Cross-polarization in satellite and earth-station antennas

Cross-plarization in axially symmetric reflector antennas can be reduced, theoretically, to zero by use of special feeds like the Huygens' source. Alternatively, paraboloidal reflectors with large f/D ratio do not deteriorate further the cross polarization level relative to the value due to the feed itself. The Cassegrainian optics is equivalent to a large f/D paraboloid. The reflector of linearly polarized off set fed antennas contribute more cross-polarization than symmetrical reflectors fed by the same feed. With symmetrical reflectors the cross-polarized component generated by the reflector vanishes in the principal planes and is confined to four main lobes that have peak values in planes at 45° to the principal planes. In the case of offset fed reflectors cross-polarization vanishes in the plane of symmetry and has its peak in the plane of asymmetry. The reflector generated cross-polarization with offset fed antennas may be reduced by use of small offset angles and large f/D ratios. Feed offsetting has but little effect on the peak level of cross-polarization. This is usually accompanied with an asymmetry in the cross-polarization radiation pattern. Feed offsetting also results in spatial tilt in the copolarized and cross-polarized lobes with the cross-polar minimum always coinciding with the main beam peak. The effect of surface errors on the antenna cross-polarization is to partly fill the cross-polar along boresight. The peak cross-polarization, however, changes but slightly.     

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.     

Axial cross polarization in reflector antennas with surface errors of large correlation diameter

Equations for boresight cross polarization and isolation of axisymmetric and offset antennas in the presence of surface errors are derived in terms of numerically computable integrals. Computations revealed that 1) for root mean square (rms) error< lambda/4cross polarization increases monotonically with increasing rms error; and 2) maximum cross polarization occurs when the correlation diameter is 0.5Dfor axisymmetric antennas (0.8Dfor offset antennas). For rms error>lambda/4boresight cross polarization remains almost constant. Furthermore, results revealed that for given correlation diameter and rms error, axisymmetric antennas offer better boresight isolation in comparison with offset antennas having the same polarization efficiency.     

On the cross polarization of asymmetric reflector antennas for satellite applications

It is well known that focussed, axial symmetrical reflector antennas collimate the co- and cross-polar components of the primary field separately, i.e., the reflector does not create a contribution to the cross polarization of the far-field. By a simple extension of a classical physical argument it is demonstrated that this separability does not depend on the symmetry of the antenna, and that it, therefore, holds even for off-set fed reflectors. A new mathematical formulation of the collimation is derived in which this is shown. Yet the separability does depend on how the co- and cross-polar fields are defined, and the cross polarization of feeds for asymmetric reflectors is discussed in detail in the light of this. It is further suggested how to design low cross polarization feeds for off-set fed antennas. As a consequence of the separate collimation such feeds will lead to low cross-polarization of the secondary fields. Two simple examples are treated. The only limitations of the results are those due to the application of the aperture field version of the physical optics approximation.     

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.     

A parametric study of the constraints related toGregorian/Cassegrain offset reflectors having negligible crosspolarization

A parametric study of the constraints related to Gregorian/Cassegrain offset reflectors having negligible cross polarization was undertaken with the aid of design equations linking the defining parameters of the primary reflectors to those of the secondary reflectors. This study resulted in the mapping of a very large number of Gregorian/Cassegrain offset reflectors having negligible cross polarization. The domains of definition of the equations derived are deduced from geometrical considerations. Physical insights into the above constraints are presented     

Reflector radiation pattern from planar near-field measurements of array feed

High gain shaped beam antennas for satellite frequency reuse applications are almost exclusively obtained by the use of complex multielement feed arrays to provide pattern control in conjunction with offset reflectors to remove blockage effects. In the design of complex multielement feed arrays for offset reflectors, the element excitations are usually synthesized using the isolated element properties. Proper performance of the array often requires that these theoretical excitations be modified to account for the effects on the feed elements due to the array environment. Near-field planar probing of the fields of the feed array have been found to provide an efficient and accurate method of predicting the secondary performances, including cross polarization and axial ratio. The nearfield measurement technique, moreover, provides an extremely effective method of determining the element performance and for determining the required compensation for desired antenna performance.     

Dual offset reflector multibeam antenna for international communications satellite applications

Dual offset reflector antenna systems offer exciting possibilities for achieving both low scan losses and low cross polarization in geosynchronous communications satellite antennas providing narrow (100 leq D/lambda leq 400) and multiple beam frequency reuse coverages over an18degconical field of view. Novel geometrical configurations for the reflectors are characterized by simultaneously achieving: 1) blockage free apertures for all element beams within the18degconical field of view, 2) compatibility with large planar feed arrays, 3) additional degrees of design freedom by orientation and shaping of reflector surfaces for depolarization and scan loss optimization, and 4) large effectivef/Dratio achieved in compact and foldable geometries. A comparison of new front-fed offset Cassegrain (FFOC) and side-fed offset Cassegrain (SFOC) systems is made.     

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.     

Offset dual-reflector multiple-beam antennas using circularly symmetric main reflectors

An investigation into the multiple-beam capability of offset antennas using circularly symmetric main reflectors is described. The efficiency and sidelobe performance of systems designed to produce beams up to 4.6° either side of boresight with reflector diameters ranging from 100 to 300 wavelengths are shown to make the antennas suitable for multiple-satellite-access earth stations. Comparison is made with a nonsymmetrical main-reflector antenna designed for the same requirements.     

Parametric design and analysis of multiple-beam reflector antennas for satellite communications

This paper presents the parametric design and analysis of multiple-beam reflector-antenna systems employed for satellite communications. It is based on extending the earlier work of Rao (see IEEE Antennas and Propagation Magazine, vol.41, no.4, p.53-59,1999) by taking into account the efficiency of the horn and pointing error of the satellite in the design of the multiple-beam antennas (MBAs), and by analyzing the edge-of-coverage directivity and co-polar isolation (C/I) performance. Design and analysis equations are developed for the multiple-beam antennas using offset parabolic-reflector antennas by including various design parameters such as the number of reflectors, the number of frequency cells, the focal-length-to-diameter (F/D) ratio, the horn efficiency, and the pointing error. The analysis employs a quasi-Gaussian beam representation for the primary and secondary patterns in order to take into account the effect of the sidelobes. Results of the analysis given in this paper agree well with rigorous computations based on physical optics analysis of the antenna radiation. Design curves showing the impact of horn efficiency on the C/I performance of multiple-beam antennas are presented for various frequency-reuse schemes.     

Design of offset-parabolic-reflector antennas for low cross-pol andlow sidelobes

The results of a computational study on the radiation properties of offset prime-focus-reflector antennas are reported. Emphasis is on reducing sidelobes and cross polarization without sacrificing gain. The influence of feed pointing is examined in detail. A simple technique is introduced for determining the feed-pointing direction. This technique yields near-minimum sidelobe levels over practical ranges of feed pointing, with only small penalties in gain and cross polarization     

Performance and design procedure of dual parabolic cylindricalantennas

The radiation characteristics of dual parabolic cylindrical antennas are studied, and the dependence of the principal plane beamwidths and the peak cross-polarization on their geometrical parameters is determined. The antenna aperture is rectangular in shape and generates an elliptical beam pattern, with a beamwidth ratio that can be controlled by the main and subreflector focal lengths. The far-field patterns are determined by an extended aperture integration method that includes the contributions of the reflected and the main diffracted rays. It is found that the cross-polarization depends of the offset angle between the axis and the direction of the normal to the subreflector surface and can be minimized by optimizing the relative angle between the reflectors. Other pattern characteristics are controlled by the antenna geometrical parameters and the feed illumination. A procedure for the design of these antennas and the expressions for determining the reflector geometries are provided     

The current state of the reflector antenna art

The current state of the reflector antenna art is presented in the form of a review article, intended primarily for engineers who are new in the field. The properties of paraboloidal antennas are related to aperture efficiency, surface tolerance, feed defocusing, and aperture blocking. Other reflector shapes considered are offset-fed paraboloids, classical and offset Cassegrain systems, shaped-dual reflectors, and scanning reflectors. Properties of contoured-beam systems are reviewed. Structures and materials are reviewed for ground stations and satellite-borne reflectors. Emphasis is placed on specific hardware examples for each principle or concept. Near-field probing, dichroic surfaces, and techniques of surface metrology are presented as examples of promising areas for new developments.     

A broad-band twist reflector

Polarization twisting reflectors, or twist reflectors, consisting of a wire grid spaced approximately3 lambda/8from a ground plane [1], [2] have a bandwidth of at most 30 percent. It is shown that the inclusion of an additional wire grid in front of the first grid makes bandwidths of 100 percent possible. Design formulas, theoretical performance in terms of cross polarization attenuation, and some experimental results for a twist reflector with two wire grids are given.     

Coaxial feeds for high aperture efficiency and low spillover of paraboloidal reflector antennas

Coaxial feeds produce an approximate sector-shaped pattern, an almost optimum pattern of a feed for high aperture efficiency and low spillover of paraboloid antennas. Such a coaxial feed consists of a central circular waveguide which is surrounded by one or more conductors with circular cross sections. Theoretical and experimental investigations on coaxial feeds excited by H11modes have shown that the first ring yields the highest increase in the aperture efficiency of paraboloid antennas illuminated by them. Measurements performed on paraboloid antennas illuminated by a coaxial feed with only one ring yielded aperture efficiencies of 68 to 75 percent for angular apertures of the paraboloidal reflector of100degto160deg. Circularly symmetric patterns in conjunction with almost linearly polarized aperture fields can be achieved by multimode coaxial feeds. The values for the aperture efficiency, which are calculated for paraboloid antennas illuminated by multimode coaxial feeds, nearly reach the theoretical optimum. The measured values are 68 to 80 percent. In addition, the multimode feeds produce very little cross polarization.     

Dual-offset antennas with a spherical main reflector for smallearth stations

The design of dual-reflector antennas with a spherical main reflector for small earth stations is considered. An analysis of the field polarization throughout the system shows that it is possible to adjust the reflectors to obtain very low cross-polarization. The use of an elliptical main reflector projection is explored in order to enhance efficiency and lower side-lobe levels     

Antenna-polarization and terrain-depolarization effects on pulse-radar return from extended areas at the near vertical

The methods now in the literature for calculating the terrain-backscattered radar-altimeter echo utilize the antenna power-gain patterns and the terrain radar cross section per unit area, usually designated σ0. In this paper, the polarization characteristics of the antennas and the echo depolarization by the terrain are taken into account for the first time. The formulas for the backscattered return are presented in terms of antenna and terrain parameters that can be measured directly or calculated from other measurements. These parameters are the four transmitting- and receiving-antenna gain patterns for the orthogonal polarization components, the two functions giving transmission- and reception-phase difference between the polarization components, the terrain-depolarization factor that accounts for the average backscattered power cross polarized with respect to the incident wave, and the radar cross section σ0per unit area. It is shown that the older formulas used for radar return from the near vertical are correct only when both antennas are linearly polarized and have the same direction of polarization. The errors that result from failure to consider polarization effects are illustrated by examples.     

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。     

Slot stripline circular-polarization antennas for a few-element antenna array for a high-accuracy receiver of signals from global navigation satellite systems

New radiators for slot stripline leaky-wave circular-polarization antennas for high-accuracy GLONASS/GPS receivers have been developed. The main technical characteristics of antennas with new radiators have been theoretically and experimentally studied. It is shown that application of the new radiators improves the axial ratio, increases suppression of the cross polarization, increases the stability of the antenna phase center, and thus can increase the GLONASS/GPS positioning accuracy. The results of the analysis of the GLONASS/GPS positioning accuracy with the use of these antennas in an angle-measuring receiver and as elements of a few-element antenna array are presented.