Offset Reflector Antenna System Research Articles

A Design Method for Mode-Selective Waveguide Couplers in Dual-Polarized Wideband Matched-Feed Antennas

A novel type of multihole directional coupler is presented as well as a design method involving novel procedures. The device couples two orthogonal TE11 modes in a circular waveguide into two orthogonal TE21 modes with specified excitations in a surrounding coaxial waveguide, which has the circular waveguide as the inner conductor. A simple, novel analytical procedure is used to determine azimuthal hole positions, such that each of the input modes couples to only the desired coaxial TE21 mode. The method is applied to a specific coupler which is designed to couple −14 dB to both higher order coaxial modes in the band from 10 to 14 GHz. The suppression of undesired modes is generally better than 40 dB. The coupler is intended for broadband dual-polarized matched feeds, which are able to compensate for the intrinsically high cross polarization of offset single reflector antenna systems.

Wideband Corrugated Conjugate Matched Feed Horn

ABSTRACTThis article presents the design of a wideband corrugated conjugate matched feed horn to cancel the high unwanted cross-polar components generated in offset reflector geometry having very low F/D and high offset angle. A new kind of symmetrical cascaded discontinuities is used in circular waveguide at the throat section of the horn for wideband operation of corrugated conjugate matched feed. The feed horn is specifically designed to mitigate the cross-polarization generated in 90° offset reflector antenna system with F/D = 0.7 and 0.4. The secondary radiation patterns have been computed. Significant improvement in the cross polarization is achieved at 55–60 GHz frequency band.

Matched feed design employing TE01 and TM11 modes in a smooth walled rectangular waveguide for cross-polar reduction in offset reflector antenna systems

Cross polar performance of an offset reflector antenna fed by a smooth walled rectangular matched feed employing TE01 and TM11 modes is presented. When studied with analytical mode field patterns, TE01+TM11 combination is found to give better cross-polar performance compared to combinations like TE01+TE11 and TE01+TE11+TM11. Physical optics (PO) has been used to compute secondary pattern. Mode coefficients and relative phases of different modes are evaluated using Particle Swarm Optimization (PSO) technique or contour plot. A shifted waveguide junction is used to generate the TM11 mode for realization of the matched feed. The desired mode coefficients are realized by proper choice of shifted waveguide dimensions and relative phase is maintained by proper choice of waveguide lengths. The performance evaluation of the complete antenna system is carried out using HFSS. A −30dB cross-polar bandwidth of 2.16% is achieved.

A Novel Shaping Design Technique for the Reflector in Hybrid Antennas

A new reflector shaping technique to improve the beam scanning capability of hybrid antennas (i.e., a reflector system illuminated by a phased array) is presented. Errors between the desired scan directions and the outgoing rays' directions in the scanning range are minimized by adjusting the reflector shape. This technique can be used to decrease the active element number of the feed array during the antenna scans, or reduce the gain loss caused by beam scanning when the number of active elements is given. An offset reflector antenna system is designed and analyzed, through which the validity of the technique is demonstrated.

Fringe-Current Effects on Reflector Antenna Crosspolarization

ABSTRACT The paper deals with an extension of PO by fringe currents at the reflector rim and at slots in the reflector surface. Such slots occur, for instance, in unfurlable reflectors composed of solid panels or in hinged solid reflectors with no electrical contacts between the panels. The fringe-current effects on the antenna far field are evaluated numerically for representative cases. It is found that they are small and primarily influence the crosspolarized radiation. They may in the case of circularly polarized offset reflector antenna systems be the dominant crosspolar contribution due to the reflector alone.

Pattern synthesis of the offset reflector antenna system with less complicated 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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Canonical examples of reflector antennas for high-power microwave applications

The suitability of various radiating systems for generating a directive high-power microwave (HPM) beam is considered. It is concluded that offset reflector antenna systems are well suited for this purpose. An HPM reflector antenna may consist of one or two reflectors that are illuminated by single or array feeds. To accurately predict the performance of an HPM reflector antenna, a versatile analysis scheme is used to compute the fields of an array feed with general configurations, and the diffraction techniques of physical optics (PO) and physical theory of diffraction (PTD) are applied to analyze reflectors of conic or shaped surfaces. Design examples of offset dual-reflector antennas for HPM applications are presented. Analysis results are shown with emphasis on both the near-field and far-field radiation characteristics. Applications of optimization (mathematical programming) techniques to the diffraction synthesis of HPM reflector antenna systems are discussed, and examples are given to demonstrate their effectiveness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Geometric-optic synthesis of dual-reflector antennas with distributed sources

The problem of synthesising a dual offset reflector antenna system when illuminated by a distributed source, to satisfy prespecified conditions on power and phase distributions over the output aperture are examined under the assumptions of geometrical optics. The formulation shows that the synthesis hinges on solving a system of two coupled nonlinear partial differential equations, one of which is the Monge-Ampère equation. The relationship of this study with previous point-source treatments is examined in detail. Uniqueness properties of the design are extracted from the equations and new features introduced by the distributed source are indicated. Possible applications to contoured-beam reflector antennas fed by real sources are envisaged.

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 ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100 \leq D/\lambda \leq 400</tex> ) and multiple beam frequency reuse coverages over an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18\deg</tex> conical field of view. Novel geometrical configurations for the reflectors are characterized by simultaneously achieving: 1) blockage free apertures for all element beams within the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18\deg</tex> conical 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 effective <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f/D</tex> ratio achieved in compact and foldable geometries. A comparison of new front-fed offset Cassegrain (FFOC) and side-fed offset Cassegrain (SFOC) systems is made.

An offset reflector antenna with low sidelobes

The corrugated conical horn is considered to be an ideal feed for low sidelobe reflector antennas because of its unique characteristics. Analysis is carried out to show that low flare horns are preferred over high flare horns to give rise to low sidelobe performance for a given offset reflector antenna system.

Preliminary study on offset scan-corrected reflector antenna system

Preliminary study on offset shaped dual reflector antenna systems has been carried out to assess the feasibility for multibeam satellite applications. The two-dimensional offset shaped reflector antenna geometry is generated by first creating the nodal points according to a bifocal condition and then connecting the nodal points by smooth curves to form the profiles of the main and subreflectors. The three-dimensional geometry is created by body revolution. The offset geometry is obtained by properly tailoring the three-dimensional geometry. This offset shaped reflector antenna system has an inherent astigmatism which can be either fully or partially compensated. For applications requiring a scan range in azimuth more than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm 5</tex> beamwidths, the offset shaped dual reflector antenna systems offer better scan performance (in terms of peak gains) than offset Cassegrain geometries at the expense of the performance of the on-axis beams. In elevation with a 16 beamwidth scan range, the shaped design provides 0.3 dB less scan loss than the Cassegrain design.

A measurement technique for modeling the effects of feed support struts on large reflector antennas

A measurement technique is described in which frequency scaled models of struts are placed in the near-field region of an offset reflector. In this compact range environment the excitation of the strut model is by plane waves, as would be encountered in the axisymmetrical reflector situation. Far-field radiation patterns are recorded, with and without the strut model in place, and, because of the low sidelobe levels associated with offset reflector antenna systems, it is possible to isolate the far-field response of the strut model. This technique is particularly useful for determining the real effects of structures that are difficult to analyze mathematically, such as latticed struts or metallic geodetic radomes.

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.