Linear array characteristics with one-dimensional reactive-regionnear-field scanning: simulations and measurements

A model for simulating the performance of a phased-array antenna with one-dimensional near-field scanning is addressed. Measurements and simulations in the reactive near-field region for a linear array with monopole elements are described. The array and near-field probe characteristics are simulated by using the method of moments. Measurements and simulations of the array near-zone received voltage, plane-wave spectrum, and far-field data, using centerline scanning at less than one wavelength distance from the antenna, are shown to be in good agreement. The presence of evanescent grating lobes in the plane-wave spectrum is observed. It is shown that a V-dipole theoretical probe antenna can accurately model a practical near-field measurement probe     

Evaluation of adaptive phased array antenna, far-field nullingperformance in the near-field region

A theory for analyzing the behavior of adaptive phased array antennas illuminated by a near-field interference test source is presented. Conventional phased array near-field focusing is used to produce an equivalent far-field antenna pattern at a range distance of one to two aperture diameters from the adaptive antenna under test. The antenna is assumed to be a linear array of isotropic receive elements. The interferer is assumed to be a bandlimited noise source radiating from an isotropic antenna. The theory is developed for both partially and fully adaptive arrays. Results are presented for the fully adaptive array case with single and multiple interferers. The results indicate that near-field and far-field adaptive nulling can be equivalent. The adaptive nulling characteristics studied in detail are the array radiation patterns, adaptive cancellation, covariance matrix eigenvalues, and adaptive array weights     

Analysis of phase-focused near-field testing for multiphase-centeradaptive radar systems

Airborne or spaceborne radar systems often require tests before deployment to verify how well the system detects targets and suppresses clutter and jammer signals. The radar antenna diameter can be large and thus the conventional far-field test distance is impractical to implement. The theory and simulations of phase-focused near-field testing for adaptive phased array antennas is discussed. With near-field source deployment, standard phased-array near-field phase focusing provides far-field adaptive nulling equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. Both main beam clutter sources and sidelobe jammer sources are addressed. The phased array antenna elements analyzed are one-half wavelength dipoles over the ground plane. Bandwidth, polarization, array mutual coupling, and finite array edge effects are taken into account. Numerical simulations of an adaptive antenna that has multiple displaced phase centers indicate that near-field and far-field testing can be equivalent     

Fully Probe-Corrected Near-Field Far-Field Transformation Employing Plane Wave Expansion and Diagonal Translation Operators

Near-field antenna measurements combined with a near-field far-field transformation are an established antenna characterization technique. The approach avoids far-field measurements and offers a wide area of post-processing possibilities including radiation pattern determination and diagnostic methods. In this paper, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near-field measurements. Compared to existing algorithms, this approach exploits the benefits of diagonalized translation operators, known from fast multipole methods. Due to the plane wave based field representation, a probe correction, using directly the probe's far-field pattern can easily be integrated into the transformation. Hence, it is possible to perform a full probe correction for arbitrary field probes with almost no additional effort. In contrast to other plane wave techniques, like holographic projections, which are suitable for highly directive antennas, the presented approach is applicable for arbitrary radiating structures. Major advantages are low computational effort with respect to the coupling matrix elements owing to the use of diagonalized translation operators and the efficient correction of arbitrary field probes. Also, irregular measurement grids can be handled with little additional effort.     

平面波谱恢复算法在平面近场测量中的应用

介绍用于天线平面近场测量的一种近远场变换新算法。该法利用被测天线的平面波谱和口径场幅相分布之间的关系,以及天线口面的约束条件,用G-P迭代算法从平面波谱的置信谱域部分恢复出置信谱域外的平面波谱。这种方法减小了较小截断角下有限扫描面对测量精度的影响,并提高了天线近场测量的效率。     

The bi-polar planar near-field measurement technique, part II:near-field to far-field transformation and holographic imaging methods

A novel customized bi-polar planar near-field measurement technique is presented in a two-part paper. This bipolar technique offers a large scan plane size with minimal “real-estate” requirements and a simple mechanical implementation, requiring only rotational motions, resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper introduced the bi-polar planar near-field measurement concept, discussed the implementation of this technique at the University of California, Los Angeles (UCLA), and provided a comparative survey of measured results. This paper examines the data processing algorithms that have been developed and customized to exploit the unique features of the bi-polar planar near-field measurement technique. Near-field to far-field transformation algorithms investigated include both interpolatory and non-interpolatory algorithms due to the a typical arrangement of the bi-polar near-field samples. The algorithms which have been tailored for the bi-polar configuration include the optimal sampling interpolation (OSI)/fast Fourier transform (FFT), Jacobi-Bessel transform, and Fourier-Bessel transform. Additionally, holographic imaging for determination of antenna aperture fields has been incorporated to facilitate antenna diagnostics. Results for a simulated measurement of an array of infinitesimal dipoles and a measured waveguide-fed slot array antenna are included. Appropriate guidelines with respect to the advantages and disadvantages of the various processing algorithms are provided     

基于FFT的近场——口径场变换方法

本文提出的近场－口径场变换方法基于探头与被测天线（AUT）的耦合方程,由Fourier变换求出AUT发射场的平面波谱的切向分量,然后由Fourier反变换求出被测天线口径场的切向分量,从而达到诊断天线口径场幅相分布的目的。口径场变换中的大部分计算采用了FFT算法,并且引入空间域的重构法,数值精度好,数据处理效率高,可以达到任意的诊断分辨率。通过数值模拟和诊断实验,说明了该方法的正确性和工程实用性。     

扫描波束天线无相位近场测量技术

以寻找高频扫描波束天线近场测量方法为目的, 提出了一种结合差分进化算法和迭代傅里叶变换算法的双平面无相位近场测量方法.首先用线极化探头在近区采集正交方向切向场幅值信息; 其次使用差分进化算法寻找合适的初始迭代相位; 再利用迭代傅里叶变换算法对一扫描面上的相位进行还原; 最后使用采样幅值和还原相位结合近远场变换理论求得天线远场方向图.为验证方法可行性, 以对称振子天线阵为模型, 对不同扫描角时的测量过程进行仿真, 均获得良好结果.     

Reflector antenna fields--An exact aperture-like approach

A new computational approach is presented which allows fast analysis of radiation from large reflector antennas. For an aperture a Fourier transform (FT) relationship does exist between far-field and aperture distribution. Accordingly, the far field can be exactly reconstructed from the knowledge of approximately one sample per lobe (Shannon-Whittaker theorem applied at Nyquist rate). The finite reflector curvature introduces an extra factor in the radiation integral so that the radiation integral is no longer a FT. In order to overcome this difficulty a new pseudosampling expansion, which explicitly takes into account the extra factor, is developed. For parabolic reflector the sampling functions are related to the Fresnel integrals, and the far field can be exactly reconstructed in terms of aperture far-field samples, which can be computed using the fast Fourier transform (FFT). Numerical computations and error analysis show the excellent performance of the method, which can be generalized to deal with arbitrary reflector surfaces and near-field evaluation.     

Far-field accuracy investigation using modulated scattering technique for fast near-field measurements

Near field measurement techniques in conjunction with near-field to far-field transformation algorithms are widely used today. Two of the most important concerns are, firstly, the degree of accuracy achieved, and secondly, the measurement duration. Although high degrees of accuracy can be obtained, the time required to scan completely the near field of an antenna using the classical near-field measurement techniques is rather long. The modulated scattering technique would offer a means to reduce this time by a factor of 10 to 100 while maintaining a reasonable degree of accuracy. Using this technique, however, one introduces further sources of inaccuracy such as the mutual coupling between the elements of the array used to probe the test antenna, and the further limitation of the available measurement dynamic range. In this paper, these two sources of inaccuracy inherent in this technique or other techniques which use a similar set-up, are explored. Multiple reflections between the test antenna and the probe array are ignored. A parabolic reflector is chosen as the test antenna, and an array of dipoles is chosen as the probe antenna in the numerical simulation.     

A plane-polar approach for far-field construction from near-field measurements

It is well-known that the far field of an arbitrary antenna may be calculated from near-field measurements. Among various possible nearfield scan geometries, the planar configuration has attracted considerable attention. In the past the planar configuration has been used with a probe scanning a rectangular geometry in the near field, and computation of the far field has been made with a two-dimensional fast Fourier transform (FFT). The applicability of the planar configuration with a probe scanning a polar geometry is investigated. The measurement process is represented as a convolution derivable from the reciprocity theorem. The concept of probe compensation as a deconvolution is then discussed with numerical results presented to verify the accuracy of the method. The far field is constructed using the Jacobi-Bessel series expansion and its utility relative to the FFT in polar geometry is examined. Finally, the far-field pattern of the Viking high gain antenna is constructed from the plane-polar near-field measured data and compared with the previously measured far-field pattern. Some unique mechanical and electrical advantages of the plane-polar configuration for determining the far-field pattern of large and gravitationally sensitive space antennas are discussed. The time convention exp (j omega r) is used but is suppressed in the formulations.     

Analysis of mode field radius calculated from single-mode fiber far-field radiation patterns

We have investigated the angular range and mesh size required to calculate accurate mode field radii (MFR) from single-mode fiber far-field radiation patterns using the inverse Hankel transform. A computer aided fiber modeling program was used to calculate near-field mode distributions for both step-index and triangular index fibers at 1.55 μm. These distributions were then transformed to the far-field and used as input "data" for the inverse transform studies. Experimental far-field data measured on fibers similar to those modeled were also used. We find that the MFR is insensitive to mesh sizes less than 0.6° but the range of far-field data required depends upon the fiber design and the criteria used to determine the MFR. This study provides guidelines for calculating accurate MFR from experimental data when using the Hankel transform.     

A technique of synthesizing the excitation currents of planararrays or apertures

A technique of synthesizing or reconstructing the excitation currents of a planar array of aperture-type antennas from the known near-field patterns of the radiating source is presented. This technique uses an exact solution to the fields radiated by the aperture antenna without disregarding the source currents. Typical numerical computations have been carried out to validate the analytical technique developed. Sensitivity and stability of the numerical computations performed have been studied. The available iterative bandlimited signal extrapolation technique is used to reconstruct the aperture excitation currents only if the far-field patterns of the radiating source are known. Far-field patterns of aperture antennas measured in the laboratory were also used to reconstruct the aperture electric field distribution in the principal plane     

Linear spiral sampling for the bipolar planar near-field antennameasurement technique

This paper investigates linear spiral sampling for bipolar planar near-field antenna measurements. This sampling scheme is, depending on range implementation, the most rapid polar near-filed data acquisition mode. The near-field to far-field transformation is performed using a modified optimal sampling interpolation (OSI)/fast Fourier transform (FFT) approach. Measured far-field pattern results for a waveguide-fed slot array antenna are presented and are shown to have excellent agreement with results obtained from a conventional bipolar measurement     

Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays

We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.     

基于幅相误差阵列的远近场混合信号超分辨测向方法

大多数的超分辨测向方法都需要掌握准确的阵列流型,然而在实际应用当中,各阵元通道对信号的幅度增益和延时往往不一致,使得真正的阵列流型和它的理论模型之间存在一定的误差,最终造成测向性能的下降.针对这个问题,论文提出了一种基于幅相误差阵列的远近场混合信号超分辨测向估计方法.首先对信号的空间谱函数进行变换判断出远场信号方向,接着根据远场信号子空间和噪声子空间的正交性估计出阵列误差并对数据进行校正,在此基础上通过矩阵分解判断出近场信号方向,同时还能够实现近场信号的定位.所提方法直接对信号空间谱函数分母的多项式求根得出信号方向和近场信号距离,回避了谱峰搜索的过程,在保证一定精度的前提下大大提高了计算速度.     

Synthetic aperture radiometry evaluated by a two-channeldemonstration model

The Technical University of Denmark (TUD) Synthetic Aperture Radiometer (SARad) is a two-channel demonstration model that can simulate a two-dimensional (2D) thinned array radiometer having an unfilled aperture populated with several small antenna elements. Aperture synthesis obtained by interferometric measurements using the antenna elements in pairs, followed by an image reconstruction based on an inverse Fourier transform, results in an imaging instrument without the need of mechanical scan. The thinned aperture and the nonscanning feature make the technique attractive for spaceborne radiometer systems, especially at low frequencies. The TUD SARad demonstration model consists of a two-channel K_u-band correlation radiometer with two horn antennas and an antenna mounting structure enabling the horns to be mounted in relevant positions within a certain aperture. A total aperture synthesis is obtained by sequentially placing the two antenna elements in all required pairs of positions and measuring the corresponding samples of the visibility function. The system has been used to demonstrate 2D synthetic aperture imaging of complex targets in outdoor ground experiments, a special feature of the system is that it uses a focused antenna system, thus enabling a short distance to the target. Set still utilizing image reconstruction algorithms identical to those used in a normal far-field situation. The aperture synthesis theory is discussed, with special emphasis on focused systems; the radiometer system is described; and images suitable for demonstration of resolution and other imaging properties are presented and discussed     

Passive Localization of Mixed Near-Field and Far-Field Sources Using Two-stage MUSIC Algorithm

Passive source localization is one of the issues in array signal processing fields. In some practical applications, the signals received by an array are the mixture of near-field and far-field sources, such as speaker localization using microphone arrays and guidance (homing) systems. To localize mixed near-field and far-field sources, this paper develops a two-stage MUSIC algorithm using cumulant. The key points of this paper are: (i) in the first stage, this paper derives one special cumulant matrix, in which the virtual ?steering vector? is the function of the common electric angle in both near-field and far-field signal models so that source direction-of-arrival (DOA) (near-field or far-field one) can be obtained from this electric angle using the conventional high-resolution MUSIC algorithm; (ii) in the second stage, this paper derives another particular cumulant matrix, in which the virtual ?steering matrix? has full column rank no matter whether the received signals are multiple near-field sources or multiple far-field ones or their mixture. What is more important, the virtual ?steering vector? can be separated into two parts, in which the first one is the function of the common electric angle in both signal models, whereas the second part is the function of the electric angle that exists only in near-field signal model. Furthermore, by substituting the common electric angle estimated in the first stage into one special Hermitian matrix formed from another MUSIC spectral function, the range of near-field sources can be obtained from the eigenvector of the Hermitian matrix. The resultant algorithm avoids two- dimensional search and pairing parameters; in addition, it avoids the estimation failure problem and alleviates aperture loss. Simulation results are presented to validate the performance of the proposed method.     

基于重加权?1范数惩罚的远近场混合源定位算法

现有信源定位方法大多假定信源是远场源或近场源,而实际定位系统中往往存在远场源和近场源共存的情况。为实现远、近场源分离及高精度信源定位,本文在稀疏信号重构理论框架下提出了一种新的远近场混合源定位算法。该算法利用阵列协方差矩阵反对角线元素和重加权l1范数惩罚获得所有信源的到达角（Direction Of Arrival, DOA）估计。在DOA估计的基础上,根据远场与近场源距离参数位于不同区间的特点利用一维搜索实现远、近场源分离以及近场源距离参数的估计。从理论角度分析了重加权l1范数惩罚算法的重构性能。本文所提算法不仅同时适用于高斯和非高斯信号,而且无需多维搜索和参数配对,也无需信源数的先验信息,同时还可以获得较好的定位精度。计算机仿真结果验证了所提算法的有效性。