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A formulation is presented for shaping dual-reflector antennas designed to offer an omnidirectional coverage. The shaping procedure is based on geometrical optics (GO) principles and assumes a uniform phase distribution for the aperture field. Two distinct dual-reflector arrangements, based on the axis-displaced Cassegrain (ADC) and ellipse (ADE) configurations, are investigated. The GO shaping results are validated using the accurate analysis provided by the method-of-moments technique. Index Terms--Omnidirectional antennas, reflector antennas, reflector shaping.

The vector aperture field of classical offset Dragonian dual-reflector antennas is derived using geometrical-optics concepts. This field then yields the equivalent paraboloid of the geometry. From these results, the conditions for an axially symmetric equivalent paraboloid, when a circular aperture is assumed, are obtained. A complete step-by-step geometrical-optics-based design procedure for optimum classical offset Dragonian antennas with circular apertures is then presented (i.e., zero geometrical-optics cross-polarization and minimum spillover). This procedure is demonstrated by two design examples. Index Terms--Dragonian antennas, multireflector antennas, offset reflector antennas, reflector antennas.

Twisting a wire pair is often used to reduce the wire's low frequency magnetic flux density impinging upon spatially adjacent circuitry. Typically, predictions of the magnetic flux density due to the twisted-wire pair are based upon mathematical formulations for infinite-length wires. In this work, expressions for predicting the quasistatic magnetic flux density near a finite-length, current-carrying twisted-wire pair are derived. These equations are numerically integrated, and the results compared with the magnetic flux density from the corresponding straight-wire pair, thereby yielding results useful for electromagnetic compatibility analyses. Additionally, approximate far-zone magnetic flux density equations are developed, and their range of validity established. These equations are useful for magnetic flux density estimations at distances further removed from the twisted-wire pair than the usual ones encountered in a typical enclosure. Also, they show that the large near-zone reduction of the magnetic flux density magnitude obtained by twisting the wire pair, relative to the straight-wire pair, is not available in the far-zone.

A geometrical optics procedure for designing electrically optimized classical offset dual reflector antennas with circular apertures is presented. Equations are derived that allow the size and spacing of the main and subreflectors of the antenna system, along with the feed horn subintended angle, to be used as input variables of the design procedure. The procedure, together with these equations, yields an optimized design, starting from general system requirements. The procedure is demonstrated by designing both an offset Cassegrain and an offset Gregorian antenna, and is validated by analyzing their radiation patterns using physical optics surface current integration on both the main and subreflectors.

An efficient algorithm for numerically evaluating diffraction integrals is presented. The algorithm employs a predictor-corrector scheme combined with Ludwig's integration procedure. The predictor-corrector eliminates the amplitude and phase ambiguities present in the real+imaginary algebra used in machine calculations and provides accuracy self-checking capabilities. The end result is a reliable and efficient integration method that does not require independent integrand phase information, can handle arbitrarily shaped integration domains, and is capable of monitoring its own accuracy as the integration proceeds. The performance of the algorithm is investigated by computing, using the Physical Optics technique, the electromagnetic field scattered by representative reflector antenna geometries. These tests demonstrate that the proposed algorithm is particularly efficient in the analysis of multi-reflector systems.

This paper presents a technique for solving the mode purity problem that exists in cryogenic microwave frequency discriminators using X-band sapphire whispering-gallery mode resonators. The sapphire cavity resonates in a pair of quasi-degenerate modes split by a few kHz due to small geometrical imperfections. Since the nature and location of the geometrical imperfections are difficult to determine, variable coupling of the modes to a fixed output port results. Resonator applications require reproducible coupling to only one of these modes. This objective can be achieved by adding a larger dominant perturbation to the resonator. Results of tests show that a noncircularly-symmetric sapphire piece suspended above the resonator can provide controllable mode coupling and simultaneous frequency tuning.

This work presents a generalized study of classical axially symmetric dual-reflector antennas. The antenna dishes are simply described by conic sections, arranged to reduce the main-reflector radiation toward the subreflector surface. The dual-reflector configuration provides a uniform-phase field distribution over the illuminated portion of the aperture, starting from a spherical-wave feed source at the antenna primary focus. All possible configurations are characterized into a total of four distinct groups. Simple closed-form design equations and the aperture field distribution are derived, in a unified way, for all these kinds of generalized antennas using the principles of geometrical optics. The formulation is applied in a parametric study to establish the configurations yielding maximum radiation efficiency (not including diffraction effects). The design procedure is exemplified in the synthesis of a novel configuration, which is further analyzed by the moment method. Index Terms--Aperture fields, design methodology, electromagnetic reflection, geometrical optics, reflector antennas.

The 2007 Decadal Survey for the National Aeronautics and Space Administration recommended an Aerosol/Cloud/Ecosystems (ACE) mission with a new advanced cloud profiling radar. In this letter, we describe ACERAD, a radar design that could meet the goals for the ACE mission. ACERAD has 35 GHz (Ka-band) and 94 GHz (W-band) channels. It would scan at Ka-band and be nadir looking at W-band. The scanning antenna geometry is a classical Dragonian illuminated by a feed array; a reduced-size prototype of the antenna has been successfully fabricated and tested. ACERAD also uses a quasi-optical transmission line at W-band for low-loss routing of the signal from the transmitter to the antenna and from the antenna to the receiver. This letter briefly describes the ACERAD instrument design and test results for some of the major technologies needed for ACERAD implementation. [ABSTRACT FROM PUBLISHER]

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Copyright of Radiologia Brasileira is the property of Radiologia Brasileira and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The vector aperture field of classical offset Dragonian dual-reflector antennas is derived using geometrical-optics concepts. This field then yields the equivalent paraboloid of the geometry. From these results, the conditions for an axially symmetric equivalent paraboloid, when a circular aperture is assumed, are obtained. A complete step-by-step geometrical-optics-based design procedure for optimum classical offset Dragonian antennas with circular apertures is then presented (i.e., zero geometrical-optics cross-polarization and minimum spillover). This procedure is demonstrated by two design examples. [ABSTRACT FROM AUTHOR]

Describes the finding of unizoic cysts of Isospora belli infection in lymphoid tissues of a Brazilian patient with AIDS. Discussion on the possibility of Isospora belli's drug resistance; Indication for the existence of intermediary or paratenic animal hosts; Cause of the infection's relapse.