Directivity of planar array feeds for satellite reflector applications

Array-fed reflector antennas are used extensively in today's contour and multiple beam satellite antennas. To determine the directivity of these antennas theoretically, the total radiated power of the array feed must be accurately computed. In this paper, a closed-form expression for the radiated power is obtained for arrays with the well-known(cos theta)q-type element patterns. The formulation is general, and takes into account polarization, nonsymmetricE- andH-plane element patterns, nonuniform element spacings and arbitrary complex excitation coefficients. Selective numerical data are presented to demonstrate the usefulness of this result. Comparisons are made with the available results obtained using direct numerical integration techniques, and with other available data based on less general formulations. Excellent agreement is observed for all cases. In particular, data are presented on the directivity of seven-element cluster feeds used in multiple beam designs, and an array feed for producing a contour beam covering the United States Eastern Time Zone (ETZ).     

Array-fed reconfigurable reflector for spacecraft systems

The authors describe a new type of reconfigurable reflector antenna for spacecraft applications in which both the reflector shape and the excitation of a small array feed combine to offer maximum coverage flexibility. To create a reconfigurable continental coverage the array is excited to produce a suitably tapered circularly symmetric pattern and the shape of the mesh reflector adjusted to yield the appropriate secondary radiation pattern. When spot beams are required within the coverage region, the mesh reflector is configured as a paraboloid and individual elements or local clusters excited within the array     

On the convex layers of a planar set

LetSbe a set ofnpoints in the Euclidean plane. The convex layers ofSare the convex polygons obtained by iterating on the following procedure: compute the convex hull ofSand remove its vertices fromS. This process of peeling a planar point set is central in the study of robust estimators in statistics. It also provides valuable information on the morphology of a set of sites and has proven to be an efficient preconditioning for range search problems. An optimal algorithm is described for computing the convex layers of S. The algorithm runs in0 (n log n)time and requiresO(n)space. Also addressed is the problem of determining the depth of a query point within the convex layers ofS, i.e., the number of layers that enclose the query point. This is essentially a planar point location problem, for which optimal solutions are therefore known. Taking advantage of structural properties of the problem, however, a much simpler optimal solution is derived.     

Compensation of spherical reflector aberrations by planar array feeds

The feasibility of correcting phase aberrations of spherical reflector antennas with planar array feeds has been investigated. This type of feed seems to be particularly attractive for applications requiring several closely spaced beams. A synthesis procedure for the array excitation has been developed which minimizes the mean-square error with respect to a prescribed reflector illumination. This method was applied to the analysis of a spherical reflector antenna with a642lambdadiameter and an effectiveFnumber of 0.9.     

Modulation bandwidth of planar circular grating distributed Bragg reflector lasers

This paper presents an analysis of the 3-dB-modulation bandwidth in planar circular grating distributed Bragg reflector lasers. The theoretical model takes into account both the spatial hole burning effect and the transverse and longitudinal field distribution. That model allows the study of the influence of real structure parameters on the 3-dB-modulation bandwidth.     

Performance characteristics of a narrow-linewidth distributed-Bragg reflector laser for optical remote sensing systems

The performance of a novel ridge-waveguide, distributed-Bragg-reflector laser was tested for spectroscopic remote sensing applications. The laser exhibits a narrow linewidth, excellent side-mode suppression, and flexible, highly repeatable wavelength tuning. These characteristics are significant improvements over current Fabry-Perot lasers, and indicate that distributed-Bragg-reflector lasers are promising candidates for implementing advanced semiconductor-based optical remote sensing systems.     

On the calculation of the directivity of planar array feeds for satellite reflector applications

A calculation of the directivity of a planar array involves a coefficientC_{mn}which is given in an integral form in the literature. A series representation forC_{mn}is derived here, which allows efficient numerical evaluation. Numerical results and comments on the behavior ofC_{mn}are discussed.     

Optimization of planar arrays

A method is developed for maximization of the gain of a planar array at a prescribed sidelobe level. The method is iterative and includes a quadratic programming routine. Numerical examples are given for an octagonal array with a quadratic array lattice and for hexagonal arrays with triangular lattices. The latter arrays are compared to the results obtained by sampling the circular Taylor distribution.     

Estimating the Euler Characteristic of a planar set from a digital image

A new estimator (approximation) for the Euler–Poincaré characteristic of a planar set K in the extended convex ring is suggested. As input, it uses only the digital image of K, which is modeled as the set of all points of a regular lattice falling in K. The key idea is to estimate the two planar Betti numbers of K (number of connected components and number of holes) by approximating K and its complement by polygonal sets derived from the digitization. In contrast to earlier methods, only certain connected components of these approximations are counted. The estimator of the Euler characteristic is then defined as the difference of the estimators for the two Betti numbers. Under rather weak regularity assumptions on K, it is shown that all three estimators yield the correct result, whenever the resolution of the image is sufficiently high.     

基于粗糙集的信息安全系统评估指标优化

信息安全系统效能评估指标体系的建立,是实现信息安全系统效能评估的前提。首先通过德尔菲法获取初始评估指标集;其次根据指标统计数据,运用粗糙集理论的知识约简方法,得到初始指标集的核集,即可剔除冗余指标,从而得到优化的评估指标体系;最后对其进行稳定性分析,验证了指标体系的科学合理性。     

Evaluation of optimal-reliability indices for electricaldistribution systems

A method to calculate the optimal values of reliability indices for a load point in an electrical distribution system is presented. The problem is formulated as an optimization problem and solved by the gradient projection method; the objective is to minimize interruption cost. The algorithm is very useful and powerful for extending the existing network and planning new networks. It has been tested on a practical system (the Port-Fouad power network), and the results are discussed. Once the optimal reliability indices are determined, modification of the system by equipment replacement and future system planning can be done in such a way that the interruption cost is minimized     

Design of a low loss SAW reflector filter with extremely widebandwidth for mobile communication systems

The design and performance of a low-loss surface acoustic wave (SAW) reflector filter are presented. The structure is a dual-track configuration incorporating two interdigital transducers (IDTs) and reflectors in each track. Extremely wide bandwidth can be obtained if chirped IDTs and reflectors are used. From the theoretical conditions for low-loss operation of the reflector filter, a design rule for the distances between the chirped components and for their lengths is derived. Because the transfer function of the filter is predominantly determined by the reflectors, a new synthesis method for chirped reflectors has been developed. For the design of the reflectors, phase-weighting and finger-width-weighting techniques were used to reduce passband distortions and improve the shape factor and stopband rejection. The filter has been fabricated on 128° YX-LiNbO₃ . The center frequency and fractional bandwidth are 200 MHz and 100%, respectively, the minimum insertion loss is 4 dB, a passband ripple is about 1 dB, and the stopband rejection is better than 35 dB     

Aperture feed for a spherical reflector

The problem of designing a feed system for illuminating a spherical reflector is examined. A method is proposed for specifying the required field distribution over the aperture of the feed system, and the primary illumination and gain resulting from this distribution are derived. The results indicate that a significantly smaller feed aperture can be employed than would be indicated by conventional ray tracing methods. Specific numerical results are obtained by taking the Arecibo antenna as an example, for which a calculated aperture efficiency of 67.5 percent is possible with approximately a 38-foot-diameter aperture feed.     

Image-region fields for a stepped reflector

A method is presented for the calculation of the field in the image space of a stepped reflector, when the incident field is a plane wave inclined at an angle to the reflector axis. Each field component is obtained as a Fourier series in the azimuthal variable of the image plane.     

Time-weighted quadratic performance indices for linear systems

Some analytical results are derived and established for the time-weighted quadratic Performance Indices (PF's) of a linear, time-invariant system in this letter. The final cost as well as the recursive formulas are developed and established in concise forms.     

Self-routing planar network for guided-wave optical switching systems

An N*N self-routing planar network suitable for fabricating optical space switches on lithium niobate substrate is presented. The network has O(N/sup 2/) cross points, good modularity and expandability. It also has no waveguide crossovers and no differential attenuation between any of the input and output pairs. A self-routing control can be applied to this network, which is indispensable for ATM switching systems.<>     

Comments on the synthesis of dual offset reflector systems

Comments are made about the theoretical significance of two approaches to the synthesis of dual offset reflector systems under geometrical optics assumptions.     

A line feed for a spherical reflector

A line feed for a spherical reflector is considered on the basis of a plane-wave spectrum of radiation angles. It is shown that a feed excited by circumferential slots results in a gain deterioration of at least 3 dB. The correct excitation of the feed is indicated. Expressions for field components in the focal region are obtained.