Application of spherical near-field measurements to microwaveholographic diagnosis of antennas

Microwave diagnosis of antennas is considered as a viable tool for the determination of reflector surface distortions and location of defective radiating elements of array antennas. A hybrid technique based on the combination of the spherical near-field measurements and holographic metrology reconstruction is presented. The measured spherical near-field data are first used to construct the far-field amplitude and phase patterns of the antenna on specified regularized u-v coordinates. These data are then utilized in the surface profile reconstruction of the holographic technique using a fast-Fourier-transform (FFT)/iterative approach. Results of an experiment using a 156-cm reflector antenna measured at 11.3 GHz are presented for both the original antenna and the antenna with four attached bumps. Several contour and gray-scaled plots are presented for the reconstructed surface profiles of the measured antennas. The recovery effectiveness of the attached bumps has been demonstrated. The hybrid procedure presented is used to assess the achieved accuracy of the holographic reconstruction technique because of its ability to determine very accurate far-field amplitude and phase data from the spherical near-field measurements     

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     

A two probes scanning phaseless near-field far-field transformationtechnique

An innovative and effective technique to determine the far-field of a radiating system from near-field intensity data is introduced, analyzed, and tested. The approach is based on the simultaneous measurement of the amplitude of the voltages received by two different probe antennas moving over a single scanning surface in the near zone and performs the phase retrieval of the near-field by assuming as unknown the plane wave spectrum of the field. The radiated field is then straightforwardly evaluated. As compared to the existing phaseless measurement techniques, the use of two different probes makes it possible to avoid the need for a second scanning surface and thus allows the use of smaller (and cheaper) anechoic chambers. Furthermore, the measurement time is essentially equal to that required by conventional techniques based on the measurement of the complex near-field. The reliability and the effectiveness' of the approach are investigated and discussed and the key factors affecting its behavior are highlighted. In particular, the relevance of the difference between the plane wave spectra (PWS) of the two probe antennas in ensuring an acceptable reliability of the solution, with respect to the starting point of the procedure, is outlined. Finally, the effectiveness of the approach is confirmed by an extensive numerical analysis, which also shows the stability of the solution against data noise     

球面近远场和远近场变换算法

天线的远场对于研究天线辐射特性具有重大意义，近场测量技术因其能够避免直接测量远场而得到广泛应用，该技术采用近远场变换获得远场，然而，检验该远场的准确性也是很重要的.为了解决此类问题，文中以球面近场测量为例，提供了一种解决方案.该方案主要探讨了球面波模式展开理论，该理论是实现球面近远场变换算法的关键，其将待测天线在空间建立的场展开成球面波函数之和，天线的加权系数既包含了远场信息也包含了近场信息.因此，不仅能够利用近场测量信息获得远场辐射特性，同样能够利用远场辐射特性反推得到近场处电场，这样就能检验由近远场变换算法得到的远场是否准确.文中首先推算得到了近远场变换公式，随后进一步推算得到远近场变换的公式，最后将本文算法计算结果与FEKO测量结果进行比较，二者吻合良好，从而证实了本文两种算法的有效性.     

基于未知源域重构的无相位近场测量技术

天线的远场对于研究天线辐射特性具有重大意义,由于远场的直接测量有着诸多限制,近场测量技术计算远场因其简洁准确的特点得到广泛应用. 然而,传统的近场测量技术要求获取近场区的幅度和相位分布才能发挥作用,随着天线频率的升高,人们想要在近场区获取准确的相位信息变得十分困难. 为了解决该技术难题,文中提出一种无相位近场测量技术. 利用一个封闭面上的幅度信息重建或猜测出包围待测天线的球面切向电场分布,并采用遗传算法进行全局优化,其最初为四组随机数据,经过数次优化后将逐渐接近准确结果. 仿真结果表明,本文方法能够在忽略相位信号的前提下,计算出准确的远场辐射特性.     

Determination of far-field antenna patterns from near-field measurements

In many cases, it is impractical or impossible to make antenna pattern measurements on a conventional far-field range; the distance to the radiating far field may be too long, it may be impractical to move the antenna from its operating environment to an antenna range, or the desired amount of pattern data may require too much time on a far-field range. For these and other reasons, it is often desirable or necessary to determine far-field antenna patterns from measurements made in the radiating near-field region; three basic techniques for accomplishing this have proven to be successful. In the first technique, the aperture phase and amplitude distributions are sampled by a scanning field probe, and then the measured distributions are transformed to the far field. In the second technique, a plane wave that is approximately uniform in amplitude is created by a feed and large reflector in the immediate vicinity of the test antenna. And in the third technique, the test antenna is focused within the radiating near-field region, patterns are measured at the reduced range, and then the antenna is refocused to infinity. Each of these techniques is discussed, and the various advantages and limitations of each technique are presented.     

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。     

Characterization of Real-World Steered-Beam Antennas from Amplitude-Only Near-Field Data

Phase-less near-field techniques are becoming increasingly more important for antenna characterization, due to the growing interest in millimeter-and sub-millimeter-wave applications, where the near-field phase is difficult or even impossible to measure. In this framework, the routine application of phase-less near-field/far-field (NFFF) transformations to real-world operational antennas is a challenging problem, recently questioned in the literature, requiring algorithms capable of providing reliable and accurate results over a large class of radiators. In the present paper, the possibility of applying phase-less near-field techniques for routine testing of antennas is discussed. We point out how -following the recent developments in the field, and by a formulation of the problem based on proper representations of unknowns and data - it is possible to gain the reliability and the accuracy required for this. Experimental tests were carried out on steered-beam antennas, which have lately been pointed out as "difficult" workbenches, to test the feasibility of operational phase-less near-field/far-field transformations. The experimental results refer to a reflectarray radiating a tilted beam, and to a phased array of large electrical dimensions, radiating a scanned beam and actually employed in real-world applications.     

A near-field to far-field transformation for spheroidal geometry utilizing an eigenfunction expansion

This work presents a near-field to far-field (NF-FF) transformation for antenna and scatterer radiation evaluation. The transformation allows practical computation by making use of a sampling surface in the near-field that is spheroidal in shape: namely a prolate or oblate spheroid. The resulting vector wave equation does not support orthogonal vector solutions in spheroidal coordinates and instead rectangular field components are solved for using the scalar wave equation in spheroidal coordinates. The new transformation only requires knowledge of the completely-specified near-field electric field along the spheroidal transformation surface and does not need any information associated with the corresponding magnetic field. The benefit of using a spheroidal near-field geometry is its ability to closely conform to both linear and planar radiating structures while still permitting evaluation of the full far-field radiation pattern. Our approach makes use of an eigenfunction expansion of spheroidal wave-harmonics to develop two distinct, yet closely related, NF-FF transformation algorithms for each type of spheroidal surface. The spheroidal NF-FF transformation is validated and performance assessed using a well-characterized radiation structure. By applying the prolate and oblate algorithms to a radiating structure with known analytical near- and far-field electric fields, viz., a filament dipole with sinusoidal current distribution, we are able to setup and conduct multiple numerical tests that serve as a proof-of-concept for the spheroidal NF-FF transformation.     

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     

Near-field qualification methodology

A methodology that is a combination of analysis, computer simulation, component certification, self-tests, and comparison tests is presented for the accuracy qualification of near-field antenna measurement ranges. The analysis uses closed-form equations to establish upper-bound far-field determination errors due to near-field measurement errors. Computer simulation is used to model the specific near-field measurement errors associated with the near-field measurement system components. The closed-form equations and computer simulations are used to form a near-field error budget for each of the near-field measurement system components. A near-field system component certification is undertaken to measure the near-field measurement system component error and establish that they are within the error budget     

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     

New method of far-field reconstruction from Fresnel field

The letter is concerned with the near-field/far-field transformation based on the pseudosampling representations of fields. In the Fresnel small-angle approximation the present procedure allows the far-field reconstruction from near-field samples without requiring a matrix inversion. The approach drastically reduces the storage requirements of the 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。     

半导体激光器近场分布测量系统

基于半导体激光器远场分布新模型和反源计算方法，建立了一套半导体激光器近场分布测量装置。系统的测量机制是利用测量的远场数据，反源计算近场分布。该装置由计算机控制，自动完成远场产及拟合处理，经反源计算得到近场分布。     

基于矩阵差分的远场和近场混合源定位方法

混合源定位在无源雷达中发挥着重要作用.针对均匀圆阵下基于相位差方法的定位精度较低的问题,该文提出基于矩阵差分的远场和近场混合源定位方法.首先,利用二维多重信号(2-D MUSIC)分类方法估计出远场源的方位角和俯仰角;随后,利用协方差矩阵差分方法提取出近场源差分矩阵,通过改进的类旋转不变估计信号参数(ESPRIT-li...     

An Effective NF-FF Transformation Technique With Planar Spiral Scanning Tailored For Quasi-Planar Antennas

An effective probe compensated near-field—far-field transformation technique with planar spiral scanning is developed in this paper. It makes use of an ellipsoidal modelling of the source, instead of the previously adopted spherical one. Such a modelling, tailored for quasi-planar antennas, allows one to consider measurement planes at a distance smaller than one half the antenna maximum size, thus reducing the error related to the truncation of the scanning surface. Moreover, it is quite general, containing the spherical modelling as particular case, and reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.      

Comparison of measured and calculated antenna sidelobe couplingloss in the near field using approximate far-field data

Computer programs presently exist to calculate the coupling loss between two antennas provided that the amplitude and phase of the far field are available. It is shown that when this far-field information is not available it is possible to specify approximate far fields from a knowledge of the sidelobe of each antenna along the axis of separation and the electrical size of each antenna. The ENVLP computer program developed by M.H. Francis and A.D. Yaghjian (ibid. vol.AP-34, p.952-5, July 1986) was modified for this purpose. Measurements of near-field coupling loss between two moderately sized microwave antennas were made to determine the effectiveness of using approximate sidelobe level data instead of the detailed far fields. Comparison of the measured and computed coupling indicates that the use of approximate far fields gives an estimate of the coupling loss with an uncertainty of about ±5 dB     

A genetic algorithm based method for source identification andfar-field radiated emissions prediction from near-field measurements forPCB characterization

A new method for predicting the far-field radiated emissions and for finding the radiation sources of a device from near-field measurements is presented. It is based on the substitution of the original device by an equivalent set of elemental dipoles, placed over the main radiating sources, which radiate the same near-field (and therefore, far-field). This equivalent set of elemental dipoles is generated using a genetic algorithm. From the position and type of the equivalent elemental dipoles, the position of the actual radiating sources is determined. Since the field produced by an elemental dipole is known, the far-field radiation of the actual radiating source can be calculated. The new method has been tested using synthetic data and real measurements from the radiation generated by a modem PCB demonstrating its viability and usefulness