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Far Field
Advantages and Disadvantages of Various Hemispherical Scanning Techniques
When performing far field or near field antenna measurements on large antennas, it is often necessary to have various types of mechanical positioning systems to achieve the required hemispheric scans. Measurement systems employing a singlearm gantry, a dualarm gantry, a fixed arch moving probe, or a fixed arch multiprobe have been paired with either an azimuth positioner or a vehicle turntable to provide hemispheric scanning of the object being tested. This paper will highlight the key characteristics of various scanning methods making comparisons between the different techniques. Positioning and system accuracy, speed, stowing ability, calibration, frequency range, upgradability, relative cost and other key aspects of the various techniques will be discussed in detail to help the end user during the system design and selection process. In addition, the paper will highlight novel hemispheric and truncated spherical scanning approaches. In many applications, the success of the entire project often centers on the judicious selection of the positioning subsystem. This paper will provide guidance toward making the proper selection of the scanning concept as well as of the positioning system.
Experimental Tests on an Effective NearField to FarField Transformation with Spherical Scan From Irregularly Spaced Data
The nearfield – farfield (NF–FF) transformation with spherical scanning is particularly interesting, since it allows the reconstruction of the complete radiation pattern of the antenna under test (AUT) [1]. In this context, the application of the nonredundant sampling representations of the electromagnetic (EM) fields [2] has allowed the development of efficient spherical NF–FF transformations [3, 4], which usually require a number of NF data remarkably lower than the classical one [1]. In fact, the NF data needed by this last are accurately recovered by interpolating a minimum set of measurements via optimal sampling interpolation (OSI) expansions. A remarkable measurement time saving is so obtained. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. As a consequence, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF–FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy [5] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in [5] to the uniform samples reconstruction in the case of cylindrical and spherical surfaces. The latter relies on the singular value decomposition method, does not exhibit the above limitation, but can be conveniently applied only if the uniform samples recovery can be reduced to the solution of two independent onedimensional problems [6]. Both the approaches have been applied and numerically compared with reference to the positioning errors compensation in the spherical NF–FF transformation for long antennas [7] using a prolate ellipsoidal AUT modelling. The goal of this work is just to validate experimentally the application of these approaches to the NF–FF transformation with spherical scanning for elongated antennas [4], using a cylinder ended in two halfspheres for modelling them. The experimental tests have been performed in the Antenna Characterization Lab of the University of Salerno, provided with a roll over azimuth spherical NF facility supplied by MI Technologies, and have fully assessed the effectiveness of both the approaches. [1] J.E. Hansen, ed., Spherical NearField Antenna Measurements , IEE Electromagnetic Waves Series, London, UK, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop. , vol. 46, pp. 351359, 1998. [3] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl ., vol. 15, pp. 755775, June 2001. [4] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Effective antenna modellings for NFFF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop ., vol. 2011, Article ID 936781, 11 pages, 2011 [5] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop ., vol. 141, pp. 7784, April 1994. [6] F. Ferrara, C. Gennarelli, G. Riccio, C. Savarese, “Far field reconstruction from nonuniform planepolar data: a SVD based approach,” Electromagnetics, vol. 23, pp. 417429, July 2003 [7] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Two techniques for compensating the probe positioning errors in the spherical NF–FF transformation for elongated antennas,” The Open Electr. Electron. Eng. Jour. , vol. 5, pp. 2936, 2011.
Equiangular Phase Shifting Holography for THz Nearfield/Farfield Prediction
A threestep equiangular (120º) phase shifting holography (EPSH) technique is proposed for THz antenna nearfield/farfield prediction. The method is attractive from the viewpoint of receiver sensitivity, phase accuracy over the entire complex plane, simplified detector array architecture, as well as reducing planarity requirements of the nearfield scanner. Numerical modeling is presented for the holographic receiver performance, using expected phase shift calibrations errors and phase shift noise. The receiver model incorporates responsivity and thermal noise specifications of a commercial Schottky diode detector. Additionally, simulated nearfield patterns at 372GHz demonstrate the convenience of the method for accurate and high dynamic range THz nearfield/farfield predictions, using a phaseshifter calibrated to ±0.1°.
Experimental Tests on an Effective NearField to FarField Transformation with Spherical Scan From Irregularly Spaced Data
The nearfield – farfield (NF–FF) transformation with spherical scanning is particularly interesting, since it allows the reconstruction of the complete radiation pattern of the antenna under test (AUT) [1]. In this context, the application of the nonredundant sampling representations of the electromagnetic (EM) fields [2] has allowed the development of efficient spherical NF–FF transformations [3, 4], which usually require a number of NF data remarkably lower than the classical one [1]. In fact, the NF data needed by this last are accurately recovered by interpolating a minimum set of measurements via optimal sampling interpolation (OSI) expansions. A remarkable measurement time saving is so obtained. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. As a consequence, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF–FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy [5] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in [5] to the uniform samples reconstruction in the case of cylindrical and spherical surfaces. The latter relies on the singular value decomposition method, does not exhibit the above limitation, but can be conveniently applied only if the uniform samples recovery can be reduced to the solution of two independent onedimensional problems [6]. Both the approaches have been applied and numerically compared with reference to the positioning errors compensation in the spherical NF–FF transformation for long antennas [7] using a prolate ellipsoidal AUT modelling. The goal of this work is just to validate experimentally the application of these approaches to the NF–FF transformation with spherical scanning for elongated antennas [4], using a cylinder ended in two halfspheres for modelling them. The experimental tests have been performed in the Antenna Characterization Lab of the University of Salerno, provided with a roll over azimuth spherical NF facility supplied by MI Technologies, and have fully assessed the effectiveness of both the approaches. [1] J.E. Hansen, ed., Spherical NearField Antenna Measurements , IEE Electromagnetic Waves Series, London, UK, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop. , vol. 46, pp. 351359, 1998. [3] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl ., vol. 15, pp. 755775, June 2001. [4] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Effective antenna modellings for NFFF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop ., vol. 2011, Article ID 936781, 11 pages, 2011 [5] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop ., vol. 141, pp. 7784, April 1994. [6] F. Ferrara, C. Gennarelli, G. Riccio, C. Savarese, “Far field reconstruction from nonuniform planepolar data: a SVD based approach,” Electromagnetics, vol. 23, pp. 417429, July 2003 [7] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Two techniques for compensating the probe positioning errors in the spherical NF–FF transformation for elongated antennas,” The Open Electr. Electron. Eng. Jour. , vol. 5, pp. 2936, 2011.
Achieving High Accuracy from a Nearfield Scanner without Perfect Positioning
We propose a technique which achieves highly accurate nearfield data as well as farfield patterns despite the positioning inaccuracy of the scanner in the antenna nearfield measurements. The method involves position sensing hardware in conjunction with data processing software. The underlying theory is provided by the Field Mapping Algorithm (FMA), which transforms exactly the measured field data on a conventional planar, spherical, or cylindrical surface, indeed on any enclosing surface, to any other surface of interest. In our modified nearfield scanning system, a position recording laser device is attached to the probe. The positions of data grid points are thus found and recorded along with the raw RF data. The raw data acquired over an irregular, imperfect surface is subsequently converted exactly to a designated, regular surface of canonical type based on the FMA and its associated position information. Once the nearfield data is determined at all required grid points, the farfield pattern per se is obtained via a conventional nearfieldtofarfield transformation. Moreover, and perhaps just as importantly, the interplay between our FMA and the freeform position/RF recording methodology just described allows us to bypass entirely the arduous task of strict antenna alignment. The freeform position/RF data are simply propagated by the FMA software to some perfectly aligned reference surface ideally adapted as a springboard for any intended farfield buildup. Our proposed marriage of a standard scanning system and a position recorder, with otherwise imperfect RF/location data restored to ideal status under the guidance of the FMA, clearly offers the advantage of high precision at minimal equipment cost. It is, simply stated, a winwin budget/accuracy RF measurement solution. Two analytic examples and one measurement case are given for demonstration. The first example is a circular aperture within an infinite conducting plane, the second is a 10 lambda x 10 lambda dipole array antenna. The measurement case involves a waveguide slot array antenna. In all three cases, the nearfield data were deliberately acquired over imperfectly located grid points. The FMA was then applied to obtain nearfield data at the preferred, regularly arranged grid points from these position compromised values. Excellent gridtogrid nearfield comparison and calculated farfield results were obtained.
FarField Reconstruction from NearField Data Collected through a Planar Spiral Scan: Experimental Evidences
In the recent years, many efforts have been spent to reduce the time required for the nearfield data acquisition, since such a time is nowadays very much greater than that required to perform the transformation. In this context, planar spiral scanning techniques exploiting continuous and synchronized movements of the positioning systems of the probe and antenna under test (AUT) have been proposed [14] to significantly reduce the measurement time. They are based on the nonredundant sampling representations of electromagnetic fields [5, 6] and use optimal sampling interpolation formulas to efficiently recover the data required by the classical planerectangular nearfield – farfield (NF–FF) transformation [7] from those acquired along the spiral. In particular, the AUT has been modelled as enclosed in a sphere in [1, 2], whereas an oblate ellipsoid has been considered in [3, 4]. When dealing with a quasiplanar AUT, this last antenna modelling results to be more effective from the truncation error and data reduction viewpoints with respect to the spherical one. As a matter of fact, it is able to reduce the redundancy induced by the spherical modelling for such a kind of antennas and allows to consider measurement planes at distances less than one half of the antenna maximum size, thus lowering the error related to the truncation of the scanning surface. The goal of this work is to experimentally validate the NF–FF transformation with planar spiral scanning which makes use of the ellipsoidal AUT modelling [3]. The experimental tests will be performed in the Antenna Characterization Lab of the University of Salerno, equipped with a planepolar NF facility system, besides the cylindrical and spherical ones, and will fully assess the effectiveness of this technique, as well as, of that based on the spherical modelling, that can be obtained as particular case from the oblate one when the ellipsoid eccentricity goes to zero. [1] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Probe compensated farfield reconstruction by nearfield planar spiral scanning,” IEE Proc. – Microw., Antennas and Propagat. , vol. 149, pp. 119–123, 2002. [2] F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Theoretical foundations of nearfield–farfield transformations with spiral scannings,” Prog. in Electromagn. Res. , vol. 61, pp. 193214, 2006 [3] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, and M. Migliozzi, “An effective NFFF transformation technique with planar spiral scanning tailored for quasiplanar antennas,” IEEE Trans. Antennas Propagat ., vol. 56, pp. 29812987, 2008. [4] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, and M. Migliozzi, “The unified theory of near–field – far–field transformations with spiral scannings for nonspherical antennas,” Prog. in Electromagn. Res. B, vol. 14, pp. 449477, 2009. [5] O.M. Bucci, C. Gennarelli, and C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop. , vol. 46, pp. 351 359, 1998. [6] O.M. Bucci and C. Gennarelli, “Application of nonredundant sampling representations of electromagnetic fields to NFFF transformation techniques,” Int. Jour. of Antennas and Propagat. , vol. 2012, ID 319856, 14 pages. [7] D. T. Paris, W. M. Leach, Jr., and E. B. Joy, “Basic theory of probecompensated nearfield measurements,” IEEE Trans. Antennas Propagat., vol. AP26, pp. 373379, May 1978.
NearField to FarField Transformation for ICs Using DipoleMoment Models on EMI Measurement
The electromagnetic compatibility (EMC) problems are becoming more challenging and noticeable due to the increasing complexity of integrated circuits (IC). Currently, most electromagnetic interference (EMI) standards specify that the measurements must be performed in the far field which is time consuming and expensive for the use of semianechoic chambers or open area test site. While nearfield measurement is usually fast and much more flexible, especially for the complex structures, the nearfield results could be obtained more efficiently for builtin ICs. The transformation between nearfield and farfield data is of great significance as long as the nearfield data is measured. Many methods including nearfield scanning method and Huygens’ equivalence method are used to complete the transformation from nearfield data to farfield radiation. However, the nearfield scanning method is inherent complex and requires strict mathematical derivation, which is difficult to handle for some practical cases. Huygens’ equivalence method is restricted by the location of observation point and the results are hardly obtained under scanning plane. In contrast, nearfield to farfield transformation based on inverse method appears to be more desirable by reconstructing a dipolemoment model instead of an IC. The dipolemoment model can be used to predict the farfield data, but also can be incorporated into a numerical fullwave tool as an equivalent source for complex systems. In this paper, the inverse method is firstly introduced. A noise source model from an IC is proposed based on an array of dipoles. These dipole moments can be extracted from the nearfield measurement in a scanning plane above the IC. Each dipole is modeled as an equivalent combined source consists of wire antennas and loop antennas. Then the radiation of IC in farfield region can be easily obtained. Finally, an example of physical IC is given to validate the approach.
NearField to FarField Transformation for ICs Using DipoleMoment Models on EMI Measurement
The electromagnetic compatibility (EMC) problems are becoming more challenging and noticeable due to the increasing complexity of integrated circuits (IC). Currently, most electromagnetic interference (EMI) standards specify that the measurements must be performed in the far field which is time consuming and expensive for the use of semianechoic chambers or open area test site. While nearfield measurement is usually fast and much more flexible, especially for the complex structures, the nearfield results could be obtained more efficiently for builtin ICs. The transformation between nearfield and farfield data is of great significance as long as the nearfield data is measured. Many methods including nearfield scanning method and Huygens’ equivalence method are used to complete the transformation from nearfield data to farfield radiation. However, the nearfield scanning method is inherent complex and requires strict mathematical derivation, which is difficult to handle for some practical cases. Huygens’ equivalence method is restricted by the location of observation point and the results are hardly obtained under scanning plane. In contrast, nearfield to farfield transformation based on inverse method appears to be more desirable by reconstructing a dipolemoment model instead of an IC. The dipolemoment model can be used to predict the farfield data, but also can be incorporated into a numerical fullwave tool as an equivalent source for complex systems. In this paper, the inverse method is firstly introduced. A noise source model from an IC is proposed based on an array of dipoles. These dipole moments can be extracted from the nearfield measurement in a scanning plane above the IC. Each dipole is modeled as an equivalent combined source consists of wire antennas and loop antennas. Then the radiation of IC in farfield region can be easily obtained. Finally, an example of physical IC is given to validate the approach.
Application of Huygens' Principle to a Dual Frequency Constant Beamwidth Reflector Operating in the Focused NearField
A technique is presented for determining the pattern of an antenna in the focused nearfield from cylindrical nearfield measurements. Although the same objective could be achieved by conventional nearfield to farfield transformation followed by a back projection, the proposed technique has an intuitive appeal and is considerably simpler and faster. The focused nearfield antenna pattern is obtained by applying Huygens’ principle, as embodied in the field equivalent principle, directly to nearfield measurements and by including an “obliquity factor” to suppress backlobe radiation. The technique was experimentally verified by comparison with farfield patterns obtained by conventional cylindrical nearfield to farfield transformation and by EM simulations. Excellent agreement in sidelobe levels and beamwidth was achieved. The technique was applied to the 25 in diameter reflector antenna of a harmonic radar operating at 5.8 GHz and 11.6 GHz. Since the operating range of this radar is less than 40 ft, the reflector is the nearfield at both frequencies. By defocusing the reflector at the harmonic frequency the beamwidths and gains at both frequencies can be made the same. The defocusing is accomplished by exploiting the frequency dependent phase center displacement of a logperiodic feed.
Beamforming Filtering for Planar NearField Antenna Measurements
It is well known that a field probe acts as a filter for the measured antenna under test (AUT) fields, whose influence can be either described in spatial or in spectral domain. Directive probes, for instance, serve to filter out signals that originate far away from the boresight axis. However, there are several drawbacks to the use of such directive probes including the possibility of multiple reflections and probe nulls. This contribution discusses the application of beamforming techniques to suppress unwanted echo signals in planar nearfield antenna measurements. The AUT is measured with a small probe antenna such as is normally used for such measurements. Neighboring measurement signals are thereafter combined in a moving average manner in order to generate the signal as would be measured by a probe array. Successive filter lengths, such as 3x3, 5x5, etc., are utilized such that the valid angle is preserved without extending the measurement plane. The generated nearfield signals are then transformed using a flexible plane wave based nearfield farfield transformation algorithm. Probe correction does not reverse the reduction in multipath signals achieved by the use of a directive probe or beamforming since sources are assumed only within the minimum sphere enclosing the AUT. Results are presented for simulated data with substantially improved results of the farfield pattern of the AUT.
Dual Polarized Near Field Probe Based on OMJ in Waveguide Technology Achieving More Than Octave Bandwidth
In classical probecorrected spherical nearfield measurements, one source of measurement errors, not often given sufficient consideration is the probe [13]. Standard nearfield to farfield (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physicallyrealizable probe is just an approximation to this ideal case. Probe excitation errors, finite manufacturing tolerances, and probe interaction with the mounting interface and absorbers are examples of errors that can lead to presence of higherorder spherical modes in the probe pattern [45]. This in turn leads to errors in the measurements. Although probe correction techniques for higherorder probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and postprocessing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern highaccuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and porttoport isolation. As a desired feature of modern probes the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. Consequently, the probe design is a tradeoff between performance requirements and usable bandwidth. High performance, dual polarized probe rely on balanced feeding in the orthomode junction (OMJ) to achieve good performance on a wide, more than octave, bandwidth [57]. Excitation errors of the balanced feeding must be minimized to reduce the excitation of higher order spherical modes. Balanced feeding on a wide bandwidth has been mainly realized with external feeding network and the finite accuracy of the external components constitutes the upper limits on the achievable performance. In this paper, a new OMJ designed entirely in waveguide and capable of covering more than an octave bandwidth will be presented. The excitation purity of the balanced feeding is limited only by the manufacturing accuracy of the waveguide. The paper presents the waveguide based OMJ concept including probe design covering the bandwidth from 1840GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTUESA Spherical NearField facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.1491979 [2]Recommended Practice for NearField Antenna Measurements, IEEE 17202012 [3]J. E. Hansen (ed.), Spherical NearField Antenna Measurements, Peter Peregrinus Ltd., on behalf of IEE, London, UK, 1988 [4]L. J. Foged, A. Giacomini, R. Morbidini, J. Estrada, S. Pivnenko, “Design and experimental verification of Kaband Near Field probe based on wideband OMJ with minimum higher order spherical mode content”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [5]L. J. Foged, A. Giacomini, R. Morbidini, “Probe performance limitation due to excitation errors in external beam forming network”, 33rd Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2011, Englewood, Colorado, USA [6]T. Laitinen, S. Pivnenko, J. M. Nielsen, and O. Breinbjerg, “Theory and practice of the FFT/matrix inversion technique for probecorrected spherical near eld antenna measurements with highorder probes,” IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2623–2631, Aug. 2010. [7]L. J. Foged, A. Giacomini, R. Morbidini, "Wideband dual polarised openended waveguide probe", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. [8]L. J. Foged, A. Giacomini, R. Morbidini, “ “Wideband Field Probes for Advanced Measurement Applications”, IEEE COMCAS 2011, 3rd International Conference on Microwaves, Communications, Antennas and Electronic Systems, TelAviv, Israel, November 79, 2011.
Combining Pattern, Polarization and Channel Balance Correction Routines to Improve the Performance of Broad Band, Dual Polarized Probes
Broad band, dual polarized probes are becoming increasingly popular options for use in nearfield antenna measurements. These probes allow one to reduce cost and setup time by replacing several narrowband probes like openended waveguides (OEWG) with a single device covering multiple waveguide bands. These probes are also ideal for production environments, where chamber throughput should be maximized. Unfortunately, these broadband probes have some disadvantages that must be quantified and corrected for in order to make them viable for high accuracy nearfield measurements. Most of these broadband probes do not have low cross polarization levels across their full operating bandwidths and may also have undesirable artifacts in the main component of their patterns at some frequencies. Both of these factors will result in measurement errors when used as probes. Furthermore, the use of a dual port RF switch adds an additional level of uncertainty in the form of porttoport channel balance errors that must be accounted for. This paper will describe procedures to calibrate the pattern and polarization properties of broad band, dual polarized probes with an emphasis on a newly developed polarization correction algorithm. A simple procedure to measure and correct for amplitude and phase imbalance entering the two ports of the nearfield probe will also be presented. Measured results of the three calibration procedures (pattern, polarization, channel balance) will be presented for a dualpolarized, broad band quadridged horn antenna. Once calibrated, this probe was used to measure a standard gain horn (SGH) and will be compared to baseline measurements acquired using a good polarization standard openended waveguide (OEWG). Results with and without the various calibration algorithms will illustrate the advantage to using all three routines to yield high accuracy farfield pattern data.
Indoor RCS measurement facilities ARCHE 3D: Influence of the target supporting mast in RCS measurement
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment. This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a mast (polystyrene or Plexiglas) mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. This paper investigates the influence of the material of the mast supporting the target under test. Across several measurement steps, we compare different RCS measurement results of canonical targets in order to eliminate the unwanted RCS measurement contribution due to the mast. The aim is to find out the mast which disturbs the least the RCS of the target under test but still compatible with the measurement facility ARCHE 3D. All these measurements are also compared to Near Field and Far Field calculations taking into account the material of the supporting mast.
Field Synthesis Using Multilevel Plane Wave Based Field Transformation
The synthesis of a specific field distribution in a certain volume with a given set of sources is an issue which arises in acoustics as well as in electromagnetics. Field Synthesis is of increasing interest for over the air (OTA) testing of multiple input multiple output (MIMO) based communication devices as arbitrary multipath communication channels can be simulated synthesizing the corresponding field distribution around the device under test (DUT). Planewave Field Synthesis methods have already been applied to improve the quality and extents of the quiet zone region of compact antenna test ranges (CATR). Furthermore, by synthesizing a plane wave field in a test region for an antenna under test (AUT), using an array of probe antennas in its nearfield region, nearfield farfield transformations (NFFFT) can be performed. Since there exists a variety of important applications for electromagnetic Field Synthesis, a Field Synthesis approach with high flexibility and low computational complexity is presented in this contribution. Usually, depending on the application, a single moving probe antenna or an array of probe antennas is used to synthesize a desired field distribution in the test zone volume where the DUT will be placed. The challenge is to determine appropriate excitation signals for the individual probe antennas. For that purpose an equation system is iteratively solved which arises from the boundary condition for the tangential field components on the surface of the test volume. As a consequence of the uniqueness theorem, equality of the desired and synthesized tangential field components induces that the desired and synthesized field distribution are identical in the source free test volume. Field testing on the surface of the test volume is performed by vector testing functions defined on a triangular mesh of the test zone surface enabling field synthesis in arbitrarily shaped test volumes. For accelerated evaluation of the coupling between probe antennas and vector testing functions, principles of the fast multipole method (FMM) are adopted. The implied plane wave expansions enables to incorporate the radiation characteristic of the probe antenna sources just by directly employing its plane wave spectrum representation which is nothing else but its farfield pattern. Additionally, the multilevel approach minimizes the number of translation operations between source and receiver boxes organized in a hierarchical octtree. Altogether the approach is applicable to arbitrarily shaped test volumes and arbitrarily arranged probe antennas and still shows a linearithmic complexity. In this contribution, detailed insight in the Field Synthesis method is given. Results for synthesized field distributions for arbitrarily shaped test volumes are presented. Finally the application of planewave Field Synthesis to NFFFT is shown for synthetic as well as for real nearfield antenna measurement data.
Distinguishing Localized and NonLocalized Scattering for Improved NearField to FarField Transformations
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of ImageBased Near FieldtoFar Field Transformations (IBNFFFT) to estimate monostatic far field radar crosssections (RCS) from monostatic near field radar measurements. The ISAR assumption states that all target scattering occurs at the location of the incident field excitations, i.e., the target is composed entirely of noninteracting localized scatters. Certain nonlocalized scattering phenomenon cannot be effectively handled by the IBNFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint timefrequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of nonlocalized scatterers. The localized scattering features are processed through the IBNFFFT as typical, which includes compensating for the R4 falloff present in the near field measured data. The nonlocalized scattering features, more appropriately scaled, are then coherently added back in to the postNFFFT localized scattering phase history. Although this does not properly transform the nonlocalized scattering features into the far field, it does avoid the overestimation error associated with improperly compensating distributed nonlocalized scattering features by a R4 power fall off based strictly on downrange position.
Distinguishing Localized and NonLocalized Scattering for Improved NearField to FarField Transformations
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of ImageBased Near FieldtoFar Field Transformations (IBNFFFT) to estimate monostatic far field radar crosssections (RCS) from monostatic near field radar measurements. The ISAR assumption states that all target scattering occurs at the location of the incident field excitations, i.e., the target is composed entirely of noninteracting localized scatters. Certain nonlocalized scattering phenomenon cannot be effectively handled by the IBNFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint timefrequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of nonlocalized scatterers. The localized scattering features are processed through the IBNFFFT as typical, which includes compensating for the R4 falloff present in the near field measured data. The nonlocalized scattering features, more appropriately scaled, are then coherently added back in to the postNFFFT localized scattering phase history. Although this does not properly transform the nonlocalized scattering features into the far field, it does avoid the overestimation error associated with improperly compensating distributed nonlocalized scattering features by a R4 power fall off based strictly on downrange position.
Spherical Scanning Measurement Challenge for Future MillimeterWave Applications
A specific setup for probefed antenna with an articulated arm has been developed by NSI with a 500mm AUTprobe distance. This paper will give an example of farfield measurement and highlight its limitations. A near field approach to filter the probe effect is investigated. First measurement results, including amplitude and phase, will be presented. Phase data will be leveraged to develop postprocessing technique to filter probe and environmental effect.
Absolute NearField Determination of the RapidScat Reflector Antenna onboard the International Space Station
Recently, the Rapid Scatterometer (RapidScat) instrument was developed to sense ocean winds while being housed onboard the International Space Station (ISS). This latest addition to the ISS, launched and mounted in September 2014, significantly improves the detection and sensing capabilities of the current satellite constellation. The dualbeam Kuband reflector antenna autonomously rotates at 18 rpm and acquires scientific data over a circular scan during typical ISS operations. Mounting such an antenna on the ISS, however, gives rise to many engineering challenges. An important consideration for any antenna onboard the ISS is the interference generated towards nearby ISS systems, space vehicles and humans due to the possible exposure to high RF power. To avoid this issue, this work aimed to characterize the antenna's absolute nearfield distribution, whose knowledge was required for a blanker circuit design to shut off the RF power for certain time slots during the scan period. Computation of these absolute nearfields is not a straightforward task and can require extensive computational resources. The initial computation of those fields was done using GRASP; however, an independent validation of the GRASP results was necessary because of safety concerns. A customized plane wave spectrum back projection method was developed to recover the absolute electric field magnitudes from the knowledge of the measured farfield patterns. The customized technique exploits the rapid computation of the Fast Fourier Transform alongside the proper normalization. The procedure starts by scaling the normalized (measured or simulated) farfield patterns appropriately to manifest the desired total radiated power. This was followed by transforming the vectors into the desired rectangular coordinate system and interpolating those components onto a regularized spectral grid. The FFT of the resulting Plane Wave spectrum was properly scaled using the sampling lengths to determine the absolute nearfield distributions. The procedure was initially validated by comparing the results with analytical aperture distributions with known farfield patterns. The properly normalized PWS approach was subsequently applied to the RapidScat Antenna using measured patterns from JPL’s cylindrical nearfield range. The resulting nearfields compare quite well between the plane wave spectrum technique and GRASP, thus validating the calculations. This work provided significant enabling guidelines for the safe operation of the ISSRapidScat instrument.
Characterization of DualBand Circularly Polarized Active Electronically Scanned Arrays (AESA) Using ElectroOptic Field Probes
The design of active electronically steered arrays (AESA) is a challenging, timeconsuming and costly endeavor. The design process becomes much more sophisticated in the case of dualband circularly polarized active phased arrays, in which CP radiating elements at two different frequency bands occupy a common shared aperture. A design process that takes into account various interelement and intraelement coupling effects at different frequency bands currently relies solely on computer simulations. The conventional nearfield scanning systems have serious limitations for quantifying these coupling effects mainly due to the invasive nature of their metallic probes, which indeed act as receiving antennas and have to be placed far enough from the antenna under test (AUT) to avoid perturbing the latter’s near fields. In recent years, a unique, versatile, nearfield mapping/scanning technique has been introduced that circumvents most of such measurement limitations thanks to the noninvasive nature of the optical probes. This technique uses the linear Pockels effect in certain electrooptic crystals to modulate the polarization state of a propagating optical beam with the RF electric field penetrating and present inside the crystal. In this paper, we will present nearfield and farfield measurement data for a dualband circularly polarized active phased array that operates at two different S and C bands: 2.1GHz and 4.8GHz. The array uses probefed, crossshaped, patch antenna elements at the Sband and dualslotfed rectangular patch elements at the Cband. At each frequency band, the array works both as transmitting and receiving antennas. The antenna elements have been configured as scalable array tiles that are patched together to create larger apertures.
On the Probe Pattern Correction in Spherical NearField Antenna Measurements
In planar and cylindrical nearfield antenna measurements the probe pattern correction is essential, since the used angular sector of the probe pattern extends over large part of the forward hemisphere. But in spherical nearfield measurements, the probe is always looking towards an antenna under test (AUT) and the used angular sector of the probe pattern is relatively small: it usually does not exceed some ±30deg, but typically is much smaller, depending on the size of the AUT and the distance to the probe. For this reason, for lowdirective probes with little pattern variation in the used angular sector, it is often said that the probe pattern correction can be omitted without introducing significant error in the calculated farfield AUT pattern. However, no specific guidelines on the value of the introduced error have been presented so far in the literature. In this paper, the error in the calculated farfield AUT pattern due to omitted probe pattern correction is investigated by simulations and confirmed by selected measurements. The investigation is carried out for two typical probes, an openended waveguide and a small conical horn, and for aperturetype AUTs of different electrical size with different distance to the probe. The obtained results allow making a justified choice on including or omitting the probe pattern correction in practical situations based on the estimated error at different levels of the AUT pattern.

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