Depolarization properties of offset reflector antennas

The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0}between the feed axis and the reflector axis as well as upon the half-angletheta_{c}subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg, measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas.     

Calculation of ring-shaped phase centers of feeds for ring-focusparaboloids

Some primary feeds such as the hat feed radiate around the feed waveguide, which also acts as an axial support tube. Such feed antennas have phase centers in the form of rings rather than points. This paper presents a formula to calculate the location of the ring-shaped phase center. The optimum reflector is a ring-focus paraboloid with the ring focus coinciding with the ring-shaped phase center. The phase center formula is applied to two versions of the hat feed and it is shown that the aperture efficiencies can be improved by up to 0.5 dB when optimum ring-focus paraboloids are used instead of point-focus paraboloids     

A comparison among 1-, 3-, and 7-horn feeds for a 37-beam MBA

A very common multiple beam antenna (MBA) configuration consists of a collimating device illuminated by an array of feeds. The collimating device is usually a reflector or a lens. The feeds are usually horn antennas with a circular aperture. The reflector is usually offset-fed to eliminate aperture blockage; the lens is center fed. The antenna's feeds are excited to produce a finite number of beams, so as to provide contiguous coverage of the field of view. The designer is forced to minimize the angular spacing between adjacent beams in order to maximize the minimum gain over the antenna's field of view. On the other hand, the feed horn's aperture gain is maximized when the feed horn spacing and its aperture diameter are equal. This results in antenna efficiency of the order of 30% when a single feed horn is excited to produce a beam. When a cluster of 3 or 7 adjacent feed horns are excited to produce a single beam, antenna efficiency can be increased to 50%. When it is tolerable, several identical antenna apertures can be used to replace a single aperture configuration. In this case, each of M apertures produces approximately N/M beams of an MBA that produces N beams. Horns producing adjacent beams do not illuminate the same aperture. This permits the use of a much larger horn aperture for a given beam spacing. This results in reduced spillover, higher gain of each beam, and increased antenna efficiency of each aperture. This paper investigates the maximization of gain for several lens antennas. It shows that antenna gain is increased as its focal length is decreased. That is, a focal length-to-diameter ratio (F/D) less than 1 is preferred     

Optimizing multimode horn feed arrays for offset reflector antennasusing a constrained minimization algorithm to reduce cross polarization

This letter presents an algorithm for suppressing the cross polarization of single offset reflector antennas illuminated by a cluster of multimode horn feeds using a constrained minimization routine. The design variables are the mode ratios for the higher order modes excited in the feeds relative to the dominant TE₁₁ mode. The goal is to systematically determine the optimum mode ratios such that the overall crosspole performance of the reflector antenna improves relative to the single TE₁₁ mode excitation. The multimode horns considered are Potter-type (dual mode) horns and trimode horns     

Scalar horn with shaped lens improves Cassegrain efficiency

It is shown that the aperture efficiency of a classical Cassegrain antenna can be considerably improved if the radiation pattern of the feed is optimally shaped. The corresponding optimum field distribution Over the aperture of the feed consists of a circular main lobe which is surrounded by concentric sidelobe rings. This optimum field distribution with one sidelobe ring included is realized by shaping of a dielectric lens which is positioned in the aperture of a corrugated horn antenna. The design can provide a theoretical aperture efficiency of 90.5 percent when subreflector diffraction and aperture shadowing are neglected, i.e., an improvement of 0.4 dB compared to an optimum conventional feed. Measurements of a model at 22.8 GHz gave a practical result of 85.6 percent. If the model is used to feed a 30 m radiotelescope the overall antenna efficiency becomes about 71 percent.     

Radiation pattern synthesis for circular aperture horn antennas

A set of radiation pattern functions, suitable for synthesis of radiation patterns from circular aperture horn antennas, is obtained by assuming an aperture distribution consisting of the fields of cylindrical waveguide modes. A technique is presented for using a linear combination of the radiation pattern functions to approximate a desired radiation pattern. Linear combinations of the radiation pattern functions resulting in maximum secondary gain, when used to illuminate a paraboloidal antenna, are obtained empirically. Using spherical wave theory, maximum performance theoretically obtainable from an antenna is derived as a function of the aperture size of the feed system; the feed efficiency resulting from these theoretical limits on performance is compared to the feed efficiency of patterns obtainable from circular aperture horn antennas, and to experimental results of attempts to realize optimum circular aperture horn patterns.     

TE11mode excitation of flanged circular coaxial waveguides with an extended center conductor

Properties of circular coaxial Waveguide antennas terminated in a plane conductor are analyzed where in the most general case the center conductor is of infinite extent. Solutions for mutual admittance and radiated fields are detailed for all asymmetric modes. An asymptotic solution is also derived for the far-zone fields. We describe results forTE_{11}mode excitation of coaxial waveguides where 1) outside the aperture the conductor is absent, and 2) the center conductor extends to infinity. These antennas are of practical interest as feeds for reflectors. Experimental results are described for several examples and excellent agreement with theory is shown. Comparison of the two cases shows that in 1) there is always an optimum frequency for good pattern symmetry and low cross polarization. The mismatch in case 2) is less than in 1) but pattern symmetry is poor if the ratio of inner to outer conductor radius is greater than about 0.2. Further, in 2) mutual coupling to adjacent annular rings is stronger and the principal plane radiation patterns has widely spaced phase centers.     

Radiation characteristics of coaxial waveguides as a small primary feed

The radiation patterns of coaxial waveguides with a TE11 exciting mode are computed numerically. It is shown that by a proper selection of the coaxial aperture dimensions the symmetry of the copolar patterns can be improved considerably. Coaxial radiators can therefore be designed as a useful small primary feed for paraboloid reflector antennas.     

The design of classical offset Dragonian reflector antennas with circular apertures

The vector aperture field of classical offset Dragonian dual-reflector antennas is derived using geometrical-optics concepts. This field then yields the equivalent paraboloid of the geometry. From these results, the conditions for an axially symmetric equivalent paraboloid, when a circular aperture is assumed, are obtained. A complete step-by-step geometrical-optics-based design procedure for optimum classical offset Dragonian antennas with circular apertures is then presented (i.e., zero geometrical-optics cross-polarization and minimum spillover). This procedure is demonstrated by two design examples.     

Beam efficiency limitations of large antennas

The beam efficiency of an antenna may be defined as the ratio of the power radiated within the main beam to the total power radiated. The beam efficiency is derived for ideal rectangular and circular apertures, as a function of the edge-to-center amplitude ratio. Random phase errors are assumed to exist across the aperture. Various types of feeds for parabolic reflectors are also considered in relation to the fraction of power which the feed directs into the paraboloid. A primary limitation on the beam efficiency of a paraboloid is shown to be produced by the surface roughness.     

First-order correction of aberrations in Cassegrainian and Gregorian antennas

Cassegrainian and Gregorian reflector arrangements are needed for multibeam ground station and satellite antennas. In these antennas one or more feeds are displaced from the focus. This displacement normally causes aberrations, due primarily to astigmatism and coma. In a previous article, astigmatism was eliminated for small feed displacements by centering the antenna aperture with respect to the equivalent paraboloid axis. In this article coma is also removed by deforming the reflectors without causing spherical aberration, and simple expressions for the deformation coefficients are derived. The analysis applies in general to an arrangement ofNreflectors. The deformations required to correct astigmatism, if the antenna aperture is not centered is also derived.     

Optimum low sidelobe high crossover multiple beam antennas

Simultaneous multiple beam antennas (MBA's) with low sidelobe high crossover beams are considered. A simplified proof is presented which shows dissipative loss of typically -3 dB must be accepted. The optimum efficiency for the constrained fed MBA is shown to be achieved by placing attenuation in the aperture of a Butler matrix (BM) in accordance with the desired tapered aperture distribution. A combination of aperture attenuation and feed design, including the use of overlapping orthogonal feed distributions, is considered for lens type MBA's. The optimum efficiency is shown to be achieved with relatively simple feeds for a broad class of desired beam shapes.     

GTD analysis of the radiation patterns of a prime focus paraboloid with shroud

The uniform geometrical theory of diffraction (UGTD) is employed to analyze the far-field radiation patterns of a prime focus paraboloid with a cylindrical shroud. The blockage of the aperture illumination of the dish by the gooseneck and the primary feed is also taken into account in the analysis. Far-field radiation patterns (for0degleqTheta leq 180deg), calculated for a typical prime focus paraboloid with 2 m aperture diameter, designed and fabricated, are compared with the experimentally derived patterns at 8.8 GHz in theE- andH-planes. There is a satisfactory agreement between the two results.     

Mutual coupling in arrays of coaxial waveguides and horns

A solution is presented for the mutual coupling between the modes of different-sized circular coaxial aperture arrays, which are used as direct radiators and feed antennas. This solution is expressed in terms of integral transforms and the mutual admittances for the cross coupling between all three mode types, TE, TM, and TEM, are obtained. The accuracy of this new integral formulation is demonstrated by comparing computed results with measured data for a three-element test array. Excellent agreement is obtained between theory and experiment for both coupling coefficients and radiation patterns. Results are also presented for a coaxial feed array for the Lovell radio telescope.     

The polarization losses of offset paraboloid antennas

In this paper the electric field in the aperture of offset front-fed paraboloid antennas and open Cassegrainian antennas, excited by an electric dipole or Huygens source in the focus, is compared with the fields of front-fed circularly symmetrical paraboloid reflector antennas and classical Cassegrainian antennas. The aperture field forms the basis of expressions to calculate the polarization efficiency of all four types of antenna. Computed results are given, showing that offset antennas can compete with front-fed paraboloids if they are excited by an electric dipole; the classical Cassegrainian antenna, however, shows better results. If offset antennas are excited by a Huygens source, the result is very unfavorable compared with the symmetrical antennas which show no cross polarization.     

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.     

Optimization of the TEM feed structure for four-arm reflectorimpulse radiating antennas

This paper considers the optimization of the feed arm geometry of four-arm crossed-coplanar plate impulse radiating antennas (IRAs) when the angular position and extent of the arms are taken as free parameters. Previously, optimization of this class of antenna considered only the symmetric case where the two pairs of crossed feed arms were perpendicular to each other. Comparison is made using the prompt aperture efficiency, and the results indicate that the efficiency of four-arm IRAs can be increased from ~25% for the perpendicularly crossed arms to ~35% for the optimum configuration. In addition to the optimization, the feed impedance of coplanar feeds is presented for general values of feed arm angle and plate width, and the optimum feed impedance is computed for each feed arm angle. The results can be used to design the optimal four-arm IRA with an arbitrary specified input impedance     

A description and the measured performance of three coaxial beam-rotating antenna prototypes

Many high-power microwave (HPM) sources utilize an azimuthally symmetric output mode, like the TM/sub 01/ circular waveguide or the coaxial TEM modes. If radiated directly, these modes produce a doughnut-shaped radiation pattern, with a boresight . Mode-conversion techniques for transforming the azimuthally symmetric mode to one with a more desirable radiated pattern are possible, but mode conversion is typically undesirable, due to inefficiencies and due to increases in system size and weight. Antenna designs have been explored that will radiate the azimuthally symmetric mode directly, but those considered to date tend to exhibit low gain, and do not radiate a boresight peak (along the longitudinal axis of the source). This article describes the measured performance of three prototype antennas, all of the coaxial beam-rotating antenna (COBRA) class. These accept directly an azimuthally symmetric mode, and radiate a high-gain, circularly polarized beam with a boresight peak. The antennas achieve this capability by varying the electrical length of a path from a focal point to the aperture plane as a function of the azimuthal angle of the aperture. A brief overview of the general theory of COBRA operation is first presented. Next, measured data, characteristic of the input impedance and far-field patterns of three COBRA prototypes, are given. The architectures of COBRA prototypes reviewed in this article include those utilizing (1) a single, stepped paraboloidal reflector; (2) a dual reflector; and (3) a dual reflector with a coaxial feed.     

Characterization of tapered slot antenna feeds and feed arrays

A class of feed antennas and feed antenna arrays used in the focal plane of paraboloid reflectors and exhibiting higher than normal levels of cross-polarized radiation in the diagonal planes is addressed. A model which allows prediction of element gain and aperture efficiency of the feed/reflector system is presented. The predictions are in good agreement with experimental results. Tapered slot antenna (TSA) elements are used as an example of an element of this type. It is shown that TSA arrays used in multibeam systems with small beam spacings are competitive in terms of aperture efficiency with other, more standard types of arrays incorporating waveguide type elements     

椭圆口径天线的一种新的设计方法

利用口径坐标变换，比较了圆口径和椭圆口径天线方向函数的差异，得出了将圆口径分布方向图特性映射到椭圆口径的条件，并将圆口径天线最佳综合方法圆Taylor综合应用于椭圆口径天线的设计。将这一条件应用于椭圆离散阵设计，给出了设计实例，比较了连续圆口径分布与椭圆离散阵的特性，扩展了椭圆阵列的常规设计方法。