A measurement technique for modeling the effects of feed support struts on large reflector antennas

A measurement technique is described in which frequency scaled models of struts are placed in the near-field region of an offset reflector. In this compact range environment the excitation of the strut model is by plane waves, as would be encountered in the axisymmetrical reflector situation. Far-field radiation patterns are recorded, with and without the strut model in place, and, because of the low sidelobe levels associated with offset reflector antenna systems, it is possible to isolate the far-field response of the strut model. This technique is particularly useful for determining the real effects of structures that are difficult to analyze mathematically, such as latticed struts or metallic geodetic radomes.     

Aperture blockage in reflector antennas

The aperture blockage effects on both the copolarized and the cross polarized components of the front-hemisphere pattern of a typical quadrupod supported primary-feed paraboloidal reflector antenna are evaluated. A detailed numerical model of the scattering process which involves the feed, the feed supporting struts, and the reflector itself is employed to compute the blockage effect.     

Reduction of sidelobe level of reflector antennas by covering feed struts with a periodic structure

A periodic structure has been investigated and optimised which can be used to cover the struts of reflector antennas in order to reduce the far sidelobes caused by scattering from the struts.     

Influence des supports sur les rayonnements copolaire et contrapolaire d’une antenne à réflecteur

Scattering by struts of triangular or elliptical cross sections for reflector antennas is rigorously studied. Conformal mapping make easier solving integral equations deduced from boundary value problem. The effects of struts on co-polarization, crosspolarization antennas performances is emphasized.     

Losses, sidelobes, and cross polarization caused by feed-supportstruts in reflector antennas: design curves

In a symmetrical reflector antenna the feed system and its support struts block the aperture and thereby deteriorate the radiation characteristics. Simple design curves are presented for the efficiency reduction, the sidelobe levels, and the cross polarization caused by strut blockage. The results are obtained from an analytical study that includes the induced field ratio (IFR) of the struts. The most significant IFR values for struts with circular cross section are calculated and plotted in a way which makes them easy to use as design curves. The use of the design curves is demonstrated by an example     

Spherical wave blockage in reflector antennas

The effects of the spherical wave blockage in reflector antennas is investigated. This problem is likely to occur in axially symmetrical feed antennas of single- and dual-reflector type in both single- and dual-reflector configurations, owing to the presence of primary feeds and their supports including struts that are normally placed between the primary source and the main reflector. The main reflector blockage due to large obstacles is estimated by the well-known null-field technique that employs flat polygonal plate models of the masking structures to define the obscured area. Although this same approach may be used to predict the spherical wave blockage due to the struts, a more rigorous but yet efficient technique is also employed, which consists of superimposing to the primary field the high-frequency scattered field from the struts. This field is calculated by using scattering coefficients that are derived by locally approximating the actual structure, by an infinite circular cylinder. This latter formulation is compared with the null-field technique and validated by an experimental campaign. The measurement setup is particularly useful for isolating spherical wave-blockage effects. It consists of a single-reflector offset antenna where a single strut is mounted with its axis parallel to that of the focusing parabola, thus, practically enforcing the plane wave blockage to vanish. Comparisons with the measurements have shown that the null-field approach is adequate for predicting the secondary pattern for large polygonal obstacle, but it is unsatisfactory to treat the strut blockage. It is found that this latter can be successfully described with the more rigorous high-frequency approach     

Effect of support strut diffraction on reflector surface profile assessment

The phase-imaging properties of microwave holographic processing are used to provide an inward-looking reconstruction of support strut diffraction and its effect on reflector surface profile assessment. Measured near-field data are employed to illustrate the contribution made by the support strut geometry to the apparent profile error for a 2.5 m reflector antenna operating at 10 GHz. The relative significance of real profile errors and those deriving from strut effects are studied by using a diffraction model. It is concluded that certain configurations of support struts can give rise to significant modification of the effective profile error distribution.     

Forward scattering from cylinders of triangular cross section

The induced-field ratios (IFR) of conducting cylinders of a triangular cross section have been calculated. When the width of the cross-section is in the range of 1-2 wavelengths, theE-wave IFR's are substantially less than the corresponding values for square or circular cross sections. From the standpoint of RF aperture blocking of a reflector antenna, the triangular cross section thus offers an advantageous cross section for feed-support struts.     

Limitations of the projected shadow method for estimating the strut influence in paraboloidal antennas

The results of two convenient methods for calculating the sidelobe level caused by the feed support struts of paraboloidal reflector antennas have been compared. These methods are the simple `projected shadow method? and the more sophisticated `equivalent current approach?. It turns out that the shadow method is applicable for the computation of the sidelobes of large antennas up to an angle of 3 to 4° from boresight and in the case of small antennas up to 8 to 10°.     

Radiation cones from feed-support struts of symmetric paraboloidal antennas

Wide-angle radiation associated with the plane-wave component of the field in the focal space of a paraboloidal reflector is computed by integrating the currents induced on the feed-support struts. This component of radiation is locally maximum on radiation cones which lie along the axis of each strut. Comparison with data measured for the Dwingeloo radio telescope indicates good agreement for the position, width, and intensity of these cones, even at levels 50 dB below the peak of the main beam.     

High Tc superconducting active antenna withreflector

A high-T_c superconducting (SC) active antenna with a corner reflector is found to be useful for detecting electromagnetic waves (f=9.55 GHz). The SC active antenna is a new type of antenna which consists of material properties and antenna properties. The microwaves with fixed polarizations are radiated to the SC active antennas, and the detectivities and the directivities were confirmed experimentally. With the use of the corner reflector, the sensitivity and the directivity can be improved     

Accurate numerical modeling of the TARA reflector system

The radiation pattern of the large parabolic reflectors of the Transportable Atmospheric RAdar system (TARA), developed at Delft University of Technology, has been accurately simulated. The electric field integral equation (EFIE) formulation has been applied to a model of the reflectors including the feed housing and supporting struts, discretised using the method of moments. Because the problem is electrically large (the reflector has a diameter of 33/spl lambda/) and nonsymmetrical, this lead to a badly conditioned linear system of approximately half a million unknowns. In order to solve this system, an iterative solver (generalized minimum residual method) was used, in combination with the multilevel fast multipole method. Because of the bad conditioning, the system could only be solved by using a huge preconditioner. A new block-incomplete LU preconditioner (ILU) algorithm has been employed to allow for efficient out-of-computer core memory preconditioning.     

A New Reflector Antenna Based on the Fresnel Principle

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Monolithic white LED based on AlxGa1-x N/InyGa1-yN DBR resonant-cavity

A monolithic white light-emitting diode (LED) with blue and yellow light active regions has been de-signed and studied. With the AlxGa1-xN/InyGa1-yN distributed Bragg reflector (DBR) resonant-cavity, the extraction efficiency and power of the yellow light are enhanced so that high quality white light can be obtained.     

激光对准快速反射镜控制系统的设计

为了校正激光发射设备中激光对准光路的偏差,设计了一种激光对准快速反射镜控制系统。采用步进电机作为驱动,控制快速反射镜在互相垂直的两个方向进行运动,校正激光光路的偏差,达到了精确控制激光光路的目的。对激光对准快速反射镜的工作原理和设计过程进行了详细阐述,并利用对准控制机箱等硬件设备对软硬件设计进行了实验验证,取得了较好的实验数据。结果表明,快速反射镜控制系统在小角度工作范围内方位误差和俯仰误差均方根都小于1,即控制精度小于1。该系统能够很好地控制快速反射镜进行2维运动,软件设计和硬件设计都是正确可靠的,能够满足激光对准控制系统精确控制激光光路的要求。     

Monolithic white LED based on Al_xGa_(1-x) N/InyGa_(1-y)N DBR resonant-cavity

A monolithic white light-emitting diode (LED) with blue and yellow light active regions has been designed and studied. With the AlxGa1-xN/InyGa1-yN distributed Bragg reflector (DBR) resonant-cavity, the extraction efficiency and power of the yellow light are enhanced so that high quality white light can be obtained.     

不同参数超宽带反射面天线系统辐射特性

在超宽带反射面天线设计中,首要问题是馈电天线的方向图与反射器匹配。以往在频域上对反射面天线的研究发现,当馈源天线方向图给出约-11 dB的边缘照射时,反射面天线可实现其最佳性能。在参数化模拟了作为馈源的透射电子显微镜(TEM)喇叭天线方向图特性的基础上,根据反射面天线的频域设计准则,相应地加入合适的超宽带反射面天线,对其远场辐射特性进行模拟计算。为了获得尽可能大的远场辐射场,采用计算机仿真技术(CST)数值模拟软件,在0 GHz~1 GHz频率范围内模拟了以不同参数TEM喇叭天线作馈源的超宽带反射面天线的远场辐射特性,并在时域上对其结果进行分析。模拟结果表明,由于其相位中心的不确定性,作为馈源的TEM喇叭天线无法与反射面天线完全匹配,其沿着反射面天线主轴移动时产生的反射面口径场相位和幅度的变化影响着远场辐射场的变化,TEM喇叭天线的遮挡效应也不容忽视。     

均匀照射赋形反射面天线设计

为提高反射面天线的照射均匀性，利用波束赋形技术设计一款工作在15 GHz的高性能偏置反射面天线.该天线由一个非均匀有理B样条（non-uniform rational B-splines，NURBS）反射面和一个介质棒馈源组成，NURBS反射面通过物理光学法结合Nelder-Mead优化算法来赋形，从而获得理想的天线辐射场.仿真结果表明，赋形后的反射面天线相对旁瓣电平低于-20 dB，增益在-2.7°~2.7°的波束范围内超过20 dBi，且浮动不超过1 dB.该天线具有结构简单、旁瓣低、增益均匀的特点，在卫星通信等领域具有重要作用.     

Performance characteristics of an injection-controlledelectron-beam pumped XeF(C→A) laser system

Characteristics of an injection-controlled electron-beam pumped XeF(C→A) laser are investigated with emphasis on efficient wideband tuning and scaling issues. Using a quasi-CW dye laser as an injection source, data are obtained that describe the laser characteristics over a wide parameter range. A high-Z electron-beam backscattering reflector inside the laser reaction cell improved the electron-beam energy deposition by 40%, resulting in an increase of the amplified laser output by more than a factor of four. Efficient and continuous wavelength tuning between 470 and 500 nm is achieved with an output energy density of ~1 J/l, and an intrinsic efficiency of ~1% throughout the entire tuning region     

Scanning properties of large dual-shaped offset and symmetricreflector 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