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Antennas and Propagation Society International Symposium, 1989. AP-S. Digest

Date 26-30 June 1989

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Displaying Results 1 - 25 of 427
  • AP-S International Symposium 1989. 1989 International Symposium Digest: Antennas and Propagation (Cat. No.CH2654-2/89)

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  • Spherical wavefunction analysis of radiation from the pyramidal horn

    Page(s): 3 - 6 vol.1
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    The pyramidal horn may be approximated as a quasi-pyramidal horn if the flare angles that describe it are measured from the same point. The quasi-pyramidal horn model is analyzed using spherical wave functions. Only the dominant TE/sup r01/ mode is considered. To simplify the boundary condition it is useful to consider the horn enclosed by a perfectly conducting sphere concentric to the horn apex. A finite spherical wave expansion (SWE) is used to describe the fields outside the sphere, with only outgoing waves needed. A comparison between the predicted and the measured far-field E-plane ( phi =90 degrees ) patterns was made with good agreement. Comparison with Jull's results (1981) shows that the SWE method is more accurate in predicting the radiation pattern than the aperture integration method.<> View full abstract»

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  • Treatment of vector potential in a three-dimensional lattice network of spatial network method

    Page(s): 7 - 10 vol.1
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    The authors assess the validity of applying the equivalent circuit for electromagnetic fields in the spatial network method to the vector potential fields in three-dimensional space and the time domain. The electric vector potential is introduced as the dual quantity of the magnetic vector potential to satisfy the property of the equivalent circuit for Bergeron's method. The validity of the treatment is shown by computing the rotating magnetic field around the straight line current from the magnetic and electric vector potentials.<> View full abstract»

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  • Radiation by a modulated sheet electron beam propagating parallel to a reflection grating under a finite magnetic field

    Page(s): 11 - 14 vol.1
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    The authors investigate two-dimensional radiation from a modulated sheet electron beam propagating parallel to a reflection grating under the influence of an externally applied magnetic field. From the wave-theoretic viewpoint, the spontaneous emission can be interpreted as the radiative leakage of the space-charge waves associated with the electron beam. The analysis is reduced to the determination of the complex eigen wavenumber of the space-charge waves perturbed by the presence of the periodic structure. The set of Maxwell equations and relativistic hydrodynamic equations for cold electrons is solved using a perturbation approach based on multiple-space scales. The transport equations governing the radiative leakage of the space-charge waves are systematically derived. Numerical results on the leakage coefficients and the phase change due to the leakage that depend on the applied magnetic field are presented.<> View full abstract»

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  • On matrix partitioning, the Sherman-Morrison-Woodbury technique and the add-on method

    Page(s): 15 - 18 vol.1
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    The moment method matrix is compared with the add-on technique for the analysis of scattering from large planar structures. It is shown that the two formulations yield the same results, provided that the physical model, the basis and testing functions, and the truncation and discretization criteria are the same. The differences lie in the numerical routes taken to arrive at those results. The numerical advantages of the add-on technique are clearly shown. Consideration is also given to the Sherman-Morrison-Woodbury technique, which has been used for adapting existing moment method codes, within their limitations, to account for changes in the configuration. It is noted, however, for the case where the changes are additions of segments to the original structure, as in the present case, that this scheme coincides with the matrix partitioning scheme discussed here and that it provides no further advantage.<> View full abstract»

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  • A uniformly valid insulated antenna theory

    Page(s): 19 - 20 vol.1
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    The authors describe a uniform insulated antenna theory, valid through transition to bare antennas. The transition of the insulated antenna solution to the bare antenna solution is characterized by two parameters: the equivalent bare antenna parameter and the Sommerfeld numerical distance. The error incurred in using WKG theory (Wu, King and Giri, 1973) is discussed.<> View full abstract»

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  • Error estimate of the Fourier-Bessel expansion in computation of field distributions

    Page(s): 21 - 24 vol.1
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    Formulation, solution, and numerical results are presented for the error estimation of the Fourier-Bessel expansion in the computation of field distribution in coaxial regions. It is shown that there exists an optimal number of terms of the expansion which gives the minimum mean square error. This optimal number of terms is a function of the eigenvalue uncertainties.<> View full abstract»

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  • The dispersion characteristics of the FD-TD method

    Page(s): 26 - 29 vol.1
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    The numerical phase and group velocity characteristics of the three-dimensional FDTD (finite-difference time-domain) algorithm are derived from the dispersion relation for wave propagation in the TEM, TE, or TM mode. From these dispersion characteristics, numerical errors due to dispersion can be easily assessed and would be used in determining the mesh size and the stability factor in order to minimize such errors.<> View full abstract»

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  • Characterization of radiation boundary conditions used in the finite-difference time-domain method

    Page(s): 30 - 33 vol.1
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    When the finite-difference time-domain (FDTD) method is applied to open region scattering and coupling problems, some artificial mechanism must be used to simulate the infinite region on the necessarily finite spatial grid. The current method of choice is to use a radiation boundary condition (RBC) based on an approximate one-way wave equation. Typically, these RBCs are tested numerically. Unfortunately, using the numerical approach, it is difficult to extract precise information relating the RBC error to the frequency and angle of incidence of the outgoing wave and to the cell and time step sizes from this type of measurement. An alternate, analytic, approach to testing RBCs is presented. This approach provides a more effective means of characterizing various RBCs and comparing their accuracy in different parameter regimes. Results are presented indicating the validity of the proposed approach.<> View full abstract»

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  • Time domain finite difference calculations using a variable step size

    Page(s): 34 - 37 vol.1
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    The authors describe a novel method of computational electromagnetics which uses a variable step size (calculation increment) in the finite-difference technique (VSSFDM). The step size is kept small around discontinuities and larger away from them, thereby using only about 1/3 of the memory required by a straight application of K.S. Yee's (1966) method. The results obtained by VSSFDM are very close to those obtained by the finite-difference method, but with a significant reduction in the required computer memory as well as CPU time. The numerical stability of the proposed approach is demonstrated, and the VSSDM is applied to a symmetric microstrip with a width to height ratio of one.<> View full abstract»

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  • On the use of conformal grids for propagation and scattering problems in finite-difference time-domain computations

    Page(s): 38 - 41 vol.1
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    The authors examine the use of grids that are conformal with the geometry of the material boundaries and, consequently, reduce the discretization errors introduced by the stair-stepped approximation. However, it is found that the introduction of the nonuniformity in the grid gives rise to at least two difficulties in the implementation of the finite-difference time-domain algorithm. The first is the loss of accuracy in the computation of finite-difference derivatives when the field points are distributed nonuniformly. The second is implementation of the appropriate boundary conditions at the material interfaces. The authors address these problems and suggest some means for eradicating them. A scheme is presented that has the attractive feature that it reduces to the conventional finite-difference time-domain method if a uniform grid is used. To illustrate the proposed algorithm the authors consider a parallel plate waveguide which supports a single propagating mode.<> View full abstract»

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  • Accurate computation of the radiation from simple antennas using the finite-difference time-domain method

    Page(s): 42 - 45 vol.1
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    The analysis of a group of three simple antennas is used to illustrate the accuracy of the FDTD (finite-difference time-domain) method and to show that various geometrical features are handled correctly by the method. Each antenna is a well-posed electromagnetic boundary value problem that corresponds to a realizable experimental model. The three antennas considered are of increasing complexity: an open-ended parallel plate waveguide, a cylindrical monopole, and a conical monopole. The FDTD calculations for these antennas were compared with analytical results (open-ended parallel plate waveguide) and accurate measurements in the time and frequency domains (cylindrical and conical monopoles). In all cases the agreement was excellent.<> View full abstract»

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  • Absorbing boundary conditions for arbitrary outer boundary

    Page(s): 46 - 49 vol.1
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    The authors propose an absorbing boundary condition that can be applied to an outer boundary of arbitrary shape, even one that is conformal to the geometry of the scatterer. In order to investigate the application of the new boundary condition, several strip and wedge type geometries were considered, both for TE and TM incidence.<> View full abstract»

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  • FDTD formulation for frequency dependent permittivity

    Page(s): 50 - 53 vol.1
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    Current finite-difference time-domain (FDTD) formulations require the permittivity, permeability, and conductivity to be constant. However, for many real materials of interest these parameters vary significantly with frequency. The effects of constitutive parameters which vary with frequency are included in a frequency dependent FDTD formulation, (FD)/sup 2/TD, by extending the traditional Yee formulation to include discrete time-domain convolution. To apply (FD)/sup 2/TD all of the frequency domain information (in the complex permittivity function) is Fourier-transformed to a time-domain susceptibility function. To demonstrate (FD)/sup 2/TD computation of wideband reflections at an air-water interface the authors consider a one-dimensional problem space consisting of 1000 cells: 499 cells were used to model free space (air) and 501 cells were used for water. They compare (FD)/sup 2/TD results after 1200 time steps and FDTD results (with 20-GHz permittivity and conductivity values constant) after 8000 time steps with the exact analytical frequency domain result. The (FD)/sup 2/TD formulation is clearly much more accurate than traditional FDTD.<> View full abstract»

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  • A simple absorbing boundary algorithm for the FDTD method with arbitrary incidence angle

    Page(s): 54 - 57 vol.1
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    An algorithm was developed by X. Zhang and K.K. Mei (1988) for simulating the propagation of the quasi-TEM wave in a microstrip structure with the wave impinging at normal incidence onto the absorbing boundary. The amount of reflection was reported to be on the order of 3 to 5%. For the present work, the authors rationalize the concept of the absorbing boundary for the finite-difference time-domain (FDTD) method in terms of phase and group velocities, improve on the algorithm with a resulting decrease of two orders of magnitude in the amount of reflection at the tuning frequency, and generalize the algorithm to encompass the case of arbitrary incidence angle. Yet, the algorithm remains very simple and very local, requiring the knowledge of the field values at points located only one space increment away from the absorbing boundary.<> View full abstract»

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  • The application of a simple absorbing boundary algorithm to cylindrical waveguide

    Page(s): 58 - 61 vol.1
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    The authors present the implementation of a very simple, yet very effective, plane absorbing boundary algorithm for a rectangular waveguide operated in the TE/sub 10/ mode analyzed with the FDTD (finite-difference time-domain) method. This absorbing boundary produces an amount of reflection smaller than 0.02% at the tuning frequency, works for arbitrary incidence angle, and is very local, meaning that the algorithm requires the knowledge of the field values at grid points located only one space increment away from the absorbing boundary. The algorithm was developed from physical understanding of the wave propagation on a discrete mesh, with the concepts of phase and group velocities. The efficacy of the absorbing boundary is demonstrated by measuring the percentage of reflection in the frequency domain.<> View full abstract»

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  • The phase retrieval by a reference source

    Page(s): 64 - 67 vol.1
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    Exploiting the knowledge of the amplitude of a reference element, the authors develop a novel numerical technique in the phase retrieval problem. An example is shown in the case of a discrete source distribution. Specifically, consideration is given, in a bidimensional geometry, to an array of 18 elements spaced lambda /2 apart plus a reference element spaced 10.5 lambda above the last element of the array.<> View full abstract»

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  • Linear array diagnostic from near-field measurements

    Page(s): 68 - 71 vol.1
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    The authors present an inversion procedure to obtain the current distribution on a linear array from a near-field measurement. Numerical simulations were performed for a 50-element linear array. Single elements are half-wavelength dipoles spaced 0.75 lambda . The measuring distance is 5 lambda and the frequency 10 GHz. The current distribution is a Taylor (one parameter) of 35 dB, with a Gaussian distributed error of sigma =1 dB and a uniformly distributed error of +or-10 degrees in phase. Three cases have been considered: no filtering of the spectrum, filtering with a Hamming window, and low-pass filtering with a cutoff frequency k'= pi /d. It is shown that from a near-field measurement it is possible to reconstruct the current distribution of a linear array with an error smaller than 0.2 dB in amplitude and 2.5 degrees in phase when filtering is performed according to element spacing. Since the linear arrays may be constituent elements of larger planar arrays, this method makes it possible to check and adjust the intermediate steps of large radiating systems.<> View full abstract»

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  • Prediction of far-field patterns using synthesized steerable spatial filtering functions

    Page(s): 72 - 74 vol.1
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    An improved version of the integral equation formulation proposed earlier by Biraud (1971, 1976) for predicting the far-field patterns of aperture antennas from measured near-field data is presented. It is shown that the numerical computational work can be significantly reduced by performing the near-field measurements over a large surface so as to realize a band-pass spatial filtering function which is inserted into the integrand. This filtering function, corresponding to the pattern of a synthesized array, can be beam-steered by means of multiplying the near-field measurement values with appropriate sets of coefficients and then summing them up vectorially. The synthesized array may have a semicylindrical or spherical surface as well as a planar one. The solution of the integral equation is based on converting it into a simple matrix equation by expanding the unknown plane wave spectrum functions of the measured antenna into sampling series with unknown coefficients. The sampling series may assume different forms according to the type of the zero-one function of the aperture.<> View full abstract»

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  • Practical aspect of spherical near-field measurement

    Page(s): 75 - 77 vol.1
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    A special custom-tailored near-field range is described. The test range is a large radome (typically 68-foot diameter) and is used to host the antenna under test (AUT) and the circular track which carries the scanning probe. The scanned surface encloses the AUT completely, thereby distinguishing itself from other near-field techniques where only partial scan coverage is possible. Alignment of reflector panels may be accomplished by holography or optical instruments such as theodolites. The spherical near-field results can also be synthesized analytically.<> View full abstract»

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  • Measurement of correction parameters for a dipole probe

    Page(s): 78 - 81 vol.1
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    A procedure for experimentally determining the correction coefficients for a dipole antenna was demonstrated. The resulting coefficients can be used to correct the measured tangential field in planar near-field measurements. The results also indicate the possibility of asymmetry in the dipole spectral response. The method described has been used to measure the far field of an isolated microstrip path using the dipole as a near field probe, in which case the measured coefficients are useful.<> View full abstract»

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  • Microwave diversity imaging of perfectly conducting object in the close near-field region

    Page(s): 82 - 85 vol.1
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    The use of frequency, angular, and polarization diversity approaches in a close near-field microwave imaging system is investigated theoretically. A two-dimensional close near-field microwave imaging system is used to illustrate the theoretical considerations. Numerical results for a perfectly conducting cylinder with a radius of 15 cm in a backward scattering arrangement using frequency and angular diversity techniques are shown.<> View full abstract»

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  • Measurements of antenna radiation patterns with a computer controlled 108-220 GHz transmitter/receiver system

    Page(s): 86 - 89 vol.1
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    Measurements of antenna radiation patterns in the D- and G-bands have been performed with a novel mm-wave transmitter/receiver system. With this system, measurements can be made at any frequency in the 108-220-GHz band. The cost of the system is considerably lower than that of systems which use separate mm-wave sources for each frequency. Measured radiation patterns of a broadband corrugated horn antenna at 140 GHz (co/crosspolar) and 170 GHz (copolar) are shown.<> View full abstract»

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  • An alternative plane wave synthesis method for Fresnel-zone antenna measurements

    Page(s): 90 - 93 vol.1
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    It has been demonstrated that the method of successive projections applied to plane wave synthesis yields a fast and effective method for analyzing Fresnel-zone data. In particular, compact range performance can be simulated without the need for a large reflector, at the cost of an increased number of azimuth scans. As an example, for a range length of 320 lambda , 21 scans may be used to achieve a 40- lambda -diameter plane wave region with root-mean-square phase and amplitude errors less than 1 degrees and 0.1 dB, respectively. Such an accurate plane wave is seldom required for normal antenna measurements, and some accuracy may be sacrificed to reduce the number of azimuth scans.<> View full abstract»

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  • Measurements of dipole, bow-tie, log-periodic and log-spiral antennas on a dielectric-filled parabola

    Page(s): 94 - 97 vol.1
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    The power patterns and input impedances of four types of planar antennas on a dielectric-filled parabola are presented. The measurements were made on a microwave scale model of a proposed submillimeter-wave receiver front end. The dipoles have the lowest input impedance (generally preferred for matching to most nonlinear elements used in low noise receivers), while the log spiral has the broadest bandwidth and the most circularly symmetric patterns. If cross polarization is a consideration, the log-periodic antennas are likely to give less satisfactory performance. The bow-tie has the most compact impedance locus but its H-plane patterns vary widely with frequency. All of the antennas slightly underilluminate the dielectric-filled parabola. The input impedances of the antenna on the dielectric-filled parabola differ from those of the same antennas in free space and in general the real part decreases.<> View full abstract»

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