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AMTA Paper Archive

Limitations of the Free Space VSWR Measurements for Chamber Validations
Zhong Chen, Zubiao Xiong, Amin Enayati, November 2016

Free Space VSWR measurement has been the de facto standard method for anechoic chamber performance evaluation for more than 50 years.  In this method, a probe antenna is kept at a fixed angle while traveling along a linear path to record the standing wave pattern.  The probe antenna is then rotated to a different angle to repeat the measurement.  Reflectivity, which is used as the chamber performance metric, is calculated for each probe rotation angle.   In this paper, we show that the reflectivity is affected by the antenna patterns of the probe antenna.  When the probe antenna is aimed at the specular reflection point of a chamber surface, measurement dynamic range is improved, and the method provides a measure of the reflectivity primarily from that surface.  When the probe is not directed at a specular point, other reflections in the chamber can contribute to the VSWR, and the chamber reflectivity becomes more dependent on the probe antenna pattern.

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.

Uniaxial Anisotropic Material Measurement using a Single Port Waveguide Probe
Alexander Knisely, Milo Hyde, Michael Havrilla, Peter Collins, November 2016

Anisotropic material characterization requires versatile sample fixtures in order to provide sufficient measurement diversity for material parameter extraction.  However, extensive sample preparation is often required prior to making a measurement, especially for anisotropic materials.  An alternative nondestructive material measurement approach using a Single Port Waveguide Probe (SPWP) is proposed to simplify measurement of uniaxial anisotropic media.  Instead of cutting a material sample to fit into a given fixture, nondestructively interrogating a sheet of material via the SPWP greatly simplifies sample preparation and measurement.  The SPWP system measures a metal-backed sample of a known thickness.  A flange with a waveguide aperture cut in the center is placed on the metal backed sample (thus forming a parallel plate region) and a length of rectangular waveguide is connected to the flange aperture. A Vector Network Analyzer port is connected to the end of the rectangular waveguide to collect calibration and sample data.  Measurements of two different thicknesses of a sample are performed to provide sufficient data for extracting the sample permittivity tensor.  The sample permittivity tensor is computed via comparison of the measured and theoretical S-parameters using a least squares minimization algorithm.  The theoretical S-parameters are derived using a magnetic field integral equation which utilizes a uniaxial parallel plate Green’s function to constitute the fields in the parallel plate region.  Love’s Equivalence Principle is used to relate the fields in the parallel plate flange region to the fields in the waveguide (assumed to be the dominant TE10 mode only).  In this paper, the SPWP theoretical development, measurement and material parameter extraction are discussed.  Measurements and simulations of isotropic and uniaxial samples are made to assess the SPWP performance.

Transfer Function Characterization for a Dual Reflector, Indoor Compact Range
Thomas Cowles, Lonny Walker, November 2016

Raytheon, El Segundo, CA chamber #2 is a dual reflector, indoor compact range that is the largest facility of its kind within the company.  A series of tests were performed to characterize the measured transfer function of the chamber because of a recent capital upgrade of the range measurement system. The purpose of this paper is to document and discuss the results of the characterization testing, review how the measured transfer function of the range was determined, and compare the current results with both past data and analytical predictions, and demonstrate how this transfer function is used for antenna and radar cross section (RCS) measurement characterization. The measured transfer function of the range is used for both antenna and RCS measurement characterization. For antenna measurements, the transfer function is used in the Friis transmission equation to determine, for example, the expected power at the receiver given the transmit power and gain of both the transmit antenna and the antenna under test. Appropriate amplification and/or attenuation can determined as part of the test planning process saving time during test setup and test execution. For RCS measurements, the transfer function was recently utilized to study the benefits and challenges of relocating our instrumentation radar from a smaller compact range to this large compact range. The motivation for the study was enhanced measurement capability for larger targets and lower frequencies. This study utilized noise equivalent RCS (NERCS) as the metric and transmit power, pulse width, and pulse integration as the study parameters to find a practical solution for optimizing NERCS.

Echo Reduction with Minimum Sampling in Spherical Near Field Measurements using Translated-SWE Algorithm
Francesco Saccardi, Lars Foged, Francesca Mioc, Per Iversen, November 2016

In Near Field (NF) measurements different echo reduction techniques can be applied to mitigate echoes or stray signals deriving from the surrounding environment. A very promising echo reduction technique is based on the so called spatial or modal filtering. The spatial filtering is very efficient in measurement scenarios with stationary AUT but necessitates an offset of the AUT in scenarios where the antenna is rotating. Unfortunately, the measurement of the AUT in an offset configuration requires the acquisition of a higher number of samples. An innovative spherical NF/FF transformation algorithm for offset measurements based on a Translated Spherical Wave Expansion (TSWE) has been recently proposed. In this paper we investigate by experiment the echo reduction properties of offset AUT measurements using TSWE.

Probe Correction Technique of Arbitrary Order for High Accuracy Spherical Near Field Antenna Measurements
Francesco Saccardi, Andrea Giacomini, Lars Foged, November 2016

Probe correction in standard spherical near field measurements are typically limited to probes with |µ|=1 spherical wave spectrum when performing spherical wave expansion. The design of such probes is often a trade-off between achievable performance, modal purity and bandwidth. Compensation techniques for probes with higher or full order modal spectrum have recently been proposed. The advantages of such techniques are more freedom in the selection of the probe for a given measurement scenario and increased bandwidth. The technique reported in this paper is valid for probes with a known modal spectrum of arbitrary order. Probe compensation is performed directly on each spherical wave function before expanding the measured field. This leads to a computationally very effective algorithm. In this paper, the accuracy of the new algorithm is validated experimentally for different higher order probes in the measurement of a standard gain horn. For each scenario, the accuracy and computational requirement of the new algorithm is compared to standard transformations.

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.

Source reconstruction by far-field data for imaging of defects in frequency selective radomes
Bjorn Widenberg, Kristin Persson, Mats Gustafsson, Gerhard Kristensson, November 2016

An inverse source reconstruction method with great potential in radome diagnostics is presented. Radomes are designed to enclose antennas to protect them, from e.g. weather conditions. Frequency selective surface (FSS) radomes are designed to conceal the antennas and provide stealth properties, by transmitting specific frequencies and be reflective for other frequencies. Ideally, the radome is expected to be electrically transparent. However, tradeoffs are necessary to fulfill properties such as aerodynamics, robustness, lightweight, weather persistency, stealth properties, etc. One tradeoff is the existence of inevitable defects. Specifically, for examples, seams in large radomes, lightning strike protection, Pitot tubes, rain caps, or lattice dislocations in frequency selective radomes. In all these examples of defects, it is essential to diagnose their influences, since they degrade the electromagnetic performance of the radomes if not carefully attended and analyzed. In this contribution, we investigate if source reconstruction can be employed to localize and image the disturbances from the defects on the surface of the radome. Employing far-field measurements remove the need for probe compensation. An artificial puck plate (APP) radome with dislocations in the lattice is investigated. An APP radome is a frequency selective surface (FSS) and it consists of a thick perforated conducting frame, where the apertures in the periodic lattice are filled with dielectric pucks. Due to the double curvature of an FSS surface, gaps and disturbances in the lattice may cause deterioration of the radome performance. Source reconstruction methods determine the equivalent surface currents close to the object of interest. The reconstructions are established by employing an integral representation in combination with an integral equation. The geometry of the object on which the fields are reconstructed is arbitrary. However, the problem is ill-posed and needs regularization. The equivalent surface currents are reconstructed on a body of revolution with the method of moment (MoM), and the problem is regularized with a singular value decomposition (SVD). The aim is to back-propagate a measured far field to determine the field components on the radome surface. The purpose is to investigate if defects on a frequency selective surface (FSS) lattice can be localized.

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.

Enabling Extremely High Dynamic Range Measurements using a Simple Correlator
Brett Walkenhorst, November 2016

In order to achieve high accuracy in measuring sidelobes and/or nulls in antenna patterns, it is necessary to use a test system with very high dynamic range. This is particularly important when the antenna has extremely high gain such as those used for certain satellite communications or radio astronomy applications or when transmit power is limited relative to range loss as is often the case in millimeter wave applications. For several years, commercially available antenna measurement receivers have offered a dynamic range as high as 135dB for such applications. This dynamic range has been made possible, in part, by a simple correlator in the receiver’s DSP chain. In this paper, we model the various sources of error in a test signal due to imperfections and uncertainties of the test equipment and the physical environment and analyze these models as they propagate through the receive chain. The results of that analysis demonstrate the correlator’s ability to reduce carrier frequency offset (CFO) and local oscillator (LO) phase noise to offer the fidelity of test signal necessary to achieve extremely high dynamic ranges of up to 135dB.

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.

Millimeter-wave Antenna Measurements Using a Novel Approach
Tom Newman, Joe Chandler, November 2016

A novel system architecture has been developed which makes measurements at N times the analyzer’s frequency, yet requires no communication with the analyzer.  Millitech’s Spartan Test Modules, STMs, splits the input signal from an analyzer, multiplies this by N for the source, and by N-1 for the LO of the receiver mixer.  The mixer downconverts to the original signal, while maintaining its phase integrity, and sends this back to the analyzer.  This scheme is straightforward for narrow bandwidth requirements, but becomes more difficult for wideband ones.  The filtering and temperature compensation requirements are high, but have been solved for these bands resulting in a dynamic range of 70 to 80 dB across 54-69 GHz for V-Band and across 69-90 GHz for E-Band, which directly relates to the side lobe resolution in an antenna pattern measurement.  The wide dynamic range doesn’t come at a cost of slowing the sweep, as in other frequency extension solutions.  This puts the Spartan system performance at the same or higher level as other mixer based systems that have much higher hardware requirements.  STMs can be used to convert any make, model or vintage of vector network, scalar network or spectrum analyzer into a millimeter-wave test station.  The small size of the STMs allows them to be mounted directly onto the back of the antennas.  Therefore, readily available, < 10 GHz cables can be used for the long run back to the analyzer.  The Spartan enables state-of-the-art antenna measurements either directly, in compact ranges, or in near-field ranges, examples will be shown.

Spherical Near-Field Alignment Sensitivity for Polar and Equatorial Antenna Measurements
Patrick Pelland, Greg Hindman, Daniël van Rensburg, November 2016

Spherical near-field (SNF) antenna test systems offer unique advantages over other types of measurement configurations and have become increasingly popular over the years as a result. To yield high accuracy far-field radiation patterns, it is critical that the rotators of the SNF scanner are properly aligned. Many techniques using optical instruments, laser trackers, low cost devices or even electrical measurements [1 - 3] have been developed to align these systems. While these alignment procedures have been used in practice with great success, some residual alignment errors always remain. These errors can sometimes be quantified with high accuracy and low uncertainty (known error) or with large uncertainties (unknown error). In both cases, it is important to understand the impact that these SNF alignment errors will have on the far-field pattern calculated using near-field data acquired on an SNF scanner. The sensitivity to various alignment errors has been studied in the past [4 - 6]. These investigations concluded that altering the spherical acquisition sampling grid can drastically change the sensitivity to certain alignment errors. However, these investigations were limited in scope to a single type of measurement system. This paper will expand upon this work by analyzing the effects of spherical alignment errors for a variety of different measurement grids and for different SNF implementations (phi-over-theta, theta-over-phi) [7]. Results will be presented using a combination of physical alignment perturbations and errors induced via simulation in an attempt to better understand the sensitivity to SNF alignment errors for a variety of antenna types and orientations within the measurement sphere. Keywords: Spherical Near-Field, Alignment, Uncertainty, Errors. References [1]     J. Demas, “Low cost and high accuracy alignment methods for cylindrical and spherical near-field measurement systems”,  in the proceedings of the 27th annual Meeting and Symposium, Newport, RI, USA, 2005. [2]     S. W. Zieg, “A precision optical range alignment tecnique”, in the proceedings of the 4th annual AMTA meeting and symposium, 1982. [3]     A.C. Newell and G. Hindman, “The alignment of a spherical near-field rotator using electrical measurements”,  in the proceedings of the 19th annual AMTA meeting and symposium, Boston, MA, USA, 1997. [4]     A.C. Newell and G. Hindman, “Quantifying the effect of position errors in spherical near-field measurements”,  in the proceedings of the 20th annual AMTA meeting and symposium, Montreal, Canada, 1998. [5]     A.C. Newell, G. Hindman and C. Stubenrauch, “The effect of measurement geometry on alignment errors in spherical near-field measurements”,  in the proceedings of the 21st annual AMTA meeting and symposium, Monterey, CA, USA, 1999. [6]     G. Hindman, P. Pelland and G. Masters, “Spherical geometry selection used for error evaluation”,  in the proceedings of the 37th annual AMTA meeting and symposium, Long Beach, CA, USA, 2015. [7]     C. Parini, S. Gregson, J. McCormick and D. Janse van Rensburg, Theory and Practice of Modern Antenna Range Measurements. London, UK: The Institute of Engineering and Technology, 2015

Inverse Scattering and Imaging of Compensated Compact Ranges by Plane Wave Analysis
Engin Gülten, Josef Migl, Thomas Eibert, November 2016

The Compensated Compact Range (CCR) 75/60 of Airbus DS GmbH is the state-of-the art indoor test facility for real-time RF measurements of satellite antennas within a frequency range from 1 to 200 GHz. The CCR is composed of a two reflector system, a main reflector and a sub-reflector, to create a cross-polar-compensated plane wave in the test zone. However, even such a sophisticated design has residual cross-polar components due to the contribution of the range feed, edge diffraction from the reflector system, as well as from the serrations and imperfect absorbers. To improve and optimize the RF performance of the CCR, detailed EM simulation models are developed in order to solve the related forward scattering problem [1, 2, 3]. In spite of this it is also of great importance to analyze the CCR in a different perspective to gain insight into the CCR. To this aim, an approach based on plane wave spectrum analysis combined with inverse scattering and imaging techniques is proposed. The proposed approach firstly computes the plane wave spectrum of the measured or simulated data taken in the quite zone by using 2D Fast Fourier Transform (FFT).  Then, the measured or simulated field is back-propagated by using an inverse scattering approach. By considering the geometrical shape information of the main reflector, the current distribution on the reflector is imaged. The reconstructed images help to clearly identify the effects of. Appropriate windowing is applied to the computed plane wave (angular) spectrum in order to locate and image the echoes. Based on the investigation carried out with the proposed approach, it turns out that the area of the main reflector should be increased to reduce the disturbing impact of the serrations. This investigation also shows that increasing the size of the sub-reflector does not help to improve the plane wave uniformity of the fields in the test zone.  In order to test the proposed method against the experimental data, which is not in a suitable format for FFT, the measured data is interpolated to equally spaced data in a Cartesian coordinate system. The experimental results, which are obtained by processing both co and cross polar measurements, show very good agreement with the results obtained by using synthetic data.      References [1] A. Geise, J. Migl, J. Hartmann, H-J. Steiner, “Full Wave Simulation of Compensated Compact Ranges at Lower Frequencies”, AMTA 33th Annual Symposium, 16 – 21 October 2011 in Englewood Colorado, USA. [2] C. H. Schmidt, A. Geise, J. Migl, H-J. Steiner, H.-H. Viskum, “A Detailed PO/ PTD GRASP Simulation Model for Compensated Compact Range Analysis with Arbitrarily Shaped Serrations”, AMTA 35th Annual Symphosium, 6 – 11 October 2013 in Colombus Ohio, USA. [3] O. Borries, P. Meincke, E. Jorgensen, C. H. Schmidt, “Design and Validation of Compact Antenna Test Ranges using Computational EM”, AMTA 37th Annual Symphosium , 11 – 16 October 2015 in Long Beach, CA, USA.

Quiet-Zone Qualification of a Very Large, Wideband Rolled-Edge Reflector
Anil Tellakula, William Griffin, Scott McBride, November 2016

Installing a large compact range reflector and electromagnetically qualifying the quiet zone is a major undertaking, especially for very large panelized reflectors. The approach taken to design the required rolled-edge reflector geometry for achieving a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz was previously presented [1]. The segmentation scheme, fabrication methodology, and intermediate qualification of panels using an NSI-MI developed microwave holography tool were also presented. This reflector has since been installed and the compact range qualified by direct measurement of the electromagnetic fields in the quiet zone using a large field probe. This paper presents the comparison and correlation between the holography predictions and the field probe measurements of the quiet zone. Installation and alignment techniques used for the multiple panel reflector are presented.  Available metrology tools have inherent accuracy limitations leading to residual misalignment between the panels.  NSI-MI has overcome this limitation by using its holography tool along with existing metrology techniques to predict the field quality in the quiet zone based on surface measurements of the panels.   The tool was used to establish go/no-go criteria for panel alignment accuracy achieved on site. Correlation of the holography predictions with actual field probe measurements of the installed reflector validates the application of the holography tool for performance prediction of large, multiple-panel, rolled-edge reflectors. Keywords: Rolled-Edge Reflector, Compact Range, Field-Probing, Quiet Zone, Microwave Holography

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.

Near to Far Field Transformation of RCS Using a Compressive Sensing Method
Christer Larsson, November 2016

Near field Inverse Synthetic Aperture Radar (ISAR) Radar Cross Section (RCS) measurements are used in this study to obtain geometrically correct images of full scale objects placed on a turntable. The images of the targets are processed using a method common in the compressive sensing field, Basis Pursuit Denoise (BPDN). A near field model based on isotropic point scatterers is set up. This target model is naturally sparse and the L1-minimization method BPDN works well to solve the inverse problem.  The point scatterer solution is then used to obtain far field RCS data. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper.  A comparison to image based near to far field methods utilizing conventional back projection is also made. The main advantage of the method presented in this paper is the absence of noise and side lobes in the solution of the inverse problem. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are, to mention some examples, constraints in terms of available equipment and considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements but in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases.   Separate features in the images containing the point scatterers can be selected using the method presented here and a processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the method described in this paper will be given.







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