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Antennas and Propagation, IEEE Transactions on

Issue 7 • Date Jul 1993

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Displaying Results 1 - 24 of 24
  • A finite-difference time-domain method applied to anisotropic material

    Page(s): 994 - 999
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    The popularity of the finite-difference time-domain (FDTD) method stems from the fact that it is not limited to a specific geometry and it does not restrict the constitutive parameters of a scatterer. Furthermore, it provides a direct solution to problems with transient illumination, but can also be used for harmonic analysis. However, researchers have limited their investigation to materials that are either isotropic or that have diagonal permittivity, conductivity, and permeability tensors. The authors derive the necessary extension to the FDTD equations to accommodate nondiagonal tensors. Excellent agreement between FDTD and exact analytic results is obtained for a one-dimensional anisotropic scatterer View full abstract»

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  • TM scattering by a metallic half plane with a resistive sheet extension

    Page(s): 910 - 917
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    An analysis is presented of the scattering by a resistive strip joined to a metallic half plane. Three different solutions are developed, one of which is based on the moment method/Green's function approach. The other two are approximations, one of which improves as the resistive strip width decreases whereas the other does the same for large strip widths. Using the moment method solution as a reference, the accuracy of each approximation is examined and guidelines for using them are developed. It is found that the validity ranges of the two approximations overlap, making them suitable for computing the TM scattering of the configuration regardless of the strip's width and resistivity View full abstract»

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  • Inaccuracies in numerical calculation of scattering near natural frequencies of penetrable objects

    Page(s): 982 - 986
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    Calculated scattering from a nonconducting cylindrical ring shows great variability with small changes in the material or geometric parameters within certain ranges of these parameters. The observations are explained by a resonance phenomenon in which the operating frequency is found to lie close to a complex natural frequency of the scattering object. This resonance is a real, observable effect predicted by analytical solutions and not the spurious numerical resonances which have been widely discussed and which one wishes to suppress. Attempts have been made to reproduce near-resonance scattering results using the method of moments and finite-difference-time-domain (FDTD) codes. These have failed despite the use of widely accepted discretization densities. Thus, the existence of such resonances requires additional care when interpreting computed results for scattering from similar nonconducting objects have electric and/or magnetic properties View full abstract»

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  • Maximum likelihood signal estimation for polarization sensitive arrays

    Page(s): 918 - 925
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (684 KB)  

    The authors consider the problem of separating and estimating the waveforms of superimposed signals received by a polarization-sensitive array. Signal estimation is accomplished by a two-step maximum likelihood procedure: (i) The directions of arrival and polarization parameters of all the signals are estimated. (ii) The estimated signal is obtained as a linear combination of the array outputs, with weights which are computed from the estimated direction/polarization parameters. The objective of this study is to analyze the quality of the estimated signal in terms of the output signal-to-interference ratio (SIR) and output signal-to-noise ratio (SNR). Closed-form expressions are derived for the output SIR and SNR of a general diversely polarized array. By evaluating these expressions for selected test cases it is shown that polarization-sensitive arrays can provide significantly higher output SIR and SNR than uniformly polarized arrays. The performance improvement is especially significant for closed spaced sources with sufficiently different polarization characteristics View full abstract»

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  • Radar cross section of symmetric parabolic reflectors with cavity-backed dipole feeds

    Page(s): 992 - 994
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    The monostatic radar cross section (RCS) of a symmetric parabolic reflector antenna with a cavity-backed dipole feed is computed using the method of moments. At frequencies below the operating frequency band of the antenna the dipole contribution is not significant; in the operating band the dipole terminal load condition only affects the RCS near boresight. The f/D ratio of the antenna is shown to have a significant effect on the RCS. By adjusting the focal length, the cavity and paraboloid scattering contributions can be made to partially cancel, yielding a reduction in RCS near boresight View full abstract»

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  • The use of Huygens' equivalence principle for solving the volume integral equation of scattering

    Page(s): 897 - 904
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    An algorithm is introduced to solve the volume integral equation of scattering. A volume scatterer is first divided into N subscatterers. Then the subscatterers are divided into four groups, and the groups are in turn divided into four subgroups and so on. By using the idea found in many fast algorithms, a smaller problem can hence be nested within a larger problem. Moreover, by way of Huygen's equivalence principle, the scattering properties of a group of subscatterers in a volume can be replaced by a group of subscatterers distributed on a surface enclosing the volume. This idea is used as the basis of an algorithm which solves the scattering problem in several stages, where at each stage the interaction matrix algorithm is first used to find the scattering solution of each subgroup of subscatterers. Subscatterers are then replaced by equivalent surface subscatterers which are used in the next stage. This results in a reduction in the number of subscatterers at every stage. This algorithm can be shown to have a CPU time asymptotically proportional to N1.5 for N subscatterers View full abstract»

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  • Efficient method for analysis and design of aperture-coupled rectangular microstrip antennas

    Page(s): 986 - 988
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    A method for the analysis and design of rectangular microstrip antennas coupled through a small aperture to a microstrip feed line in a two-layer configuration is described. The technique is numerically efficient, making it suitable for computer-aided design. The analysis takes into account higher order modes excited underneath the patch. Analysis results were implemented in two computer programs: an analysis program and a design program based on optimization techniques. Experimental results are presented to validate this method View full abstract»

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  • On the use of SVD-improved point matching in the current-model method [EM scattering]

    Page(s): 926 - 933
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    An approach which uses the singular value decomposition (SVD) to improve the accuracy of the numerical solution obtained with fictitious current models is introduced. In this approach, the SVD is essentially facilitating a systematic way to optimally reduce the generalized inverse matrix used in the solution to a submatrix of smaller rank. This reduction strikes a balance between the fulfillment of the boundary conditions at the matching points and that between them. Clearly, the boundary conditions errors at the matching points are no longer strictly zero. However, the previously discernible errors between the matching points are markedly suppressed. The approach is efficacious not only when the impedance matrix is inherently singular or highly ill conditioned, but also when this matrix is entirely well conditioned. It can be generalized and implemented in any method of moments code which uses point matching for testing. The approach has been incorporated into an existing solution based on the current-model method for the problem of scattering from periodic sinusoidal surfaces, and is shown to render the solution more accurate View full abstract»

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  • Optimization of a conical antenna for pulse radiation: an efficient design using resistive loading

    Page(s): 940 - 947
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (668 KB)  

    The conical monopole antenna with a section of continuous resistive loading is considered as a radiator for temporally short, broad-bandwidth pulses. The geometrical details of the coaxial feed and the resistive loading are varied to optimize this structure for pulse radiation. Compared with the perfectly conducting cone, the optimized resistive cone radiates a better reproduction of the pulse excitation with no loss in amplitude, and has internal reflections that are much smaller in amplitude. Graphical displays of the field surrounding the antenna are used to give insight into the physical processes for transient radiation from this antenna. Experimental models were constructed to verify the optimization and demonstrate the practicality of the design. Measurements of both the reflected voltage in the feed line and the time-varying radiated field are in excellent agreement with the theoretical calculations View full abstract»

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  • Propagation measurements for satellite radio reception inside buildings

    Page(s): 954 - 961
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (668 KB)  

    Swept CW signals (from 700 to 1800 MHz) were received inside six buildings of brick, corrugated sheet-metal, wood-frame, mobile-home, and reinforced concrete-wall construction. A transmitter antenna was mounted outdoors on top of an 18 m tower to simulate a satellite, and a linearly scanned directional receiver antenna was used to probe the spatial, spectral, and temporal variability of the signal indoors. Levels were found to have much structure in the spatial and frequency domain, but were relatively stable in time. Typically, people moving nearby produced variations of less than 0.5 dB, whereas a person blocking the transmission path produced fades of 6 to 10 dB. Severe losses (17.5 dB) were observed in the concrete-wall building, which also exhibited the longest multipath delays (>100 ns). Losses inside a mobile home were even larger (>20 dB) and were independent of antenna orientation. The power-frequency distortion increased with the logarithm of the bandwidth, but could be reduced by moving to a position of higher power. Only the losses showed a clear frequency dependence, but they could be mitigated by moving the antenna View full abstract»

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  • A waveguide transverse slot for array applications

    Page(s): 845 - 850
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (352 KB)  

    The characteristics of a radiating rectangular transverse slot in a rectangular waveguide have been studied. A moment method solution is used with entire basis expansion and testing functions (Galerkin) including the effect of wall thickness. The results are presented in terms of normalized resistance and reactance versus slot length and frequency. Excellent agreement with a previous pulse basis solution is found, as well as with measured data. An array of resonantly spaced transverse slots radiates large grating lobes. A spatial filter using baffles to suppress the grating lobes is described, suppressed. The effect of the baffles on the transverse slot impedance is analyzed for the case of baffles with infinite height View full abstract»

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  • Calculation of Mueller matrices and polarization signatures for a slab of random medium using vector radiative transfer

    Page(s): 851 - 862
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    The Mueller matrix which characterizes a slab of random medium containing spherical particles is calculated by using the vector radiative transfer theory. The vector radiative transfer equation is solved for arbitrarily polarized incident waves. The background refractive index of the slab is allowed to be different from the surrounding media. The scattering specific intensities for four independent polarized incident waves are calculated and used to construct the Mueller matrix, which contains multiple scattering due to the randomly distributed particles governed by the vector radiative transfer theory. The calculated are found to be symmetrical, and there are eight nonvanishing matrix elements. Polarization signatures are obtained in the backscattering direction from the Mueller matrix of the reflection side View full abstract»

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  • A novel ray tracing on general paraboloids of revolution for UTD applications

    Page(s): 934 - 939
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    Analytical expressions have been obtained for the ray-geometry parameters on the general paraboloid of revolution (GPOR) using a novel technique known as the geodesic constant method. These ray parameters are in a one-parameter form and can be readily employed in high-frequency EM antenna scattering problems. As an example, the uniform theory of diffraction (UTD) mutual admittance results are presented for two antennas located on the GPOR arbitrarily, in three dimensions View full abstract»

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  • A practical tropospheric scatter model using the parabolic equation

    Page(s): 905 - 909
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    A simple method that accounts for tropospheric scatter using the parabolic equation portion of the radio physical optics (RPO) model is described. RPO is a hybrid propagation model that combines ray-optics and parabolic-equation methods to assess realistic range-dependent tropospheric effects at frequencies from 100 MHz to 20 GHz. A semiempirical scatter model adds a random refractive-index fluctuation to the mean refractive-index value at each height considered by the parabolic equation method. The results of this scatter model are compared with those of another scatter model and with a few sample radio measurements View full abstract»

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  • Scattering from conducting open cavities by generalized ray expansion (GRE)

    Page(s): 989 - 992
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    The problem of electromagnetic scattering by metallic open cavities is addressed here using a generalized ray expansion (GRE) method. The wave coupled into the cavity is modeled as a combination of a number of spherical waves, the propagation of such waves being described in terms of geometrical optics. A brief description of the method including some details about aperture discretization as well as results for three-dimensional problems such as circular and rectangular cross-section cavities are presented. The GRE method gives results that agree well with those of modal analysis and allow, like ray methods, for greater flexibility in geometric modeling. Furthermore, the ray-tracing process must be done only once for different incident waves, so its computational cost is lower than that of the classic ray methods, where the incoming wave is described as a different bundle of parallel rays for each direction of incidence View full abstract»

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  • Pyramidal horn gain calculation with improved accuracy

    Page(s): 884 - 889
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    Previously published methods for pyramidal horn gain calculation have used an approximate expression for the path length error that gives rise to aperture quadratic phase error. In this study pyramidal horn gain is calculated without approximating the path length error. These improved accuracy gain calculations give results equal to the previous approximate calculations for large apertures (A or B>50 λ) or small peak aperture phase error in wavelengths (S or T<0.2). For intermediate aperture size (5 λ⩽A⩽8 λ or 5 λ⩽B⩽8 λ) and intermediate peak aperture errors in wavelengths (0.2⩽S⩽0.6 or 0.2⩽T⩽0.6) the improved accuracy calculations will always yield higher gain than the previous approximate calculations by as much as a few tenths of a decibel, and by as much as 0.6 dB in the E plane. For large values of peak phase error in wavelengths (S or T>0.6) the improved accuracy method can yield lower gain than the previous calculations, especially for an E-plane flare View full abstract»

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  • The finite-element modeling of three-dimensional electromagnetic fields using edge and nodal elements

    Page(s): 948 - 953
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    An efficient and accurate finite-element method is presented for computing transient as well as time-harmonic electromagnetic fields in three-dimensional configurations containing arbitrarily inhomogeneous media that may be anisotropic. To obtain accurate results with an optimum computational efficiency, both consistently linear edge and consistently linear nodal elements are used for approximating the spatial distribution of the field. Compared with earlier work, the formulation is generalized by adding a method for explicitly modeling the normal continuity along interfaces that are free of surface charge. In addition, the conditions for efficiently solving time-harmonic problems using a code designed for solving transient problems are discussed. A general and simple method for implementing arbitrary inhomogeneous absorbing boundary conditions for modeling arbitrary incident fields is introduced View full abstract»

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  • Surface integral representation radiation boundary condition for the FDTD method

    Page(s): 890 - 896
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (544 KB)  

    A radiation boundary condition for the finite-difference-time-domain (FDTD) method is presented. It is based on a time-domain integral representation of the electromagnetic fields outside the calculation volume in terms of the known fields on a surface surrounding that volume. Numerical evaluation of the method shows that its performance is superior to that of existing radiation boundary conditions View full abstract»

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  • Superconducting microstrip antennas: an experimental comparison of two feeding methods

    Page(s): 967 - 974
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    The recent discovery of high-temperature superconductors (HTSs) has generated a substantial amount of interest in microstrip antenna applications. However, the high permittivity of substrates compatible with HTS causes difficulty in feeding such antennas because of the high patch edge impedance. Two methods for feeding HTS microstrip antennas at K- and Ka-band are examined. Superconducting microstrip antennas that are directly coupled and gas-coupled to a microstrip transmission line have been designed and fabricated on lanthanum aluminate substrates using Y-Ba-Cu-O superconducting thin films. Measurements from these antennas, including input impedance, bandwidth, efficiency, and patterns, are presented and compared with published models. The measured results demonstrate that usable antennas can be constructed using either of these architectures, although the antennas suffer from narrow bandwidths. In each case, the HTS antenna shows a substantial improvement over an identical antenna made with normal metals View full abstract»

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  • High-frequency scattering from a wedge with impedance faces illuminated by a line source. II. Surface waves

    Page(s): 877 - 883
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    For pt.I see ibid., vol.37, no.2, p.212-18 (1989). In Part I a rigorous integral representation for the field scattered at a finite distance from the edge of an impedance wedge when it is illuminated by a line source was derived. It was shown that the total field can be expressed as the sum of the geometrical optics (GO) field, the field diffracted by the edge, and terms related to the excitation of surface waves. The double spectral integral representation for the diffracted field was asymptotically evaluated there, in the case in which no surface wave can be supported by the two faces of the wedge. In particular, the high-frequency solution was expressed in the special format of the uniform geometrical theory of diffraction (UTD). Here, field contributions related to the surface wave excitation mechanism are examined. By a convenient asymptotic approximation of the integrals, a high-frequency solution which is uniform with respect to aspects of both incidence and observation is obtained. Moreover, this solution has useful symmetry properties so that it explicitly exhibits reciprocity. Numerical results are presented to show the relevance of the surface wave terms in the evaluation of the field View full abstract»

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  • Finite element analysis of axisymmetric radomes

    Page(s): 975 - 981
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (464 KB)  

    A two-step technique for the analysis of axisymmetric radomes is presented. Initially, an axisymmetric finite-element approach is employed, together with an absorbing boundary condition for mesh truncation, to determine the near fields scattered by an empty radome illuminated by a distant source. Next, the reciprocity theorem is invoked to determine the far-field pattern of an antenna encased by the radome, by computing the interaction between the current distribution on the antenna and the near-field data determined in the first step. The details of the formulation are presented along with numerical results for two different arrays enclosed by radomes of varying permittivities View full abstract»

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  • Three-dimensional Gaussian beam reflection from short-circuited isotropic ferrite slab

    Page(s): 962 - 966
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    The plane wave spectrum analysis is extended to the study of three-dimensional Gaussian beam propagation and scattering. The reflection of a three-dimensional circular Gaussian beam from a parallel-sided ferrite slab, backed by a ground plane, is then investigated. The beam field is represented by an angular continuous spectrum of plane waves. Using the Fresnel reflection coefficients of the short-circuited slab for both perpendicular and parallel polarizations, a paraxial approximation for the reflected beam field is derived. It is found that after reflection from the short-circuited slab, the circular Gaussian beam becomes, in general, an elliptical Gaussian beam, and the beam axis is displaced from the position predicted by ray optics. For the thin slab case, approximate formulas for the phase center difference and the lateral shift are determined. The relevance of these results to a new method of ferrite measurement is explained View full abstract»

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  • A monopole antenna loaded with a modified folded dipole

    Page(s): 871 - 876
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    A traveling-wave distribution of current can be produced on a linear antenna by inserting a resistance of suitable magnitude one-quarter wavelength from its end. In this study the resistor is replaced with a modified folded dipole which has a radiation resistance approximately equal to that of the matching resistor. Thus the input section has a traveling wave distribution up to the inserted antenna, as before, but now the power that was previously dissipated in the resistor is also radiated. Input impedance, current distribution, and radiation patterns of this antenna are computed, and input impedance and radiation patterns are also measured. Horizontally polarized patterns are similar to those of a horizontal dipole over a ground plane, and vertically polarized patterns in a plane orthogonal to the folded element are similar to those of a monopole over a ground plane. Coverage is also obtained in the zenith direction in the plane of the folded element, as long as it is not an integral number of half wavelengths above the ground plane. The peaks and nulls can be controlled by adjusting the monopole height accordingly and it is possible to achieve near hemispherical coverage View full abstract»

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  • Propagation analysis of signal fading for basic exchange radios

    Page(s): 863 - 870
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    Although empirical multipath fading models are available for microwave links above 2 GHz, these models are not directly applicable to basic exchange radio (BEXR) links because of the substantial differences in frequency, antenna beamwidth, and radio path clearance. A method for obtaining a scaling factor which accounts for the differences between BEXR and microwave links is presented. First, the terrain scattering is studied using a rough surface model, and the atmospheric refraction is studied using a ray tracing approach. Then, the received signal powers of a microwave link and two BEXR links on the same path under the same propagation condition are calculated. The signal characteristics are investigated and used to simulate the fading distributions for all three links. From the simulation results, a scaling factor is derived and used to modify the existing microwave multipath fading models for BEXR application. The predictions of the modified model agree well with measured BEXR data. This study shows that the probability distribution of signal fading on BEXR links is a strong function of antenna height and beamwidth View full abstract»

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IEEE Transactions on Antennas and Propagation includes theoretical and experimental advances in antennas.

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Editor-in-Chief                                                 Kwok W. Leung