Complementary field component of radiation from a Cassegrain subreflector

The effect of the induced currents on the subreflector can be evaluated by the method of stationary phase and gives, to a first approximation, the specularly reflected radiation. The radiation at an angle of interest, as thus calculated, is found to be supplemented by the backlobe of the radiation directed at the complementary angle.     

Aperture blockage by a subreflector in a Cassegrain antenna system

The problem of aperture blockage by a subreflector in a Cassegrainian antenna system is discussed. The Lorentz reciprocity theorem is used to derive an expression for the far field radiated by the Cassegrain system. It is suggested that the field scattered by the subreflector can be computed using Keller's geometrical theory of diffraction.     

On the numerical analysis of the subreflector of an offset Cassegrain microwave antenna

The physical optics analysis of the scattering from the subreflector of an offset Cassegrain microwave antenna is formulated in a novel way using the geometrical properties of the conic sections. The resulting integral is computed by means of an algorithm involving quadratic approximation of the phase and polynomial approximation of the remainder of the integrand. The formulation allows for displacement of the phase center of the feed pattern representation. Examples are presented which provide a comparison with previously reported results obtained using the geometrical theory of diffraction rather than physical optics.     

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

Many existing large ground reflector antennas have been designed as Cassegrain systems-i.e., paraboloid/hyperboloid combinations. Other large ground antennas are simply paraboloid designs. Upgrading the gain of these systems to a gain comparable to that obtainable with a dual shaped reflector antenna system has been an important and costly objective of many such ground stations. A potentially economic method for such an antenna upgrade is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a parabaloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna that was first presented by Galindo-Israel and Mittra in 1977. In that approximate formulation, the geometrical optics (GO) energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller (sub) reflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.     

The effects of subreflector diffraction on the aperture efficiency of a conventional Cassegrain antenna--An analytical approach

An asymptotic theory is presented with which the reduction in aperture efficiency caused by diffraction from a subreflector edge can be calculated for any dual-reflector system. The theory is applied to conventional Cassegrain antennas, for which approximate analytical effieiency formulas are derived. These formulas show that subreflector diffraction may represent a significant efficiency loss even for subreflector diameters as large as 20 wavelengths. The formulas are used to obtain an optimum subreflector size which represents the best trade-off between losses due to subreflector diffraction and geometrical shadowing.     

Design of low VSWR axisymmetric subreflector

The design of a low VSWR axisymmetric subreflector using the method of moments is described. To reduce the power reflected back into the waveguide horn the surface of the hyperbolic subreflector is deformed using raised-cosine functions. The method of moments technique is then validated by comparing experimental and theoretical results.<>     

Coma correction of a wobbling subreflector

Radio astronomical observations at millimeter wavelengths with Cassegrain telescopes often use on-source/off-source measurements with a wobbling subreflector to eliminate the influence of the Earth's atmosphere. The tilt of the subreflector produces the required beam throw, but also a coma-like wavefront deformation noticeable as a degradation of the beam pattern and a reduction of the main beam power. We demonstrate, with measurements made with the IRAM 30-m telescope at 1.3 mm wavelength (230 GHz) of the quasi-point-like source Mars and the extended source Saturn, that it is possible to eliminate (to a large extent) this wavefront deformation when using a near-focus corrector, on the 30-m telescope conveniently located on the co-rotating Nasmyth mirror     

Design of a Modified Cassegrain System Used in Joint Transform Correlator

In order to detect and recognize infrared target with joint transform correlator,a modified Cassegrain optical system is designed.The main advantages of the system are large field-of-view,infrared dual-band common optical path and compact structure.In the modified Cassegrain optical system,the working wavelengths are 3.7~4.8μm and 8~12μm,the field-of-view is 4° and the aperture is 240mm.The paraboloidal primary mirror and hyperboloidal secondary mirror are all replaced by spherical surfaces.So the problems ...     

Improved efficiency with a mechanically deformable subreflector

Like most conventional radio telescopes, the 140-ft at the National Radio Observatory (NRAO), Green Bank, WV, is limited in its short-wave performance by gravitational deformations, whose main part is of an astigmatic shape with elevation-dependant amplitude. This surface degradation could be corrected if a subreflector were deformed in a similar shape by the same amount. For testing this possibility an experimental Cassegrain subreflector was built which can be deformed in an astigmatic mode by simple means: two stiff diagonals and four points in-between where a motor pushes or pulls normal to the surface, servo-controlled by the on-line computer. Although the amount of deformation is too limited in the present setup, good results have already been obtained. Seven unresolved radio sources were observed at the water vapor line (22.3 GHz,lambda = 1.345cm), at various telescope pointing, and with different amounts of subreflector deformation, scanning in both directions for complete beam maps. The astigmatic deformation of the subreflector gave considerable improvements, especially fat south and east: fairly strong secondary beams disappeared; the beamshape became more symmetrical, narrow, and round; and at20degelevation the aperture efficiency increased by factors between two and three.     

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.     

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。     

含衍射透镜的卡塞格林红外光学系统设计

该设计将衍射透镜引入到卡塞格林红外光学系统中。设计结果表明,衍射透镜的引入增大了卡塞格林光学系统的视场和相对孔径,改善了成像质量;同时也降低了加工工艺的要求,大大节约了成本。     

Design of wide-band corrugated conical horns for Cassegrain antennas

Design procedures based on extensive theoretical and experimental investigations are given for hybrid-mode corrugated conical horns having near-constant beamwidth, low cross-polarized sidelobes and low reflection for continuous bandwidths up to 2.1:1. The design techniques are applicable for horn flare semi-angles to about30deg, which includes most feed horns in dual-reflector antennas.     

Broad-band VSWR measurements of a Cassegrain antenna

Broadband VSWR measurements are reported on a smallF-number Cassegrain antenna. Cyclical variations of VSWR between 1.00 and 1.25 with a 350-MHz frequency interval between unity VSWR values were observed over the 8.5-12.4 GHz frequency range. These variations correspond to a change in the power transmission ratio of 1.231 percent between the unity and 1.25 values of VSWR.     

高功率近场卡塞格伦天线的设计

系统地给出了高功率近场卡塞格伦天线的设计.刺用内壁光滑的指数型多模喇叭满足高功率微波天线对馈源高功率容量和良好辐射特性的要求,并将介质透镜加栽于喇叭馈源口面,以保证对天线副面进行平面波照射.通过优化设计,得到X频段高功率近场卡塞格伦天线的结构尺寸,使其理论功率容量大于600 mW.实测结果与仿真结果吻合良好,证明该设计方案的可行性.     

UTD analysis of a shaped subreflector in a dual offset-reflectorantenna system

The geometrical theory of diffraction (GTD) is known as an efficient high-frequency method for the analysis of electrically large objects such as a reflector antenna. However it is difficult to obtain geometrical parameters in order to apply GTD to an arbitrary shaped reflector, especially a subreflector. The geometrical parameters of an arbitrary shaped subreflector for the uniform theory of diffraction (UTD) analysis are derived based on differential geometry. The radiation patterns of various subreflector types, including hyperboloidal and a shaped subreflector, are evaluated by UTD. The computed result for the hyperboloidal reflector agrees well with that obtained by uniform asymptotic theory (UAT)     

卡塞格伦光学接收天线的设计

自由空间光通信是一种通过在自由空间传输激光信号来实现点对点、点对多点或多点对多点间语音、数据、图像信息的双向通信技术。光学接收天线是自由空间光通信系统的关键部件。丈中分析了卡塞格伦系统作为自由空间光通信系统中光学接收天线的可行性,讨论了其设计参数间的相互关系,并在数值计算的基础上给出一个具体的设计方案。     

GTD analysis of a hyperboloidal subreflector with conical flange attachment

A geometrical theory of diffraction (GTD) analysis of the principal plane far-field radiation patterns of a hyperboloidal subreflector with a conical flange attachment (HWF) fed by a primary feed located at its focus is presented. While using the uniform geometrical theory of diffraction (UGTD) for evaluating the nonaxial fields, the method of equivalent currents is used in the axial region. In this paper, both the diffraction by the wedge formed between the hyperboloid and the conical flange and the diffraction by the edge of the flange are considered. While considering the diffraction by the edge due to the diffracted ray from the wedge in theH-plane, the slope diffraction technique has been used. The computed diffracted farfields of a typical HWF illuminated by a high performance primary feed shows good agreement with the available measured data and with the results based on the method of physical optics (PO). The sharp cutoff and the low spillover characteristics of the HWF are highlighted by comparing its radiation pattern with that of a hyperboloid without a flange. Further, the effects of the different parameters of the HWF on its radiation pattern are also studied and plotted, so that these results can be utilized in the design of the HWF for a specific requirement.     

GTD analysis of the radiation patterns of a shaped subreflector

A technique of analyzing the principal plane radiation patterns of a subreflector shaped using the method enunciated by Collins [1] has been presented. The analysis is based on the uniform geometrical theory of diffraction (UGTD) [2], and a simplified procedure has been adopted in the determination of the principal plane radii of curvature of the subreflector. The numerical results obtained based on UGTD correlate well with those obtained using the method of physical optics (PO).     

Design and implementation of an error-compensating subreflector forthe NRAO 12-m radio telescope

Describes the measurement of the surface shape of the primary reflector of the NRAO 12-m radio telescope and the use of the measurements to machine the shape of a secondary mirror to compensate for the primary reflector errors. The method known as holography was used with the 38.04-GHz transmitter on the geosynchronous Lincoln Experimental Satellite to map the main reflector. The machining of the secondary compensated for the errors in the primary shape, and as a result, at 345 GHz, the axial gain of the instrument was increased by about 50% and the antenna pattern correspondingly improved