Showing papers by "Yahya Rahmat-Samii published in 1990"

Array feeds for reflector surface distortion compensation: concepts and implementation

Abstract: The development of a compensation algorithm based on the use of array feeds and a focal-plane conjugate field approach is discussed. Results of numerical simulations performed to verify the accuracy of the computer programs are examined, and an experimental study is described. Both the numerical and experimental results show that the array will compensate for the slowly varying surface distortion. In particular, for some typical, slowly varying thermal or gravitational surface distortions, a 19-element array feed can improve the reflector performance considerably. Additionally, it is demonstrated that it will be possible to compensate for the surface distortion without actual knowledge of the distortion itself. >

Fan beam generated by a linear-array fed parabolic reflector

Abstract: The theoretical background and the results of computer simulations and experimental studies for a parabolic reflector fed by a linear array are detailed. The concept of using a parabolic reflector antenna fed by a small linear array to generate fan-beam patterns is validated. Large angle scan along the broad-beam direction of the fan beam can be achieved by offsetting the linear array laterally. It is both empirically and numerically demonstrated that the array feed must be displaced in the reflector's axial direction to an optimum location from the focal plane in order to achieve the best antenna gain performance. As a result, the linear-array fed parabolic reflector can be used in place of a long planar array in a multifunctional reflector antenna system. >

Analysis of antennas with elliptical apertures using Fourier-Bessel expansion

Abstract: The Fourier-Bessel expansion technique for elliptical apertures is described. The elliptical coordinate system is introduced and the radiation integral of a reflector antenna is expressed in terms of radial and angular Mathieu functions. However, the integrals can be simplified, and the final integrals are written in terms of Bessel functions only. An effort is also made to compare the Jacobi-Bessel expansion and Fourier-Bessel techniques and to find the convergence behaviors of both methods for various sizes and shapes of the elliptical aperture. The procedures discussed are also applicable to any elliptically shaped aperture-type antennas with known aperture distributions. >

Application of FFT and DFT for image reconstruction of planar arrays in microwave holographic diagnostics

Abstract: It is demonstrated that, for a planar array of discrete elements, the accuracy of the reconstruction is more dependent upon the location of the reconstruction points than on the actual resolution (i.e., spacing) of the reconstruction points. The surface field distribution of a planar array of infinitesimal dipoles is reconstructed from complex far-field and near-field data with the use of microwave holographic metrology. A comparison is made between the discrete Fourier transform (DFT) and the more traditional fast Fourier transform (FFT) to reconstruct the surface distribution. The former technique gives the user more freedom in choosing the location of the reconstruction points. A 22 by 22 element planar array of infinitesimal dipoles was simulated. The FFT required less computer time to perform the reconstruction than the DFT, but the increased accuracy provided by the DFT warrants its use in cases where the element anomalies are small or a more accurate determination of the amplitude and phase needs to be made. >

Microwave holographic metrology for antenna diagnosis

Abstract: Advances in antenna diagnostic methodologies have been very significant in recent years. In particular, microwave holographic diagnostic techniques have been applied very successfully in improving the performance of reflector and array antennas. These techniques use the knowledge of the measured amplitude and phase of the antenna radiated fields and then take advantage of the existing Fourier transform relationships between the radiated fields and the effective aperture or current distribution to eventually determine the reflector surface or array excitation coefficients anomalies. In this paper an overview of the recent developments in applying microwave holography is presented. The theoretical, numerical and measurement aspects of this technique is detailed by providing representative results.

Beam efficiency of large reflector antennas subject to correlated random surface errors

Abstract: The beam efficiency of a large reflector antenna is characterized by its random surface errors and the antenna aperture distribution. An improved mathematical model is presented to handle two-dimensional surface errors with zero mean and a relatively short correlation length. Pattern degradations and beam efficiency decrease due to surface errors are illustrated in the results explicitly. It is shown that tighter surface tolerances are required when low sidelobe level is desired. The linear surface errors will cause not only pattern degradation but also main beam broadening. Beam efficiency decrease is sensitive to both uniform and linear surface errors, but has little dependence on the F/D ratio. >

Comparison between different rectangular horn elements for array antenna applications

Abstract: The coupling and directivity characteristics of an array with rectangular open-ended waveguide elements are considered. In particular waveguides with soft and hard boundaries are analyzed and discussed with respect to array applications. A soft waveguide or horn (e.g. corrugated horn) is characterized by zero intensity of the aperture field at the wall, while a hard waveguide or horn is characterized by a uniform aperture field distribution. The latter has higher directivity than the former, at the expense of higher sidelobes. Applications of hard and soft horn elements are discussed. >

Analysis of near-field Cassegrain reflector: plane wave versus element-by-element approach

Abstract: A near-field Cassegrain reflector (NFCR) is an effective way to magnify a small phased array into a much larger aperture antenna for limited scan applications. Traditionally the pattern wave approach, i.e. the field from the feed array incident on the subreflector is approximated by a truncated collimated beam with planar phase and tapered amplitude distribution. This approach simplifies the computation tremendously, but fails to provide design information about the most critical component of the whole antenna system, namely, the feed array. With the help of today's computers, it is now feasible to calculate the pattern of a NFCR by a more exact element-by-element approach. Each element in the feed array is considered individually and the diffraction pattern from the subreflector is calculated by the geometrical theory of diffraction (GTD). The field contributions from all elements are superimposed at the curved main reflector surface, and a physical optics integration is performed to obtain the secondary pattern. >

Numerical and experimental results for the reflectivity of complex mesh surfaces

Abstract: Wire mesh knit reflecting surfaces have been analyzed by moment methods using a piecewise triangular basis function with a Floquet mode description of the periodic structure. Numerical and experimental results obtained from the complex weave are presented. In particular, the effects of wire loops and junction connectivity are examined. Calculations for the mesh used on the Tracking and Data Relay Satellite System (TDRSS) single access antennas have been carried out, and results are given. >

Quiet zone evaluation of serrated compact range reflectors

Abstract: A Fourier transform method developed for the quiet-zone field analysis of serrated compact range reflectors is presented. The analysis relies upon a variable spectrum sampling rate which increases with the separation between the reflector aperture and the quiet-zone plane. Errors caused by rim approximation inherent in techniques involving numerical integration are avoided by employing a closed-form expression for the plane wave spectrum of a uniformly illuminated aperture which is bounded by a polygon. The method has been validated for the case of uniform illumination by computing the quiet-zone fields of several apertures, and its utility was demonstrated for several reflectors which are representative of commercial compact range designs. >

Generalised formulas for beam squint prediction in conic-section reflector antennas

Abstract: The beam squint phenomenon is of importance when antenna pointing performance is considered. This phenomenon is characterised in paraboloidal, Cassegrain and Gregorian antennas. Squint-predicting formulas are presented, squint-free conditions are outlined and an extension of these formulas to general multi-reflector antennas is suggested.

Elliptical and rectangular dielcore horns for reflector antenna applications

Abstract: A novel class of dielectric-loaded feed horns, so-called dielcore horns, for generating elliptical radiation patterns is presented. The dielcore horns are used as feeds in an offset parabolic reflector with an elliptical aperture. Their characteristics are high feed efficiency for elliptical beams and high cross-polar isolation for dual-linear and for dual-circular polarization. Compared to corrugated rectangular or elliptical horns they have considerably larger cross-polar bandwidth and potentially lower production costs. The calculated feed patterns in the narrowest pattern planes of the open-ended waveguides are shown for both the fundamental orthogonal modes. Copolar and cross-polar patterns in the two major planes of an offset parabolic reflector with an elliptical projected aperture fed by a rectangular open-ended dielcore waveguide are also shown. >

Lateral shift of a Gaussian beam reflected at a dielectric-chiral interface

Abstract: The reflection of a Gaussian beam at a dielectric-chiral interface is examined theoretically, and numerical results are presented. It is shown that for a dielectric-chiral interface a critical angle occurs when each of the two normal modes in the chiral media becomes evanescent. In addition, when the incident wave is polarized perpendicular to the plane of incidence both polarizations (parallel and perpendicular) are reflected. Therefore, for either polarization of the incident beam, two beams are expected to be reflected. Phenomena similar to the Goos-Hanchen shift are found to exist for both reflected beams for angles of incidence between the two critical angles. >

A comparative study among GTD and PTD techniques for circular discs

Abstract: Two groups of techniques based on the geometric theory of diffraction (GTD) and the physical theory of diffraction (PTD) are studied by using a canonical problem consisting of a conducting circular disc and an infinitesimal dipole feed. The GTD group includes the GTD, the uniform geometrical theory of diffraction, and the uniform asymptotic theory. The PTD group includes Mitzner's incremental length diffraction coefficients, Michaeli's equivalent edge currents, and Ando's modified physical theory of diffraction, which are modifications of Ufimstev's PTD. These solutions are compared with those obtained by the physical optics and the method of moments. Features of the far-field radiation patterns that are of particular interest include the performances at reflection boundaries, boresight caustics, and near theta =90 degrees as well as the prediction of cross-polarized fields, sidelobe levels, and nulls. Although only far-field radiation is considered, these methods can also be used for near-field calculations. The bistatic radar cross section calculated by these techniques is also examined. >