Dual-reflector Antenna System Research Articles

Low-Cost 3-D Millimeter-Wave Concealed Weapons Detection Using Single Transceiver Affine Synthetic Arrays

The problem of detecting concealed weapons with a low-cost 3-D imaging radar system is addressed. The concept of affine phase center translations to generate a synthetic array is proposed as an alternative to conventional synthetic aperture and multistatic imaging radars. This enables the generation of a synthetic 2-D array with a dynamic dual reflector antenna system and a single transceiver. The dual reflector antenna is composed of a 3-D printed ellipsoidal sub-reflector and a conic main reflector. The phase front of the horn feed is shown to be translated to different locations by controlling the relative positions of the two reflectors. The reflectors are fabricated using a simple protocol that keeps the cost very low but introduces significant phase aberrations. These phase aberrations are corrected by developing a novel geometrical optics based optimized calibration routine. The imaging system is tested using a 76.5–84.5 GHz stepped frequency transceiver in different concealed weapons detection scenarios. As a demonstration, a metallic handgun and a nonmetallic pipe-bomb are concealed on a mannequin which serves as a human surrogate with similar skin reflectivity. At a target distance of 2.8 m the achieved depth resolution and lateral resolution are 2 and 1.5 cm, respectively. The 3-D images constructed using back-projection are of high enough quality to successfully identify the weapons while maintaining the privacy of the individual.

Method for Calculating Radiation Characteristics of Dual-Reflector Antennas with Resonance Size Reflectors of Finite Thickness and Conductivity

The paper presents a method for calculating the radiation patterns (RP) of dual-reflector antenna systems taking into account the electrodynamic interaction between reflectors. This method is suitable for use in the resonance range of wavelengths with respect to the main dimensions of reflectors. The method is based on the iteration procedure proposed by the authors. Each iteration involves the use of the method for calculating the scattering of electromagnetic wave by unclosed screens of finite thickness and conductivity that is suitable for use in resonance range. The specified method is based on applying the E-field integral equation that takes into account the Leontovich approximate boundary conditions on impedance surface and makes it possible to take into account the real thickness and conductivity of antenna’s reflectors. The paper presents the results of calculating RP of dual-reflector antennas built in accordance with the Gregorian scheme for different sizes of small reflector aperture and different arrangements of feeder at the fixed size of large reflector. The paths of reducing the level of the first side lobes of RP are shown. The specified reduction is achieved at the expense of the feeder proper placing with respect to the small reflector of antenna.

A Reconfigurable Reflector Antenna System With a Hybrid Scanning Method: Imaging antennas for simultaneous multiple spot and wide coverage beams

On-orbit beam reconfiguration and beam scanning are the two most important features for future satellite antenna payloads. There are several ways of achieving the desired flexibility using different types of antennas and different scanning methods. This article presents single- and dual-reflector antenna systems where reconfiguration is achieved using high-gain multiple beams (HGMBs) employing imaging reflector configurations. Beam scanning over the global coverage is achieved using a combination of precise electronic scanning over a small coverage region and a coarse mechanical scanning over a wide global coverage. This method results in significant improvement in gain relative to conventional methods that employ element beams and can simultaneously provide both HGMBs and wide-area coverage beams with low-cost payloads.

Phase error analysis of displaced-axis dual reflector antenna for satellite earth stations

In a classical Cassegrain geometry, the sub-reflector partly blocks reflected rays from the main reflector. Feed antenna of the subreflector also blocks some of the radiated energy towards main reflector. In a properly designed displaced-axis dual-reflector antenna (DADRA) system based on axially-displaced ellipse (ADE) sub-reflector configuration, these blockage losses are minimized. Thus, with its high gain and low return-loss, DADRA system is a strong candidate for large scale earth station antennas. In general, to achieve maximum gain using a single-reflector system, the phase center of the feed antenna should be placed at the focus of the paraboloidal reflector. However, phase center of a feed antenna moves with the change of frequency. Phase error loss is not avoidable, especially in wide band applications. In this work, phase error losses due to axial defocusing of a DADRA system designed for satellite earth station are analysed in Ku-band. The system is fed by a corrugated horn. Phase center stability of the feed over a wide frequency band is exhibited resulting in negligible loss levels. The results are compared with that of a single-paraboloidal reflector fed by the same horn. Measured and simulated patterns of DADRA system are presented.

Ray tracing technique for shaping a dual reflector antenna system

In radio astronomy applications, antennas having the highest aperture efficiencies are requested. In order to meet this request, a reector shaping program for dual reector antennas has been developed. The design procedure is based on the geometrical optics where differential equations have been formulated to dene the ray trace of the antenna system. Recently, the fourth generation programming language MATLAB has become well known for its numerical computing environment and very useful three-dimensiona data visualization functions. The authors wished to make a dual reector shaping program by a ray tracing method in MATLAB. In this paper, details of the program are explained, and antenna ray tracing results are shown visually by three-dimensiona viewgraphs. The numerical results are compared with the theoretical ones to conrm the correctness of the developed shaping program for a Cassegrain dual reector antenna system. For uniform aperture distribution therst side lobe level is numerically obtained as {17.59 dB, which can be further reduced through shaping to {24.67 dB and {30.65 dB for parabolic and squared parabolic, respectively.

Analytic transient analysis of radiation from hyperboloid reflector antennas via surface curvature continuation of ellipsoidal surfaces

A transient analysis of a hyperbolic reflector antenna is performed based on a mathematic continuation of surface curvatures of an ellipsoidal reflector antenna with the analysis previously performed. This work makes the time-domain analysis useful for the design of reflector antennas since both ellipsoidal and hyperbolic reflectors are widely used as sub-reflectors for dual-reflector antenna system. In particular, this work interprets the scattering phenomena in terms of reflected and diffracted fields with respect to the frameworks of geometrical theories of diffraction, and allows one to analyse the reflector based on the local scattering mechanism.

Reflector Antenna Distortion Compensation Using Sub-Refl ectarrays: Simulations and Experimental Demonstration [AMTA Corner

This paper demonstrates main-reflector antenna-distortion compensation using a novel sub-reflectarray compensation technique, through extensive simulations and measurements on a symmetric dual-reflector antenna system at Ku band. A sub-reflectarray Cassegrain system was used for the study. The distortion was created by adding an annular metallic ring to the main refl ector's surface. This ring-type distortion created pattern degradation and performance deterioration of the dual-reflector system. Back-projection holography was used to study the effects of the distortion on the antenna's performance. The conjugate-fi eld-matching method was used to obtain phase excitations at the sub-refl ectarray element locations for distortion compensation. A prototype microstrip-patch-type sub-refl ectarray was designed and fabricated. Radiation-pattern measurements and holographic diagnostics demonstrated the compensating technique, and showed that the sub-refl ectarray drastically improved the antenna's performance.

Offset Dual-Reflector Antenna System Efficiency Predictions Including Subreflector Diffraction

A simple analytical formula for the prediction of diffraction efficiency in offset dual-reflector antenna systems is presented, and its performance evaluated by comparison to commercial software physical optics and method-of-moments simulations. The formula only requires knowledge of the symmetry plane physical configuration of the reflectors and an analytical approximation of the feed radiation pattern, and is therefore very straightforward to implement and use as a design aid. Some example applications of the formula are presented and discussed.

The polarisation ellipse and associated properties

The ellipse is well known to students of mathematics at an advanced high school level and at an undergraduate level through its associations with, say, conic sections and their deployment as hardware in dual reflector antenna systems. Perhaps less well known is its appearance on the software side, so to speak, for example in RF (Radio Frequency) engineering in the context of polarisation, where it is known as the polarisation ellipse. It is the purpose here to discuss this manifestation and dwell particularly on the concept of a circumscribing rectangle for this ellipse because such aspects are of interest to the RF engineer who may be involved with measurements to determine its properties. This exercise affords an interesting insight into a very practical application of tertiary level mathematics and whilst it is of course helpful to appreciate the physical concepts involved, a lack of such knowledge does not detract from an appreciation of the underlying sums, which should be within the grasp of the above, interested student.

Extension of Gaussian Beam Techniques for the Rapid Analysis of Electromagnetic Radiation/Scattering from Dielectric Reflector Antennas

A highly efficient Gaussian beam (GB) method was recently developed to provide a relatively rapid analysis of large, perfectly conducting reflector antennas. This GB method is very efficient because it represents the feed radiation in terms of a set of very few rotationally symmetric GBs and provides an essentially closed form solution for the reflection and diffraction of each GB incidence on the reflector surface. This method completely avoids the time-consuming numerical integrations in the evaluation of radiated fields over the large reflector surfaces that are generally required in conventional approaches of the physical optics and aperture integration techniques. This paper extends previous works to treat reflector antennas with dielectric surfaces that derive from many practical applications, including, for example, the analysis of the scattering from sub-reflectors having frequency selective surfaces in a dual-reflector antenna system. Numerical examples are used to validate this extended GB approach.

Scanning of an offset dual-reflector antenna pattern through subreflector movement: Translation versus rotation

In this paper, a comparison is made of the effectiveness of sub-reflector translation and rotation as a means of steering the beam of a classical dual-reflector antenna system. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 338–342, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20628

Design of a dual-reflector antenna with nonuniform amplitude and phase distributions

A synthesis method based on geometrical optics to design a dual-reflector antenna system with maximum power transfer efficiency is presented. The system has nonuniform phase and amplitude distributions on the aperture. In our design, the effect of aperture blocking is also considered. © 1996 John Wiley & Sons, Inc.

Scanning properties of large dual-shaped offset and symmetric reflector antennas

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss.< <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.

Scattering Analysis of Dichroic Subreflectors

ABSTRACT This paper presents an efficient technique developed to analyse dual reflector antenna systems operating with double curved dichroic subreflectors. The analysis is based on the Physical Optics integration of proper equivalent currents computed on the subreflector surface. The possibility of determining the optimum position of the dichroic subreflector in the antenna system, taking conveniently into account its finite thickness, is also discussed and evidenced through a meaningful example of design analysis. The same example gives to the reader an idea about the high performance achievable by dichroic antennas.

An offset-fed reflector antenna with an axially symmetric main reflector

A design method for an offset-fed, dual reflector antenna (Cassegrain type or Gregorian type) system with an axisymmetric main reflector is presented. Geometrical optics (GO) and the geometrical theory of diffraction (GTD) are used to find the surface-current density on the main reflector. A modified Jacobi-Bessel series (JBS) method is used to find the far-field pattern for the physical optics (PO) integral. In the defocused mode of operation, a new technique is developed to find the reflection point on the subreflector corresponding to the defocused feed and a general field point on the main reflector. Two sample systems are designed.

Analysis of subreflectors for dual reflector antennas

The paper presents various methods for the calculation of the field reflected from a subreflector in a dual reflector antenna system. It is demonstrated that the physical-optics (PO) solution agrees well with the geometrical theory of diffraction (GTD) for the copolar component. Significant discrepancies may appear for the crosspolar component, and it is necessary to introduce additional fringe currents in the PO solution. If the subreflector is located in the near field of the feed, special precautions must be taken. One can either subdivide the feed aperture into a number of smaller subapertures for each of which standard GTD can be applied or an alternative and more efficient method is to use complex ray analysis (CRA), where the directive feed is represented by a point source located in the complex co-ordinate space. Both methods are compared with PO solutions taking the near-field effects into account. The theoretical results are verified experimentally for a near-field illuminated offset hyperboloidal subreflector.

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.

Dual reflectors for shaped beams and zero cross polarisation

Offset dual doubly-curved reflector-antenna systems are designed on the basis of geometrical optics to produce a shaped beam (cosec2 pattern) in the vertical plane and a narrow beam in the transverse plane with zero cross polarisation. Two configurations of ray-path geometry are identified and the main parameters are investigated. Blockage effects can be removed by judicious choice of parameter values.