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We propose a technique which achieves highly accurate near-field data as well as far-field patterns despite the positioning inaccuracy of the scanner in the antenna near-field 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 near-field 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 near-field data is determined at all required grid points, the far-field pattern per se is obtained via a conventional near-field-to-far-field transformation. Moreover, and perhaps just as importantly, the interplay between our FMA and the free-form position/RF recording methodology just described allows us to bypass entirely the arduous task of strict antenna alignment. The free-form position/RF data are simply propagated by the FMA software to some perfectly aligned reference surface ideally adapted as a springboard for any intended far-field 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 win-win 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 near-field data were deliberately acquired over imperfectly located grid points. The FMA was then applied to obtain near-field data at the preferred, regularly arranged grid points from these position compromised values. Excellent grid-to-grid near-field comparison and calculated far-field results were obtained.
Legacy methods for testing the performance of radome panels and finished radomes have always been in isolation from the system antenna, for many reasons. The legacy method of testing employed horn antennas at relatively close distances, a fixed-frequency signal source, and primitive receivers. More modern systems used much better receivers capable of measuring both phase and amplitude, and these gave way to automatic network analyzers. The network analyzer system also replaces the fixed-frequency source, because it has its own step-frequency source. The rest of the setup remains the same. A network analyzer can itself be calibrated, but that calibration cannot include the probe antennas, nor can it account for interactions, particularly at normal incidence. With increasing demands on performance, it is essential that the interaction effects of the probe antennas with the radome be removed. The micorwave integrated circuit industry has the identical problem. The circuit probes that are used to reach into the circuit assemblies have very small tips, and the internal elements to accomplish this size reduction make probe mataching difficult. Thus the probe parameters become embedded into the overall measured response. The circuit testing community has developed a process to de-embed these probes, yielding the S-parameters of the circuit under test in isolation from surroundings. This paper investigates a method for applying this closed-system technique to open-system testing, such as panel-measuremsnt tables, by using a secondary calibration technique that is adapted to open systems. This effectively extends the calibration of the analyzer system to encompass the probes, thus improving accuracy.
There are many methods of aligning feeds on a dual cylindrical parabolic sub-reflector compact range. Presented in this paper is a laser tracker and Field probe method that was used to align the RF feed to the sub-reflectors. The laser tracker provides real time positional error measurements that are mapped and these results are used to fine tune the alignment of RF feed to the phase centers of the dual cylindrical parabolic sub-reflectors. Field probe test scans are performed to verify QZ performance of various alignment positions measured comparing scans of amplitude, phase and taper. The laser tracker alignment method provides an efficient and a highly accurate method to achieving precision alignment of the RF feed to the sub-reflector system installed into the dual reflector compact range. High accuracy antenna measurements in a compact range require precision alignment of the RF feed to the sub-reflectors phase center. The quality and size of the RF plane wave field of the quiet zone (QZ) performance is affected by the alignment of the RF feed and sub-reflector system combination. This alignment is accomplished through mechanical adjustments of the x-y-z axis RF feed positioning system. Measurements of both mechanical and electrical bore site is performed and compared across the full measurement spectrum to verify the compact antenna test range (CATR) system positioning accuracy.
A popular method for microwave characterization of materials is the free-space focused beam technique, which uses lenses or shaped reflectors to focus energy onto a confined region of a material specimen. In the 2-18 GHz band, 60 cm diameter lenses are typically spaced 30 to 90 cm from the specimen under test to form a Gaussian focused beam with plane-wave like characteristics at the focal point. This method has proved popular because of its accuracy and flexibility. Another free-space measurement technique that has been employed by some is the use of dielectrically loaded antennas that are placed in close proximity to a specimen. In this alternate technique, the dielectrically loaded antennas are smaller than lenses, making the hardware more compact and lower cost, however this is done at the expense of potentially reduced accuracy. This paper directly compares a standard laboratory focused beam system to a measurement system based on some recently developed RF spot probes. The spot probes are specially designed antennas that are encapsulated in a dielectric and optimized to provide a small illumination spot 3 to 8 cm in front of the probe. Experimental measurements of several dielectric, magnetic, and resistive specimens were measured by both systems for direct comparison. With these data, uncertainty analysis comparisons were made for both fixtures to establish measurement limits and capability differences between the two methods. Understanding these uncertainties and measurement limits are key to implementing compact spot probes in a manufacturing setting for quality assurance purposes.
The development of the Configurable Robotic Millimeter-Wave Antenna facility (CROMMA) by the antenna metrology lab at the National Institute of Standards and Technology in Boulder Colorado has brought together several important aspects of 6-degree-of-freedom robotic motion, positioning and spatial metrology useful for high frequency antenna characterization. In particular, the ability to define a unified coordinated metrology space, which includes all the motion components of the system is at the heart of this facility. We present the details of integrating robotics that have well defined kinematic models, advanced spatial metrology techniques, and millimeter wave components which make up the CROMMA facility. From this, a high level of precision, accuracy, and traceability that is requisite for performing high frequency near-field antenna pattern measurements can be achieved. Emphasis is placed on the ability to precisely characterize and model the movement patterns of the robot positioners, and probe and test antenna apertures using state-of-the-art full 6-degree-of-freedom spatial metrology, while being able to manipulate this information in a unified measurement space. The advantages of using a unified coordinated metrology space as they pertain to complex antenna alignments, scan geometry, repeatability analysis, traceability, and uncertainty analysis will be discussed. In addition we will also discuss how the high level of positioning, and orientation knowledge obtainable with the CROMMA facility can enable the implementation of sophisticated near-field position correction algorithms and precisely configurable scan geometries.
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.
For the purposes of antenna or radome measurement, a gimbal may be thought of as a compact, two or three axis antenna positioner with mutually orthogonal, intersecting axes. The unrelenting demand for higher accuracy in positioners of this type is driving innovation in mechanical architecture and design. A new position feedback technique, reflecting an enhanced understanding of position errors, and delivering unprecedented native encoder accuracy, has been developed and tested. New mechanical architecture has been created that allows for fully-featured two-axis gimbals to exist in the restricted confines behind an aircraft radome. The principal result of these developments is increasingly accurate and capable systems, particularly in the field of radome measurements. These new applications, techniques, architectures, and their results are explored in the following pages.
In Compact Antenna Test Range (CATR) applications, better cross polar discrimination is often the main motivation for choosing the more complex and expensive compensated dual reflector system as opposed to the simpler and cheaper single reflector system. Other than reflector geometry adjustment, different options have been presented in the literature to improve the cross polar performance of the single reflector CATR [1-4]. One solution is the insertion of a polarization selective grid between the feed and the reflector. The shape of the grids curved strip geometry is determined from the geometry of the reflector and each polarization has a different shape. This approach has been demonstrated to provide Quit Zone (QZ) cross polar performances similar to the dual reflector system on a decade bandwidth. The drawback of this solution is that orthogonal polarizations components cannot be measured simultaneously since a different polarizer grid is required for each polarization [1-2]. Other techniques aim at improving both amplitude/phase taper and cross polarization are based on measurement post processing. Processing techniques have been proposed based on numerical modelling of the range [3] or by de-convoluting the measured pattern with a predetermined range response based on QZ probing [4]. The drawback of these methods are the finite accuracy of the post processing, increased measurement complexity and the difficulty to measure active antenna systems. Recently, the application of conjugated matched feeds for reflector systems aimed at cross polar reduction in space application have received attention in the literature [5-10]. Recognizing, that the cross polar contribution induced by the offset reflector geometry has a focal plane distribution very similar to the higher order modes in feed horns, various techniques have been devised to excite compensating feed modes. Although a very elegant technique, the achievable bandwidth is limited and only single polarized solutions have been presented. A different concept of conjugated matched excitation, overcoming the dual polarization limitation has been introduced in [11-12] based on a patch array feed system. However, this implementation is aimed at applications with different beam-width in the principle planes. In this paper we will introduce a new feed horn concept, based on conjugate matched feeding, aiming at cross polar cancellation in single reflectors CATR systems. The proposed feed system is dual polarized and has an operational bandwidth of 1:1.5. The feed concept is introduced and the demonstrator hardware described. The target QZ <40dB cross polar discrimination is demonstrated by QZ probing of a standard single reflector CATR. References: [1] C. Dragone, "New grids for improved polarization diplexing of microwaves in reflector antennas," Antennas and Propagation, IEEE Transactions on , vol.26, no.3, pp.459-463, May 1978 [2] M.A.J. Griendt, V.J. Vokurka, “Polarization grids for applications in compact antenna test ranges”, 15th Annual Antenna Measurement Techniques Association Symposium, AMTA, October 1993, Dallas, Texas [3] W. D. Burnside, I. J. Gupta, "A method to remove GO taper and cross-polarization errors from compact range scattering measurements," ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM (APSURSI), June 1989, San Jose, California [4] D. N. Black and E. B. Joy, “Test zone eld compensation,” IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, vol. 43, no. 4, pp. 362–368, Apr. 1995. [5] K. K. Shee, and W. T. Smith, “Optimizing Multimode Horn Feed Arrays for Offset Reflector Antennas Using a Constrained Minimization Algorithm to Reduce Cross Polarization”, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 45, No. 12, December 1997, pp. 1883-1885. [6] S. B. Sharma, D. Pujara, Member, S. B. Chakrabarty,r. Dey, "Cross-Polarization Cancellation in an Offset Parabolic Reflector Antenna Using a Corrugated Matched Feed", IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009, pp. 861-864. [7] S. B. Sharma, D. A. Pujara, S. B. Chakrabarty, and V. K. Singh, “Improving the Cross-Polar Performance of an Offset Parabolic Reflector Antenna Using a Rectangular Matched Feed”, IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009, pp. 513-516. [8] S. K. Sharma, and A. Tuteja, “Investigations on a triple mode waveguide horn capable of providing scanned radiation patterns”, ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM (APSURSI), July 11-17, 2010 [9] K. Bahadori, and Y. Rahmat-Samii, “Tri-Mode Horn Feeds Revisited: Cross-Pol Reduction in Compact Offset Reflector Antennas”, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, No. 9, September 2009. [10] Z. Allahgholi Pour, and L. Shafai, “A Simplified Feed Model for Investigating the Cross Polarization Reduction in Circular- and Elliptical-Rim Offset Reflector Antennas”, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, No. 3, March 2012, pp. 1261-1268. [11] R. Mizzoni, G. Orlando, and P. Valle, “Unfurlable Reflector SAR Antenna at P-Band”, Proc. of EuCAP 2009, Berlin, Germany. [12] P. Valle, G. Orlando, R. Mizzoni, F. Heliere, K. van ’t Klooster, “P-Band Feedarray for BIOMASS”, Proc. of EuCAP 2012, Prague, Czech Republic.
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
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.
The US NEXRAD weather surveillance Doppler radar (WSR-88D) was recently upgraded to polarimetric capability. This upgrade permits identification of precipitation characteristics and type, thus providing the potential to significantly enhance the accuracy of radar estimated rainfall, or water equivalent in the case of frozen hydrometeors. However, optimal benefits are only achieved if errors induced by the radar hardware are properly accounted for through calibration. Hardware calibration is a critical element in delivering accurate meteorological information to the forecast and warning community. The calibration process must precisely measure the gain of the antenna, the Polarimetric bias of the antenna, and the overall gain and bias of the receive path. The absolute power measurement must be accurate to within 1 dB and the bias between the Polarimetric channels must be known to within 0.1 dB. These requirements drive a need for precise measurement of antenna characteristics. Engineers and scientists with the NEXRAD program employ solar scanning techniques to ascertain the absolute gain and bias of the 8.53 m parabolic center fed reflector antenna enclosed within a radome. They are also implementing use of daily serendipitous interference strobes from the sun to monitor system calibration. The sun is also used to adjust antenna gain and pedestal pointing accuracy. This paper reviews the methods in place and under development and identifies some of the challenges in achieving the necessary calibration accuracies.
Within RF measurement systems, engineers commonly wish to know how much phase ripple will be present in a signal based on a given signal-to-noise ratio (SNR). In a past AMTA paper (Measurement Considerations for Antenna Pattern Accuracy, AMTA 1997), John Swanstrom presented an equation which demonstrated how the bound on the phase error could be calculated from the peak SNR value. However, it can be shown that the Swanstrom bound is broken when the signal has a peak SNR value of less than approximately 15 dB. This paper introduces a new equation that bounds the maximum phase error of a signal based on the signal’s peak SNR value. The derivation of this new bound is presented, and comparisons are made between the old Swanstrom bound and the new bound. In addition, the inverse relationship (i.e., calculating the SNR value of a signal from phase-only measurements) is investigated. In the past, analytical equations for this relationship have been presented by authors such as Robert Dybdal (Coherent RF Error Statistics in IEEE Trans. on Microwave Theory and Techniques) and Jim P.Y. Lee (I/Q Demodulation of Radar Signals with Calibration and Filtering in a Defense Research Establishment Ottawa publication). The analytical equations for calculating the SNR value using phase-only measurements are reviewed and discussed, and a brand new numerical relationship based on a polynomial curve fitting technique is proposed.
Inter-comparisons and validations of antenna measurement ranges are useful tools allowing the detection of various problems in the measurement procedures, thus leading to improvements of the measurement accuracy and facilitating better understanding of the measurement techniques. The maximum value from a validation campaign is achieved when a dedicated Validation Standard (VAST) antenna specifically designed for this purpose is available. The widespread use of the known VAST-12 (12 GHz) antenna, developed by the Technical University of Denmark (DTU) in 1987 and operated by the DTU-ESA Facility since 1992, demonstrates the long-term value of the dedicated VAST antennas [1]. The driving requirements of VAST antennas are their mechanical stability with respect to any orientation of the antenna in the gravity field and thermal stability over a given operational temperature range. The mechanical design shall ensure extremely stable electrical characteristics with variations typically an order of magnitude smaller than the measurement uncertainty. At the same time, it must withstand high g-loads under frequent transportations and it shall also support convenient handling of the VAST antenna (practical electrical and mechanical interfaces, low mass, attachment points for lifting, etc.). Today, there is a well identified need for increased operational frequencies to get access to large bandwidth for broadband communication. Upcoming satellite communication services utilize up/down link at K/Ka-bands, while the use of Q/V bands is contemplated for the feeder links in the coming years. In response to this need, a millimeter-wave VAST (mm-VAST) antenna is currently under development in a collaboration between the DTU and TICRA under contract from the European Space Agency. In this paper, the electrical and mechanical requirements of the DTU-ESA mm-VAST antenna are discussed and presented. Potential antenna types fulfilling the electrical requirements are briefly reviewed and the baseline design is described. The emphasis is given to definition of the requirements for the mechanical and thermal stability of the antenna, which satisfy the stringent stability requirement for the mm-VAST electrical characteristics. [1] S. Pivnenko et al., “Comparison of Antenna Measurement Facilities with the DTU-ESA 12 GHz Validation Standard Antenna within the EU Antenna Centre of Excellence”, IEEE Trans. Antennas Propagat., 2009, vol. 57, no. 7. pp. 1863-1878.
Antenna, Radome, and RCS testing systems rely on high-fidelity positioner systems to provide high-precision positioning of articles for RF testing. Historically, the industry has relied on linear PID control techniques in torque, velocity, and position control loops on individual axes to drive the positioners. Recently, advancements have been made in the use of advanced control hardware including multiple-DOF laser and optical feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation. Advanced control techniques including single-axis error correction, multi-axis global error compensation, and multi-axis coordinated motion have been implemented to improve positioner accuracy. Here, a survey is conducted of control technologies in other industries such as machine tools and industrial robotics. An assessment is conducted on the viability of other advanced techniques to provide insight into the potential future control and capabilities of positioning systems in the RF testing industry. Candidate advanced techniques include gain scheduling and sliding-mode control which could provide improved accuracy over a wider range of conditions including varying loads and operating points caused by differing movement speeds or large variations in static loading. Dynamic input-shaping and feed-forward techniques could help suppress dynamic vibrations and improve dynamic tracking behavior for improved continuous-measurement scanning accuracy. Adaptive and non-linear control techniques might improve disturbance and error rejection for improved accuracy while managing dynamic-behavior drift allowing for adaptation to long-term positioner changes without re-tuning.
The need to maintain very low observability, along with the need to manipulate the model through a large range of motion, result in a challenging set of problems. These have been effectively addressed over decades of RCS equipment design. In recent years however, RCS applications have become much more demanding. Models are ever larger and heavier, with length exceeding 150 feet, and with weight up to 50,000 lbs. Required accuracy with some applications has increased to ±0.01°, an increase of 67% as compared to legacy values. MI Technologies has developed products that significantly expand the structural and operational envelopes of rotator/pylon systems to meet the demand for higher performance. This paper presents the various challenges encountered in RCS Rotator and Pylon design, and the innovative solutions that have arisen from recent engineering efforts.
Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Abstract— Several gain measurement techniques exist for near-field antenna ranges. These include Comparison-gain, Direct-gain and Three-antenna gain methods. Each technique has its own unique advantages and disadvantages in terms of accuracy, cost and measurement time. Range operators must understand the differences between these techniques in order to properly configure their test system to best suit their requirements. This paper surveys each of the gain techniques and identifies the relative advantages of each. As part of the survey, all three techniques were performed on three types of near-field antenna measurement systems: Planar, Cylindrical and Spherical. The results of this paper provide the reader with a practical understanding of each technique, the formulas required, and real-world examples for the trade-offs needed to outfit a range for fast and accurate gain measurements while balancing cost and schedule.
Abstract -We propose a method of utilizing near-field spherical measurements so as to obtain the back lobes of high gain antennas without sacrificing the accuracy of the far-field, high-gain main lobe prediction. While a spherical scan is perfectly adequate to gauge the relatively broad back lobes, it is in general inadequate to capture the required details of a sharp forward peak. We overcome this difficulty through recourse to our Field Mapping Algorithm (FMA), which latter allows us to assemble planar near-field data based upon the spherical measurements actually acquired. In particular, planar data of this sort on the forward, main-lobe side offers the standard route to predicting the desired, high-gain, far-field pattern. Our spherical-to-planar FMA near-field data manufacture showed excellent agreement with direct planar near-field measurements for a slot array antenna, each one of them, naturally, underlying a common, far-field, high-gain pattern.
This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure [8] and can be used on small, and large, plane-polar near-field antenna test systems.