Aperture feed for a spherical reflector

The problem of designing a feed system for illuminating a spherical reflector is examined. A method is proposed for specifying the required field distribution over the aperture of the feed system, and the primary illumination and gain resulting from this distribution are derived. The results indicate that a significantly smaller feed aperture can be employed than would be indicated by conventional ray tracing methods. Specific numerical results are obtained by taking the Arecibo antenna as an example, for which a calculated aperture efficiency of 67.5 percent is possible with approximately a 38-foot-diameter aperture feed.     

Studies of the Focal Region of a Spherical Reflector: Polarization Effects

The polarization of the reflection of a uniform plane wave incident on a spherical reflector is analyzed using the current distribution method for scattered fields. The current distribution on the reflector is derived. For reflectors subtending about60degor less, the radiation scattered in the direction of the circle of least confusion has essentially the same polarization as that reflected specularly from the tangent plane. The effective current, the component of surface current density radiating toward the focal region, is derived in several representations. Assuminghat{i}incident polarization, contour plots are provided forhat{i},hat{j}, andhat{k}components in spherical coordinates. Next, general formulas are derived for thehat{i},hat{j},hat{k}components of the reflected fields, in terms of the direction cosines of the normal to the reflecting surface. These are displayed in terms of projections, and apply directly to the spherical reflector.     

A method for producing a shaped contour radiation pattern using asingle shaped reflector and a single feed

Eliminating the corporate feed network in shaped contour beam antennas will reduce the expense, weight, and RF loss of the antenna system. One way of producing a shaped contour beam without using a feed network is to use a single shaped reflector with a single feed element. For a prescribed contour beam and feed, an optimization method for designing the reflector shape is given. As a design example, a shaped reflector is designed to produce a continental-United-States (CONUS) coverage beam. The RF performance of the shaped reflector is then verified by physical optics     

La synthèse d’antennes à deux réflecteurs conformés à alimentation décalée

A potentially economic method for upgrading the gain of the large earth reflector antenna Cassegrain system to a gain comparable to that obtainable with a dualshaped reflector antenna system is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a paraboloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna where the geometrical optics energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller subreflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.     

A line feed for a spherical reflector

A line feed for a spherical reflector is considered on the basis of a plane-wave spectrum of radiation angles. It is shown that a feed excited by circumferential slots results in a gain deterioration of at least 3 dB. The correct excitation of the feed is indicated. Expressions for field components in the focal region are obtained.     

Center-fed parabolic reflector antenna cross polarization due to slightly right-left asymmetric feed patterns

Slightly right-left (R-L) asymmetric feed pattern effects on a center-fed parabolic reflector antenna cross polarization have been described with some numerical computation examples, noting that measured feed patterns, both in theEplane and in theHplane, are generally not completely R-L symmetric and have a slightly different R-L field intensity which produces asymmetrical reflector surface currents. It is shown from the numerical computations that even a slight feed asymmetry, in both amplitude and phase, gives rise to a cross polarization in the secondary prinicpal planes and that, in particular, phase R-L asymmetry incurs two-dimensional movement of principal- and cross-polarized patterns resulting in the rapid increase of principal-plane cross polarization as well as the maximum cross polarization, principal-pattern beamshift, and the sidelobe variations.     

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

Many existing large ground reflector antennas have been designed as Cassegrain systems-i.e., paraboloid/hyperboloid combinations. Other large ground antennas are simply paraboloid designs. Upgrading the gain of these systems to a gain comparable to that obtainable with a dual shaped reflector antenna system has been an important and costly objective of many such ground stations. A potentially economic method for such an antenna upgrade is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a parabaloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna that was first presented by Galindo-Israel and Mittra in 1977. In that approximate formulation, the geometrical optics (GO) energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller (sub) reflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.     

Studies of the focal region of a spherical reflector: Stationary phase evaluation

The geometric optics and polarization properties of a spherical reflector are used to develop an integral representation of its focal region fields. These integrals are evaluated by the extended method of stationary phase for field points off the caustics, on the axial caustic, on the caustic surface, and at the paraxial focus. The contributions to the field at a field point are shown to arise respectively from three ordinary stationary points: a stationary ring and a stationary point at the vertex; an ordinary stationary point and a caustic type stationary point; and a fourth-order stationary point. The resulting formulas are used to compute the value of the focal region fields. The computed results are then compared to measured data.     

A shaped reflector antenna for 60-GHz indoor wireless LAN accesspoints

This paper addresses the design of a 60-GHz shaped reflector antenna that has to illuminate a predefined circular area without substantial spatial variation. At the boundary of this coverage area, the field strength has to fall off rapidly. Such efficient and confined illumination may be required in wireless networks that have to cope with a stringent link budget and/or require low channel dispersion. Typical examples are the emerging broadband wireless networks for customer premises and residential applications that will operate in the millimeter-wave frequency region. The particular property of the considered reflector is its diverging surface instead of the commonly applied converging shape. It is shown that the spatial variation within the defined coverage area can be within 1 dB, provided that the antenna is perfectly constructed. It is also shown that practical imperfections such as axial and lateral feed displacement and mispointing of the feed on top of effects due to blocking by the feed can contribute to spatial field variations on the order of a few decibels     

Analysis of reflector antenna systems with arbitrary feed arraysusing primary field superposition

In contrast to secondary pattern superposition, where the fields reflected from the main reflector arising from each element are superimposed in the far field of the reflector, the approach presented here sums the primary fields at the reflector surface before the physical optics radiation integral is performed. The method allows each feed array element to have arbitrary position, orientation, pattern, and excitation (magnitude and phase). In addition, it is inherently efficient because evaluation of only one time-consuming radiation integral is required, rather than one per feed element as in secondary superposition. The method allows for accurate calculation of the power radiated from the feed, permitting the reflector gain and spillover efficiency to be determined within the context of a single computer program. The accuracies and characteristics of this method are demonstrated with several examples     

Study of the field around the focal region of spherical reflectorantenna

V.P. Maslov's (1981) method is applied to derive the expression for the wave reflected by a spherical reflector when the plane wave is incident along the focal axis of the reflector. The expression is reduced to integrals with respect to a single variable. A relatively simple expression is derived which is valid in the caustic region. An asymptotic-ray expression is also derived which can be used in the region far from the caustic. Field distributions around the cusp region are calculated numerically and the results are compared with those obtained by other methods when available. Agreement among them is fairly good. Equi-contours of the field amplitude are obtained in the plane perpendicular to the axis which will serve to determine the optimum feed pattern and location     

Synthesis of the primary fields of a transmitting feed for a circular reflector

A two-dimensional electromagnetic boundary-value problem consisting in the synthesis of the primary fields of a transmitting feed antenna for a circular-cylindrical reflector is considered. A primary field that corrects for the phase aberration of the reflector is synthesized as a sum of circular waves whose common origin is displaced from the center of curvature of the reflector. Limitations on the gain of the feed-reflector combination as a function of the number of waves and the displacement are discussed. The synthesized fields are compared with those obtained from geometrical optics.     

Novel Gaussian beam method for the rapid analysis of largereflector antennas

A relatively fast and simple method utilizing Gaussian beams (GBs) is developed which requires only a few seconds on a workstation to compute the near/far fields of electrically large reflector antennas when they are illuminated by a feed with a known radiation pattern. This GB technique is fast, because it completely avoids any numerical integration on the large reflector surface which is required in the conventional physical optics (PO) analysis of such antennas and which could take several hours on a workstation. Specifically, the known feed radiation field is represented by a set of relatively few, rotationally symmetric GBs that are launched radially out from the feed plane and with almost identical interbeam angular spacing. These GBs strike the reflector surface from where they are reflected, and also diffracted by the reflector edge; the expressions for the fields reflected and diffracted by the reflector illuminated with a general astigmatic incident GB from an arbitrary direction (but not close to grazing on the reflector) have been developed in Chou and Pathak (1997) and utilized in this work. Numerical results are presented to illustrate the versatility, accuracy, and efficiency of this GB method when it is used for analyzing general offset parabolic reflectors with a single feed or an array feed, as well as for analyzing nonparabolic reflectors such as those described by ellipsoidal and even general shaped surfaces     

Analysis of numerically specified multireflector antennas bykinematic and dynamic ray tracing

A technique for tracing rays and fields with several numerically specified reflectors by using geometrical optics (GO) is described. The ray paths are determined by launching individual rays from the feed point and following them by reflection from all the reflector surfaces to the output aperture of the last reflector. This procedure is referred to as kinematic ray tracing. Thereafter, the amplitude, phase and polarization of the E-field is traced along the ray paths to the aperture; this is referred to as dynamic ray tracing. The aperture field is then integrated to find the aperture efficiency, which is factorized into convenient subefficiencies. The technique has been implemented in a computer code that has been used to analyze the proposed new shaped-offset dual-reflector feed for the spherical reflector antenna at the Arecibo Observatory     

A Dual-Mode Millimeter-Wave Folded Microstrip Reflectarray Antenna

A dual-mode folded microstrip reflectarray antenna was developed and demonstrated in this paper. The proposed folded reflectarray antenna contains three parts: a planar main reflector, a planar subreflector, and printed feed antennas. The main reflector is used to produce twisted reradiated fields and to provide phase compensation for focusing. The subreflector parallel with the main reflector is made of a substrate printed with high-density metal grid lines, which is transparent to perpendicularly polarized fields, but would reflect the parallel ones. Three fixed-position patch antennas with polarization parallel to the grid lines are created for the radar mode, so that the radiation beam is switchable. Another patch with perpendicular polarization is designed for communication. A simple approach was proposed for simulating and designing the folded reflectarray. Measured results show good agreement with the calculated ones.     

Effect of defocus on the prompt response of a reflector IRA

Impulse radiating antennas (IRAs) are an emerging class of antenna that are designed to radiate extremely short electromagnetic pulses with multiple decades of instantaneous bandwidth. The most common IRAs are made with a transverse electromagnetic (TEM) transmission line feeding a paraboloidal reflector. The IRA is usually constructed so that the electrical feed point of the TEM transmission line coincides with the focal point of the paraboloid. The paraboloidal reflector converts the spherical wave emanating from the feed point into a plane wave (i.e., a spherical wave centered at -/spl infin/). In many practical cases, the feed point and focal points are not exactly aligned, producing some defocus of the reflector. In this paper, we model the case of hyperbolic defocus because of its analytic form and compare this model with experimental measurements. A hyperboloidal reflector fed from one focus converts the expanding spherical wave into a second expanding wave which appears to emanate from the second focal point of the hyperboloid (which is behind the reflector). Hyperboloidal defocus is roughly equivalent to moving the electrical feed closer to the reflector than the optical focal point. Previous theoretical results from in-focus IRAs predicted that the E- and H-plane temporal responses should be symmetric with respect to the temporal center of the response. The results shown here demonstrate that the defocusing causes these responses to become asymmetric. The new results are in better agreement with experimental measurements of IRAs and provide a physical explanation for experimental results that differ from the original theory.     

Optimization of microstrip feed geometry for prime focus reflectorantennas

The radiation characteristics of a circular microstrip antenna are studied numerically. Surface integral equations are used to formulate the problem from the boundary conditions and moment methods are used to reduce the integral equations to a matrix equation. An analytic method is used to design a microstrip feed and to achieve symmetric radiation patterns with low cross polarization and backlobe levels. The backlobe level is reduced by adding a quarter-wavelength choke to the side wall or the ground plane of the antenna and the bandwidth is improved by stacking two layers. The performance of the feed with the reflector antenna is also considered. One of the feeds was fabricated and tested. Satisfactory agreement between the computed results and the measurement data was obtained. The microstrip feed has a very small size which should reduce its blockage of the reflector aperture     

Diffraction analysis of line feeds for spherical reflectors

The diffraction limitations of the line feeds for spherical reflector antennas are analyzed by means of an asymptotic transition region theory (TRT). It is shown that diffraction from the ends of the line feed causes a broad 6-dB-deep central dip in the aperture field of the reflector. The corresponding reduction in aperture efficiency and increase in spillover are also calculated by means of the TRT. The accuracy of the theory is checked by numerical evaluations of the array sum expression for the radiation field and of the secondary aperture-field and spillover integrals. The results are applicable to the line feeds of the spherical reflector antenna of the Arecibo Observatory     

Improvement of prompt response from impulse radiating antennas by aperture trimming

Reflector impulse radiating antennas (IRA) traditionally have been constructed by terminating a self-reciprocal, transverse electromagnetic (TEM) transmission-line feed structure into a paraboloidal reflector. The section of the paraboloid used is usually circular in cross-section, with the outer boundary coinciding with the circle of symmetry of the TEM feed. The reflector converts the spherical TEM mode on the feed line into an approximate plane wave in the near field by geometric optics. The prompt radiated electric field in the direction of focus is given in the physical optics approximation in terms of the integral of the electric field of the TEM mode over the aperture plane inside the reflector boundary. Balanced feed structures have TEM modes that provide both positive and negative contributions to this integral in the aperture plane. Determination of the contour where the principal component of the electric field in the TEM mode is zero identifies portions of the aperture that contribute destructively to the integral. These portions are removed, thereby increasing the prompt radiated field without altering the feed structure or the applied voltage waveform. Furthermore, decreasing the size of the TEM feed relative to the aperture size, followed by appropriate aperture trimming, allows an even greater increase in radiated field. Results are presented that predict an increase in prompt radiated fields for all electrode configurations. Improvements are largest for electrode angles that are large (with respect to the vertical). The trends predicted by the numerical results are verified by an experiment conducted on a time-domain antenna range.