A Novel Beam Squint Compensation Technique for Circularly Polarized Conic-Section Reflector Antennas

Beam squint generally exists in offset reflector antennas with circularly polarized feeds. It is manifested by a small beam shift of the radiation pattern in the plane perpendicular to the principal offset plane, which can significantly affect the beam pointing accuracy. In this paper a practical and widely applicable compensation technique for the beam squint is proposed. Simulation results show that a small lateral feed displacement in the perpendicular plane can effectively minimize or eliminate the linear phase shift caused by the depolarization effect, thus compensating for the beam squint effect. This is in practice very useful for offset reflector antennas where the previously suggested method based on feed tilting may not be proper. A simple formula is derived to quickly estimate the optimal feed displacement for both right- and left-hand circularly polarized feeds. Three representative examples: a single offset parabolic reflector, a suboptimal offset Cassegrain reflector, and an axially symmetric Cassegrain reflector with an off-focus feed, are presented to validate the proposed method. Satisfactory results are achieved for all three examples.      

Preliminary study on offset scan-corrected reflector antenna system

Preliminary study on offset shaped dual reflector antenna systems has been carried out to assess the feasibility for multibeam satellite applications. The two-dimensional offset shaped reflector antenna geometry is generated by first creating the nodal points according to a bifocal condition and then connecting the nodal points by smooth curves to form the profiles of the main and subreflectors. The three-dimensional geometry is created by body revolution. The offset geometry is obtained by properly tailoring the three-dimensional geometry. This offset shaped reflector antenna system has an inherent astigmatism which can be either fully or partially compensated. For applications requiring a scan range in azimuth more thanpm 5beamwidths, the offset shaped dual reflector antenna systems offer better scan performance (in terms of peak gains) than offset Cassegrain geometries at the expense of the performance of the on-axis beams. In elevation with a 16 beamwidth scan range, the shaped design provides 0.3 dB less scan loss than the Cassegrain design.     

Some important geometrical features of conic-section-generated offset refelector antennas

Geometrical characteristics of conic-section-generated offset reflectors are studied in a unified fashion. Some unique geometrical features of the reflector rim constructed from the intersection of the reflector surface and a cone or cylinder are explored in detail. It is found that the intersection curve (rim) of the rotationally generated conic-section reflector surface and a circular cone with its tip at the focal point is always a planar curve and has a circular projection on the focal plane only for the offset parabolic reflector. Furthermore, in this case, the line going through the center of the circle, parallel to the focal axis, and the central axis of the cone do not intersect the reflector surface at the same point. Numerical results are presented to demonstrate some unique features of offset parabolic reflectors.     

Pattern synthesis of the offset reflector antenna system with lesscomplicated phased array feed

The performance degradation of an offset reflector antenna with off-axis scanning feed and distorted reflector surface can be improved by using a phased array feed. Generally, both analog attenuators and phase shifters are used in phased array feed. Yet, it seems to make the feed system very complicated. In this paper, a phase-only gradient search (POGS) algorithm is developed to optimize the performance of offset reflector antenna systems using phased array feed that is equipped with phase shifters only. This technique not only can greatly simplify the complexity of phased array feed, but also can provide the reflector antenna with better antenna gain and sidelobe level control capability. Simulation and experimental results are presented to illustrate the excellent performance of offset reflector, with limited beam scanning capability, can be obtained by using this less complicated phased array feed     

Offset dual-shaped reflectors for dual chamber compact ranges

The application of the theory of the synthesis of offset dual-shaped reflectors to the design of compact ranges is examined. The object of the compact range is to provide a uniform plane wave with minimum amplitude and phase ripple over as large a volume as possible for a given size reflector. Ripple can be lowered by reducing the edge diffraction from the reflector producing the plane wave. This has been done either by serrating or rolling the edge. An alternative approach is to use dual offset-shaped reflector synthesis techniques to produce a reflector aperture distribution that is uniform over most of the aperture, but with a Gaussian taper near the edge. This approach can be used together with rolling and/or serration if desirable. The amount of phase and amplitude ripple obtained with two different dual-shaped reflector designs is studied as a function of position in the plane wave zone and reflector size in wavelengths. The amount of both transverse and longitudinal (z-component) cross polarization is studied     

A design procedure for classical offset dual reflector antennaswith circular apertures

A geometrical optics procedure for designing electrically optimized classical offset dual reflector antennas with circular apertures is presented. Equations are derived that allow the size and spacing of the main and subreflectors of the antenna system, along with the feed horn subintended angle, to be used as input variables of the design procedure. The procedure, together with these equations, yields an optimized design, starting from general system requirements. The procedure is demonstrated by designing both an offset Cassegrain and an offset Gregorian antenna, and is validated by analyzing their radiation patterns using physical optics surface current integration on both the main and subreflectors     

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.     

Beam squint in a linearly polarised offset reflector antenna

A beam squint has been found to occur in the field radiated by an offset paraboloid antenna illuminated by a feed with linear polarisation in the plane of offset. The squint, whose existence has been confirmed by computed and measured radiation patterns, occurs in the offset plane, and is typically 0.03 of a beamwidth for a reflector of 6-wavelengths diameter. An assessment is given of the likely effect of the squint on the performance of an offset reflector radiating a circularly polarised field     

Analysis of the radiation patterns of reflector antennas

The development and application of a numerical technique for the rapid calculation of the far-field radiation patterns of a reflector antenna from either a measured or computed feed pattern are reported. The reflector is defined by the intersection of a cone with any surface of revolution or an offset sector of any surface of revolution. The feed is assumed to be linearly polarized and can have an arbitrary location. Both the copolarized and the cross polarized reflector radiation patterns are computed. Calculations using the technique compare closely with measured radiation patterns of a waveguide-fed offset parabolic reflector. The unique features of this technique are the freedom from restrictive feed assumptions and the numerical methods used in preparing the aperture plane electric field data for integration.     

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     

偏置抛物面天线圆极化版图成形波束工程理论的研究

本文根据矩形波导中隔片式圆极化器传输波型的渐变过程,导出了圆极化馈源的辐射场公式,并利用R.Mittra等作者提出的物理光学辐射积分Jacobi-Bessel级数展开方法对偏置抛物面天线圆极化版图成形波束的工程理论进行了研究。它包括:圆极化馈源辐射场公式;偏置抛物面电流等效投影孔径布分;二次辐射场计算公式;版图成形波束等效各向同性辐射功率(EIRP)工程计算公式;天线坐标、版图经纬度坐标与卫星坐标之间的变换关系;给出了七元馈源阵成形的我国西部版图圆极化波束的数值结果,其EIRP等电平曲线是计算机自动打印的,本文研究的理论方法可作为研制版图成形波束天线各阶段的基本分析工具。     

Impulse-radiating antenna with an offset geometry

An offset impulse-radiating antenna (IRA) is numerically analyzed and compared with a typical centered IRA. In the typical centered IRA, the transverse electromagnetic (TEM) feed arms block the aperture because they are located at the center of the aperture. This blockage causes multiple reflections inside the antenna and, thus, ripples in the tail of the radiated waveform. In the offset IRA, the TEM feed arms are removed from the aperture, lowering the tail ripples caused by multiple reflections between the TEM feed arms and the reflector. The boresight gains and the impulse amplitudes are seen to be essentially the same for both IRAs. The monostatic radar cross section of the offset IRA is significantly lower than that of the centered IRA for the plane wave incident from the boresight direction because the wave incident to the offset IRA is diverted toward the focal point of the reflector, which is away from the boresight direction. The offset IRA has a shadow behind the reflector. This feature can be useful in bistatic radar applications because the antennas can be placed in the shadows of each other.     

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 measurement technique for modeling the effects of feed support struts on large reflector antennas

A measurement technique is described in which frequency scaled models of struts are placed in the near-field region of an offset reflector. In this compact range environment the excitation of the strut model is by plane waves, as would be encountered in the axisymmetrical reflector situation. Far-field radiation patterns are recorded, with and without the strut model in place, and, because of the low sidelobe levels associated with offset reflector antenna systems, it is possible to isolate the far-field response of the strut model. This technique is particularly useful for determining the real effects of structures that are difficult to analyze mathematically, such as latticed struts or metallic geodetic radomes.     

Offset parabolic reflector antennas

The paper provides a tutorial review of a number of offset parabolic reflector configurations including both single and double-reflector geometries. The author commences by describing some basic techniques which can be applied to predict the vector radiation fields and provides some indication of the validity of these methods. The formulation of a relatively simple analytical model for the offset reflector antenna is described based upon the physical-optics approximation. The electrical performance of the single-offset reflector is examined by comparison of predicted and measured data. The particular problems arising from the choice of polarisation and reflector dimensions are highlighted, and some practical applications involving multiplebeams, shaped and contoured beams, monopulse tracking and low sidelobes are briefly reviewed. Practical primary-feeds for offset-reflector antennas are discussed and the matched-feed concept is outlined, the matching of the electric fields in the primary-feed aperture to the reflector focal fields being illustrated. The advantages and disadvantages of dual-reflector antennas are then examined, with particular emphasis upon the open Cassegrainian configuration and the optimised doubleoffset configuration which offers, in principle, both freedom from blockage and low levels of cross-polarised radiation.     

Compact antenna test range reflector edge treatment

The physical optics technique is used to compare the performance of single offset compact antenna test ranges with different reflector edge treatments and rim shapes. A comparison between reflector edge taper and rim serrations in controlling edge diffraction is demonstrated     

The effects of reflector antenna diffraction on the interference cancellation performance of coherent sidelobe cancellers

While adaptive antenna technology has undergone significant development, little attention has been given to the impact of antenna design on the performance of the adaptive system. A need exists to factor the details of the antenna system response into the analysis of adaptive system performance, particularly in the case where adaptive cancellation is required over a broad bandwidth. At extremely high frequency (EHF) where wide bandwidth allocations exist, reflector antenna technology used with an adaptive sidelobe canceller design is appropriate. This paper uses a simple diffraction model to compare the adaptive performance differences between Cassegrain and offset reflector designs. The reduced diffraction of the offset reflector design results in improved cancellation performance. These analyses also provide the opportunity to explore the impacts of antenna design parameters and interference power levels and arrival directions.     

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.     

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.     

An imaging beam waveguide feed

A pseudo-frequency-independent beam waveguide feed has been designed and built to accommodate a new liquid-helium-cooled millimeter wave radio astronomy receiver in the side cab of the Crawford Hill 7-m antenna. This enables the antenna to be tilted without tilting the liquid-filled receiver. Comparison with the old vertex-cab feed indicates very little measured transmission loss through the beam waveguide. The frequency independence is based upon the Fresnel zone imaging principle. Design procedures and practical bandwidth limitations are explained. An explicit expression for the third-order term of an offset reflector surface clarifies the approximation of a lens by an offset reflector.