Comments on "The analysis of deployable umbrella parabolic reflectors"

In a recent paper (see abstr. B28664 of 1972), Ingerson and Wong show, using geometrical relations, that the entire deployable reflector can be thought of as being made up of an infinite number of parabolas. They give an expression for the focal length of this infinite collection of parabolas and then state incorrectly that `The infinite collection of parabolas has a continuous distribution of foci. Thus, if a primary feed is placed in focus for one parabola, it is out of focus for the rest of the collection'. This statement is evidently the result of improper application of geometrical optics (surface normals are not taken into account) and provides a wrong physical picture of the focusing and the axial defocusing in the umbrella parabolic reflectors     

Experimental verification of the analysis of umbrella parabolic reflectors

Gored umbrella parabolic reflectors with symmetric aperture illumination have been treated analytically in a recent paper. This communication extends the analysis to the case where the primary feed illumination is not symmetric, because real primary feeds usually have unequalE- andH-plane beamwidths. The gore loss and the shift in the defocusing curve obtained using the modified gain expression are in excellent agreement with values measured for a smooth parabolic reflector and a gore parabolic reflector of the same diameter.     

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.     

Focal-plane field distribution of parabolic reflectors

The principal component of the parabolic-reflector focal-plane electric-field distribution is derived from scalar wave theory in a form providing a simple analytic representation of the field in terms of available tabulated functions not restricted to large focal-length/diameter ratios.     

Beam-scanning primary feed for parabolic reflectors

A system is described which achieves wide-angle beam scanning from a parabolic reflector antenna by movement of an adaptive primary-feed system about a defined locus. The feed system performs a sampled spatial Fourier transformation of the focal-plane field in order to obtain a uniform power distribution prior to phase compensation and summation. Scanning of ±15 beamwidths with less than 0.5dB loss in gain and negligible beam distortion has been demonstrated experimentally.     

Scan limits of off-axis fed parabolic reflectors

The scan limits of point-fed paraboloidal and line-fed parabolic-cylinder reflector antennas are derived for large reflectors of arbitrary focal-length-to-diameter ratio (F-number). A numerical approach is employed which incorporates the effects of all higher order spatial distortions (ray aberrations) normally neglected in previous analyses. The results demonstrate that paraboloids of largeF- number have substantially less scan capability than could be inferred from earlier analyses.     

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     

Parametric study of the crosspolarisation efficiency of parabolic reflectors

The crosspolarisation efficiency of a reflector antenna is the ratio between the peak crosspolarisation from the complete antenna to that from the feed. The results of a parametric study into this factor for parabolic reflectors fed by a mathematical model of a common class of feeds are presented, and it is concluded that the efficiency is typically ?4 to ?6 dB.     

Wide field of view imaging with parabolic cylindrical reflectors

Parabolic cylindrical reflectors (PCRs) can have a large field of view, for example a 15 m reflector can access an area of 1/spl deg/ by 120/spl deg/ at 1.4 GHz. It is seen that the radiation pattern is asymmetric. This causes problems in situations where the radiation pattern rotates with time, such as radio astronomy. The rotation of the radiation pattern limits the field of view. In this paper a normalized signal to noise ratio is calculated for a full observation and this is then used to determine the field of view. The greatest variation occurs where the radiation pattern of a single feed element is used. The variation is reduced by RF beamforming. If the radiation pattern has an aspect ratio of 2.5:1 the observed field of view is approximately constant and has a value that is three times larger than that of an equivalent parabolic dish.     

Dual parabolic cylindrical reflectors employed as a compact range

Compact ranges using dual parabolic cylindrical reflectors are investigated, and the dependence of the aperture field on the feed pattern and system geometrical parameters is studied. A uniformity factor is defined to indicate the aperture field uniformity and the significance of the diffracted fields is explored. Offset configurations are also considered and studied. It is shown that their feed location and orientation can be optimized to minimize the geometrical-optics cross polarization. The effects of offset angle on the edge diffraction and aperture shape are also studied     

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     

Uniformity factor of a compact range using dual parabolic cylindrical reflectors

For a dual parabolic cylindrical reflector configuration employed as a compact range, a uniformity factor is defined to indicate the uniformity of the aperture field. The effects of the reflector geometrical parameters on this uniformity factor are studied and some representative results are presented.     

The extended aperture method for the determination of the shadow region radiation of parabolic reflectors

In this paper the problem of the shadow region radiation of reflectors with circular apertures is examined. The approach is based on an extension of the aperture method of calculating antenna patterns. Using this extended aperture method and assuming that the reflector is an absorbing screen, the shadow region pattern within small angles from the axis of the reflector is determined. These methods are applicable only to cases where the wavelength is small with respect to the dimensions of the reflector. The main conclusions are the following: the shadow region field is primarily dependent on the illuminating field which is present on the outer edges of the reflector. For a uniform illumination the front to back ratio is of the order of[(2pi/lambda)rho]^{2}(wherelambdais the wavelength andrhothe radius of the aperture).     

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 results of electronic compensation for surface profile errors in parabolic reflectors

This letter describes the basic principles of a proposed system for compensating for the effect of surface profile errors in parabolic reflectors. It also describes some experimental results which verify the technical feasibility of the proposals and discusses some implications of its future development.     

The rapidly deployable radio network

The rapidly deployable radio network (RDRN) is an architecture and experimental system to develop and evaluate hardware and software components suitable for implementing mobile, rapidly deployable, and adaptive wireless communications systems. The driving application for the RDRN is the need to quickly establish a communications infrastructure following a natural disaster, during a law enforcement activity, or rapid deployment of military force. The RDRN project incorporates digitally controlled antenna beams, programmable radios, adaptive protocols at the link layer, and mobile node management. This paper describes the architecture for the RDRN and a prototype system built to evaluate key system components     

The AM umbrella antenna

One of the greatest limiting factors in locating an AM broadcast tower is the significant amount of acreage occupied by the buried radials. The authors describe a novel method of feeding a grounded tower which overcomes this difficulty, since essentially no land is required for the earth connection. Computer-predicted values of the radiated electric-field intensity are competitive with those generated by conventional techniques. A scale model has been constructed, and measured results are discussed in comparison with computed values     

多模块可展机构运动学分析及驱动设计

由于可折展机构在空间任务中的展开需要,其运动学分析和驱动设计至关重要。根据空间多模块可展开机构中单个模块的周向周期性及多模块连接的非完全周期性特点,首先基于复数法和机构单元的拆分思想进行了单个模块的运动学建模,同时进行了满足曲面拟合条件的多模块展开机构的几何建模,并进行了多模块可展开机构的运动学分析,得到多模块联动过程中运动部件的位置与模块间偏角的关系曲线,随着滑块位移的增大,外层模块与内层模块的偏角增大,最大值为1.22°。基于可展开机构的运动学分析,得到不同驱动方式下滑块和铰链在机构展开过程中的受力变化曲线,进行展开机构的驱动分析及低质量和高可靠性的驱动设计,为展开机构的顺利展开提供保障。     

Diffraction analysis of line feeds for spherical reflectors

The diffraction limitations of the line feeds for spherical reflector antennas are analyzed by means of an asymptotic transition region theory (TRT). It is shown that diffraction from the ends of the line feed causes a broad 6-dB-deep central dip in the aperture field of the reflector. The corresponding reduction in aperture efficiency and increase in spillover are also calculated by means of the TRT. The accuracy of the theory is checked by numerical evaluations of the array sum expression for the radiation field and of the secondary aperture-field and spillover integrals. The results are applicable to the line feeds of the spherical reflector antenna of the Arecibo Observatory