Rim loaded reflector antennas

A general theory of reflector antennas loaded by surface impedances is presented. Spatial variation of primary illumination is taken into account using a generalized slope diffraction coefficient. The theory is experimentally checked on surface loaded square plate scatterers and then used for computing the radiation diagram of parabolic and hyperbolic dishes. Computer programs and computed diagrams refer to the case of focal illumination and negligible tapering of primary illumination.     

一维菲涅尔区相位修正平面反射器的聚焦特性研究

在平面上沿某一个方向进行1/P波长的相位修正,可以构成一维菲涅尔区相位修正平面。由此而得到的平面反射器可作为抛物柱面反射器的替代物来构成扇形波束天线。本文用物理光学法分析了平面波入射时该结构的聚焦特性,并给出了一组焦散区场分布的数值计算结果。     

Large lateral feed displacements in a parabolic reflector

The radiation patterns of a parabolic reflector with large lateral-feed displacements are computed utilizing both the vector current method and scalar aperture theory, and compared to experimental results. The theory is general enough to include asymmetric primary pattern illumination. The scalar and vector solutions are derived from the same initial equation so that the approximations used in obtaining the scalar solution are clearly displayed. Results from the vector and scalar theories are compared and the range of validity of the approximate analysis is indicated.     

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.     

Modified diffraction coefficients for focusing reflectors

A modification to the halfplane diffraction coefficients is given for a plane wave incident on a curved screen. The result is used to calculate the near field of a parabolic reflector under plane-wave illumination. Comparison with the physical-optics method shows excellent agreement.     

Efficient computation of the far field of parabolic reflectors by pseudo-sampling algorithm

The newly developed pseudo-sampling representation is applied for computing the far field of an offset parabolic reflector with a cluster feed illumination. A new comb-type fast Fourier transform (FFT) algorithm is used in a computer program. Both precision and computational time are analyzed, demonstrating the excellent performance of the method.     

Some examples of generalized cassegrainian and gregorian antennas

Some theoretical generalizations are given of two-reflector, rotationally symmetric microwave antennas fed by a plane wave across the feed aperture. So far as geometrical optics apply, the proposed designs offer 1) no reflection of energy back into the feed and 2) arbitrary illumination over the antenna aperture. The general solution is exhibited in terms of quadratures, and the reflector shapes for some simple cases are worked out in detail and plotted. Two families of antennas are found which image the feed aperture onto the secondary reflector. These antennas might be expected to have low spillover losses due to diffraction, but none of them is of practical proportions. It is shown in general that such imaging is incompatible with efficient illumination of the secondary aperture.     

Radar cross section of symmetric parabolic reflectors withcavity-backed dipole feeds

The monostatic radar cross section (RCS) of a symmetric parabolic reflector antenna with a cavity-backed dipole feed is computed using the method of moments. At frequencies below the operating frequency band of the antenna the dipole contribution is not significant; in the operating band the dipole terminal load condition only affects the RCS near boresight. The f/D ratio of the antenna is shown to have a significant effect on the RCS. By adjusting the focal length, the cavity and paraboloid scattering contributions can be made to partially cancel, yielding a reduction in RCS near boresight     

Focal region fields of distorted reflectors

The problem of the focal region fields scattered by an arbitrary surface reflector under uniform plane wave illumination is solved. The physical-optics (PO) approximation is used to calculate the current induced on the reflector. The surface of the reflector is described by a number of triangular domain-wise fifth-degree bivariate polynomials. A two-dimensional Gaussian quadrature is employed to numerically evaluate the integral expressions of the scattered fields. No Fresnel or Fraunhofer approximations are made. The relation of the focal fields problem to surface compensation techniques and other applications are mentioned. Several examples of distorted parabolic reflectors are presented and discussed     

Finite difference analysis of electrically large parabolicreflector antennas

A reflector antenna is analyzed using the finite-difference method (FD). The induced current densities on an axially symmetric parabolic reflector are rigorously calculated. The measured equation of invariance (MEI) is used to terminate the FD mesh very close to the reflector surface. To take advantage of the axial symmetry, the theory of coupled-azimuthal potentials (CAPs) is employed. Illustrative results are obtained for reflector antennas with different aperture dimensions. Results by physical optics (PO) approximation are also included for comparison. The purpose of this paper is not to replace ray optics (RO) and PO in the design of reflector antennas, but to demonstrate the advancement in the FD method, which hitherto was limited to low-frequency and closed-boundary regime. The calculated surface current densities of a reflector antenna do show that the normal component of the current densities at the edges exhibits high standing waves which are missing in PO, and which we know should be there. The standing wave of current densities may not affect the main beam, but certainly will have an effect on side lobes and have a major impact in estimating the loss of the antenna     

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.     

Dual reflector antenna with narrow broadside beam for omnidirectional coverage

A new type of high gain omnidirectional antenna is presented, particularly suited for higher frequencies (e.g. millimetre waves): it is obtained with an axially symmetric dual reflector system, fed with an unconventional feed with conical pattern. The feed radiated field, after reflection on the parabolic subreflector and on the conical main reflector, is uniformly scattered in the plane perpendicular to the axis of the system, with vertical polarisation and narrow vertical beamwidth.<>     

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     

The analysis of deployable umbrella parabolic reflectors

The radiation characteristics of an umbrella-type reflector are studied in detail. When the supporting ribs of the quasiparabolic reflector are parabolic in shape and the surface between any two adjacent ribs is the surface of a parabolic cylinder, the deviation of the surface from the true parabolic shape has the effect of spreading the focal point of the parabolic reflector into a focal region, the limits of which can be calculated from the knowledge of the reflector parameters. The best feed position can be accurately determined by requiring that the phase error over the surface be minimized. For the cosine to the powernillumination, numerical results showing the gain degradation, the shift in optimum focal point, and the change in secondary patterns, due to the deviation of the reflector surface from the true parabolic surface, are presented.     

Triple corner reflector antenna and its performance in H-plane

An experimental investigation of a novel corner reflector antenna with improved performance is reported. The corner reflector antenna has been structurally modified by attaching two more reflector elements. The parameters such as the primary corner angle, position, width and angle of the secondary elements have significant effects on the H-plane radiation characteristics of the antenna. Certain symmetric configurations of this triple corner reflector (TCR) antenna provide sharp beams with a gain of the order of 3 dB over that of the conventional corner reflector (CR) antenna     

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.     

Lateral-feed displacement in a paraboloid

The beam shift and degradation of a paraboloidal reflector with an offset feed is analyzed by the scalar plane wave theory. Higher order coma terms are included with the feed at its optimum axial position. The beam characteristics for a tapered circularly symmetric illumination are presented. The range of validity of the approximate analysis is indicated.     

改善单偏置天线交叉极化劣化的C-band馈源

描述了用波纹喇叭中的 HE_(21)模抵消单偏置抛物面天线非对称平面交叉极化分量的基本原理,介绍了 TE_(21) 模激励器、波纹喇叭的设计方法和展宽馈源抑制单偏置天线交叉极化分量频带宽度的设计方法,给出了 C 波段 2．4m 单偏置抛物面天线的实验测试数据,单偏置抛物面天线非对称平面交叉极化电平改善了10dB,抑制交叉极化分量的馈源相对工作频带宽度大于8％。这种 TE_(21)模激励技术和与主模 TE_(11)模相位差的调整方法,对馈源的电压驻波比几乎没有影响。     

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.      

Reflector surface deviations in large parabolic antennas

One of the important factors affecting the efficiency of parabolic reflector antennas is the degree to which the surface of the reflector deviates from the true parabolic shape. For a given reflector surface it is also important to locate the focal point of the best-fit paraboloid relative to the existing feed support structure. A simple method is presented for making these measurements, and results are given for a particular 60-ft diameter reflector. Static measurements were made with the antenna axis pointed vertically upwards, and changes were also measured as a function of elevation angle and wind. When the surface errors have a Gaussian distribution relative to the focal point, simple theory indicates that the received power relative to that from a perfect reflecting surface isP/P_{0} = exp -(2pi sigma / lambda)^{2}wheresigmais the standard deviation of the surface errors andlambdais the operating wavelength.