Subreflectarrays for Reflector Surface Distortion Compensation

With the increasing interest in the applications of large deployable reflector antennas operating at high frequencies, the requirement on the reflector surface accuracy becomes more demanding. Thermal effects inevitably cause certain reflector surface distortions, thus degrading the overall antenna performance. This paper introduces a novel reflector surface distortion compensation technique using a subreflectarray and presents detailed discussions. A microstrip reflectarray is used as a subreflector, illuminated by a primary feed. By properly adjusting the additional phase shift provided by the subreflectarray, the aperture phase errors caused by the main reflector surface distortions are compensated, resulting in a considerably improved antenna performance. As an example, a distorted 20-m offset parabolic reflector antenna operating at X-band is successfully compensated by a subreflectarray, and the simulation results are compared with those obtained by array feed and shaped subreflector compensation techniques. The microstrip subreflectarray is low-profile, lightweight, and cost-effective. Only one primary feed is required, and a reconfigurable design can be achieved if electronically reconfigurable reflectarray elements are adopted.      

A generalized diffraction synthesis technique for high performancereflector antennas

Stringent requirements on reflector antenna performances in modern applications such as direct broadcast satellite (DBS) communications, radar systems, and radio astronomy have demanded the development of sophisticated synthesis techniques. Presented in the paper is a generalized diffraction synthesis technique for single- and dual-reflector antennas fed by either a single feed or an array feed. High versatility and accuracy are achieved by combining optimization procedures and diffraction analysis such as physical optics (PO) and physical theory of diffraction (PTD). With this technique, one may simultaneously shape the reflector surfaces and adjust the positions, orientations, and excitations of an arbitrarily configured array feed to produce the specified radiation characteristics such as high directivity, contoured patterns, and low sidelobe levels, etc. The shaped reflectors are represented by a set of orthogonal global expansion functions (the Jacobi-Fourier expansion), and are characterized by smooth surfaces, well-defined (superquadric) circumferences, and continuous surface derivatives. The sample applications of contoured beam antenna designs and reflector surface distortion compensation are given to illustrate the effectiveness of this diffraction synthesis technique     

Array feeds for reflector surface distortion compensation: conceptsand implementation

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     

A pattern synthesis technique for array feeds to improve radiationperformance of large distorted reflector antennas

Technologies exist for construction of antennas with adaptive surfaces that can compensate for many of the larger distortions caused by thermal and gravitational forces. However, as the frequency and size of the reflectors increase, the subtle surface errors become significant and degrade the overall electromagnetic performance. Electronic compensation through an adaptive feed array offers a means for mitigation of surface distortion effects. A pattern synthesis approach for electromagnetic compensation of surface error is presented. The pattern synthesis approach uses a localized algorithm in which pattern corrections are directed toward specific portions of the pattern requiring improvement. The pattern synthesis techniques uses radiation pattern data to perform the compensation     

A fast hybrid asymptotic and numerical physical optics analysis of very large scanning cylindrical reflectors with stacked linear array feeds

A novel hybrid combination of an analytical asymptotic high-frequency method with a numerical physical optics (PO) procedure is developed to efficiently and accurately predict the far zone fields of extremely long, scanning, very high gain, offset cylindrical reflectors of arbitrary cross-section, with large stacked finite periodic linear phased array feeds, for spaceborne applications. In this method, the field generated by each finite length linear feed array is represented as a spectral integral and the induced current on the cylindrical reflector surface due to this illumination is obtained via the PO approximation. The reflector surface is divided into thin, long, piecewise planar strips along the generator of the cylindrical reflector, and the radiation integral over each strip is evaluated asymptotically in closed form, yielding an eight-term ray solution for the radiated fields. What remains is simply the superposition of the contributions of each strip and linear array in the feed stack. The proposed approach is shown to be extremely efficient and accurate as compared to the conventional PO integration technique. In addition, the method is sufficiently versatile to account for the reflector edge treatments (e.g., using resistive cards), as well as to account for a twist in the reflector surface due to thermal distortions in space.     

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     

Characterization of effects of periodic and aperiodic surface distortions on membrane reflector antennas

The focus of this paper is to characterize the effects of periodic and aperiodic surface distortions on the performance of membrane reflector antennas. Since the surface of this class of reflector antennas is very thin, it is susceptible to various types of periodic and aperiodic distortions. The particular antenna dimensions used for this study are similar to the specifications for the JPL/UCLA half scale model of second generation precipitation radar (PR-2) mission reflector. Analytical expressions are introduced to model periodic and aperiodic surfaces and based on these models the effects of distortions on the radiation performance of the antenna are simulated. Aperiodic distortions are more realistic cases of distortions due to the fact that the period of the distortions is not constant through out the reflector surface. For each case, far-field patterns of the reflector are simulated and it is shown that closed-form expressions can then be derived which result in a very efficient computational method to predict some of the unique features of these patterns including location and level of observed grating lobes. Furthermore, based on spatial Fourier analysis of the surface distortion, it is shown that deviation from periodicity in the distortions of reflector surface results in lowering these grating lobes. Parametric studies have been performed to provide design guidelines for acceptable surface behavior for large deployable membrane reflector antennas for future space borne missions.     

An efficient neural network algorithm for reflector surface errorcompensation

A neural network algorithm for electromagnetic compensation of reflector surface error effects is formulated. Sets of trained neural networks are used to compute the compensation excitations for array feeds. The networks were trained using data generated with the constrained least squares (CLS) compensation method. Once trained, the calculation of the excitations is accomplished in significantly less time than required by the original constrained least squares algorithm. The surface error profile for a distorted reflector antenna is expanded using bivariate surface basis functions. Each of the trained networks corresponds to one of the expansion functions. Excitations computed using the neural networks are superposed to produce composite compensation excitations for the distorted reflector. The compensation results for a distorted reflector are presented, and the neural network algorithm performance is compared to the original CLS technique     

Accurate simulation of reflector antennas by the complexsource-dual series approach

The radiation from circular cylindrical reflector antennas is treated in an accurate manner for both polarizations. The problem is first formulated in terms of the dual series equations and then is regularized by the Riemann-Hilbert problem technique. The resulting matrix equation is solved numerically with a guaranteed accuracy, and remarkably little CPU time is needed. The feed directivity is included in the analysis by the complex source point method. Various characteristic patterns are obtained for the front and offset-fed reflector antenna geometries with this analysis, and some comparisons are made with the high frequency techniques. The directivity and radiated power properties are also studied     

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     

Beam diffraction by planar and parabolic reflectors

In the complex source point technique, an omnidirectional source diffraction solution becomes that for a directive beam when the coordinates of the source position are given appropriate complex values. This is applied to include feed directivity in reflector edge diffraction. Solutions and numerical examples for planar strip and parabolic cylinder reflectors are given, including an offset parabolic reflector. The main beams of parabolic reflectors are calculated by aperture integration and the edge diffracted fields by uniform diffraction theory. In both cases, a complex source point feed in the near or far field of the reflector may be used in the pattern calculation, with improvements in accuracy in the lateral and spillover pattern lobes     

Numerical analysis of small parabolic antennas using the method of current and change integral equations

The method of current and charge integral equations [1] is applied to numerical electrodynamic analysis of radiation and impedance matching characteristics of parabolic antennas with reflector diameter from 0.5 to 10λ. As a result as opposed to the current method relying on physical optics approximation the influence of feed on reflector’s radiation pattern, the influence of reflector on feed’s impedance matching and the contribution of feed’s rear radiation into reflector’s radiation pattern are all accounted for. A new model of representing a parabolic surface in the form of its square approximation is suggested, which provides almost uniform partitioning grid and has at least 2.5 times less boundary elements that the common revolution surface representation while having the same sampling coefficient. Dependences of antenna’s directivity on reflector dimensions (0.5–10λ) are calculated for six different focus distance to reflector’s diameter ratios using the developed by the authors crystal_U software package. The calculated results are confirmed by good matching with well-known experimental results.     

Effect of surface distortions on the crosspolarisation performance of paraboloidal reflectors

The effects on the crosspolarisation and the radiation patterns of circular paraboloidal reflectors produced by (a) reflector distortions which are periodic in the circumferential direction and (b) a lateral offset of the feed have been evaluated for a wide range of reflector and feed parameters. It is shown that the use of a balanced hybrid-mode feed minimises crosspolarisation introduced by reflector distortions.     

Array feed synthesis for correction of reflector distortion andvernier beamsteering

An algorithmic procedure is described for the synthesis of a planar-array for paraboloidal reflectors to provide simultaneously electronic correction of systematic reflector surface distortions as well as a vernier electronic beamsteering capability. Several f/D ratios and distortion models were examined that are typical of large paraboloidal reflectors. Numerical results are presented showing that, for the range of distortion models considered, significant on-axis gain restoration can be achieved with a one-ring (seven-element) array. However, with seven elements, the array parameters that maximize system gain, do not provide uniform beam-steering (±1 BW) and an additional ring (19 elements) is required. For arrays either 7 or 19 elements, the results indicate that the use of high-aperture-efficiency elements in the array yields higher system gain than can be obtained with elements having lower aperture efficiency. Contour plots of the focal-plane fields are also presented for various distortion and beam-scan-angle cases, showing the dynamic nature of the problem     

Analytical method of pattern analysis for cluster-fed reflectors with random feed displacements

An innovative analytical method for the evaluation of the directivity patterns of cluster-fed reflector antennas with random feed element position and orientation displacements is presented. In this approach, the sensitivities of the incident magnetic field, far electrical field and directivity are derived with respect to the feed element position and orientation displacements. The effect of random position and orientation displacements on the directivity pattern is investigated via the numerical characteristics of the directivity with pre-calculated sensitivities; meanwhile, the upper and lower bounds of the directivity pattern are illustrated. A seven-element cluster-fed offset reflector is utilised as a numerical example to show the application of this method with different feed element position and orientation displacements. Comparison with the Monte Carlo method demonstrates the effectiveness and applicability of the proposed method.     

A surface distortion analysis applied to the hoop/column deployablemesh reflector antenna

A practical approach is demonstrated for the deterministic analysis of surface distortions in reflector antennas, based on a first-order approximation to the aperture field phase. Measured pattern results from the 15-m-diameter hoop/column deployable mesh reflector antenna are used to demonstrate the accuracy which can be obtained with this surface distortion analysis. The only practical limitation of the first-order approximation is determined by the slope derivations of the distorted surface from the best-fit paraboloid     

Directivity optimization of a reflector antenna with cluster feeds: A closed-form solution

The directivity of a reflector antenna deteriorates as the feed moves away from the focal point for beam scanning. This deterioration can be substantially reduced if a cluster feed instead of a single feed is used to control a beam. A closed-form solution is presented for the cluster excitation to achieve the optimum directivity. For an offset108 lambdaparabolic reflector scanning 10 beamwidths, the optimum-directivity achieved by a 19-element (seven-element) cluster is 12 dB (8 dB) higher than that of a single element. Comparison of the optimum-directivity design and the popular conjugate field matching design is made. When the cluster spacingdis greater than1 lambda, it is found that the optimum directivity is higher than that of conjugate field matching (CFM) scheme by an insignificant amount, although the excitations of two designs can be quite different. Ford < 0.5 lambda, the optimum design may exhibit the supergain phenomenon, namely, extremely high directivities achieved by an oscillatory cluster excitation.     

Focal shifts in parabolic reflectors

Given a parabolic reflector, the maximum directivity is not always achieved by placing the feed at the focal point. Depending on the nature of the feed, the maximum directivity can be obtained by axially displacing the feed either toward or away from the reflector. For low-tapered feeds, the shift should be toward the reflector. This result is similar to an optical phenomenon called the focal shift. We find that this positive shift depends mainly on the Fresnel number of the reflector. For highly tapered feeds, the shift should be away from the reflector. This negative shift becomes significant when the reflector aperture is small, in units of wavelength. A unified view is presented to explain both the positive shift and the negative shift in terms of spillover, aperture illumination efficiency and phase asynchronism. For a system with optimum aperture edge taper, no focal shift can exist.     

Analysis of circular reflector antenna covered by concentricdielectric radome

The radiation from a two-dimensional cylindrical reflector antenna with a concentric dielectric radome is analyzed in an accurate manner for both H and E polarization cases. The problem is first formulated in terms of the dual series equations, and then it is regularized by using Riemann-Hilbert problem technique. The resulting matrix equation is solved numerically, with a guaranteed accuracy, and sufficiently little CPU time is needed. The feed directivity is included in the analysis by the complex source point method. Various characteristic patterns are obtained for the front-fed reflector antenna geometries in this study