由极平面近场测量确定天线远区辐射场

文章讨论了极平面近场测量确定天线远区辐射场的基本公式，采用Ｊａｃｏｂｉ－Ｂｅｓｓｅｌｓ级数展开方法求解电磁流模系数。通过对１．２ｍ反射面天线极平面近场扫描的实测结果与远场测量结果的比较，证明了该方法的有效性。     

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 patterns of spaceborne antennas from plane-polar near-field measurements

Certain unique features of a recently constructed plane-polar near-field measurement facility for determining the far-field patterns of large and fragile spaceborne antennas are described. In this facility, the horizontally positioned antenna rotates about its axis while the measuring probe is advanced incrementally in a fixed radial direction. The near-field measured data is then processed using a Jacobi-Bessel expansion to obtain the antenna far fields. A summary of the measurement and computational steps is given. Comparisons between the outdoor far-field measurements and the constructed far-field patterns from the near-field measured data are provided for different antenna sizes and frequencies. Application of the substitution method for the absolute gain measurement is discussed. In particular, results are shown for the 4.8-m mesh-deployable high-gain antenna of the Galileo spacecraft which has the mission of orbiting Jupiter in 1988.     

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     

A plane-polar near-field to far-field transformation with the fast Hankel transform method

It is known that a direct radial integration, used to compute the far-field from uniformly spaced plane-polar near-field measurements requires the evaluation of a large amount of Bessel functions and hence CPU time. Up to 1985 only unequally spaced fast Hankel algorithms were available. Hansen [3] developed an algorithm that was usable for equally spaced measurements points, but only for order zero. His theory is generalised in this paper and applied to a plane-polar near-field to far-field transformation.     

Nonuniform sampling techniques for antenna applications

Recent investigations have demonstrated that uniform sampling techniques can be effectively applied for construction of far-field patterns of antennas. There are, however, many circumstances for which it may not be practical to directly utilize uniform sampling techniques. A two-dimensional sampling technique which can employ irregularly (nonuniformly) spaced samples (amplitude and phase) in order to generate the complete far-field patterns is presented. The technique implements a matrix inversion algorithm which depends only on the nonuniform sampled data point locations and with no dependence on the actual field values at these points. A powerful simulation algorithm is presented to allow a real-life simulation of many reflector/feed configurations and to determine the usefulness of the nonuniform sampling technique for the co-polar and cross-polar patterns. Additionally, an overlapped window concept and a generalized error simulation model are discussed to identify the stability of the technique for recovering the field data among the nonuniform sampled data. Numerical results are tailored for the pattern reconstruction of a 20-m offset reflector antenna operating atL-band. This reflector is planned to be used in a proposed measurement concept of large antennas aboard the space shuttle, whereby it would be almost impractical to accurately control the movement of the shuttle with respect to the radio frequency (RF) source in prescribed directions in order to generate uniform (u, v) sampled points. Also, application of the nonuniform sampling technique to patterns obtained using near-field measured data is demonstrated. Finally, results of an actual far-field measurement are presented for the construction of patterns of a reflector antenna from a set of nonuniformly distributed measured amplitude and phase data.     

Near-field antenna measurements using nonideal measurementlocations

We introduce a near-field to far-field transformation algorithm that relaxes the usual restriction that data points be located on a plane rectangular grid. Computational complexity is O(Nlog N) where N is the number of data points. This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable to other measurement approaches such as plane-polar scanning, where data are collected intentionally on a nonrectangular grid     

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.     

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     

Sampling in plane-polar coordinates

A number of details are clarified regarding the sampling-reconstruction theorem for near-field scanning in plane-polar coordinates. The rigorous sampling-reconstruction theorem is applied to the near-field measurement of a circular aperture test antenna offset from the plane-polar axis of rotation, so that a large number of angular modes are necessary to represent the fields of the test antenna. An algorithm is described for computing accurately and rapidly the required zeros of Bessel functions of arbitrary integer order     

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。     

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     

Simplified Calculation of Antenna Patterns, with Application to Radome Problems

Generally, the calculation of antenna far-field patterns from known near-field distributions is tedious and may require the use of a large computer. The calculations are simplified for certain types of antennas having separable near fields. This simplifying assumption is found to yield satisfactory results with pyramidal horns and parabolic reflector antennas. Calculations are further simplified by approximating a complex line integral with two real summations. Measured and calculated far-field patterns are included to indicate the accuracy of the calculations. Results are presented for horns and parabolic antennas and for a horn covered with a hollow dielectric wedge. The method is applicable to both E-plane and H-plane pattern calculations. The main lobe of a far-field pattern is calculated in less than one hour on a desk calculating machine by the simplified method. In radome work an important feature is that it permits rapid evaluation of the far-field distortion associated with any given near-field distortion in any given small region in the near field.     

A measurement technique for modeling the effects of feed support struts on large reflector antennas

A measurement technique is described in which frequency scaled models of struts are placed in the near-field region of an offset reflector. In this compact range environment the excitation of the strut model is by plane waves, as would be encountered in the axisymmetrical reflector situation. Far-field radiation patterns are recorded, with and without the strut model in place, and, because of the low sidelobe levels associated with offset reflector antenna systems, it is possible to isolate the far-field response of the strut model. This technique is particularly useful for determining the real effects of structures that are difficult to analyze mathematically, such as latticed struts or metallic geodetic radomes.     

On the use of antenna weight functions in field strength prediction and interference reduction

In this paper two methods for calculating the received electromagnetic field by a single-parabolic reflector antenna in the shadow region behind a finite-width screen are proposed and analysed. The first one is referred to as the far-field approach and treats the obstacle and reflector antenna diffraction separately. The antenna simply is replaced by a point source having the receiving properties of the reflector antenna considered. The second method is called the near-field approach and considers the combined effect of obstacle and antenna diffraction. It is shown that considerable differences between the results of both methods may exist, even for an obstacle-antenna separation large compared to the Rayleigh distance of the antenna, and both for a CW and broadband analysis of the communications channel. It is concluded that the near-field method gives the best results and can be applied to many practical problems such as interference reduction and searching the optimal position of VSATs in urban environments.     

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     

Microwave holography of large reflector antennas--Simulation algorithms

The performance of large reflector antennas can be improved by identifying the location and amount of their surface distortions and then by correcting them. Microwave holography techniques are finding considerable applications as viable tools for performing this task. In these techniques, the complex (amplitude and phase) far-field pattern of the antenna is measured, using a reference antenna. Then, the Fourier transform relationship, which exists between the far field and a function related to the induced current, is invoked to result in the identification of the surface distortions. To critically examine the accuracy of the constructed surface profiles, simulation studies are required to incorporate both the effects of systematic and random distortions, particularly the effects of the displaced surface panels. In this paper, different simulation models are investigated with emphasis given to a model based on the vector diffraction analysis of a curved reflector with displaced panels. The simulated far-field patterns are then used to reconstruct the location and amount of displacement of the surface panels by employing a fast Fourier transform (FFT)/iterative procedure. The sensitivity of the microwave holography technique based on the number of far-field sampled points, level of distortions, polarizations, illumination tapers, etc., is also examined. In addition, the relationships between Az-El andu-vspaces are addressed in the Appendix. Most of the data are tailored to the dimensions of the NASA/JPL Deep Space Network (DSN) 64-m reflector antennas for which the result of a recent measurement is also presented.     

Microwave holographic technique for reflector antenna profile measurement

A short-range microwave holographic technique for the measurement of reflector antenna surface profile is described. The technique uses a linearly scanned transmit/receive CW probe located on-axis at approximately two focal lengths from the reflector vertex. Rotation of the reflector provides a plane-polar data set which can be reconstructed by a fast algorithm to provide a surface profile error map. No special precautions are required with regard to the reflectivity of the environment. Practical results are provided to illustrate the performance of the system.