偏轴卡塞格伦天线的二次赋形

针对采用旋转副镜方式切换频段的大型卡塞格伦天线,提出根据等光程条件和主反射镜的射线描迹对副反射镜进行二次赋形,以修正偏轴对光程造成的影响。通过物理光学法计算了二次赋形后的卡塞格伦天线及正馈和偏焦的对称型卡式天线远场方向图,并比较了馈源相心在不同横偏距离时二次赋形后的天线方向图。结果表明:副反射镜的二次赋形有效地修正了偏轴对天线辐射方向图的影响。     

A technique to combine the geometrical theory of diffraction and the moment method

A technique is presented in which the moment method (MM) is combined with the geometrical theory of diffraction (GTD). Since diffraction solutions exist for only relatively few structures, it is very desirable to have a means of obtaining the diffracted field for additional structures. Solutions for many structures can be obtained from this combination of techniques, and thus one is able to handle a wide variety of new problems which could not have been solved previously. The approach is developed and applied to a variety of structures in order to illustrate the approach and its validity.     

A physical optics version of the geometrical theory of diffraction

The authors derive a diffraction coefficient which is suitable for calculating the filed diffracted by the vertices of perfectly conducting objects. This diffraction coefficient is used to calculate the field scattered by the corner of a metallic sheet. Two diffraction coefficients, one for edges and one for vertices, are derived by solving the appropriate canonical problems using the physical optics (PO) approximation. The diffraction coefficients are calculated by first using the PO approximation which consists of calculating the total field on the surface of an object from the incident field according to the laws of geometrical optics, and then calculating the scattered field by employing this total surface field in a vector diffraction integral. The validity of the diffraction coefficients has been investigated by comparing their predictions with experimental measurements of the scattered field from a single corner of a rectangular metal sheet, and good agreement was found     

A hybrid technique for combining moment methods with the geometrical theory of diffraction

A technique for combining moment methods with the geometrical theory of diffraction (GTD) is presented, which permits the application of the method of moments to a larger class of problems. The fundamental idea used to develop the hybrid technique is to modify the usual impedance matrix that characterizes, for example, a wire antenna such that a metallic body or discontinuity on that body is properly accounted for. It is shown in general that one can modify the impedance matrix for any basis and/or weighting functions if one can compute the correct modification to the impedance matrix element. The modification is readily accomplished using the geometrical theory of diffraction and/or geometrical optics. Several example problems are considered to illustrate the usefulness of the technique. First, the canonical problem of a monopole near a conducting wedge is investigated. Second, a monopole at the center of a four-sided and an eight-sided flat plate is considered. Impedance results for the latter case are in good agreement with measurements. Third, a monopole at the center of a circular disc is examined and compared with experimental measurements in the literature, and fourth, the problem of a monopole near a conducting step is solved and the dependence of the input impedance upon the step height shown.     

Design of Cassegrain antennas employing dielectric cone feeds

A design procedure is described for a Cassegrain antenna employing a dielectric cone feed. Modal methods are used to determine the cone geometry, while a geometric-optics method is used to determine the subreflector profile. Good agreement is observed between measured and predicted radiation patterns for a 1.2 m-diameter parabolic antenna utilising the feed and operating at a frequency near 11 GHz.     

The geometrical theory of diffraction for axially symmetric reflectors

The geometrical theory of diffraction (GTD) (cf. [1], for example) may be applied advantageously to many axially symmetric reflector antenna geometries. The material in this communication presents analytical, computational, and experimental results for commonly encountered reflector geometries, both to illustrate the general principles and to present a compact summary of generally applicable formulas.     

Time-domain uniform geometrical theory of diffraction for a curvedwedge

A time-domain version of the uniform geometrical theory of diffraction (TD-UTD) is developed to describe, in closed form, the transient electromagnetic scattering from a perfectly conducting, arbitrarily curved wedge excited by a general time impulsive astigmatic wavefront. This TD-UTD impulse response is obtained by a Fourier inversion of the corresponding frequency domain UTD solution. An analytic signal representation of the transient fields is used because it provides a very simple procedure to avoid the difficulties that result when inverting frequency domain UTD fields associated with rays that traverse line or smooth caustics. The TD-UTD response to a more general transient wave excitation of the wedge may be found via convolution. A very useful representation for modeling a general pulsed astigmatic wave excitation is also developed which, in particular, allows its convolution with the TD-UTD impulse response to be done in closed form. Some numerical examples illustrating the utility of these developments are presented     

The geometrical theory of diffraction applied to antenna pattern and impedance calculations

The geometrical theory of diffraction is applied to the calculation of the radiation pattern and impedance of a monopole antenna on a perfectly conducting circular ground plane of limited extent. In this calculation, the radiation problem is resolved into two components, one being the monopole contribution and one the edge contribution. The impedance problem is resolved into the components of a reflection from the monopole in an infinite ground plane and a reflection from the circular edge as seen through the antenna. The known solutions of these individual components then permit the calculation of the overall radiation pattern and impedance by superposition. The techniques described are general and are considered applicable to a large class of similar radiating structures.     

A uniform geometrical theory of diffraction for an imperfectly conducting half-plane

Diffraction tensors are presented in the context of the uniform geometrical theory of diffraction (UTD) for the high frequency scattering by an impedance half-plane at normal and oblique (skew) incidence. These are based on the exact Wiener-Hopf solution and were derived according to the UTD ansatz. In addition, unlike previous uniform diffraction coefficients, the ones given here reduce to the known UTD diffraction coefficients for the perfectly conducting case. The coefficients are explicit and therefore appropriate for practical applications. Several scattering patterns are also presented and compared to a previous heuristic solution.     

An analysis of the radiation from apertures in curved surfaces by the geometrical theory of diffraction

In this paper the geometrical theory of diffraction is extended to treat the radiation from apertures or slots in convex perfectly conducting surfaces. It is assumed that the tangential electric field in the aperture is known so that an equivalent infinitesimal source can be defined at each point in the aperture. Surface rays emanate from this source which is a caustic of the ray system. A launching coefficient is introduced to describe the excitation of the surface ray modes. If the field radiated from the surface is desired, the ordinary diffraction coefficients are used to determine the field of the rays shed tangentially from the surface rays. The field of the surface ray modes is not the field on the surface; hence if the mutual coupling between slots is of interest, a second coefficient related to the launching coefficient must be employed. In the region adjacent to the shadow boundary, the component of the field directly radiated from the source is represented by Fock-type functions. In the illuminated region the incident radiation from the source (this does not include the diffracted field components) is treated by geometrical optics. This extension of the geometrical theory of diffraction is applied to calculate the radiation from slots on elliptic cylinders, spheres, and spheroids.     

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

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.     

Aperture radiation from an axially slotted elliptical conducting cylinder using geometrical theory of diffraction

The equatorial radiation pattern of a parallel-plate TEM mode axial slot on an infinite cylinder of elliptical cross section is computed using wedge-diffraction and creeping-wave theory. The wedge-diffracted fields are obtained by the diffraction interaction from a set of infinite wedges approximating the parallel-plate-cylinder geometry. Surface-wave propagation on curved bodies commonly used in scattering is employed for the creeping-wave contribution. The total field in the lit region is approximated using superposition of wedge-diffracted and creeping-wave fields and in the shadow region solely by creeping-wave fields. The computed patterns are compared with experimental results since boundary-value solutions are not readily available. Good agreement between theory and experiment is indicated.     

Application of geometric diffraction theory to scattering from cones and disks

The ability of the geometrical theory of diffraction to predict the radar cross section (RCS) of a perfectly conducting, right circular cone as a function of viewing angle is evaluated by comparison of computed and measured values of RCS. Both vertical and horizontal polarization have been considered for cones ranging from 0.98 to 2.87 wavelengths in diameter at the base and having half angles of 4°, 15°, and 90°; the latter case corresponds to a disk. It is shown that for cones having normalized base circumference (ka) of 8 or 9 the predicted and measured RCS agree very well except when the cone is observed within about 30° of nose-on with vertical polarization, in which case large errors occur for some as yet unknown reason. For smaller cones having diameters about equal to the wavelength (ka around 3), the computed RCS is generally predicted within 5 dB, but the form of the RCS pattern is not predicted very accurately. Backscattering from the base of the cone is very nearly the same as backscattering from a disk of the same diameter for viewing angles within 60° of the normal to the base.     

Compensation of the beam squint in axially symmetric, large dualreflector antennas with large-ranging laterally displaced feeds

The beam squint in axially symmetric reflector antennas with laterally displaced feeds is manifested by a small shift of the radiation patterns for left- and right-hand circular polarization. It is caused by the cross-polarized component in the reflected field. The authors' interest in this effect, and especially in how to minimize or to compensate for it, was stimulated by some measurements with the 100-m radio telescope of the Max-Planck-Institut fuer Radioastronomie. They have carried out a series of computer simulations for different geometrical antenna configurations for the Cassegrain optics. The presented results show that depending upon various geometrical antenna parameter values, while keeping the laterally displaced feed in a gain optimized orientation, i.e., directed to the primary focus, the angular shift between the squinted beams can even reverse. This offers the chance to compensate the beam squint for large range lateral displacement of the feed by choosing an appropriate antenna/feed configuration     

The use of the geometrical theory of diffraction in the calculation of antenna patterns of submillimeter and millimeter-wave radiometers

The Geometrical Theory of Diffraction is applied in the calculation of the antenna pattern of a submillimetre/millimetre-wave radiometer. Axial symmetry of the radiometer is shown to lead to considerable simplification of the expressions to be evaluated. It is also shown that the sequence of apertures in the radiometer can be chosen to give extremely high rejection of signals incident at off-axis angles greater than 20^o.     

A hybrid method combining the multitemporal resolution time-domain method of moments with the time-domain geometrical theory of diffraction for thin-wire antenna problems

In this paper, a hybrid method combining a multitemporal resolution (MTR) enhanced time-domain method of moments (TD-MoM) with the TD geometrical theory of diffraction (GTD) is presented, which allows to efficiently calculate the transient fields radiated by antennas in presence of large objects. The MTR scheme tailors the time step size-and thus implicitly the duration of the temporal basis function-on the basis of the distance between source and test element. Additionally, the voltage induced by source elements far away from the test element is interpolated in time. The hybrid method is applied to calculate the radiation properties of thin-wire antennas in presence of perfectly conducting flat scatterers to demonstrate its basic features and advantages.     

Analysis of aperture radiation from an axially slotted circular conducting cylinder using geometrical theory of diffraction

The wedge-diffraction theory in combination with the creeping wave theory is used to compute the radiation pattern of an axial slot on a circular conducting cylinder. The slot is excited by a parallel-plate waveguide operating in the TEM-mode. The total field in the "lit" region is obtained by the superposition of the wedge-diffracted and creeping wave fields. The total field in the "shadow" region is obtained solely from the creeping wave contribution.     

On the role of the geometrical optics field in aperture diffraction

The asymptotic solution of a high-frequency electromagnetic field transmitted through a finite aperture is studied. In applying Keller's geometrical theory of diffraction (GTD), a basic yet unanswered question for an observation point in the lit region is: "Should the geometrical optics field on a direct incident ray be included in the total field solution?" By studying a test problem and utilizing the newly developed uniform asymptotic theory (UAT), we have deduced simple and explicit rules for the role of the geometrical optics field and the regions of validity for GTD in a general aperture diffraction. The rules reveal that the success of GTD in treating aperture problems in the literature depends critically on the assumption that both the source and the observation points are infinitely far away from the aperture. Had either point been a finite distance away, Keller's GTD, in general, would fail and UAT must be used. The paper also demonstrates a physical phenomenon: the diffusion of the incident field as the observation point moves away from the aperture.