Design of superdirective endfire antenna arrays

Efficient superdirective array functions can be generated using an optimized polynomial technique. Methods for the generation of these functions are investigated, and the effects of changing the number of elements and array length on the different array parameters are studied. Legendre polynomials have been used for the first time as optimized polynomials, and results are compared to Chebyshev polynomials where higher directivities were obtained without degradation to the array performance. An example of an 11-element two-wavelength array was used to emphasize the possible improvements from using the proposed techniques. The difficulties facing the realization of superdirectivity in practice have been analyzed for the arrays designed. The optimum separation between the array elements was identified for the endfire case     

The theory of endfire arrays

The general King-Sandler array theory has been examined in detail for the case of endfire arrays. Since it is not necessary to assume identical current distributions, a distinction is made between specification of the base voltages and currents. The driving-point impedances for specified base voltages and currents are presented for arrays of up to 25 elements. The effect of interaction between the element currents in distorting the radiation pattern is shown for the 15-element endfire array. The results indicate that the unequal current distributions have a pronounced effect in determining the driving-point impedances, sidelobe levels and back-to-front ratios of endfire arrays.     

Modified progressive phasing for endfire arrays

The general problem of modifying antenna array patterns by adjusting only the phase of the elements is discussed. It is shown that, in the case of endfire arrays, considerable reduction of the sidelobe levels can be obtained by using small departures from progressive phasing. Simple formulas for the required phasing are derived. An alternative method, using only a single pair of "sidelobe-reducing" elements, is also given.     

On the optimum illumination of arrays

A unified method for the optimization of receiving arrays is presented. The optimization is performed by a linear transformation (coordinate transformation) method. Arrays of lossless arbitrarily arranged in space, but uniformly oriented, elements are analyzed.     

Optimum spacing for Cheby?shev arrays

For a Cheby?shev array with a prescribed sidelobe level and given number of elements, an optimum spacing is shown to exist, for any arbitrarily prescribed steering angle, for which the beamwidth is the least obtainable. An expression for the optimum spacing is given.     

Derivative constrained optimum broad-band antenna arrays

The optimization problem corresponding to the design of a presteered broadband processor with maximally flat second-order spatial response in the look direction is discussed. The constraints corresponding to necessary and sufficient conditions for second-order flatness are presented. The nonlinearly constrained optimization approach is unable to consistently locate the global minimum. An approach to solving this problem that essentially converts the nonconvex optimization problem into a parameterized set of convex problems is presented. In the case of 2-D scenarios, the global optimum is determined by finding the roots of a cubic function. The characteristics of the constraints, including the minimum number required and the dependence on the choice of coordinate systems, are examined     

Phase-locked injection laser arrays with nonuniform stripe spacing

A new type of phase-locked injection laser array which utilises nonuniform spacing between the lasing stripes has been studied. The variation in spacing alters the stripe-to-stripe coupling which modifies the array mode selection. Arrays that operate with a single far-field lobe have been fabricated.     

Phase-locked injection laser arrays with variable stripe spacing

A phase-locked injection laser array is described which utilizes variations in spacing of identical lasing elements to vary the coupling between them. In general, phase-locked arrays have been fabricated as periodic structures with uniform coupling, although arrays with different element sizes have been described. An important advantage of the variable spacing array (VSA) structure over those designs is that the fundamental array mode can be readily matched to a uniform pumping profile across the entire array aperture. This allows the variable spacing arrays to be optimized for high-power operation. A coupled-mode analysis indicates that excellent matching of fundamental array mode to a uniform gain distribution can be obtained. However, for some array geometries the operation of array modes other than conventional 0 and 180° phase shift modes is enhanced. Variable spacing arrays have exhibited phase-locked behavior to CW outputs as high as 80 mW (7 elements) and single longitudinal mode operation to powers > 50 mW. Observation of the array emission patterns confirms the results of the coupled-mode analysis.     

The optimum grid to cathode spacing in planar triodes

The behavior of planar triodes with close grid-to-cathode spacings is investigated theoretically and experimentally. Special attention is paid to island formation dimensional variation initial electron velocity and cathode coating resistance. A formula is derived for the optimum grid-to-cathode spacing which yields maximum transconductance, maximum power gain-band product and minimum noise figure.     

Dynamically optimum lookup-table spacing for power amplifier predistortion linearization

The lookup-table-based digital adaptive predistortion (DAPD-LUT) approaches are low cost and effective for power amplifier (PA) linearization in wireless applications. However, most existing DAPD-LUT schemes are sub-optimum because they adopt uniformly spaced LUTs regardless of the system state information (SSI), i.e., the PA characteristics and the input signal statistics. Other existing DAPD-LUT schemes assume either full or partial knowledge of the SSI to optimize and then to freeze the LUT spacing. Without prior knowledge of the SSI, we propose an SSI-learning low-complexity procedure to optimize the LUT spacing for a DAPD-LUT scheme. The proposed procedure is capable of online adapting the LUT spacing for PAs with various nonlinear characteristics, for input signals with various statistics, and for wireless environments with various time-varying properties.     

Assessment of performance limitations of Pon retrodirective arrays

We examine the behavioral characteristics of Pon type heterodyne retrodirective arrays (RDAs) (see Pon, C.Y., IEEE Trans. Antenna Propagat., vol.12, p.176-80, 1966) and provide a simple, yet comprehensive, modeling equation that allows the retransmission response of the RDA to be determined. Using this modeling philosophy, the optimum array size required for a prespecified beam pointing error is determined. Then, deleterious effects, such as mixer-leakage, phase-conjugate circuit gain imbalance and cable phase error, that occur due to the presence of real components, are quantified experimentally for a four-element RDA. These are then correlated with theory. Specific recommendations on the permissible tolerances allowable for these parameters are given in order to assist design specification adherence. In addition, it is shown in theory, and confirmed by measurement, how radiation element aberration due to diffraction and mutual coupling can be used to enhance the azimuthal tracking response of the array. This paper allows the first comprehensive, practical, and theoretical appraisal of the design limitations that exist for this type of array. The insights provided by this paper should be of value to practicing antenna engineers.     

Bandwidth limitations of log-periodic microstrip patch antenna arrays

Results are presented for the frequency dependence of the propagation constant of uniform microstrip patch arrays that allow the bandwidth limitations of log-periodic patch arrays to be deduced. It is found that direct coupling of the patch to the feed line limits the log-periodic bandwidth to about a 2:1 frequency range. However, the introduction of series reactance into the equivalent circuit such as by the use of overlaid patches indicate that log-periodic action over a very wide bandwidth is possible which will then be limited by the uniform substrate and production tolerances.     

Constraints for maximally flat optimum broadband antenna arrays

Frequency response shaping for the direct form pre-steered broadband (PB) antenna array processor is often achieved by imposing look direction constraints on the weights of the processor. This results in a linearly constrained optimization problem. To ensure a maximally flat spatial response of a specified order in the look direction of the PB processor, additional constraints known as derivative constraints can be further imposed on the weights. In general, derivative constraints corresponding to necessary and sufficient (NS) conditions for a maximally flat spatial power response can result in a quadratic equality constrained optimization problem. We transform the quadratic NS derivative constraints to parameterized linear forms. These parameterized linear forms allow the global optimum of the quadratic equality constrained optimization problem to be obtained easily. They also provide a general framework for deriving new sets of derivative constraints which correspond only to sufficient conditions for a maximally flat spatial power response. These sufficient derivative constraints are useful for real-time processing because of their reduced computational requirements and because they ran deliver performance comparable to the NS derivative constraints     

Wavelength fluctuations in pulsed variable spacing phase-locked laser diode arrays

Spectra taken for a pulsed variable spacing phase-locked laser diode array (VSA) were recorded and processed to determine the magnitude of wavelength fluctuations present in the diode's output field. Analysis revealed that a pulsed VSA emits primarily into a single longitudinal mode.     

The optimum directivity of uniformly spaced broadside arrays of dipoles

The optimum directivity of various types of uniformly spaced broadside arrays of dipoles is investigated theoretically in this paper. The formulation is processed with the aid of an array matrix. The expression for the optimum directivity and the corresponding excitation are expressed directly in terms of the elements of the array matrix. The computed values are assembled in several sets of curves, and the results are compared with the directivity of uniformly excited arrays.     

Calculation of optimum spacing for a three coil axially symmetric metal detector

When designing an in-line metal detector, a three coil axially symmetric design is often employed. For maximum sensitivity to small particles, the spacing of the coils is an important factor. A formula is derived which predicts the signal produced by a metal sphere when moved along the axis of such a detector and from this a graph is produced to determine the optimum coil spacing.<>     

圆形口径平面天线阵列的多约束稀布优化方法

针对圆形口径平面稀布阵列的多约束优化设计问题,以均匀同心圆环阵列的阵元位置分布特性为基础,构造了稀布圆阵的满足多个优化约束的可行初始解.提出了一种个体元素的间接表示法,设计了一种新的交叉算子和变异算子,运用改进的实数遗传算法优化设计天线阵的阵元位置.优化约束包括阵元数约束、口径约束和最小阵元间距约束,优化目标是使阵列响应的峰值旁瓣电平最小.运用这种改进实数遗传算法可以充分利用阵元布阵的自由度,同时能减小搜索空间,提高计算效率。仿真试验证实了算法的稳健性和有效性.     

一种稀布矩形平面阵的多约束优化方法

针对矩形孔径平面稀布阵的多约束优化问题(包括阵元数、阵列孔径和最小阵元间距约束), 提出了一种基于矩阵映射的差分进化算法.该方法把差分进化算法的优化变量与阵元位置坐标按照特定的关系进行矩阵映射, 使含有多约束的阵元分布优化问题转换为仅含差分进化算法优化变量上、下限约束的优化问题, 从根本上避免了进化过程中的不可行解.通过抑制阵列峰值副瓣电平进行仿真实验, 结果显示了该算法的高效性和稳健性, 且能获得比现有方法更好的优化结果.     

Application of near-field optimum microphone arrays to hands-free mobile telephony

This paper discusses the application of fixed microphone arrays to speech pickup in mobile telephone applications. Array optimization techniques are used to design two broad-band beamformers for speech pickup in the near field. The first beamformer provides optimum gain for spatially incoherent noise while the second beamformer provides optimum gain in spherically isotropic noise. Array performance was measured using vehicular noise recorded under realistic driving conditions. Results obtained are in agreement with theoretical predictions for a spherically isotropic noise field and are comparable to previously reported results obtained using adaptive beamforming algorithms.