FFT applications to plane-polar near-field antenna measurements

The four-point bivariate Lagrange interpolation algorithm was applied to near-field antenna data measured in a plane-polar facility. The results were sufficiently accurate to permit the use of the FFT (fast Fourier transform) algorithm to calculate the far-field patterns of the antenna. Good agreement was obtained between the far-field patterns as calculated by the Jacobi-Bessel and the FFT algorithms. The significant advantage in using the FFT is in the calculation of the principal plane cuts, which may be made very quickly. Also, the application of the FFT algorithm directly to the near-field data was used to perform surface holographic diagnosis of a reflector antenna. The effects due to the focusing of the emergent beam from the reflector, as well as the effects of the information in the wide-angle regions, are shown. The use of the plane-polar near-field antenna test range has therefore been expanded to include these useful FFT applications     

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.     

Surface accuracy measurement of a deployable mesh reflector byplanar near-field scanning

Using a near-field antenna measurement facility, it is possible to simultaneously evaluate the surface accuracy of a reflector antenna as well as the far-field pattern of the antenna for a short time. The surface errors of a 2-m deployable mesh reflector for satellite use were measured by a planar near-field system. As a result, the influence of periodic structures, due to the antenna ribs, is clearly observed. Also, the surface accuracy obtained with the near field scanning technique coincides well with that obtained by an optical measurement technique     

An antenna for dual-wavelength radiometry at 21 and 32 GHz

Accurate multiwavelength remote sensing of the atmosphere requires antennas with the same beamwidth at the various frequencies of operation. A single offset antenna with a corrugated feed which meets this criterion at 20.6 and 31.65 GHz is described. The planar near-field (PNF) scanning facility at the National Bureau of Standards (NBS) was utilized to measure the near-field patterns of the overall antenna for various feed positions, and with an apodizer placed on the reflector. Comparison of the far-field patterns, calculated using PNF methods, yielded the optimum configuration. In addition, the facility was used as a far-field range to measure the radiation pattern of the feed. The antenna is presently installed at Stapleton International Airport, Denver, CO, in a dual-channel radiometric system which continuously remotely senses water vapor and liquid, and it is performing satisfactorily.     

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.     

Far-field errors due to random noise in cylindrical near-fieldmeasurements

A full characterization of the far-field noise obtained from cylindrical near-field to far-field transformation, for a white Gaussian, space stationary, near-field noise is derived. A possible source for such noise is the receiver additive noise. The noise characterization is done by obtaining the autocorrelation of the far-field noise, which is shown to be easily computed during the transformation process. Even for this simple case, the far-field noise has complex behavior dependent on the measurement probe. Once the statistical properties of the far-field noise are determined, it is possible to compute upper and lower bounds for the antenna radiation pattern for a given probability. These bounds define a strip within the radiation pattern with the desired probability. This may be used as part of a complete near-field error analysis of a particular cylindrical near-field facility     

Near- and Far-Field Characterization of Planar mm-Wave Antenna Arrays with Waveguide-to-Microstrip Transition

We present the design and characterization of planar mm-wave patch antenna arrays with waveguide-to-microstrip transition using both near- and far-field methods. The arrays were designed for metrological assessment of error sources in antenna measurement. One antenna was designed for the automotive radar frequency range at 77 GHz, while another was designed for the frequency of 94 GHz, which is used, e.g., for imaging radar applications. In addition to the antennas, a simple transition from rectangular waveguide WR-10 to planar microstrip line on Rogers 3003? substrate has been designed based on probe coupling. For determination of the far-field radiation pattern of the antennas, we compare results from two different measurement methods to simulations. Both a far-field antenna measurement system and a planar near-field scanner with near-to-far-field transformation were used to determine the antenna diagrams. The fabricated antennas achieve a good matching and a good agreement between measured and simulated antenna diagrams. The results also show that the far-field scanner achieves more accurate measurement results with regard to simulations than the near-field scanner. The far-field antenna scanning system is built for metrological assessment and antenna calibration. The antennas are the first which were designed to be tested with the measurement system.     

Near-field diagnostics of small printed antennas using theequivalent magnetic current approach

A near-field to far-field transformation based on the antenna representation by equivalent magnetic current (EMC) sources has been proposed and validated experimentally on large high-directivity antenna arrays. In this paper, the use of EMC is extended to the diagnostics of low-directivity printed antennas. The limitation of the near-field to far-field transformation applied to EMC models of low-directivity antennas, caused by the finite dimensions of the antenna ground plane, is demonstrated. A method to partially overcome this limitation by including the contribution of diffracted rays is implemented, and its effectiveness is demonstrated with antenna prototypes. It is shown that the agreement between the far-field patterns measured in an anechoic chamber and the patterns computed from the EMC model obtained from the near-field measurements is significantly improved upon, within a sector of ±90° with respect to the antenna boresight in the E plane. The influence of the near-field sampling density and topology of the EMC model on the accuracy of the predicted far-field pattern is examined     

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     

Planar near-field to far-field transformation using an array ofdipole probes

A method is presented for computing far-field antenna patterns from measured near-field data measured by an array of planar dipole probes. The method utilizes the near-field data to determine some equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna. These currents are then used to find the far fields. The near-field measurement is carried out by terminating each dipole with 50 Ω load impedances and measuring the complex voltages across the loads. An electric field integral equation (EFIE) is developed to relate the measured complex voltages to the equivalent magnetic currents. The mutual coupling between the array of probes and the test antenna modeled by magnetic dipoles is taken into account. The method of moments with Galerkin's type solution procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient-fast Fourier transformation (CG-FFT) method exploiting the block Toeplitz structure of the matrix. Numerical results are presented for several antenna configurations to show the validity of the method     

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.     

Far-field uncertainty due to random near-field measurement error

Recent planar near-field scanning tests with ultralow-sidelobe antennas have confirmed that random near-field measurement errors will ultimately limit the accuracy of far-field patterns. A formulation is outlined for estimating the spectral signal-to-noise ratio (SNR) arising from noncorrectable near-field random measurement errors. The formulation applies to arbitrarily directive test antennas and probes-even nulling probes. A far-field parameter, called the scan plane coupling factor, may be computed directly from the near-field data, and then used to form the spectral SNR. The accuracy of the spectral SNR is confirmed by simulation and by actual tests with low-sidelobe AWACS array antenna     

亚毫米波段折叠光路的准光近场照射天线设计与应用

介绍了一种新颖的折叠光路近场准光照射天线设计方法,并成功应用在微波黑体定标源微弱散射的双站测量中. 在双站测量中,传统的小口径天线适用于低频范围,被测目标足够小才能保证远场条件. 与传统基于远场条件出发的小口径天线设计不同,本文所提出的准光照射天线的设计目的是在目标区降低链路衰减,实现相位平坦、幅值边缘衰减的聚焦照射,并可在近场区域内测得目标的远场散射特征. 设计基于准光学设计方法:由主反射镜和平板反射镜共同构成紧凑的折叠光路,首先确定馈源喇叭天线的等效高斯波束参数,然后基于高斯波束传播理论设计主反射镜,再通过全波仿真验证其折叠路径. 仿真分析表明即使馈源采用基本的圆锥喇叭馈源,也可实现良好的聚焦效果. 双站实测数据表明该天线设计达到了设计目的,为亚毫米波近场双站散射测量的开展提供了关键性的波束控制,具备广泛的应用价值.     

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     

High-precision outdoor cylindrical near-field antenna test facility

A new antenna test range is described. Although the facility is located outdoors, it allows precision probing of the cylindrical near-field of large and very complex antenna systems, with turning diameters up to 20 m and frequencies up to 20 GHz. The facility includes a 36 m high concrete scanner tower with low radar cross-section. Other significant features of the facility are the comprehensive air-conditioning system for all accuracy-dependent components, a permanent auto-alignment system, which ensures precision cylindrical measurements, and an interleaved high-speed data collection system, which delivers high-speed data collection within a minimum time frame. Test results, including a comparison with the patterns of a reference antenna measured on two other ranges, demonstrate the measurement integrity of the new facility     

On the characterization of a reflector impulse radiating antenna (IRA): full-wave analysis and measured results

There is a growing demand for impulse radiating antennas (IRAs) to receive and transmit short pulses. The basic concepts of IRA are reviewed and the far-field pattern versus frequency of an ideal IRA is characterized based on the fundamental properties of IRA. It is shown that the transmitted pulse is ideally in the form of a time derivative of the input pulse. The physical optics simulation results show that the far-field characteristics of a parabolic reflector are very close to an ideal IRA if it is fed properly. The reflector IRA was constructed, analyzed and measured at UCLA. The near-field and far-field characteristics of the reflector IRA are studied using both the method of moments (MoM) full-wave simulations and the frequency domain measurements. In this paper, the radiation mechanism of the reflector IRA is studied using a detailed current distribution on the parabolic reflector and the feeding structure at different frequencies. Applying either the calculated current distribution on the reflector IRA or the measured near-field results, it is seen that the aperture field intensity of the parabolic reflector is not the same in the two principle planes and as a result the beam-widths in the two principle planes are different. The far-field patterns of the antenna are measured and the calculated far-field patterns support the measured results. The calculated current distribution results provide a guideline on how to properly change the feeding structure to achieve a more uniform aperture field and increase the antenna radiation efficiency.     

Multilevel Plane Wave Based Near-Field Far-Field Transformation for Electrically Large Antennas in Free-Space or Above Material Halfspace

A recently presented fully probe-corrected near-field far-field transformation employing plane wave expansion and diagonal translation operators enables near-field far-field transformation for arbitrary measurement contours and arbitrary antennas. A multilevel extension, inspired by the multilevel fast multipole method, is presented that is suitable for the efficient transformation of electrically large antennas with a size of tens or even hundreds of wavelengths. The measurement points are grouped in a multilevel fashion and translations are carried out to the box centers on the highest level only. The plane waves are processed through the different levels to the measurement points using a disaggregation and anterpolation procedure resulting in a reduced overall complexity. In the second part of this paper, the influence of perfectly conducting ground planes and dielectric halfspaces, as an approximation for ground effects in a real measurement setup, is investigated. As such ground reflected waves are assumed, which propagate from the investigated antenna to the field probe and add to the direct wave contributions. The far-field conditions required for these assumptions are achieved by a source box grouping scheme. By this extension ground effects are directly considered within the near-field far-field transformation. Transformation results using simulated and measured near-field data are shown.      

球模式展开理论近远场变换及快速算法

基于球模式展开理论的近远场变换是天线球面近场测量系统实现的关键,它将待测天线在空间建立的场展开成球面波函数之和,由于其计算公式复杂,因而计算耗费时间长。该文在实际计算中利用快速傅里叶变换及矩阵的思想可以大幅度提高程序运行速度,节省计算时间。采用该方法对角锥喇叭天线的近远场数据进行仿真验证,结果表明外推远场的结果和理论值吻合良好,说明了该方法在保证计算精度的同时,可缩短计算时间。     

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.     

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