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     

超低副瓣天线平面近场测量取样方式的新准则

采用实验的方法给出了用平面近场技术测量超低副瓣天线时收发天线之间的近区测试距离以及取样密度的选取准则 ,提出了超低副瓣天线测量对测试系统温度漂移的要求 ,并给出了满足系统温度漂移要求的测试方式。依据新的选取准则 ,实测了最佳角锥喇叭和波纹喇叭天线的 E面方向图。测试结果说明 ,该选取准则具有良好的重复性 ,且重复精度为 60 d B± 2 d B     

Spatial sampling and filtering in near-field measurements

A sample spacing criterion and a data minimization technique for measurements made over the surface of a plane in the near field of an antenna are presented. The sample spacing is shown to depend on the distance from the antenna to the measurement plane, and on the extent to which evanescent waves can be neglected. The near-field data minimization technique utilizes two-dimensional spatial filtering to effect a significant reduction in computational effort required to calculate selected portions of the far-field pattern. Far-field patterns of anXband antenna calculated from near-field measurements are presented and compared with those measured on a standard far-field range. The far-field calculations are repeated for several near-field sample spacings and for various post-filter sample rates.     

Antenna coupling and near-field sampling in plane-polar coordinates

A probe-corrected vector transmission formula and a rigorous sampling-reconstruction theorem for near-field antenna measurements in plane-polar coordinates are derived from three fundamental theorems of antenna theory: the mutual coupling function between two antennas satisfies the homogeneous wave equation; a receiving antenna can be represented as a differentiator of the incident field; and the mutual coupling function is virtually bandlimited. The rigorous sampling equations are applied to compute the far fields of a circular-aperture antenna sampled in the near field at half-wavelength radial spacing     

Alignment procedures for planar near-field positioners

The alignment of a planar near-field scanner is important for achieving low phase error in measurements and, hence, good pattern data. Non-planarity phase errors can be measured and removed in the data-collection software. However, better mechanical alignment translates into better sidelobe measurements and a higher frequency of operation. This paper describes a procedure for installation and alignment of a planar-scanner positioner, and illustrates the procedure using a box-frame-type scanner. Alignment of the scan plane is independent of the drive system used, since the positional errors associated with the drive system are defined as positional errors     

Electro-optic near-field mapping of planar resonators

A comprehensive experimental and theoretical study for the determination of the electric near-field above planar resonators is presented. The transverse component of the electric field is mapped by external electro-optic (EO) sampling technique with high spatial and temporal resolution. The evolution of the near-field radiation pattern of the investigated 7-GHz planar resonator to the onset of the far-field pattern is traced by measurements at various heights above the sample. Frequency-dependent measurements allow to characterize the field pattern changes when the frequency is swept through the main resonance. Additional undesired resonances are identified by the detected mode pattern. The experimental data are reproduced by simulations based upon an electric field integral equation (EFIE) method     

Effect of random errors in planar near-field measurement

Expressions that relate the signal-to-noise ratio in the near field to the signal-to-noise ratio in the far field are developed. The expressions are then used to predict errors in far-field patterns obtained from near-field data. A technique for measuring the noise in the calculated far-field pattern by calculating the spectrum in the evanescent region from a single-dimensional oversampled scan is also described     

Prediction of far-field from uniform and non-uniform under-sampled near-field data in planar near-field antenna measurements

In near-field antenna measurements various forms of uniform and non-uniform sampling techniques have been widely deployed. Considering the fact that the near-field pattern of any antenna is a spatially quasi-band-width-limited function of space coordinates, Shannon's theorem simply defines the sampling frequency. Based on the sampling theorem, in order to precisely reconstruct a band-limited signal from its samples, the sampling frequency must be at least twice as much as the signal's bandwidth. Through the simulations and theoretical evaluations this research shows that if the near-field pattern is either uniformly or non-uniformly under-sampled due to any practical reasons, yet a good estimation of far-field pattern can be obtained especially if the antenna under test (AUT) is a directive high-gain or super high-gain antenna. Also the time efficiency of far-field prediction from under-sampled near-field data is discussed and the advantages and disadvantages are highlighted.     

Error analysis techniques for planar near-field measurements

A combination of techniques is described for reliably estimating the magnitude of each error arising in planar near-field measurements. They include mathematical analysis, computer simulation, and measurement tests. There are three primary applications for these tests: in designing a measurement facility, the requirements of each part of the measurement system can be specified to meet a given level of accuracy; during actual measurements, the experimenter can identify, and reduce where necessary, potential sources of error in the measurement; and when a measurement has been completed, the estimated uncertainty in the measurement can be obtained with confidence and ease. The latter application has been used in many measurements to verify that the planar near-field technique produces high-accuracy results competitive with any other measurement technique     

Phaseless bi-polar planar near-field measurements and diagnosticsof array antennas

Antenna near-field measurements typically require very accurate measurement of the near-field phase. There are applications where an accurate phase measurement may not be practically achievable. Phaseless measurements are beginning to emerge as an alternative microwave antenna measurements technique when phase cannot be directly measured. There are many important aspects for successful implementation of a phaseless measurement algorithm. This paper presents appropriate phaseless measurement requirements and a phase retrieval algorithm tailored for the bi-polar planar near-field antenna measurement technique. Two amplitude measurements and a squared amplitude optimal sampling interpolation method are integrated with an iterative Fourier procedure to first retrieve the phase information and then construct both the far-field pattern and diagnostic characteristics of the antenna under test. In order to critically examine the methodologies developed in this paper, phaseless measurement results for two different array antennas are presented and compared to results obtained when the near-field amplitude and phase are directly measured     

Formulation of probe-corrected planar near-field scanning in thetime domain

Probe-corrected planar near-field formulas in the time domain are derived for both acoustic and electromagnetic fields, so that a single set of near-field measurements in the time domain yields the fields of the test antenna directly in the time domain. The time-domain probe-corrected formulas are first derived by taking the inverse Fourier transform-of the corresponding frequency-domain formulas, and then by using a time-domain expansion for the fields of the test antenna and a time-domain receiving characteristic of the probe. Because these general formulas, which involve a double integral over the scan plane and an infinite time-convolution integral, are rather complicated, we consider a special probe whose output due to an incoming time domain plane wave is proportional to the time derivative of the field of that plane wave. For this special “D-dot probe”, the probe-corrected formulas simplify to give the time-domain far-held pattern as a double spatial integral of the time-domain output of the probe over the scan plane multiplied by the angular dependence of the inverse receiving characteristic of the probe. Time-domain reciprocity relations are derived for reciprocal probes, and their time-domain receiving characteristics are related to their far fields. Finally, a time-domain sampling theorem is derived and a numerical example illustrates the use of the time-domain probe-corrected formulas     

近场电磁散射特点及其对建模要求

介绍了近场电磁散射问题,从电磁散射角度阐释当前对近场特性认识上的分歧,分析近场电磁散射问题中局部照射与探测器及动态过程关联密切等特点.根据近场特点提出了对建模中目标模型、电磁算法以及计算特性类型等的一般要求,同时提出了一种基于基础近场散射问题与探测器去相关的建模方式,并针对典型弹载雷达探测情况,给出了近场动态散射建模中运动模型、天线模型以及动态采样等具体要求.     

Scan-plane reduction techniques for planar near-field antenna measurements

In this work, two planar near-field scan-plane reduction techniques are considered and results are presented. It is shown how truncation based on field-intensity contours, instead of simple geometric truncation, can in some cases improve the efficiency of the truncation process. Both techniques are applied to measured data sets, and it is shown how these methods can be used to reduce data-acquisition times, while also assessing the impact of the total acquisition surface reduction on the far-field radiation-pattern integrity.     

Numerical beam alignment procedure for planar near-field phaseretrieval

The authors demonstrate mathematically that the direction of an antenna beam (antenna pointing) can be determined by the centre-of-gravity of its beam intensity. The technique is validated using measurements on a 1.118 m 94 GHz Cassegrain reflector and is seen as pivotal in the future application of the phase retrieval technique for near-field/far-field prediction     

基于Gerchberg-Papoulis算法的平面近场截断误差修正方法研究

在平面近场天线测量中,有限扫描面截断是影响测量精度的主要误差源之一,找到解决截断误差的方法是天线测量的研究重点之一.文中将平面近场天线测量中由有限区域内的场求平面波谱的过程抽象为带限函数外推的数学模型,从实际测量中的近远场变换理论出发,论证了GP（Gerchberg-Papoulis）算法应用在平面近场测量中在理论上是切实可行的.将GP算法应用在平面近场天线测量中,并分析了不同迭代次数算法的修正情况.结果表明,随着算法迭代次数的增多,可信角域外计算方向图与理论方向图差别明显减小.因此,本文的方法能够明显减小平面近场测量中截断误差的影响.除此以外,还分析了误差对算法收敛性的影响,结果表明,误差对算法修正效果影响较大.     

时域近场测量采样平面选择分析

利用平面波谱展开理论和时域有限差分法对几种典型天线的远场方向图建模计算,分析和比较了不同采样平面对远场的影响,以及平面波谱理论对强、弱方向性天线的适用性,为进一步误差分析修正奠定了基础。     

模式滤波技术在平面近场天线测量中的应用

为了修正平面近场测量中的多次反射误差,介绍了模式滤波修正技术在平面近场测量中的应用,提出了一种合适的模式滤波函数.推导出模式滤波修正技术的相关公式并进行了数值仿真,仿真结果表明通过利用模式滤波技术对平面近场天线测量结果进行后处理能够有效地改善测量结果.     

天线时域平面近场测试的误差分析

天线时域近场测试技术对误差体系研究的缺失,导致测试结果的不确定度分析一直无法完成.为解决这一问题,以天线时域平面近场测试为例,对时域近场测试的误差进行研究,给出时域测试区别于频域测试技术的四个误差项:探头调制误差、信号源稳定度误差、时间采样间隔误差、时间采样长度误差.在给出误差项后,对误差的产生机理进行了讨论,通过仿真和实测给出了误差对测试结果的影响.     

"时间窗"对天线时域平面近场测试结果的影响

天线时域近场测试技术是一种新兴的、测试宽带场和工作在窄脉冲激励下天线辐射场的高效的测试技术.因为它可以利用"时间窗"技术进行信号处理,使其相对频域测试具有独特的优势.本文在已建立的天线时域近场测试系统的基础上,从实验的角度,对"时间窗"参数在天线时域平面近场测试中的影响进行验证分析.举例了三个波段标准天线方向图的测试分析结果,并得出初步结论.     

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