Offset Parabolic Reflector Research Articles

Dual-band reflector feed system for classical Cassegrain radio telescopes

The authors describe a dual-band feed system, operating at S- and X-band, that can be temporarily mounted and used in a classical Cassegrainian-type radio telescope without removing the existing feed systems. The feed system, designed to give as high efficiency as possible in both bands, consists of two offset primary-fed parabolic reflectors, one for each frequency band. The X-band reflector, located behind the S-band reflector, radiates through a plane dichroic surface that replaces the central part of the S-band reflector. The feed system is to be used for space geodesy utilizing the very-long-baseline interferometry (VLBI) technique and has been tested and used at a radio telescope in Chile. Results of both a theoretical design study and measurements are given. >

Reduction of lower sidelobe levels in a large deployable mesh reflector antenna as a means of decreasing the effective mass of communication equipment

Reducing sidelobe levels and lowering the effective mass of the equipment are important measures in the design of large deployable mesh reflector multibeam antennas used for mobile communication satellites with frequency reuse. In this paper, a method of converting from sidelobe levels to effective mass is described. Then the effects produced by the sidelobe level on the mass of the mesh reflector and the communication equipment are analyzed in the case of a cluster-fed offset parabolic mesh reflector antenna. The surface density of the mesh reflector decreases with an increase in the aperture diameter, and the output efficiency of the solid-state power amplifier decreases with an increase in output power. Next, assuming one transponder for each beam and using the EIRP of all the beams as a parameter, the relations between the sidelobe level, the minimum communication equipment mass, and the antenna aperture are described. The relation between the gain and the number of beams when the communication equipment mass is minimum is derived and the ratio of mesh reflector mass to the total mass of communication equipment is determined.

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Beam squint in parabolic reflector antennas with circularly polarized feeds [Antenna Designer's Notebook

The beam squint phenomenon is an important design issue for modern reflector antennas, because it can affect the antenna pointing performance. For long-distance communications, like satellite or deep-space communications, this effect must be care- fully taken into account in order to avoid any degradation. In some design considerations, one may use the squint phenomenon to one's advantage in separating two RHC- and LHC-polarized beams. Squint was previously observed in offset parabolic reflectors with an on-focus circularly polarized feed [l], such as that shown in Figure 1. It must be emphasized that this squint only occurs for antennas with circularly polarized feed, but not for those for which the feed is linearly polarized.

An efficient plane wave spectral analysis to predict the focal region fields of parabolic reflector antennas for small and wide angle scanning

An efficient approach is described for calculating the field distribution in the focal region of an electrically large, symmetric or offset parabolic reflector antenna with an arbitrary rim contour, when the concave reflector surface is fully illuminated by an obliquely incident arbitrary electromagnetic plane wave. The dominant contribution to the focal-region fields is found by transforming the physical-optics integral over the reflector surface into a plane-wave spectral (PWS) integral. The PWS integral is evaluated rapidly via the fast Fourier transform (FET) algorithm to furnish, in only a single computation, the field for every place in the focal plane (or any plane parallel to it) within the focal region for a given direction of the incident wave. The correction to the physical-optics field is relatively small in the focal region and may therefore be neglected. Numerical results based on this PWS approach are presented, and their accuracy is established by comparison with results based on other approaches. >

Generation of elliptical beams of an arbitrary beam ellipticity and low cross-polarization using offset dual parabolic cylindrical reflectors

It is shown that dual parabolic cylindrical reflectors can be designed to yield elliptical beam shapes with an arbitrary beam ellipticity, defined as the ratio of beam widths in the principal planes. The only limitation affecting the beam ellipticity is the practical geometry of the antenna in a particular use, where certain feed or sub-reflector dimensions may not be desirable. To confirm computations, an experimental unit was designed to yield a beam ellipticity of about 12, and was fed by a dielectric rod feed to reduce the aperture blockage. The measured patterns closely match the computations. An offset configuration which may be used to eliminate feed blockage is also investigated. Its performance is also verified by an offset experimental unit.

A dual polarized line source feed for cylindrical reflector antennas

A line source feed for an offset cylindrical parabolic reflector antenna is described. The line source consists of two transverse and longitudinal dipole arrays over a ground plane. The elements of each array are fed separately from a feed waveguide through a coaxial line which is connected to a probe inserted into the waveguide. Within each waveguide these probes form a tapered array. The probes are treated as an N-port network to determine their penetration depth for a required Taylor distribution along the line source. The line source is then constructed and tested for both aperture field distribution and the input impedance. Its performance as a feed for an offset cylindrical parabolic reflector antenna is also studied.

Comparison of induced current and aperture field integrations for an offset parabolic reflector

The fields predicted by integration of the physical optics induced currents on the reflector (ICM) and the geometrical optics aperture fields (AFM) on the surface that caps the reflector are compared numerically for offset parabolic reflectors. It is found that the AFM solutions are very close to ICM solutions for the main beam and the first few sidelobes but discrepancy remains for the far-out sidelobes. This discrepancy is found to come mainly from the difference of polarization matrix for each method.

A linearly-polarized dual-band diplexer in an offset reflector

This paper assesses the performance of a dual linearly polarized 45° incidence frequency selective diplexer in an offset parabolic reflector. Three different plane frequency selective surfaces have been studied—single layers of concentric rings and double squares, and a four—layer surface consisting of capacitive patch arrays. All three give operating bands near 22 GHz and 32 GHz. The peak linear crosspolar levels of the diplexer alone and in the offset reflector are discussed, together with the copolar sidelobe performance of the complete antenna.

PERFORMANCE AND DESIGN PROCEDURE OF OFFSET DUAL PARABOLIC CYLINDRICAL REFLECTORS

ABSTRACT The offset dual parabolic cylindrical reflectors are analyzed. They generally produce elliptical beam patterns with unequal beamwidths in the two principal planes. The dependence of the principal plane beamwidths, the peak cross-polarization level, the antenna gain and the first sidelobe level on the antenna geometrical parameters, and in particular the offset angle, is studied. The main effect of the offset angle is increasing the cross-polarization level. However, for any offset configuration, the angle between the two reflectors has a strong effect on the cross-polarization. Optimizing this angle, the peak cross-polarization may become negligibly small, and as low as -60dB. A design procedure for such an offset configuration is provided and a design example Is given.

Reconfigurable dual feed antenna for direct broadcast satellites

Direct Broadcast Satellites covering large countries such as Canada, require more than one spacecraft, located at different orbital positions, as well as the use of multiple shaped beams. This would minimize eclipse requirements over several time zones, provide increased capacity by frequency reuse and permit the use of cost effective ground receivers. Two such satellites are envisaged, one covering Eastern Canada, the other Western Canada, using two different sets of three highly shaped beams. This paper is the result of a feasibility study of a satellite antenna designed such that while at either orbital location it can be reconfigured in orbit, by ground command and hence can save the cost of one additional spare spacecraft. An offset parabolic reflector is proposed for the 12 GHz downlink, with a switched “dual feed” structure, consisting of two separate but contiguous sets of pyramidal horns and their associated beam forming networks (BFN). Only one BFN set is used at any one orbital location. Detailed radiation patterns demonstrate good beamshaping capabilities, with coverage efficiencies of the order of 94%. Other considerations such as the effect of orbital locations, gain equalization and TWTA standardization are discussed. It is concluded that a satellite, reconfigurable in orbit with a “dual feed” antenna, is feasible and cost effective, for a DBS spare as well as for the main spacecraft.

The ohio state university compact radar cross-section measurement range

The ElectroScience Laboratory at The Ohio State University has recently installed a new radar cross-section measurement system. The system uses a Scientific Atlanta compact range reflector (an offset parabolic reflector) to generate a far-field plane wave in the confines of a 40 \times 20 \times 60 ft anechoic chamber. The system uses a computer controlled microwave frequency synthesizer and a multichannel computer controlled receiver. The target support/positioning system and an optical target alignment system are also interfaced to the computer. The parameters of the system are 1) operation from 1 to 30 GHz (eventual operation to 96 GHz has been confirmed by field probing); 2) a plane wave volume (test area volume) of 1.3 m diameter; and 3) a sensitivity of -50 dBsm. Of particular importance is the ability of the system to measure phase as well as amplitude. This permits vector subtraction of the background and system calibration using a reference (sphere) target. The development of this system and the performance characteristics obtained so far will be discussed. Some results which demonstrate the system performance will be shown. Of particular interest is the broad-band measurement of both amplitude and phase. This permits conversion of the results to the time domain. Examples will be shown in which the various system components (antennas, reflector, walls, ceiling, etc.) are separated in the time domain by this technique. The development of the range is continuing and planned future improvements will also be discussed.

A comparison between GO/aperture-field and physical-optics methods of offset reflectors

Both geometrical optics (GO)/aperture-field and physical-optics (PO) methods are used extensively in the diffraction analysis of offset parabolic and dual reflectors. An analytical/numerical comparative study is performed to demonstrate the limitations of the GO/aperture-field method for accurately predicting the sidelobe and null positions and levels. In particular, it is shown that for offset parabolic reflectors and for feeds located at the focal point, the predicted far-field patterns (amplitude) by the GO/aperture-field method will always be symmetric even in the offset plane. This, of course, is inaccurate fur the general case and it is shown that the physical-optics method can result in asymmetric patterns for cases in which the feed is located at the focal point. Representative numerical data are presented and a comparison is made with available measured data.

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 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.

Performance of offset parabolic reflectors illuminated by matched feeds

The theoretical concept of applying matching primary feed devices to illuminate offset parabolic reflectors is studied. The physical interpretation of how this new class of feeds compensates for the geometric asymmetry of the offset system is explained. An economical technique to investigate the corresponding improvements in the near-axis radiation is given by suitably extending the modified stationary phase method, suggested in the literature, to the added modal behaviour of these feeds. The analytical developments and their respective novel results are accumulated relative to the main and basic parameters of offset antenna systems. The presentation is given in some detail so that it may be a useful paper for a wider group of readers.

Fields in the focal region of offset parabolic antennas

An analysis which provides the electric field distribution in the focal region of an offset parabolic reflector as a function of small scan angle has been performed. Formulae which predict both the principal and cross polarized fields have been obtained. These formulae are relatively simple and compact, provide a useful design tool, and allow efficient computer calculation. The results obtained from these formulae compare favorably with results obtained by other authors.

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.

Multiple-beam antennas: Offset reflectors with offset feeds

Two simple mathematical models are described for the prediction of the vector radiation fields from offset parabolic reflector antennas with offset feeds. Experimental support for the predictions obtained from the models has been obtained by comparisons with measured data from antenna systems operating at 30 GHz. The principal radiation characteristics of the offset-reflector with offset-feed configuration are discussed.

Radiating Mechanisms in a Reflector Antenna System

The radiating mechanisms of a reflector antenna system are discussed. Means of computing the various component fields are given, including aperture radiation, direct feed radiation, and diffracted radiation. An offset parabolic reflector antenna system fed with a corrugated horn is treated as an example. Means of reduction of the undesired components of radiation are suggested.