GTD analysis of near-field and far-field patterns of a parabolic subreflector illuminated by a plane wave

Analysis of subreflectors for dual reflector antennas

Abstract: 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.

Radar-invisibility on axis of rotationally symmetric reflectors

Abstract: It has been observed that the monostatic radar cross section (RCS) of parabolic perfect electric conductor (PEC) reflectors vanishes at certain frequencies for broadside wave incidence. It has also been noticed that hyperbolic reflectors behave similarly, with very low RCS minima. This behavior is explained by using physical optics (PO) and verified via a numerical full-wave analysis. Relations between the physical dimensions of the reflector and the characteristic values of the RCS (notch frequencies, minimum and maximum value), as well as a comparison of different canonical reflectors, are given. The interest in the phenomenon described is at present essentially speculative, since, as pointed out in this letter, there are some significant limitations which restrict its practical utilization.

Analysis of Electrically Small Hyperbolic Subreflectors Utilizing the Geometrical Theory of Diffraction

Abstract: Cassegrain antennas have been utilized in applications where small size is critical. It has been observed that for antennas having subreflectors of the order of only several λ in diameter, near-in sidelobe levels rise beyond normally predicted values. E-plane sidelobes are often several dB higher than those in the H-plane. Disturbances in the illumination of the primary reflector due to diffraction limitations of the electrically small subreflector are a possible cause of the sidelobe phenomenon. In this work, the results for 3 to 8 λ -diameter hyperboloidal subreflectors, using the geometrical theory of diffraction (GTD) are presented and compared with the physical optics approach and other data. Degradation of the scatter patterns are observed as the subreflector size is decreased, and the differences in the E-and H-plane phase and amplitude patterns are observed in the GTD solution.

A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface

Abstract: A compact dyadic diffraction coefficient for electromagnetic waves obliquely incident on a curved edse formed by perfectly conducting curved ot plane surfaces is obtained. This diffraction coefficient remains valid in the transition regions adjacent to shadow and reflection boundaries, where the diffraction coefficients of Keller's original theory fail. Our method is based on Keller's method of the canonical problem, which in this case is the perfectly conducting wedge illuminated by plane, cylindrical, conical, and spherical waves. When the proper ray-fixed coordinate system is introduced, the dyadic diffraction coefficient for the wedge is found to be the sum of only two dyads, and it is shown that this is also true for the dyadic diffraction coefficients of higher order edges. One dyad contains the acoustic soft diffraction coefficient; the other dyad contains the acoustic hard diffraction coefficient. The expressions for the acoustic wedge diffraction coefficients contain Fresenel integrals, which ensure that the total field is continuous at shadow and reflection boundaries. The diffraction coefficients have the same form for the different types of edge illumination; only the arguments of the Fresnel integrals are different. Since diffraction is a local phenomenon, and locally the curved edge structure is wedge shaped, this result is readily extended to the curved wedge. It is interesting that even though the polarizations and the wavefront curvatures of the incident, reflected, and diffracted waves are markedly different, the total field calculated from this high-frequency solution for the curved wedge is continuous at shadow and reflection boundaries.

Evaluation of edge-diffracted fields including equivalent currents for the caustic regions

Abstract: The fields diffracted by a body made up of finite axially symmetric cone frustums are obtained using the concepts of the geometrical theory of diffraction. The backscattered field for plane-wave incidence on such a target is obtained with particular emphasis on those regions that are usually avoided, namely, the caustic region and its immediate vicinity. The method makes use of equivalent electric and magnetic current sources which are incorporated in the geometrical theory of diffraction. This solution is such that it is readily incorporated in a general computer program, rather than requiring that a new program be written for each shape. Several results, such as the cone, the cylinder and the conically capped cylinder, are given. In addition, the method is readily applied to antenna problems. An example which is reported consists of the radiation by a stub over a circular ground plane. This present theory yields quite good agreement with experimental results reported by Lopez, whereas the original theory given by Lopez is in error by as much as 10 dB.

Applications of probe-compensated near-field measurements

Abstract: The theory of near-field measurements for antenna practitioners is summarized, and the measurement procedures in three coordinate systems, namely rectangular, cylindrical, and spherical are outlined. Specific topics include probe characterization, measurement systems, data reduction, and attendant accuracies. The results of recent studies are also summarized, and some brief remarks on future applications of near-field measurements in the laboratory, the production line, and in field testing and evaluation conclude the paper.

Spectral analysis of high‐frequency diffraction of an arbitrary incident field by a half plane—Comparison with four asymptotic techniques

Abstract: The knowledge of high-frequency diffraction of an arbitrary wave incident on an edge is important in many applications, such as antennas mounted on aircraft and reflector antennas illuminated by complex feeds. In this paper the problem of a half plane illuminated by a nonplanar wave is investigated using the concept of the plane wave spectral representation. For large wave number k, a new higher-order asymptotic solution for the total field up to and including terms of order k−5/2 relative to the incident field is derived. The behavior of the solution for the observation points which coincide with shadow boundary directions of a multipole line source is discussed in detail. Furthermore, numerical solution of the field integral representation is constructed for the observation angles in the transition regions. The results are compared with those of the Geometrical Theory of Diffraction (GTD), the Uniform Asymptotic Theory (UAT), the Uniform Theory of Diffraction (UTD) and the Modified Slope Diffraction (MSD).

A beam-waveguide feed having a symmetric beam for Cassegrain antennas

Abstract: The asymmetric reflectors are unavoidable in the composition of a beam-waveguide feed and generally cause asymmetry of the beam from the feed. By means of the combination of two suitable asymmetric reflectors a rotationally symmetric beam can be obtained on the basis of geometrical optics.