Be/spl acute/zier representations for the multiobjective optimization of conformal array amplitude weights

For conformal phased arrays, generally the excitation amplitude of the array elements must be adjusted in order to maintain low sidelobes as the array is scanned. While the desired phase weights for maximum gain are deterministically set by the array geometry and scan angle, the representation of optimum low sidelobe amplitude weights remains an open problem. Following up on prior work using the efficiency-constrained optimization of a modified Bernstein polynomial for low sidelobe conformal array synthesis, a Be/spl acute/zier surface is shown to provide a good representation of the optimized amplitude weights with a reduced number of parameters, while demonstrating /spl epsiv/-constraint multi-objective optimization of conformal aperture efficiency versus sidelobe level. These results are extended to include a Be/spl acute/zier volume representation for the multiobjective optimization of conformal aperture efficiency versus both sidelobe level and scan angle.     

Millimeter-wave Low Sidelobe Time Modulated Linear Arrays with Uniform Amplitude Excitations

The study of millimeter-wave low sidelobe linear arrays with time modulation technique is presented in this paper. Special emphasis is on the realization of low sidelobes with uniform amplitude excitations, which are easier for the practical implementation. Two types of time modulated linear arrays with uniform amplitude excitations, namely, the variable aperture size (VAS) linear arrays with optimized time sequences and the linear arrays with optimized random time sequences were studied. The optimized time sequences are obtained via the differential evolution (DE) algorithm and the genetic algorithm (GA). Many factors that affect the array performance are discussed.     

Statistically designed density-tapered arrays

The design of "thinned" planar array antennas is considered in which the density of elements located within the aperture is made proportional to the amplitude of the aperture illumination of a conventional "filled" array. Density tapering permits good sidelobe performance from arrays of equally radiating elements. The selection of the element locations is performed statistically by utilizing the amplitude taper as the probability density function for specifying the location of elements. The statistical design procedures and the theoretical prediction of performance are given. Application to a 50 wavelength diameter planar aperture is discussed and the results compared to conventional amplitude-taper designs. Examples of computed patterns are shown for density tapers modeled after 25, 30, 35, and 40 db circular Taylor distributions. The properties of a planar array of 10,000 elements are examined for "natural" thinning and for 70 per cent and 90 per cent of the elements removed. The sidelobes are determined more by the number of remaining elements than by the model amplitude taper. Statistically designed density-tapered arrays are useful when the number of elements is large and when it is not practical to employ an amplitude taper to achieve low sidelobes.     

Sidelobes Due to Periodic Amplitude and Phase Errors in the Aperture Field of a Radial Waveguide Pin-Fed Array Antenna

The location and level of the sidelobes of a radial waveguide pin-fed array antenna, which stem from periodic amplitude and phase errors in the actual antenna aperture field, are derived based on a simple model of a line source as well as the pertinent model of a circular aperture. The theoretical results are verified by comparisons with numerical and measured data     

Transmit patterns for active linear arrays with peak amplitude andradiated voltage distribution constraints

Distribution functions used in array antenna design typically synthesize specified pattern characteristics without consideration for either the peak amplitude of the radiating elements or the aperture radiated power. There do exist applications, however, in which the pattern synthesis must employ such constraints. In the transmit mode of active array antennas, for example, it is desirable to radiate as much power as possible subject to a per-element peak amplitude constraint while simultaneously suppressing the outer sidelobes. This paper discusses the design considerations of the constrained least squares (CLS) distribution function. In the CLS distribution, most of the radiating elements near the array center are set to their maximum value while only a few of the outer elements are tapered. A method for generating CLS distributions given constraints on both the peak element amplitude and the total effective radiated voltage (ERV) is discussed. The design involves specifying the desired ERV and a weighting function that allows selectively suppressing sidelobes in specified regions. The effects of these design parameters on the far-field patterns are explored     

Interleaved thinned linear arrays

This paper presents three approaches to improving the efficiency of an array aperture by interleaving two arrays in the same aperture area. The interleaved arrays have aperiodic spacings that are integer multiples of a set minimum spacing and are optimized to reduce the maximum sidelobe level. Fully and partially interleaved sum arrays operating at the same frequencies are demonstrated as well as interleaved sum and difference arrays for a monopulse system. A genetic algorithm is used to optimize arrays of isotropic point sources as well as arrays of dipoles modeled using the method of moments. Narrow beamwidths are possible while avoiding high sidelobes. The available aperture area is efficiently used.     

Low sidelobe electronically scanned antenna using identicaltransmit/receive modules

A microwave network is described for use with identical transmit/receive modules such that amplitude-tapered aperture distributions are obtained with no loss. High-efficiency low-sidelobe patterns are calculated for the line source case with sidelobes as low as -27.5 dB. An optimum minimum-loss design is described for arbitrarily low sidelobes. The effects of errors on sidelobes are shown to be the same as in any array. The extension of the technique to the planar array case is presented     

Generating a plane wave with a linear array of line sources

Creating a plane wave across an antenna under test is important for accurate antenna measurements. This paper optimizes the location and weightings of an array of line sources in order to approximate a plane wave at a given location in space. The amplitude and phase ripples across a desired test aperture are optimized to be much less than that of a uniform array with the same number of elements. Results are presented for a nine-element array with optimized amplitude and phase weights, with optimized weights and spacing in the x-direction, and with optimized weights and spacing in the x and z directions. The optimized approximate plane wave is a significant improvement over a uniform array or a single line source.     

基于谱外推的虚拟孔径阵列旁瓣抑制技术

分裂发射虚拟孔径阵列是MIMO成像雷达中常采用的一种阵列结构,其在稀疏分布的情况下存在高旁瓣问题。阐述了MIM0雷达的空间谱成像理论,并从谱填充的角度分析了在分裂发射虚拟孔径阵列中高旁瓣产生的原因,进而提出一种基于谱外推的分裂发射虚拟孔径阵列旁辨抑制方法。该方法利用AR模型在二维空间谱域进行数据外推处理,从而得到完整的空间谱支撑域,在保持分辨率的同时有效地抑制了旁瓣。仿真数据和实测数据处理验证了该方法的有效性。     

Efficient ultrasonic synthetic aperture imaging

Synthetic aperture imaging employing a single active array channel in data acquisition, enables low cost systems to be realised, but produces poor image quality. An investigation is presented into a new synthetic aperture system which uses a single active transmit element and two active receive elements to carry out data acquisition. This approach improves the image quality by suppressing the grating and sidelobes, while preserving the simplicity of the front-end hardware     

Beam Forming with Nonsinusoidal Coded Waveforms

The principle of beam forming based on a line array of sensors for nonsinusoidal signals with rectangular time variations is advanced to include beam forming of coded nonsinusoidal signals. The energy patterns of coded waveforms using complementary codes have a narrow main beam for small angles of incidence, a large number of sidelobes forming minor beams for large angles of incidence, and a constant value of 1/N, where Nis the number of sensors in the line array. Arrays with a large number of sensors N ? 64 have array gains that reduce the large amplitudes of the sidelobes significantly, eliminate the sidelobes with small amplitude, and decrease the final value to 1/N. The resolution angle e can be reduced by increasing the array length L or by increasing the bandwidth ?f = 1/2T, where T is the nominal pulse duration of the coded waveform.     

Sidelobe reduction for a microstrip travelling-wave antenna array using partially random weighting

The sidelobe performance of a microstrip travelling-wave antenna array is limited by the crosspolarised lobes produced if a comb-line is used with a large range of stub widths.1.2 The amplitude taper which can be applied is thereby limited to ?6?7.5 dB, so that very low sidelobes (< ?30 dB) cannot readily be achieved. Here a technique is described which offers copolar sidelobe reduction without excessive crosspolarisation, with calculated sidelobes <30 dB for a 32-element array. An amplitude taper of 20 dB is achieved using a split comb-line (giving 12?15 dB taper) plus partially random weighting.     

Using continuous aperture illuminations discretely

An iterative procedure is described for fitting a continuous aperture illumination function to a small discrete element array such that the far-field antenna pattern has prescribed sidelobe behavior. Control of successive sidelobe levels in the array factor pattern is used to minimize an error expression comprised of the sum of the squares of the differences between calculated and specified levels for a selected number of sidelobes. The coefficients of a finite series defining the continuous aperture illumination are the variables employed in a Newton-Raphson solution. A sum beam aperture excitation for meeting the 20/30 dB sidelobe requirements of the 1973 OTP manual is derived for a small array and shown to be a useful approximation for a larger array situations. The procedure has been extended to difference beam illumination functions. Extensions to other aperture illuminations are indicated.     

Low-sidelobe arrays fed by a uniform-distribution feeding network

Low-sidelobe arrays typically use feeding networks with appropriate amplitude and phase distributions, which, in some cases, can be lossy and narrow-band as a result of their complexity. In the new approach presented in this paper, a simple uniform distribution feeds the array elements, which are not uniformly positioned in the traditional uniform grid. The location of the array elements is such that the sidelobes of the array factor are “pushed” in the end-fire direction. There, the element pattern is low enough in amplitude to effectively suppress the array-factor sidelobes to the required level. The combination of the simple uniform-distribution feeding network and the spatial distribution of the array elements results in a simple design for a low-sidelobe array     

Sidelobe suppression in 76-GHz post-wall waveguide-fedparallel-plate slot arrays

Sidelobe suppression is demonstrated for 76-GHz post-wall waveguide fed parallel-plate slot arrays for car radar applications. Taylor distribution illumination control is successfully confirmed experimentally in the slot array on the parallel-plate waveguide. The amplitude distribution in the transverse direction is controlled by shaping a quasi-TEM mode and that in the longitudinal direction is realized by the slot coupling control, both in an oversized waveguide. Measured sidelobes are below -22 dB for one-dimensional (1-D) Taylor tapering while those for two-dimensional (2-D) Taylor distribution are below -18 dB. The gain reduction in comparison with that for uniform illumination is less than 1 dB both in the experiment. All these verify the potential of aperture illumination control of the parallel-plate slot arrays     

Focal region characteristics of focused array antennas

The effects of focused aperture amplitude tapering, both direct and inverse, on axial forelobes and aftlobes are investigated. Taylor tapers give low sidelobes but high forelobes and aftlobes, plus a modest gain degradation. Inverse tapers give low forelobes and aftlobes but high sidelobes and large gain degradation. Uniform excitation gives comparable axial and transverse subsidiary lobes, and is probably the best choice. Focal shift, where the peak axial power density occurs between the aperture and the phase (geometric) focus, due to the1/R^{2}factor, is accurately calculated for a uniform square aperture. Since peak axial power density available from a focused aperture depends only on aperture size in wavelengths and on distance, these focal shift results allow system trade-offs to be made.     

基于格理论的非均匀稀疏线阵旁瓣结构的分析方法

 在有限个阵元的情况下,非均匀稀疏线阵能得到更大的阵列孔径。但由于其是对空间信号的非均匀采样,不能通过常规的傅立叶变换方法求得其峰值旁瓣解析表达式。本文提出了一种基于格理论的非均匀稀疏线阵的旁瓣结构分析方法。首先建立了阵列流形格的数学模型并对其物理含义进行了仿真分析,然后推导了阵列流形格最近格点与峰值旁瓣的对应关系,从而将非均匀稀疏线阵峰值旁瓣结构分析问题转化为求距阵列流形格原点最近格点问题。该方法可以准确地确定非均匀稀疏线阵旁瓣中增益大于门限电平的旁瓣个数及其各自的方位。计算机仿真结果表明了该方法的有效性和准确性。     

Reduction of sidelobes in uniformly excited arrays with element pattern control

Uniformly excited arrays of identical elements suffer from high close-in sidelobes. Suppression of sidelobe levels can be achieved by tapering the aperture amplitude distribution, but a more complex feed network results. A novel form of amplitude tapering is described, in which element pattern control in uniformly excited arrays is shown to theoretically reduce close-in sidelobe levels.     

Meandering long slot leaky-wave waveguide-antennas

A theoretical study is made of the aperture field distributions, the far-field amplitude patterns, and the mutual coupling characteristics of several long leaky-wave slot antennas cut on the broadwall of a rectangular waveguide mounted in an infinite ground plane. Straight long slots have high inner sidelobes in the far-field amplitude patterns, which reduces their utility as high-performance antennas. To reduce these sidelobes, a long slot may be meandered from waveguide centerline to sidewall and back to centerline in such a way to produce the desired radiation patterns. Calculated results for antennas of several meander patterns are presented and discussed     

最小冗余阵旁瓣抑制算法研究

最小冗余阵作为智能天线中的一种,能以较少的阵元数获得比均匀阵大得多的阵列孔径,从而提高分辨率、简化系统、降低成本。但阵列的非均匀性导致最小冗余阵的旁瓣电平过高,为此文中提出了一种基于子空间的迭代算法抑制其旁瓣。仿真结果表明：此方法收敛速度快,可控旁瓣个数多,并且利用低旁瓣的最小冗余阵,对角度靠近观察方向的干扰进行抑制时,抗干扰能力增强,信号噪声干扰比得到提高。