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Polarization
The Boresight Roll for Antenna Range Characterization and Diagnostics
Jason Jerauld,Justin Dobbins, November 2015
The boresight roll scan is a simple yet powerful tool for antenna range characterization and diagnostics. In this type of measurement a linearly-polarized antenna with high axial ratio (such as a standard gain horn) is rotated about its mechanical boresight axis while magnitude and phase data are collected. Post-processing of these data provides a wealth of information about the source polarization characteristics, and can also be used to diagnose common problems such as receiver compression, mechanical misalignment, drift, and flexing cables. This paper describes the theory and implementation of the boresight roll scan, and provides examples of how different types of range errors manifest in the processed data.
Practical Considerations for Coordinate System Rotations in Mode-Space
Ryan Cutshall, Jason Jerauld, Justin Dobbins, November 2016
Rotating the coordinate system of an antenna pattern can be problematic due to the need to interpolate complex data in spherical coordinates. Common approaches to 2D interpolation often introduce errors because of polarization discontinuities at the spherical coordinate system poles. To overcome these difficulties, it is possible to transform an antenna pattern from field-space into spherical mode-space, perform the desired coordinate system rotation in mode-space, and then transform the modes in the rotated coordinate system back into field-space. This method, while more computationally intensive, is exact and alleviates all of the interpolation-related issues associated with rotations in field-space. Although rotations in mode-space have been implemented in commercially available software (e.g., the ROSCOE algorithm provided by TICRA), these algorithms may not be well understood by the general antenna measurement community. Therefore, the first goal of this paper is to present an easy-to-understand algorithm for performing rotations in mode-space. Next, the paper will address the challenge of computing the rotation coefficients, which are required by the mode-space coordinate system rotation algorithm. Although J. E. Hansen presented a method for recursively computing the rotation coefficients, this method is numerically unstable for large values of N (where N is the upper limit of the polar index). Therefore, this paper will present a numerically stable method for the recursive computation of the rotation coefficients. Finally, this paper will show the relationships between Euler angles and both Az-over-El angles and El-over-Az angles. These relationships are quite useful because it is often desired to rotate an antenna pattern based on Elevation and Azimuth angles, whereas the inputs for the mode-space rotation algorithm are Euler angles. Knowing these relationships, the Euler angles may be computed from the Azimuth and Elevation angles, which can then be used as the inputs to the mode-space rotation algorithm.
A Rotating Source Polarization Measurement Technique Using Two Circularly Polarized Antennas
Herbert Aumann, Kristan Tuttle, November 2016
This paper combines the standard two-antenna gain measurement technique with the rotating source method for measuring the gain as well as the polarization ratio and tilt angle of the polarization ellipse of a circularly polarized antenna.   The technique is illustrated with two identical helical antennas, one for the source and one for the antenna-under-test (AUT), facing each other. Measurements of the voltage transfer ratio are made over one full 360 degree on-axis rotation of the source while the AUT remains stationary. The rotation causes the phase of the electric field of the principal polarization to rotate in one direction and the phase of the cross polarization to rotate in the opposite direction. A Fast Fourier Transform (FFT) of the data from a single rotation is insufficient to resolve the two polarization components. Leakage from the principal polarization will most likely cover up the low-level opposite polarization signal. However, the FFT resolution can be artificially increased by appending to the measured data, precisely M-1 copies of the data. Now the polarization components will be separated by 2M rotations. Application of a heavy weighting function to the augmented data and a phase compensation to the FFT allows an unambiguous decomposition of the measured voltage transfer ratio into a principal and a cross polarization component. These are then used to calculate antenna polarization characteristics.   The technique was verified in an anechoic chamber with two 6-turn 5.8 GHz helical antennas separated by 4 feet. There was very good agreement between electromagnetic simulations and measurements of the polarization ellipse tilt angle and a -20 dB polarization ratio.
Dual-polarized Monolithic Leaky Wave Antenna Enabled by Additive Manufacturing
Esteban Menargues, Maria Garcia-Vigueras, Emile de Rijk, Juan R. Mosig, November 2016
The use of additive manufacturing (AM) techniques to manufacture microwave and mm-wave passive components has recently been demonstrated through various examples [1]. The term AM comprises all techniques based on the successive building of thin layers of material one on top of each other to create a device. When properly implemented, AM offers the possibility to manufacture light-weight and highly complex devices without generating significant costs increase. Among all AM techniques, Stereo-Lithography (SLA) is the most interesting one for the production of mm-wave components. In SLA, the materials are non-metallic epoxy-based polymers, that require a metallic coating to allow them to become RF functional. In contrast to other AM techniques, SLA manufacturing tolerances and surface roughness permit the design of devices up to 300 GHz. SWISSto12 has recently reported the successful performance of metal plated SLA devices, based on a proprietary chemical plating technology enables the processing of monolithic devices. In this contribution, we aim at exploiting the previously described SWISSto12’s AM-SLA technique [1] to obtain a monolithic directional dual-polarized high-directive Leaky-Wave Antenna (LWA) operating at mm-wave frequencies. The LWA consists of a square cross section waveguide perforated with crossed slots in its top aperture [2]. Moreover, the antenna already includes a side-arm orthomode transducer (OMT) and a smooth waveguide  twist, specifically co-designed with the LWA. The squared waveguide supports the propagation of the two first orthogonal modes, which are radiated through the cross-shaped slots. Thus, the vertically (horizontally) polarized mode inside the waveguide produces theta-polarized (phi-polarized) radiation. The pointing angle is approximately 50°, the same for both beams. The simulated cross-polarization values are very low according to the simulations. Moreover, the directivity of each orthogonal beam is controlled by the dimensions of the cross-shaped slot. Weather observation radars are considered as a privileged potential application of this kind of systems. Two different prototypes of this LWA+OMT subsystem (one operating at 30 GHz and the other one at 60 GHz, both achieving gains above 15 dB) are currently being manufactured by SWISSto12. The prototypes and their performance will be included in the final paper. [1] de Rijk, E.; Silva, J.S.; Capdevila, S.; Favre, M.; Billod, M.; Macor, A.; von Bieren, A.; "Additive Manufactured RF components based of Stereo-Lithography", in Antenna and RF Systems for Space Science 36th ESA Antenna Workshop, 6-9 Oct 2015 [2] M. Garcia-Vigueras, M. Esquius-Morote and J.R.Mosig, "Dual-polarized one-dimensional leaky wave antenna," 9th European Conference on  Antennas and Propagation (EuCAP), Lisbon, Portugal, 13-17 April 2015, pp.1-2.
Measurements and Numerical Simulations to Enhance the Assessment of Antenna Coupling
Lars Foged, Lucia Scialacqua, Andrea Giacomini, Francesco Saccardi, Francesca Mioc, November 2016
The possibility to use Near Field (NF) representation of antenna measurements in terms of equivalent currents, implemented in the commercial tool INSIGHT, is recently available in most CEM solvers. This method allows to use measured data to enhance numerical simulations in complex and/or large scenarios where antennas are installed. In the past this approach has been investigated and validated by determining the antenna radiation pattern in different antenna placement conditions. The aim of this paper is to present how this method can be extended for simulation of antenna coupling. Indeed using this innovative approach, after antennas are measured, their measured models can be imported in CEM tools and coupling with other radiators in arbitrary configurations can be simulated. No information about mechanical and/or electrical design of the measured antenna model are needed by the CEM tool, since the measured NF model in terms of equivalent currents already fully represents the antenna. Investigations have been performed on a H/V polarized array of three identical elements. Only the radiation pattern of the central element of the array has been measured, then starting from the measured data, the coupling between the other elements has been simulated by numerical tools. Accuracy of the procedure has been checked comparing the simulated results with the measured data of the entire array antenna. The testing procedure combining measurements and simulations consists of the following stages: ·      Measurement of the single element of the array and creation of the measured NF source representation. ·      Importing NF source in the CEM tool and placement in the array configuration. ·      Numerical simulation of the antenna coupling between the measured model and the other two elements of the array. Each element has two feeding ports implementing the dual H/V polarization. Preliminary analysis of the coupling is simulated and comparison with the measured data of the entire array agreement is acceptable. This study is currently under development for improving the accuracy of the results and including new test cases of different complexity.
A 6-40 GHz Antenna System for CubeSat Radiometer
Jiu-Kun Che, Chi-Chih Chen, November 2016
A high-gain 6-40 GHz circularly polarized antenna system has been designed for a CubeSat Radiometer Radio Frequency Interference (RFI) Technology Validation mission, which is to demonstrate wideband (6-40GHz) RFI mitigating backend technologies vital for future space-borne microwave radiometers. In stowed configuration, the antenna system needs to fit within a small volume of 10cm (L) by 8cm (W) by 5cm (H). The deployed length of the antenna is 25cm.  The total antenna payload including deployment mechanism needs to be less than 0.2kg. The desired gain is 14 dBic gain at 6 GHz and linearly increased to 22 dBic at 40 GHz in order to minimize the coverage footprint on earth.  The proposed antenna system include three continuous-taper helical antennas due to its simple feeding, circular polarization (CP), and wide bandwidth. They also have desirable light weight and flexible structures. The three helical elements operate at 6-11 GHz, 11-22 GHz and 21-40 GHz, respectively. The diameter of each helical antenna is specially profiled as a function of height to achieve the desired linear gain vs. frequency property. Since the three antenna elements are co-located within a small cavity, their positions were carefully investigated to minimize mutual coupling and coupling to cavity.  This paper presents the antenna design specifications, simulated performances, and preliminary measurement data.
A Reconfigurable Antenna Construction Toolkit with Modular Slotted Waveguide Elements for Arbitrary Pattern Designs
R. Geise, G. Zimmer, B. Neubauer, E. Gülten, A. Geise, November 2016
This contribution presents a universal antenna construction toolkit with slotted waveguide elements that can flexibly combined to form a reconfigurable antenna array capable of providing arbitrary symmetric radiation patterns. The design and the arrangement of radiating elements allow adjusting arbitrary real amplitudes of single radiating elements in a solely mechanical way without any electrical feeding network. Additional modular connecting elements even allow two dimensional and conformal antenna designs with circular and multiple polarizations. With a single toolkit in the Ku-band several design and measurement examples are presented, such as a linear array forming a desired main lobe down to -20dB, and a universal two dimensional antenna array that can switch between vertical, horizontal, LHC and RHC polarization. Given a desired antenna pattern the design procedure allows an automated generation of the physical antenna layout that can mechanically be combined without the need of additional full wave simulations. The waveguide toolkit is easily scalable to any other frequency band just being limited in the upper frequency by manufacturing issues. Another major benefit is that the modular concept of connecting and radiating elements eases the manufacturing where otherwise integral waveguide antennas require much more demanding processes. Different physical realizations of the modular waveguide concept are presented and discussed in the paper and related to the antenna performance. Beside several applications for the universal antenna toolkit, such as investigating illumination issues in scattering theory, educational aspects of teaching group antenna theory are also discussed in this contribution.
Improving the Cross-Polar Discrimination of Compact Antenna Test Range using the CXR Feed
Andrea Giacomini, Lars Foged, Antonio Riccardi, Jörg Pamp, Rasmus Cornelius, Dirk Heberling, November 2016
Compact Antenna Test Range (CATR) provide convenient testing, directly in far-field conditions of antenna systems placed in the Quiet Zone (QZ). Polarization performance is often the reason that a more expensive, complex, compensated dual reflector CATR is chosen rather than a single reflector CATR. For this reason, minimizing the QZ cross-polarization of a single reflector CATR has been a challenge for the industry for many years. A new, dual polarised feed, based on conjugate matching of the undesired cross polar field in the QZ on a full wave-guide band, has recently been developed, manufactured and tested. The CXR feed (cross polar reduction feed) has shown to significantly improve the QZ cross-polar discrimination of standard single reflector CATR systems. In previous papers, the CXR feed concept has been discussed and proved using a limited scope demonstrator and numerical analysis. In this paper, for the first time, the exhaustive testing of the dual polarised feed operating in the extended WR-75 waveguide band (10-16 GHz) is presented. Accuracy improvements, achieved in antenna cross-polar testing, using this feed is also illustrated by measured examples.
Correcting Polarization Distortion in a Compact Range Feed
Brett Walkenhorst, David Tammen, November 2016
A high quality antenna feed is an essential element of a compact antenna test range (CATR) in order to ensure the range can achieve the necessary stability in beam width, phase center and the necessary purity of polarization throughout the range’s quiet zone. In order to maintain the requisite quality, such feeds are typically 1) single-port and 2) cover a relatively limited band of frequencies. It is desirable to have a single dual ported, broadband feed that covers multiple waveguide bands to eliminate the need for a polarization positioner and avoid the difficulty associated with changing feeds for a single antenna measurement. Though some such feeds exist in the market, with such feeds, we often see a reduction in polarization purity across the band of interest relative to the more band limited feeds. Previous attempts to utilize dual-port probes and/or extend the bandwidth of the feed have resulted in degraded performance in terms of beam pattern and polarization purity. In an attempt to overcome some of the deficiencies above, the authors have applied polarization processing to dual-pol antennas to correct for the impurity in polarization of the antenna as a function of frequency. We present here a broadband CATR feed solution using a low-cost, dual-port sinuous feed structure combined with polarization processing to achieve low cross-pol coupling throughout the quiet zone. In the following paper, the feed structure, polarization theory, and processing algorithm are described. We also present co- and cross-pol coupling results before and after correcting for the polarization distortion using data collected in two CATRs in Atlanta, GA and Asia.
Minimum Scattering Probe for High Accuracy Planar NF Measurements
Andrea Giacomini, Lars Foged, Roberto Morbidini, Luca Tancioni, John Estrada, Jim Acree, November 2016
Dual polarized probes are convenient for accurate and time efficient Planar Near Field (PNF) antenna testing. Traditional probe designs are often bandwidth limited and electrically large leading to high scattering in PNF measurements with short probe/AUT distances. In this paper, an octave band probe design with minimum scattering characteristics is presented. The scattering minimization is largely obtained by a very small axially symmetric aperture of 0.4? diameter at the lowest frequency. The aperture also provide a near constant directivity in the full bandwidth and very low cross polar. The probe is fed by a balanced ortho-mode junction (OMJ) based on inverted quad-ridge technology and external feeding circuitry to obtain high polarization purity.
Indoor 3D Spherical Near Field RCS Measurement Facility: 3D RADAR Images From Simulated And Measured Data
Pierre Massaloux, Pierre Minvielle, November 2016
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 polystyrene mast 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 details a RCS measurement technique and the associated-post processing of raw data dedicated to the localization of the scatterers of a target under test. A specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. Compared to classical radar images, the main issue is linked with the variation of polarization induced by the near-field 3D RCS facility. This method is based on a fast and efficient regularized inversion that reconstructs simultaneously HH, VV and HV 3-D scatterer maps. The approach stands on a simple but original extension of the standard multiple scatterer point model, closely related to HR polarimetric characterization. This algorithm is tested on simulated and measured data from a metallic target. Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Implementation of a VHF Spherical Near-Field Measurement Facility at CNES
Gwenn Le Fur, Guillaume Robin, Nicolas Adnet, Luc Duchesne, Daniel Belot, Lise Feat, Kevin Elis, Anthony Bellion, Romain Contreres, November 2016
Needs of antenna measurements at low VHF range require the development of specific facilities. Costs saving could be found by reusing existing chambers and extending the frequency band down to few tens of MHz, especially if the implementation of such a system is performed in undersized chambers with already existing absorbers. CNES began such an adaptation in the 2000’s by adding a VHF measurement probe (80-400 MHz) in their CATR chamber which allows performing spherical single probe Near Field measurement thanks to the existing positioner. In the past four years, intensives studies have been led to reduce uncertainties onto measurements results and to wide again the lower frequency down to 50 MHz. Major error terms were identified and both a new measurement probe and post processing tools have been designed and implemented. This paper focuses on the hardware and software upgrades. Details will be first provided on the mechanical upgrades of the probe positioner, aiming to improve the accuracy and the repeatability of the positioning, as well as the ergonomic usage for saving installation time. A dedicated reference antenna in gain and polarization has been developed and validated. Such reliable reference antennas at this frequency range are a key point to reduce uncertainties onto measurement results. Finally, optical tool for aligning the measurement probe and the AUT as well as the post processing tool will be presented.
Phaseless Near-Field Antenna Measurement Techniques – An Overview
Olav Breinbjerg, Javier Fernández Álvarez, November 2016
For near-field antenna measurement it is sometimes desirable or necessary to measure only the magnitude of the near-field - to perform so-called phaseless (or amplitude-only or magnitude-only) near-field antenna measurements [1]. It is desirable when the phase measurements are unreliable due to probe positioning inaccuracy or measurement equipment inaccuracy, and it is necessary when the phase reference of the source is not available or the measurement equipment cannot provide phase. In particular, as the frequency increases near-field phase measurements become increasingly inaccurate or even impossible. However, for the near-field to far-field transformation it is necessary to obtain the missing phase information in some other way than through direct measurement; this process is generally referred to as the phase retrieval. The combined process of first measuring the magnitudes of the field and subsequently retrieving the phase is referred to as a phaseless near-field antenna measurement technique. Phaseless near-field antenna measurements have been the subject of significant research interest for many years and numerous reports are found in the literature. Today, there is still no single generally accepted and valid phaseless measurement technique, but several different techniques have been suggested and tested to different extents. These can be divided into three categories: Category 1 – Four magnitudes techniques, Category 2 – Indirect holography techniques, and Category 3 -Two scans techniques. This paper provides an overview of the different phaseless near-field antenna measurement techniques and their respective advantages and disadvantages for different near-field measurement setups. In particular, it will address new aspects such as probe correction and determination of cross-polarization in phaseless near-field antenna measurements. [1] OM. Bucci et al. “Far-field pattern determination by amplitude only near-field measurements”, Proceedings of the 11’th ESTEC Workshop on Antenna Measurements, Gothenburg, Sweden, June 1988.
A Novel Customized Spline-Profiled mm-Wave Horn Antenna for Emerging High Performance CubeSats
Vignesh Manohar, Joshua Kovitz, Yahya Rahmat-Samii, November 2016
The miniaturization of modern electronics has led to the development of a new class of small satellites called CubeSats. The small size facilitates launching the CubeSats as secondary payloads, significantly reducing launch costs. The scientific community is actively investigating the potential of deployable reflectors, reflectarrays and membrane antennas to accommodate the high data rate and resolution requirements for future CubeSat missions. The development of such deployable high gain antennas significantly broadens the horizons for advanced CubeSat missions at low costs. Our goal is to develop novel, practical antenna concepts that can support these emerging applications. Horn antennas are frequently used as feeds for deployable reflector antennas. With the reflector itself occupying significant space within the CubeSat, it is critical that the feed occupies minimal volume. The horn aperture dimensions are usually fixed in satisfying the -10dB edge illumination requirements set by the reflector design. For pyramidal or conical horns, the length is limited by the quadratic phase error at its aperture. Special techniques must be used to achieve desired performance when horn length is a major constraint. Potter horns use a stepped profile to create a dual-mode distribution to provide low cross polarization at the cost of reduced bandwidth and complexity of prototyping. Corrugated horns are also capable of providing low sidelobes and cross polarization, but are expensive to fabricate and are typically heavier.  Optimization techniques offer the possibilities of handling multiple design parameters, while allowing the designer to put more emphasis on critical constraints. We employ a novel spline-profiled smooth walled horn design that strikes a balance between ease of fabrication, desired radiation characteristics and overall volume. Particle Swarm Optimization (PSO) was used to optimize the horn profile for the desired beamwidth, length, cross polarization level and backlobe level. Detailed study of the aperture field distributions further illustrate the novelty of our design. The performance of the designed horn is validated using UCLA’s tabletop bipolar planar near field measurement facility. Thus, the power of optimization and elegance of monotonic splines was used to design a key component for future deployable reflector systems in CubeSats.
A Polynomial Approximation for the Prediction of Reflected Energy from Pyramidal RF Absorbers
Vince Rodriguez, Edwin Barry, November 2016
Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials, however, the bi-static or off angle performance must also be known. Some manufacturers provide factors at discrete electrical thickness for a discrete range of incident angles. This approximation is based on the curves presented in [1]. In reference [2], a polynomial approximation was introduced. In this paper, a more accurate approximation is introduced. Pyramidal RF absorber is modeled using CST’s frequency domain solver. The numerical results are compared to results from other numerical methods. The highest reflectivity of the two principal polarizations for a given angle of incidence and thickness of material is calculated. Different physical thickness pyramids are modeled. Once the worst case reflectivity is calculated, a polynomial curve fit is done to get a set of equations that provide the bi-static performance for absorber as a function of angle of incidence and thickness of material. The equations can be used to predict the necessary RF absorber to treat the walls of an indoor range.
Dependence of Antenna Cross-polarization Performance on Waveguide-to-Coaxial Adapter Design
Vince Rodriguez, Edwin Barry, Steven Nichols, November 2016
Antennas utilized as probes, sources, and for gain comparison are typically specified to have excellent cross polarization levels, often on the order of 50 dB below the primary polarization component. In many cases, these antennas are fed with a waveguide-to-coaxial adapter, which can be sourced from a multitude of vendors. Depending on the design and construction of the adapter, and the distance from the excitation probe to antenna aperture, the adapter itself can contribute significantly to the degradation of the polarization purity of the antenna. These adapters typically use one of several methods to achieve a good impedance match across their bandwidths, including tuning screws, posts and stubs. These tuning elements may be arranged asymmetrically and can cause the waveguide to be overmoded locally. Additionally, there is wide variance in the separation of the adapter excitation probe and waveguide electrical flanges, which may not be long enough to suppress the higher order modal content. In this paper, we study the effects of adapter to antenna aperture coupling, including the coupling of fields local to the current probe as well as those that are induced by design asymmetries. The results of the analysis lead to a number of rules of thumb which can be used to ensure that the antenna polarization purity is optimized.
Precise Determination of Phase Centers and Its Application to Gain Measurement of Spacecraft-borne Antennas in an Anechoic Chamber
Yuzo Tamaki, Takehiko Kobayashi, Atsushi Tomiki, November 2016
Precise determination of antenna phase centers is crucial to reduce the uncertainty in gain when employing the three-antenna method, particularly operated over a short range-such as a 3-m radio anechoic chamber, where the distance between the phase centers and the open ends of an aperture antenna (the most commonly-used reference) is not negligible, compared with the propagation distance. An automatic system to determine the phase centers of aperture antennas in a radio anechoic chamber has been developed and the absolute gain of horn antennas have been thereby evaluated with the three-antenna method. The phase center of an X-band horn was found to migrate up to 55 mm from the open end. Uncertainties in the gain were evaluated in accordance with ISO/IEC Guide 93-3: 2008. The 95% confidence interval of the horn antenna gain was reduced from 0.39 to 0.25 dB, when using the phase center location instead of the open end. Then the gains, polarization, and radiation pattern of space-borne antennas were measured: low-, medium-, and high-gain X-band antennas for an ultra small deep space probe employing the polarization pattern method with use of the horn antenna. Comparison between the radiation properties with and without the effect of spacecraft bus was carried out for low-gain antennas. The 95% confidence interval in the antenna gain decreased from 0.60 to 0.39 dB.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
Andreas Olk, Kais-Ben Khadhra, Thiemo Spielmann, October 2017
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.
RCS Measurements and Imaging on Arrays of Retro-reflectors
Pax Wei, October 2017
For features of very weak scattering while masked by background and clutter, care must be taken in the measurement design as well as data processing, in order to extract the true RCS values. A good example of flush-mounted fasteners for a low-observable (LO) aircraft, arranged in an array, was reported by Lutz, Mensa, and Vaccaro [1]. In the Boeing 9-77 range, retro-reflectors (called retros) were routinely taped on a test-body for monitoring its 3-D locations and angles during measurements. Though the retro’s RCS may be several orders below that of a test-body, a challenge was to discover their exact values. In the millimeter wave range (MMWR), we measured 2-D arrays of retros arranged in both square and hexagonal lattices taped onto a flat metal surface pitched 20o down. RCS measurements were made as a function of frequency and aspect angle. From the 2-D FFT images, the nominal RCS for a retro in VV polarization was found to be -75 dBsm, independent of the geometry and number of retros even down to one unit-cell [2]. But for the HH polarization, there is no backscattering from such a flat metal surface. References [1]. J. Lutz, D. Mensa, and K. Vaccaro, "RCS measurements of LO features on a test body," Proc. 21st AMTA, pp. 320-325 (1999). [2]. P. S. P. Wei and J. P. Rupp, " RCS measurements on arrays of weak scatterers enhanced by diffraction," Proc. 26th AMTA, pp. 263-268 (2004). ---------------------------------------------- ** Sam Wei is at: 4123 - 205th Ave. SE, Sammamish, WA 98075-9600. Email: paxwei3@gmail.com, Tel. (425) 392-0175
Ka-Band Measurement Results of the Irregular Near-Field Scanning System PAMS
Alexander Geise, Torsten Fritzel, Maurice Paquay, October 2017
The portable antenna measurement system PAMS was developed for arbitrary and irregular near-field scanning. The system utilizes a crane for positioning of the near-field probe. Inherent positioning inaccuracies of the crane mechanics are handled with precise knowledge of the probe location and a new transformation algorithm. The probe position and orientation is tracked by a laser while the near-field is being sampled. Far-field patterns are obtained by applying modern multi-level fast multipole techniques. The measurement process includes full probe pattern correction of both polarizations and takes into account channel imbalances. Because the system is designed for measuring large antennas the RF setup utilizes fiber optic links for all signals from the ground instrumentation up to the gondola, at which the probe is mounted. This paper presents results of the Ka-band test campaign in the scope of an ESA/ESTEC project. First, the new versatile approach of characterizing antennas in the near-field without precise positioning mechanics is briefly summarized. The setup inside the anechoic chamber at Airbus Ottobrunn, Germany is shown. Test object was a linearly polarized parabolic antenna with 33dBi gain at 33GHz. The near-fields were scanned on a plane with irregular variations of over a wavelength in wave propagation. Allowing these phase variations in combination with a non-equidistant grid gives more degree of freedom in scanning with less demanding mechanics at the cost of more complex data processing. The setup and the way of on-the-fly scanning are explained with respect to the crane speed and the receiver measurement time. Far-fields contours are compared to compact range measurements for both polarizations to verify the test results. The methodology of gain determination is also described under the uncommon near-field constraint of coarse positioning accuracy. Finally, the error level assessment is outlined on the basis of the classic 18-term near-field budgets. The assessment differs in the way the impact of the field transformation on the far-field pattern is evaluated. Evaluation is done by testing the sensitivity of the transformation with a combination of measured and synthetic data.


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