in the application of own-ended coaxial antennas for invasive treatment of cancer by microwave hyperthermia.

The determination of the surface impedance of an obstacle from measurements of the far field pattern

Abstract: We consider the problem of determining the surface impedance of an obstacle from measurements of the far field pattern of a scattered time harmonic acoustic wave. It is shown that this problem can be stabilized by assuming that the impedance is Holder continuous with an a priori bound given on the impedance and its Holder coefficient. A constructive method for determining the impedance is given through the use of the Backus–Gilbert method for solving improperly posed moment problems.

Transient electromagnetic response of a sphere in a layered medium

Abstract: The integral equation for the electromagnetic response of a sphere in a layered medium may be solved as follows. First, the unknown time harmonic electric field in the sphere is expanded in spherical vector waves. Secondly, the coefficients for these wave functions are found by a set of equations. The equations are found by multiplying the integral equation throughout by each wave function and integrating over the spherical conductor. Once the unknown coefficients have been determined, then the transient response may be found by taking the inverse Fourier transform. In carrying out the Fourier transform one learns that for most of the time range used in prospecting, only the lowest order vector wave function is significant. A study of the singularities of the spectrum of the transient shows that, for the time range considered, only a single branch cut is significant. There are no pole type responses. That is, the field does not decay exponentially. Previous studies of a sphere in free space reported only pole type responses. That is, at the later stages, the field decays exponentially. This study shows that, in order to model satisfactorily the effect of the host rock on transient electromagnetic fields, the sphere must be placed in layered ground.

Research topics in electromagnetic wave theory

Abstract: Reviews diverse current applications of electromagnetic wave theory, particularly those of interest in aerospace and defense research. Uses the topic of random media to model the ionosphere and the earth terrain with applications to pulse distortion and microwave remote sensing. Studies inhomogeneous plasmas and magnetized ferrites for the propagation, reflection, and transmission of electromagnetic waves with a variational formulism and a coordinate invariant method, and from the point of view of linear mode conversion. Treats result sensing based on microwave radiometer for earth terrain and line-of-sight method for atmosphere parameters. Examines the interaction of electromagnetic fields with biological bodies. In the antenna field, coverage encompasses the topics of broadband antenna arrays, dipole radiation in stratified media, insulated linear antennas, and offset fuel reflector antennas.

Near-Field Absorption in Prolate Spheroidal Models of Humans Exposed to a Small Loop Antenna of Arbitrary Orientation

Abstract: The power absorption characteristics of the prolate spheroidal model of an average man have been studied when the model is exposed to the near fields of an arbitrarily located small loop antenna. An integral equation is formulated and the fields radiated by the loop are expanded in terms of the vector spherical harmonics. This equation is then solved using the extended boundary condition method (EBCM,). For three different loop-spheroid configurations, the power distribution and the average SAR have been calculated as a function of the frequency and the separation distance. It is shown that the results obtained for separation distances larger than lambda /2 agree well with those obtained from the plane wave exposure case. Furthermore, the average SAR value calculated as a function of separation distance for the case where the magnetic dipole moment is aligned parallel to the major axis of the spheroid are found to oscillate around the constant value obtained from the H-polarized plane wave exposure case. On the other hand, the average SAR values for the E-polarization case (magnetic dipole is parallel to the spheroidal minor axis) are found to increase monotonically with the decrease in separation distance. It is also shown that despite the complicated nature of the near fields, the absorption characteristics can still be explained in terms of the variations of the incident radiation. These loop results, together with those obtained from other simple soures, can be used as building blocks in arriving at a qualitative understanding of the near-field absorption characteristics for more general exposure cases.

The near field of dipole antennas, part I: Theory

Abstract: The theoretical and experimental evaluation of the electromagnetic fields in the immediate vicinity of resonant dipole antennas is presented. This type of antenna is widely used with portable and mobile radio transmitters. The work presented herein has been motivated by the concern that future Radio Frequency Protection Guides with respect to human exposure to nonionizing electromagnetic radiation might be expressed strictly in terms of the intensity squared of the electric or magnetic fields. It is shown in the results that it is possible to detect relatively high intensity electromagnetic (EM) fields in close proximity to resonant dipoles even for very low levels of radiated power (1 mW and less). The paper is divided into a theoretical section and an experimental section because its goals are twofold. First, the formulas for the correct evaluation of the EM fields in the close proximity to dipole antennas are established. Second, it is shown that such EM fields, which can be theoretically predicted and experimentally verified with satisfactory accuracy, are indeed strong enough to violate proposed Radio Frequency Protection Guides even for very low levels of radiated power. Thus portable radios are rendered virtually useless, although the same guides permit exposures to much higher levels of power in the far field. Part I of the paper is essentially theoretical and expresses the fields near dipole antennas in terms of cylindrical waves, which lend themselves to closed form integration. The asymptotic expressions of some components of the field are particularly simple for close distances (in terms of wavelength) from the antenna. The correctness of the solution is checked by evaluating how closely boundary conditions are satisfied. Results have shown that previously used formulas for evaluating field intensity very near dipole antennas can give incorrect values.

Free Electron Lasers Based on High Current Electron Beam

Abstract: The stimulated radiation generation of intense electron beam with 1 MeV energy and current to 13 kA has been investigated. The radiation to 20 MW power in 7 + 13 mm band with the pump by helical magnetic field of the undulator and to 6 MW in 3 cm band with the pump of 10 cm electromagnetic wave has been received. The dependences of output power on pump field, transverse focusing magnetic field and interaction field length have been studied.

Diffraction losses and mode structure of equivalent TEM(00) optical resonators.

Abstract: A general description is given which allows one to determine the diffraction losses and mode structure (intensity and phase distribution) of any arbitrary TEM00 laser resonator lying in the stable region of the stability diagram. By means of equivalence relations and adaption of the aperture size to the size of the undisturbed Gaussian beam it is possible to reduce the number of characterizing parameters to two: the adapting factor s, the product (g1 · g2). Numerical results are shown. The intensity pattern of the far field is discussed.

Measurements of the RF Power Absorption in Spheroidal Human and Animal Phantoms Exposed to the Near Field of a Dipole Source

Abstract: Experimental results for the average specific absorption rate (SAR) in scaled spheroidal phantoms of human and animal models exposed to near-field radiation are presented. Prolate spheroidal phantoms filled with saline solution simulating muscle tissue were used to measure average SAR values in different models. To control the exposure conditions, simple sources of known radiation characteristics, namely, short electric dipoles, were used. The accuracy of the experimental procedure was evaluated by making several average SAR measuremens at large distances (0.6 ?) from the dipole. The results obtained are found to be in good agreement with those available in the Radiofrequency Radiation Dosimetry Handbook [2]. Near-field SAR measurements for different models are presented as a function of the distance from the source. It is shown that even for the complex radiation fields in the near zone of the source, the average SAR below the resonance frequency can be explained in tenns of the magnitude and direction of the incident fields in the same way that the plane wave absorption characteristics are explained.

Microstrip dipoles on cylindrical structures

Abstract: The fundamental solution for printed circuit antennas on cylindrical substrates is presented in this paper. Exact expressions are obtained for the electromagnetic field both inside the substrate as well as the surrounding free space produced by an arbitrarily oriented printed circuit dipole. Asymptotic results are derived for a cylindrical thin substrate whose diameter is large compared to wavelength.

Equations of motion for a free-electron laser with an electromagnetic pump field and an axial electrostatic field

Abstract: The equations of motion for a free-electron laser (FEL) with an electromagnetic pump field and a static axial electric field are derived using a Hamiltonian formalism. Equations governing the energy transfer between the electron beam and each of the electromagnetic fields are given, and the phase shift for each of the electromagnetic fields is derived from a linearized Maxwell wave equation. The relation between the static axial electric field and the resonant phase is given. Laser gain and the fraction of the electron energy converted to photon energy are determined using a simplified resonant particle model. These results are compared to those of a more exact particle simulation code.

An Empirical Relationship for Electromagnetic Energy Absorption in Man for Near-Field Exposure Conditions

Abstract: An empircal relationship is presented for the whole-body-average electromagnetic energy absorption in a 180-cell block model of man for near-field exposure conditions. Consideration is restricted to near fields with P polarization (no component of E directed arm-to-arm) in which the magnitude of the incident electric field is maximum immediately in front of the abdominal region. A highlight of this work is the considerably reduced whole-body average energy absorption for near-field partial-body exposures as compared to that obtained under plane-wave irradiation conditions.

Electromagnetic induction of a three-dimensional conductivity inhomogeneity in the two-layered earth

Abstract: The calculation of the elements of Green's tensor function is presented for solving the problem of the electromagnetic induction by means of a vector integral equation. A two-layered Earth is considered as the medium, the surface layer including a three-dimensional conductivity inhomogeneity. Use is made of the boundary condition requiring the vertical component of the electric current to be zero at the Earth's surface which partly simplifies the theoretical computation. Long-period asymptotics of the individual complicated functions, occurring in Green's tensor function as well as in the tensor function required to calculate the components of the anomalous magnetic field at the surface of the halfspace, were effected. With the aid of these asymptotics one can obtain estimates of the functions occurring in the theoretical analysis of the problem.

Transient electromagnetic field generated by a vertical electric dipole on the surface of a dissipative earth

Abstract: The problem of electromagnetic pulse propagation over a dissipative earth surface, excited by a vertical electric dipole located on the earth surface, is investigated. By deforming related integrals in the complex frequency domain it is shown that the scattering contribution of the earth, which is formally expressed in terms of several double infinite integrals, can be efficiently computed from some single finite integrals and is amenable to physical interpretation. Dispersion of electromagnetic waves eminating from an impulse current is presented as a function of both the observation distance and the earth conductivity.

Integrated antenna-radome structure that functions as a self-referencing interferometer

Abstract: An antenna-radome structure that functions as a self-referencing interferometer, including a radome having a leading edge and including a plurality of longitudinally disposed slabs of dielectric material having a front edge adjacent the leading edge of the radome; and a corresponding number of antennas disposed normal to the longitudinal axis of the radome and respectively embedded in or placed on the surface of each corresponding dielectric slab. Each antenna is located at a distance from the front end of the corresponding dielectric slab corresponding to a maximum of intensity in an intensity fringe pattern produced by interference between free space waves of a predetermined frequency incident upon the dielectric slab and waves guided by the dielectric slab in response to the incident waves. The thickness of each dielectric slab is tapered for reducing sidelobe levels in the far field pattern of the antenna. A metal foil covers the rear end of each slab for reducing backlobes in the far field antenna pattern and for reducing reflection of the guided waves. Wires are embedded in or placed on the surface of each dielectric slab for defining the shape of the far field of the antenna and for reducing sidelobe levels in the far field pattern of the antenna. Additional antennas may be disposed normal to the longitudinal axis of the radome.

EM Sensor for determining impedance of EM field

Abstract: A method and device for determining the impedance of the electric and magic fields in the near and far fields associated with a radiating antenna. The electric and magnetic field components are sensed in a far field location and the system is calibrated using a known relationship between the electric and magnetic field components. The sensed signals are transmitted optically via a fiber optic link to an optical receiver. The received optical signals are modulated by an audio signal generator and input into a ratio detector via variable attenuators which are used to calibrate the system. Multiple subsequent ratios are measured and recorded as a function of location of the sensor.

Near‐field surface motions excited by radiation from a slip zone of arbitrary shape

Abstract: A ray method is used to calculate the near-field ground acceleration excited by sliding on a fault plane. The sliding action is modeled by a propagating mode II crack. The crack is planar and it has an edge of arbitrary but smooth curvature. Attention is focused upon the case in which the expanding crack abruptly changes its rate of growth to a slower one. This change in propagation speed causes a burst of wave motion to be emitted. The interaction of the high-frequency (or wave front) part of the emitted acceleration pulse with the free surface is studied. The near field, where body waves dominate the wave motion at the surface, is considered in some detail.