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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 near-field 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 near-field signals are then transformed using a flexible plane wave based near-field far-field 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 far-field pattern of the AUT.
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable 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 higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port 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 trade-off 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 [5-7]. 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 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.149-1979 [2]Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 [3]J. E. Hansen (ed.), Spherical Near-Field 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 Ka-band 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 probe-corrected spherical near- eld antenna measurements with high-order 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 open-ended 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, Tel-Aviv, Israel, November 7-9, 2011.
Fieldprobing is often the tool of choice for validating the characteristics of a quiet zone (QZ). Some of the main disadvantageous of fieldprobing are the expense and stability of the setup, e.g. a stable non-reflective linear axis has to be build. Furthermore regular 1-dimensional fieldprobing is not very suited for detecting extraneous reflections in the measurement chamber. Former work has shown that using a second linear axis below the AUT positioner (which is sometimes present for Antenna Pattern Comparison (APC) measurements) can improve the detection, but further increases the cost factor. Using Spherical Near-Field scanning [FRANCIS,WITTMANN,BLACK,JOY] most of these disadvantageous are solved, only a rather simple, although sturdy, beam is built on top of the roll-over-azimuth positioner, placing the antenna on a sphere surrounding the QZ. Using only one measurement, for each frequency, a complete analysis of the measurement chamber can be performed. It can be used for both looking inside the QZ, i.e. chamber reflectivity and outside on extraneous reflections. This paper will show both actual spherical near-field and fieldprobing measurements of the CATR at the Institute of High Frequency Technology (IHF) of the RWTH Aachen, and compare both results.
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable 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 higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port 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 trade-off 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 [5-7]. 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 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.149-1979 [2]Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 [3]J. E. Hansen (ed.), Spherical Near-Field 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 Ka-band 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 probe-corrected spherical near- eld antenna measurements with high-order 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 open-ended 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, Tel-Aviv, Israel, November 7-9, 2011.
Broad band, dual polarized probes are becoming increasingly popular options for use in near-field antenna measurements. These probes allow one to reduce cost and setup time by replacing several narrowband probes like open-ended 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 near-field 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 port-to-port 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 near-field probe will also be presented. Measured results of the three calibration procedures (pattern, polarization, channel balance) will be presented for a dual-polarized, broad band quad-ridged 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 open-ended waveguide (OEWG). Results with and without the various calibration algorithms will illustrate the advantage to using all three routines to yield high accuracy far-field pattern data.
Numerical modeling within Computational Electromagnetics (CEM) solvers is an important engineering tool for supporting the evaluation and optimization of antenna placement on larger complex platforms. While measurements are still required for final validation due to the conclusiveness and high reliability of measured data, numerical modeling is increasingly used in the initial stages of antenna placement investigation, optimization and to ensure that final testing, often a complex procedure, has a positive outcome. In some cases, the full-wave representation of the source antenna is unavailable to the designer in the format required by the CEM solver. This is often the case if the source antenna is from a third party. To overcome this problem, an equivalent computational model of the antenna must be constructed, bearing in mind that CEM solvers require an accurate source representation to achieve reliable results. Equivalent sources or currents implemented in the commercial tool INSIGHT have been adopted as an efficient diagnostics and echo reduction tool in general antenna measurement scenarios as discussed in [1-6]. The INSIGHT processing of measured antenna data was initially developed as a numerical representation of antennas in complex environment analysis for CEM solvers [7-10]. The main obstacle for widespread use of this method was the handling of the proprietary format of the equivalent currents. Commercial CEM providers are currently investigating and implementing domain decomposition techniques based on the near field description of the local domain. This development also provides a direct link between INSIGHT processing of measured antenna data and numerical simulation opening a range of interesting applications for using measured antennas in commercial numerical simulation tools as discussed in [11-12]. In flush-mounted antenna applications the measurement and subsequent INSIGHT processing has to be carefully performed. This paper discusses guidelines for the correct source antenna measurement, post processing and successive link to the commercial numerical tools for simulation. Application examples of the link using CST STUDIO SUITE® software [14-17] with flush mounted antennas and comparison with measurements of the full structure will be provided. [1] http://www.satimo.com/software/insight [2] J. L. Araque Quijano, G. Vecchi. Improved accuracy source reconstruction on arbitrary 3-D surfaces. Antennas and Wireless Propagation Letters, IEEE, 8:1046–1049, 2009. [3] J. L. A. Quijano, G. Vecchi, L. Li, M. Sabbadini, L. Scialacqua, B. Bencivenga, F. Mioc, L. J. Foged "3D spatial filtering applications in spherical near field antenna measurements", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. [4] L. Scialacqua, F. Saccardi, L. J. Foged, J. L. Araque Quijano, G. Vecchi, M. Sabbadini, “Practical Application of the Equivalent Source Method as an Antenna Diagnostics Tool”, AMTA Symposium, October 2011, Englewood, Colorado, USA [5] J. L. Araque Quijano, L. Scialacqua, J. Zackrisson, L. J. Foged, M. Sabbadini, G. Vecchi “Suppression of undesired radiated fields based on equivalent currents reconstruction from measured data”, IEEE Antenna and wireless propagation letters, vol. 10, 2011 p314-317. [6] L. J. Foged, L. Scialacqua, F. Mioc,F. Saccardi, P. O. Iversen, L. Shmidov, R. Braun, J. L. Araque Quijano, G. Vecchi" Echo Suppresion by Spatial Filtering Techniques in Advanced Planar and Spherical NF Antenna Measurements ", AMTA Symposium, October 2012, Seattle, Washington, USA [7] E. Di Giampaolo, F. Mioc, M. Sabbadini, F. Bardati, G. Marrocco, J. Monclard , L. Foged, “Numerical modeling using fast antenna measurements”, 28th ESA Antenna Workshop on Space Antenna Systems and Technologies, June 2005 [8] L. J. Foged, F. Mioc, B. Bencivenga, E. Di Giampaolo, M. Sabbadini “High frequency numerical modeling using measured sources”, IEEE Antennas and Propagation Society International Symposium, July 9-14, 2006. [9] F. Mioc, J. Araque Quijano, G. Vecchi, E. Martini, F. Milani, R. Guidi, L. J. Foged, M. Sabbadini, “Source Modelling and Pattern Enhancement for Antenna Farm Analysis”, 30th ESA Antenna Workshop on Antennas for Earth Observation, Science, Telecommunication and Navigation Space Missions, May 2008 ESA/ESTEC Noordwijk, The Netherlands [10] L. J. Foged, B. Bencivenga, F. Saccardi, L. Scialacqua, F. Mioc, G. Arcidiacono, M. Sabbadini, S. Filippone, E. di Giampaolo, “Characterisation of small Antennas on Electrically Large Structures using Measured Sources and Advanced Numerical Modelling”, 35th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2013, Columbus, Ohio, USA [11] L. J. Foged, L. Scialacqua, F. Saccardi, F. Mioc, D. Tallini, E. Leroux, U. Becker, J. L. Araque Quijano, G. Vecchi, “Bringing Numerical Simulation and Antenna Measurements Together”, 8th European Conference on Antennas and Propagation, EuCAP, April 2014, Den Haag, Netherlands [12] L. J. Foged, L. Scialacqua, F. Saccardi, F. Mioc, D. Tallini, E. Leroux, U. Becker, J. L. Araque Quijano, G. Vecchi “Innovative Representation of Antenna Measured Sources for Numerical Simulations”, IEEE International Symposium on Antennas and Propagation and USNC/URSI, July 2014, Memphis, Tennese, USA [13] L. J. Foged, B. Bencivenga, F. Saccardi, L. Scialacqua, F. Mioc, G. Arcidiacono, M. Sabbadini, S. Filippone, E. di Giampaolo, “Characterisation of small Antennas on Electrically Large Structures using Measured Sources and Advanced Numerical Modelling”, 35th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2013, Columbus, Ohio, USA [14] CST STUDIO SUITE™, CST AG, Germany, www.cst.com [15] T. Weiland: "RF & Microwave Simulators - From Component to System Design" Proceedings of the European Microwave Week (EUMW 2003), München, Oktober 2003, Vol. 2, pp. 591 - 596. [16] B. Krietenstein, R. Schuhmann, P. Thoma, T. Weiland: "The Perfect Boundary Approximation Technique facing the big challenge of High Precision Field Computation" Proc. of the XIX International Linear Accelerator Conference (LINAC 98), Chicago, USA, 1998, pp. 860-862. [17] D. Reinecke, P. Thoma, T. Weiland: "Treatment of thin, arbitrary curved PEC sheets with FDTD" IEEE Antennas and Propagation, Salt Lake City, USA, 2000, p. 26.
In this paper the inverse-source technique or source reconstruction technique has been applied as diagnostic tool to determine the complex excitation at sub array and single element level of a measured array antenna [1-5]. The inverse-source technique, implemented in the commercially available tool “INSIGHT” [5], allows to compute equivalent electric and magnetic currents providing exclusive diagnostic information about the measured antenna. By additional processing of the equivalent currents the user can gain insight to the realized excitation law at single element and sub-array level to identify possible errors. The array investigated in this paper is intended as part of the European Navigation System GALILEO and is a pre-development model flying on the In-Orbit Validation Element the GIOVE-B satellite. The antenna, developed by EADS-CASA Espacio, consists of 42 patch elements, divided into six sectors and is fed by a two level beam forming network (BFN). The BFN provide complex excitation coefficients of each array element to obtain the desired iso-flux shaped beam pattern [6-7]. The measurements have been performed in the new hybrid (Near Field and Compact Range) facility in the ESTEC CPTR as part of the installation and validation procedure [8]. The investigation has been performed without any prior information of the array and intended excitation. The input data for the analysis is the measured spherical NF data and the array topology and reference coordinate system. References [1] J. L. Araque Quijano, G. Vecchi. Improved accuracy source reconstruction on arbitrary 3-D surfaces. Antennas and Wireless Propagation Letters, IEEE, 8:1046–1049, 2009. [2] L. Scialacqua, F. Saccardi, L. J. Foged, J. L. Araque Quijano, G. Vecchi, M. Sabbadini, “Practical Application of the Equivalent Source Method as an Antenna Diagnostics Tool”, AMTA Symposium, October 2011, Englewood, Colorado, USA [3] J. L. Araque Quijano, L. Scialacqua, J. Zackrisson, L. J. Foged, M. Sabbadini, G. Vecchi “Suppression of undesired radiated fields based on equivalent currents reconstruction from measured data”, IEEE Antenna and wireless propagation letters, vol. 10, 2011 p314-317. [4] L. J. Foged, L. Scialacqua, F. Mioc,F. Saccardi, P. O. Iversen, L. Shmidov, R. Braun, J. L. Araque Quijano, G. Vecchi " Echo Suppresion by Spatial Filtering Techniques in Advanced Planar and Spherical NF Antenna Measurements ", AMTA Symposium, October 2012, Seattle, Washington, USA [5] http://www.satimo.com/software/insight [6] A. Montesano, F. Monjas, L.E. Cuesta, A. Olea, “GALILEO System Navigation Antenna for Global Positioning”, 28th ESA Antenna Workshop on Space [7] L.S. Drioli, C. Mangenot, “Microwave holography as a diagnostic tools: an application to the galileo navigation antenna”, 30th Annual Antenna Measurement Techniques Association Symposium, AMTA 2008, Boston, Massachusetts November 2008 [8] S. Burgos, M. Boumans, P. O. Iversen, C. Veiglhuber, U. Wagner, P. Miller, “Hybrid test range in the ESTEC compact payload test range”, 35th ESA Antenna Workshop on Antenna and Free Space RF Measurements ESA/ESTEC, The Netherlands, September 2013
The typical antenna measurement system setup working above 20 GHz makes use of frequency multipliers and harmonic mixers, usually working in standard waveguide bands, and thus several parts need to be procured and interchanged to cover several frequency bands. In this paper, we investigate an alternative solution which makes use of a standard wideband VNA without external frequency conversion units. The operational capability of the Planar Near-Field (PNF) Antenna Measurement Facility at the Technical University of Denmark was recently extended to 60 GHz employing an Agilent E8361A VNA (up to 67 GHz). The upgrade involved procurement of very few additional components: two cables operational up to 65 GHz and an open-ended waveguide probe for tests in U-band (40-60 GHz). The first tests have shown good performance of the PNF setup: 50-60 dB dynamic range and small thermal drift in magnitude and phase, 0.06 dB and 6 degrees peak-to-peak deviations over 4 hours. A PNF measurement of a 25 dBi Standard Gain Horn was carried out and the results were compared to those from the DTU-ESA Spherical Near-Field Facility with a good agreement in the validity region. Uncertainty investigations regarding cable flexing effects at 60 GHz have shown that these introduce an uncertainty of about 0.02 dB (1 sigma) around the main beam region indicating a very good performance of the PNF setup.
Measurement post-processing techniques based on spatial filtering have been presented as promising tools for the mitigation of echo’s deriving from the measurement environment in regular Near Field (NF) measurement scenarios [1]. The adaptation of these tools into standard measurement procedures depends on the possibility to demonstrate the real effectiveness in a given measurement scenario. The standard validation approach is to introduce a known disturbance into a measurement scenario and show the efficiency of the techniques in attenuating this disturbance. While highly effective as a functional demonstration of this approach the benefit of the echo reduction on an actual measurement scenario should still be evaluated on a case by case basis. A hybrid Near Field (NF) system has recently been installed in the existing dual reflector Compact Payload Test Range of ESTEC [2-3]. The installed system has been designed to perform spherical, cylindrical and planar NF measurements. Despite the design effort to optimize the NF system position in the chamber some interaction with the dual reflectors in the range were expected and for the PNF system in particular [4]. During the hybrid system acceptance measurements have been performed on the space array antenna intended as part of the European Navigation System GALILEO. The antenna is a pre-development model flying on the In-Orbit Validation Element, GIOVE-B satellite, developed by EADS-CASA Espacio [5-6]. This L-band antenna is particularly important test case for ESTEC since the PNF system will later be used in the final testing at space craft level on the GALILEO Satellites. This paper presents the preliminary finding of the MV-Echo post processing validation for PNF measurements in the hybrid range. The GALILEO array antenna has been measured in different configuration, with and without echo reduction processing and the results compared. The purpose of the activity was to quantify the benefits of the MV-Echo processing. Since the array is working in circular polarization it was possible to identify the major echo contributions as 2’nd order reflections. References [1] L. J. Foged, L. Scialacqua, F. Mioc, F. Saccardi, P. O. Iversen, L. Shmidov, R. Braun, J. L. Araque Quijano, G. Vecchi, “Echo Suppression by Spatial Filtering Techniques in Advanced Planar and Spherical NF antenna Measurements”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [2] S. Burgos, M. Boumans, P. O. Iversen, C. Veiglhuber, U. Wagner, P. Miller, “Hybrid test range in the ESTEC compact payload test range”, 35th ESA Antenna Workshop on Antenna and Free Space RF Measurements ESA/ESTEC, The Netherlands, September 2013 [3] S. Burgos, P. O. Iversen, T. Andersson, U. Wagner, T. Kozan, A. Jernberg, B. Priemer, M. Boumans, G. Pinchuk, R. Braun, L. Shmidov, “Near-Field Hybrid Test Range from 400 MHz to 50 GHz in the ESTEC Compact Payload Test Range with RF upgrade for high frequencies”, EUCAP 2014 [4] Paper on position of NF system in the range – was it astrium that did it? [5] L.S. Drioli, C. Mangenot, “Microwave holography as a diagnostic tools: an application to the galileo navigation antenna”, 30th Annual Antenna Measurement Techniques Association Symposium, AMTA 2008, Boston, Massachusetts November 2008 [6] A. Montesano, F. Monjas, L.E. Cuesta, A. Olea, “GALILEO System Navigation Antenna for Global Positioning”, 28th ESA Antenna Workshop on Space [7] J. E. Hansen, Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd. On behalf of IEE, London, United Kingdom, 1988. [8] F. Jensen, A. Frandsen, “On the number of modes in spherical wave expansion”, AMTA Symposium, October 2004, Stone Mountain, GA, USA.
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.
Measurement post-processing techniques based on spatial filtering have been presented as promising tools for the mitigation of echo’s deriving from the measurement environment in regular Near Field (NF) measurement scenarios [1]. The adaptation of these tools into standard measurement procedures depends on the possibility to demonstrate the real effectiveness in a given measurement scenario. The standard validation approach is to introduce a known disturbance into a measurement scenario and show the efficiency of the techniques in attenuating this disturbance. While highly effective as a functional demonstration of this approach the benefit of the echo reduction on an actual measurement scenario should still be evaluated on a case by case basis. A hybrid Near Field (NF) system has recently been installed in the existing dual reflector Compact Payload Test Range of ESTEC [2-3]. The installed system has been designed to perform spherical, cylindrical and planar NF measurements. Despite the design effort to optimize the NF system position in the chamber some interaction with the dual reflectors in the range were expected and for the PNF system in particular [4]. During the hybrid system acceptance measurements have been performed on the space array antenna intended as part of the European Navigation System GALILEO. The antenna is a pre-development model flying on the In-Orbit Validation Element, GIOVE-B satellite, developed by EADS-CASA Espacio [5-6]. This L-band antenna is particularly important test case for ESTEC since the PNF system will later be used in the final testing at space craft level on the GALILEO Satellites. This paper presents the preliminary finding of the MV-Echo post processing validation for PNF measurements in the hybrid range. The GALILEO array antenna has been measured in different configuration, with and without echo reduction processing and the results compared. The purpose of the activity was to quantify the benefits of the MV-Echo processing. Since the array is working in circular polarization it was possible to identify the major echo contributions as 2’nd order reflections. References [1] L. J. Foged, L. Scialacqua, F. Mioc, F. Saccardi, P. O. Iversen, L. Shmidov, R. Braun, J. L. Araque Quijano, G. Vecchi, “Echo Suppression by Spatial Filtering Techniques in Advanced Planar and Spherical NF antenna Measurements”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [2] S. Burgos, M. Boumans, P. O. Iversen, C. Veiglhuber, U. Wagner, P. Miller, “Hybrid test range in the ESTEC compact payload test range”, 35th ESA Antenna Workshop on Antenna and Free Space RF Measurements ESA/ESTEC, The Netherlands, September 2013 [3] S. Burgos, P. O. Iversen, T. Andersson, U. Wagner, T. Kozan, A. Jernberg, B. Priemer, M. Boumans, G. Pinchuk, R. Braun, L. Shmidov, “Near-Field Hybrid Test Range from 400 MHz to 50 GHz in the ESTEC Compact Payload Test Range with RF upgrade for high frequencies”, EUCAP 2014 [4] Paper on position of NF system in the range – was it astrium that did it? [5] L.S. Drioli, C. Mangenot, “Microwave holography as a diagnostic tools: an application to the galileo navigation antenna”, 30th Annual Antenna Measurement Techniques Association Symposium, AMTA 2008, Boston, Massachusetts November 2008 [6] A. Montesano, F. Monjas, L.E. Cuesta, A. Olea, “GALILEO System Navigation Antenna for Global Positioning”, 28th ESA Antenna Workshop on Space [7] J. E. Hansen, Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd. On behalf of IEE, London, United Kingdom, 1988. [8] F. Jensen, A. Frandsen, “On the number of modes in spherical wave expansion”, AMTA Symposium, October 2004, Stone Mountain, GA, USA.
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). Plane-wave 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 near-field region, near-field far-field 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 far-field pattern. Additionally, the multilevel approach minimizes the number of translation operations between source and receiver boxes organized in a hierarchical oct-tree. 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 plane-wave Field Synthesis to NFFFT is shown for synthetic as well as for real near-field antenna measurement data.
This paper introduces a new spherical near-field sampling grid, which we shall refer to as the equally-spaced sampling grid, where the sample points are spread over the surface of the measurement sphere using an almost-uniform triangular tessellation of the unit sphere. By spacing the points evenly over the surface of the sphere, the grid uses 50% the number of sample points as the classical grid. In addition, it can be shown that the equally-spaced grid requires the use of fewer points than required by the commonly cited “Nyquist criteria” at certain values of ka. Since the spherical modes required for the SNF-to-FF transform cannot be obtained from the equally-spaced grid through traditional FFT methods, the solution is demonstrated by other means. The details behind a brute-force matrix inversion method and a brief discussion on the condition number of the equally-spaced sampling grid will be included. Some comparisons will be shown between results obtained using the brute-force matrix inversion method and results obtained using the more computationally-efficient conjugate gradient method (as proposed by Wittmann, Alpert, and Francis).
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of Image-Based Near Field-to-Far Field Transformations (IB-NFFFT) to estimate monostatic far field radar cross-sections (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 non-interacting localized scatters. Certain non-localized scattering phenomenon cannot be effectively handled by the IB-NFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint time-frequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of non-localized scatterers. The localized scattering features are processed through the IB-NFFFT as typical, which includes compensating for the R4 fall-off present in the near field measured data. The non-localized scattering features, more appropriately scaled, are then coherently added back in to the post-NFFFT localized scattering phase history. Although this does not properly transform the non-localized scattering features into the far field, it does avoid the over-estimation error associated with improperly compensating distributed non-localized scattering features by a R4 power fall off based strictly on downrange position.
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of Image-Based Near Field-to-Far Field Transformations (IB-NFFFT) to estimate monostatic far field radar cross-sections (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 non-interacting localized scatters. Certain non-localized scattering phenomenon cannot be effectively handled by the IB-NFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint time-frequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of non-localized scatterers. The localized scattering features are processed through the IB-NFFFT as typical, which includes compensating for the R4 fall-off present in the near field measured data. The non-localized scattering features, more appropriately scaled, are then coherently added back in to the post-NFFFT localized scattering phase history. Although this does not properly transform the non-localized scattering features into the far field, it does avoid the over-estimation error associated with improperly compensating distributed non-localized scattering features by a R4 power fall off based strictly on downrange position.
If a planar near-field (PNF) scanner is large and there is insufficient temperature regulation in the chamber to keep ordinary thermal expansion/contraction from causing unacceptable position errors, then consideration must be given to compensation techniques that can adjust for the changes. Thermal expansion/contraction will affect almost everything in the chamber including the floor, the scanner structure, the encoder or position tapes, the AUT support, and the mount for any extra instrument(s) used to measure and correct for position error. Since the temperature will generally cycle several times during a lengthy acquisition, error-correction solutions must account for the dynamic nature of the temperature effects. This paper describes a new automated tracking-laser compensation subsystem that has been designed and developed for very large horizontal PNF systems. The subsystem is active during the acquisition to account for both static and dynamic errors and compensates for those errors in all three dimensions. The compensation involves both open-loop corrections for repeatable errors with high spatial frequency and closed-loop corrections for dynamic errors with low spatial frequency. To close the loop, laser data are measured at a user-defined interval between scans and each scan that follows the laser measurements is fully compensated. The laser measurements are fully automated with no user interaction required during the acquisition. The challenges, goals, and assumptions for this development are listed, high-level implementation considerations are provided, and resulting measured data are presented.
We present a microwave near-field imaging method using Fractional Fourier Transform (FrFT). Since the FrFT of finite chirp signal including a Fresnel integral, the scattering field of target in Fresnel zone can be described in the FrFT form. It is a valuable expression of scattering, because it unified the expression of scattering field in three different range along with distance, Rayleigh zone, Fresnel zone, and Fraunhofer zone. Then, one can get the target image from the Fresnel scattering field using the inverse FrFT (IFrFT) technology. The fast algorithm of FrFT or IFrFT has the same algorithm complexity as Fast Fourier Transform (FFT), which made a fast near-field imaging method compare to conventional method like Back Projection (BP), Time Domain Correlation (TDC), etc. Two near-field imaging results, one simply target and one complex target, will be presented to prove the method.
The technique is validated with a 14-element printed circuit 2-12 GHz log-periodic antenna with known phase center, and applied to a 4-element printed circuit 2.4 GHz Yagi-Uda antenna with unknown phase center. A conventional phase center determination based on phase curvature and the proposed technique yield substantially the same results. It is shown that the phase center of a Yagi antenna is not located at the feed but is located closer to the center of the physical array.
Abstract—It has been shown that it is possible to get a good estimation of the location of the largest centers of reflection causing ripple in the quiet zone using spherical near-field scanning of the quiet zone in combination with back projection to far-field. This method however, suffers from poor resolution at lower frequencies making it hard to distinguish small contributions from the main beam if they are closely spaced. For this purpose the CLEAN algorithm has been adapted and is presented here.