A scanning spherical tri-reflector antenna with a moving flatmirror

Spherical reflector systems can achieve pattern scanning without rotation of the main reflector through the use of multiple subreflectors that can move. Also, two subreflectors can be shaped to correct for spherical aberration and to control the aperture distribution on the spherical main reflector. In a previous paper (see ibid., vol.41, p.778, no.6, 1993) we introduced a method that offers both aperture phase and intensity control and scans the main beam without an accompanying movement of the illuminated area over main reflector. The method can overcome the poor aperture utilization problem common in spherical reflector antenna systems; however, it requires motion of the entire subreflector system, including the feed, during scan. In this paper we discuss a method that does not require motion of the subreflector system during scan. This method employs a flat mirror that creates a virtual image of the subreflector system. The motion of the subreflector system in the previous design is replaced by the motion of the virtual image that is controlled by the motion of the flat mirror. The new design offers simplified mechanical motion, while maintaining beam efficiency performance comparable to that of traditional spherical tri-reflector scanning antennas, but with some sacrifice in aperture efficiency and cross-polarization performance     

High efficiency and low sidelobe design for a large aperture offset reflector antenna

A design method giving high efficiency and low sidelobes is discussed for large aperture offset reflector antennas. A new reflector shaping technique using the subreflector and the beam waveguide reflector with the parabolic main reflector is proposed to simplify the main reflector manufacturing process. The effectiveness of the technique is confirmed by the model experiments. One problem with this reflector shaping technique is that the subreflector edge level cannot be controlled independently of the main reflector edge level. By investigating the relation between the gain reduction and the subreflector edge level, which affects the wide-angle sidelobe levels, the realizable characteristics of antennas are studied. In order to decrease the subreflector edge level without reducing the aperture efficiency, a technique using an extended reflector is also proposed. Its effectiveness is shown by theoretical and experimental investigations.     

Designing classical Dragonian offset dual-reflector antennas fromcombinations of prescribed geometric parameters

This paper proposes a simple procedure for the design of classical offset-Dragonian dual-reflector antennas from combinations of prescribed geometric parameters. This procedure has already been applied to classical Cassegrain and Gregorian antennas, and to classical displaced-axis Cassegrain and Gregorian antennas. We provide a list of 20 parameters from which the antenna system is fully characterized, but only five of these parameters need to be provided by the antenna designer, as the remaining 15 parameters can be derived in closed-form using the procedure described. We consider that the main reflector (MR) has a circular aperture, while the subreflector (SR) has an elliptical aperture     

Designing classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters

This paper proposes a simple procedure for the design of classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters. This procedure has already been applied to classical Cassegrain and Gregorian antennas, to classical displaced-axis Cassegrain and Gregorian antennas, and to classical offset Dragonian antennas. The antenna systems can be fully characterized by 21 parameters, of which only five need to be provided by the antenna designer, as the remaining 16 parameters can be derived in closed form using the procedure described here. In this paper, we assume that the main reflector has a circular aperture, while the subreflector has an elliptical aperture All the antenna geometries presented satisfy the Mizugutch condition (1976), which is the geometric-optics condition for zero cross-polarized radiation. This procedure is very close to the one used for offset Dragonian systems, but all the relevant information is repeated here for completeness.     

Subreflector shaping for antenna distortion compensation: an efficient Fourier-Jacobi expansion with GO/PO analysis

Technological demands have brought a renewed interest in the application of large reflector antennas with steadily increasing operating frequencies and antenna dimensions. The high surface accuracy of the main reflector required by these antennas can often not be achieved with available manufacturing technologies. The utilization of a shaped subreflector for main reflector-distortion compensation is considered an effective measure to enhance the overall radiation performance of an antenna system. In the process of evaluating the suitability of the subreflector shaping, however, it is crucial to accurately assess the most suitable subreflector shape within a reasonable amount of computational time. This is especially true for electrically large reflectors, where simple analysis of the radiation characteristics already creates a serious computational burden, moreover, since reflector shaping is a synthesis process that necessitates repeated computation of the radiation characteristics. In this paper, the development of an efficient computational tool for subreflector shaping is presented. The subreflector shaping is performed through a combination of geometrical optics (GO) and physical optics (PO) on the subreflector and the main reflector, respectively. To significantly limit the number of parameters subject to optimization, the subreflector surface is parameterized by the coefficients of a global, orthogonal Fourier-Jacobi set (related to Zernike polynomials), which allows us to accurately represent a surface with only a small number of coefficients. The incorporation of this surface expansion into a GO/PO synthesis technique is detailed, representative results are given for a computationally challenging reflector configuration, and the tolerances for the shaped subreflector surface are studied.     

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.     

The effects of subreflector diffraction on the aperture efficiency of a conventional Cassegrain antenna--An analytical approach

An asymptotic theory is presented with which the reduction in aperture efficiency caused by diffraction from a subreflector edge can be calculated for any dual-reflector system. The theory is applied to conventional Cassegrain antennas, for which approximate analytical effieiency formulas are derived. These formulas show that subreflector diffraction may represent a significant efficiency loss even for subreflector diameters as large as 20 wavelengths. The formulas are used to obtain an optimum subreflector size which represents the best trade-off between losses due to subreflector diffraction and geometrical shadowing.     

Designing axially symmetric Cassegrain or Gregorian dual-reflectorantennas from combinations of prescribed geometric parameters

A procedure to design axially symmetric Cassegrain or Gregorian dual-reflector antennas from various combinations of prescribed geometric parameters is presented. From these input parameters, the overall geometry of the antenna is derived in closed form. This procedure can be used as the starting point of a synthesis procedure, where both main reflector and subreflector are shaped to create the desired aperture field distribution     

A transform technique for computing the radiation pattern of prime-focal and Cassegrainian reflector antennas

An efficient numerical method based on the use of the fast Fourier transform (FFT) algorithm is developed for computing radiation patterns of aperture antennas with given aperture distributions. The method is also readily applicable to the problem of computing the radiation pattern of paraboloidal reflector antennas when the induced surface currents on the surface of the reflector are known. Using an efficient launching and scanning scheme for subreflector analysis, the method is extended to a Cassegrainian reflector antenna system.     

A shaped single reflector offset antenna with lowcross-polarization fed by a lens horn

A novel compact primary-fed offset reflector antenna with the potential of radiating circular as well as elliptical beams with low cross-polarization, is proposed. The reflector is fed by a horn with a phase-correcting lens in the aperture. Compared to the dual-reflector offset antenna, the concept is easier to assemble and mechanically more robust. The antenna has been synthesized and analyzed by computer programs resulting from modifications of corresponding PO programs for dual-reflector offset antennas. The synthesized antenna exhibits similar cross-polarization, side-lobe level, and aperture efficiency as those of dual-reflector offset antennas, although the one-to-one correspondence between zero cross-polarization and conformal mapping from the feed to the aperture, is not exactly valid for this approach     

Aperture blockage by a subreflector in a Cassegrain antenna system

The problem of aperture blockage by a subreflector in a Cassegrainian antenna system is discussed. The Lorentz reciprocity theorem is used to derive an expression for the far field radiated by the Cassegrain system. It is suggested that the field scattered by the subreflector can be computed using Keller's geometrical theory of diffraction.     

Amplitude aperture-distribution control in displaced-axistwo-reflector antennas

A design method for improving the efficiency and reduction of sidelobes in displaced-axis two-reflector antennas is presented. The method is based on an analysis of the geometrical-optics field transformation in the displaced-axis two-reflector arrangement. The principal difference of the displaced-axis antenna from the Cassegrain/Gregory two-reflector antenna is pointed out. While decreasing the level of illumination of the subreflector edge in the Cassegrain/Gregory antenna leads to decreasing the sidelobe levels, and, respectively, the efficiency of the antenna system, in the displaced-axis antenna, decreasing the level of illumination of the subreflector leads to an increased level of sidelobes and an essentially nonuniform amplitude distribution in the aperture. The aperture-amplitude-distribution dependence of the illumination level of the subreflector edge in displaced-axis antennas is much stronger than in Cassegrain/Gregory antennas     

Generalized classical axially symmetric dual-reflector antennas

This work presents a generalized study of classical axially symmetric dual-reflector antennas. The antenna dishes are simply described by conic sections, arranged to reduce the main-reflector radiation toward the subreflector surface. The dual-reflector configuration provides a uniform-phase field distribution over the illuminated portion of the aperture, starting from a spherical-wave feed source at the antenna primary focus. All possible configurations are characterized into a total of four distinct groups. Simple closed-form design equations and the aperture field distribution are derived, in a unified way, for all these kinds of generalized antennas using the principles of geometrical optics. The formulation is applied in a parametric study to establish the configurations yielding maximum radiation efficiency (not including diffraction effects). The design procedure is exemplified in the synthesis of a novel configuration, which is further analyzed by the moment method     

Antenne tribande pour faisceaux hertziens numériques à grande capacité

The introduction on microwave networks of digital high spectral efficiency systems needs the use of very high performances antennas. Principally a high cross-polarization discrimination. Multiband antennas are suitable solutions to the overload of the towers or pylones. A dichroic subreflector is the proposed solution to design such an antenna. The access of several frequency bands on the same antenna feeder is realized with the use of band diplexers and circulators which must have a very low intermodulation level.     

Zooming and Scanning Gregorian Confocal Dual Reflector Antennas

The zooming and scanning capabilities of a Gregorian confocal dual reflector antenna are described. The basic antenna configuration consists of two oppositely facing paraboloidal reflectors sharing a common focal point. A planar feed array is used to illuminate the subreflector allowing the antenna to scan its beam. The resulting quadratic aberrations can be compensated by active mechanical deformation of the subreflector surface, which is based on translation, rotation and focal length adjustment. In order to reduce the complexity of the mechanical deformation, least squares fit paraboloids are defined to approximate the optimal correction surface. These best fit paraboloids considerably reduce scanning losses and pattern degradation. This work also introduces two different zooming techniques for the Gregorian confocal dual reflector antenna: the first consists of introducing a controlled quadratic path error to the main reflector aperture; and the second is based on reducing the size of the radiating aperture of the feeding array.      

Design and experimental investigation of stripline antennas

The results of development and experimental investigation of several centimeter-wave planar antennas are presented. The antennas contain a multichannel stripline power divider and a 2D periodic array of slot radiators excited by symmetric strip lines. A single-polarization antenna, an antenna with two linear polarizations, and an antenna with two circular polarizations are analyzed. The results of measuring the basic parameters of these antennas (the gain, the radiation pattern, and the cross-polarization level) are presented. Specific features of the design of such antennas and possible means of improvement of their performance characteristics are discussed.     

Planar millimeter-wave antennas using SiNx-membranes onGaAs

Planar aperture coupled microstrip antennas for 77 GHz are demonstrated for the first time. As far as possible standard GaAs monolithic microwave/millimeter-wave integrated circuit (MMIC) technology is used to realize the antennas. The antenna patches are suspended on a thin dielectric SiN_x membrane on GaAs substrate. Therefore a novel plasma-enhanced chemical vapor deposition (PECVD) process step for the fabrication of the membranes is developed and described. The single antenna patches are coupled to a microstrip line through an aperture in the ground metallization. The method of moments in spectral domain is applied to design the patches. The feed network of a 3×1 antenna array for homogeneous excitation is simulated and optimized with a microwave design system (MDS). From reflection measurements the operation frequency of this triple patch antenna is determined to be 77.6 GHz. The farfield antenna characteristics are measured in an anechoic chamber, showing good agreement between simulated and measured results and a co- to cross-polarization isolation better than 30 dB     

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.     

Subreflector extension for improved efficiencies in Cassegrain antennas--GTD/PO analysis

Both offset and symmetric Cassegrain reflector antennas are used in satellite and ground communication systems. It is known that the subreflector diffraction can degrade the performance of these reflectors. A geometrical theory of diffraction/physical optics (GTD/PO) analysis technique is used to investigate the improving effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and farfield patterns. Representative numerical results are shown for an offset Cassegrain reflector antenna with different feed illumination tapers and subreflector extensions. It is observed that for subreflector extensions as small as1 lambdanoticeable improvements in the overall efficiencies can be expected. Useful design data are generated for the efficiency curves and far-field patterns.     

Focused aperture antenna using a spherical main reflector with (Cassegrain) convex feed

A focused aperture antenna has been realised using a (Cassegrain) convex subreflector with a spherical main reflector. The design was based on geometrical optics, and good agreement was obtained between the measured and calculated field in the focal region.