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     

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.     

Determination of aerial far-field radiation patterns from near-field measurements

It is shown that the far-field radiation patterns of microwave aerials can be obtained from the solution of an integral equation. An aerial-synthesis technique, which is shown to reduce the computational work considerably, is utilised. An experimental result is given for an H plane sectoral horn.     

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.     

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.     

Far-field predictions from near-field measurements using an exactintegral equation solution

This paper presents a new approach to derive far-field data needed in antenna and EMI/EMC testing from near-field measurements. An exact integral equation solution to the wave propagation problem is used to transform the near-field data to the far field. The method requires near-field measurements on two closed surfaces enclosing all sources and inhomogeneities. The approach is validated with numerical simulation of measurements of fields radiated from a known antenna     

Overview of an image-based technique for predicting far-field radar cross section from near-field measurements

For the last 18 years, our group has been developing a variety of near-field-to-far-field transformations (NFFFTs) for predicting the far-field (FF) RCS of targets from monostatic near-field (NF) measurements. The most practical and mature of these is based on the reflectivity approximation, commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is also the most computationally efficient because - despite its theoretical underpinnings - it does not explicitly require image formation as part of its implementation. This paper presents a formulation and implementation of the image-based NFFFT that is applicable to two-dimensional (2D) spherical and one-dimensional (1D) circular near-field measurement geometries, along with numerical and experimental examples of its performance. We show that the algorithm's far-field RCS pattern-prediction performance is quite good for a variety of frequencies, near-field measurement distances, and target geometries. In addition, we show that the predicted RCS statistics remain quite accurate under conditions where the predicted far-field patterns have significantly degraded due to multiple interactions and other effect.     

A novel hybrid approach for far-field characterization from near-field amplitude-only measurements on arbitrary scanning surfaces

A novel hybrid procedure is proposed in this paper for far-field reconstruction from phaseless near-field data. A basically interferometric approach is adopted to retrieve the near-field phase from amplitude-only measurements, which are collected by a simple microstrip circuit used in conjunction with two identical probes moving on the scanning surface. A certain number of sets of complex near-field data is obtained, apart from constant phase-shifts to be computed, one for each set. A nonredundant representation based on the introduction of the reduced field is then adopted to evaluate these shifts, with an accurate and fast convergence to the solution. In order to validate the proposed technique, an X-band prototype using two flanged WR-90 waveguides is successfully designed and tested on a cylindrical geometry for a standard pyramidal horn.     

Near-field far-field predictions from single-intensity-planar-scan phase retrieval

Simulations are presented which demonstrate the feasibility of far-field predictions from a single near-field intensity planar scan using a phase-retrieval algorithm with spectral propagation.<>     

Integrated microstrip probe for phaseless near-field measurementson plane-polar geometry

An integrated probe performing phaseless near-field measurements on a polar surface is presented. The unknown near-field phase is computed by an interferometric technique used in conjunction with the minimisation of a suitable functional. A microstrip patch antenna for synthetic aperture radar (SAR) applications is considered to validate the method     

Modal structure from far-field measurements

It is shown how and to what extent the modal structure of a multimode fiber may be found from the far-field radiation pattern. Although specific inverse integral coefficient type formulas are not obtained for each modal weight, the analysis shows how to mathematically isolate the L from m modes using a polarizer, and then deduce the modal coefficients from the result     

平面近远场变换的快速算法

基于考虑探头补偿的平面近远场变换理论 ,根据实际需要 ,提出了一种工程实用的平面近远场变换快速算法。通过该算法由近场测量数据变换得到的天线远场方向图 ,既能达到任意分辨率 ,又能节约计算内存和提高计算速度。     

An approximate expression to estimate signal-to-noise ratioimprovement in cylindrical near-field measurements

A very simple approximate expression for the process gain (PG) for the cylindrical case is derived. The different approximations and assumptions required to obtain this expression are shown. This expression might be useful for most practical cylindrical near-field measurements, providing a very simple mean to assess the near-field dynamic range requirements to obtain a desired far-field signal-to-noise ratio (SNR)     

Antenna pattern comparison between an outdoor cylindrical near-field test facility and an indoor spherical near-field antenna test facility

A new spherical near-field probe-positioning device has been designed and constructed, consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility, located at Alenia Marconi Systems, on the Isle of Wight, UK. As part of the near-field qualification, testing was performed on a ground-based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility, also located on the Isle of Wight [F. Steiner et al., Jan. 1994]. These measurements included multiple-frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program, and includes discussions on the error budgets for the two ranges, along with a discussion of the mutual error budget between the two ranges.     

On cylindrical near-field scanning techniques

The agreement between the coupling equations obtained in the literature by using the reciprocity theorem and the scattering matrix formulation is demonstrated. The field is expanded in cylindrical vector wave functions and the addition theorem for these functions is used. The communication may serve as a tutorial introduction to the cylindrical scanning techniques.     

Improved algorithm for probe-corrected spherical near-field/far-field transformation

The technique for computation of antenna far fields from spherical near-field measurements has been improved, allowing large antennas to be treated. The efficiency and accuracy is demonstrated for an antenna without rotational symmetry and about 50 wavelengths in diameter.     

Accurate estimation of single-mode-fibre characteristics from near-field measurements

We propose a simple procedure to process data from a near-field measurement to obtain accurately an appropriately defined `spot size? in single-mode fibres and hence estimate almost all the relevant fibre characteristics like dispersion, splice loss and microbending loss.     

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     

Planar near-field to far-field transformation using an equivalentmagnetic current approach

An alternative method is presented for computing far-field antenna patterns from near-field measurements. The method utilizes the near-field data to determine equivalent magnetic current sources over a fictitious planar surface that encompasses the antenna, and these currents are used to ascertain the far fields. Under certain approximations, the currents should produce the correct far fields in all regions in front of the antenna regardless of the geometry over which the near-field measurements are made. An electric field integral equation (EFIE) is developed to relate the near fields to the equivalent magnetic currents. The method of moments is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method, and in the case of a rectangular matrix, a least-squares solution for the currents is found without explicitly computing the normal form of the equation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions. Numerical results are presented for several antenna configurations