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AMTA Paper Archive

RCS Calculations and Measurements of a Spherical Drone Based Calibration Device
Spencer Wallentine, CJ Reddy, Joel Cannon, R. Jerry Jost, October 2022

Ultra-Wideband (UWB) calibration of RCS measurement radar systems, particularly outdoor, ground-to-air dynamic signature measurement radars, are conventionally accomplished using calibration devices (CD) attached to static towers, tethered from balloons, dropped from helicopters or other air vehicles, or with active RF repeater systems. These methods all contain errors from background-target interactions, or are operationally compromised, or are expensive. For high accuracy RCS radar calibration, a revolutionary methodology and architecture is mandatory to support demanding new radar metrology requirements. Within this paper, an update of ongoing efforts toward developing an autonomous, airborne drone-based CD is provided. The radar reflectivity of such a device must have a highly reproducible RCS, as manifested by a narrow, well defined probability distribution function (PDF), and should be independent of viewing aspect. Computational electromagnetic simulations were used to predict the RCS of a 60.9-cm diameter autonomous Spherical Passive/Active Calibration Device (SPARCS) with periodic, hexagonal “honeycomb” electromagnetic screens for inlet/outlet ports of the propulsion system. Three methods were used to predict the RCS, including Large Element Physical Optics (LE-PO), Physical Optics (PO) and Method of Moments (MoM) using the Adaptive Cross-Approximation (MoM-ACA). All methods show reduced cross section at angles centered on the periodic hexagonal RF screens. LE-PO resulted in a 30-fold decrease in computational expense relative to PO, but had a higher standard deviation. MoM-ACA calculations are ~57-fold more computationally expensive than PO results, but provide the full wave solution. PDFs determined from UWB RCS measurements have a narrow distribution and show reasonable agreement with calculations. These results validate the RCS signatures of the autonomous, airborne, spherical CD incorporating inlet/outlet RF screens as a valid calibration target.

Low Frequency Solutions in a Compact Range
Marlow Coronado Rumreich, Sean Raffetto, October 2022

The Boeing 9-77 Compact Radar Range has utilized low-frequency solutions since the 1990s. However, compact radar ranges have innate challenges when it comes to low-frequency measurements, typically due to facility size limitations. Due to increasing demand for more reliable data across a broad set of frequencies, an upgrade to the existing Ultra High Frequency (UHF) antenna feeds was designed and implemented in July 2020. This antenna was developed with field quality improvements, reliability, repeatability, and maintainability in mind. Unlike the previous design, this antenna was designed as an array with weighted feeds to complement the characteristics of the pre-existing range Gregorian reflector system. This new UHF antenna array leveraged the Weighted Element Method (WEM) along with extensive electromagnetic modeling and trade studies to achieve an efficient design at a minimum size. As a result of these design choices, the new antenna has doubled the efficiency in the band of interest. In addition, the frequency bandwidth of the antenna was improved while also reducing calibration and background drift. Lastly, this array has significantly improved the field quality of the quiet zone compared to the previous antenna system and improved the signal-to-noise ratio. This paper describes the UHF Antenna Array design process and the compact range measurements results to demonstrate the benefit of the WEM for feed arrays in a compact range. Additionally, the authors present an evaluation of methods used to create a digital twin of the UHF Antenna Array and a summary of best practices for future development of weighted antenna arrays for compact radar ranges.

Reflectivity reconstruction from only amplitude non-redundant near-field data: numerical validation
Florindo Bevilacqua, Amedeo Capozzoli, Claudio Curcio, Francesco D'Agostino, Flaminio Ferrara, Rocco Guerriero, Angelo Liseno, Massimo Migliozzi, Yiannis Vardaxoglou, October 2022

The imaging of the reflectivity of a target from Near-Field (NF) scattered data is nowadays well established. Generally speaking, these techniques require complex data, i.e., amplitude and phase acquisitions. In this paper, the use of only-amplitude acquisitions is investigated. We propose an approach to image the reflectivity profile of a target using only amplitude NF data under a monostatic measurement configuration. To cope with phaseless data, a phase retrieval problem is settled and dealt with as a quadratic inverse one. The phaseless procedure requires two sets of independent squared amplitude measurements of the scattered field, collected on two different scanning surfaces. A solution of the problem is reached by the search for the global minimum of an appropriate quartic functional. A proper representation of unknowns and data, exploiting the available information on the target and on the scanning geometry, allows to improve the reliability and the accuracy of the optimization process. First, a representation of the unknown target reflectivity using Prolate Spheroidal Wave Functions is used. Furthermore, properly acquiring the phaselesss NF requires a significantly high sampling rate when performed with a standard approach. From this point of view, a non-redundant sampling can be employed to drastically reduce the amount of requested NF data. Following the use of the non-redundant sampling, a two-dimensional optimal sampling interpolation expansion can be employed to accurately recover the NF scattered amplitude data on the classical Cartesian grid. Numerical results to assess the effectiveness of the proposed approach will be presented. The approach has proved to be capable, besides imaging the unknown reflectivity, to accurately reconstruct the amplitude and phase NF on a third NF test plane. In the shown example, a reduction of 95% NF amplitude-only samples is achieved.

Flat Lens Antenna Technology for Free Space Material Measurements
J.W. Schultz, B.P. Petrie, C.L Bethards, J.G. Maloney, J.G. Calzada, J.T. Welter, October 2021

Free space material measurements at VHF and UHF bands require antennas that are necessarily large and heavy to accommodate the long wavelengths in these bands. Large antennas make measurement less practical and more expensive. This paper presents a new flat lens antenna technology, which enables significant reductions in size and weight compared to conventional wide bandwidth horn antennas. These new antennas utilize artificial dielectric loading combined with lossy materials to give directivities similar to much larger and heavier horns. This paper also presents the direct application of these antennas for free space dielectric material characterization. Example measurements of dielectric specimens are shown with a pair of 200 MHz to 4 GHz antennas.

On Convergence of the Upper Bound on the Ratio of Gain to Quality Factor
Alex J. Yuffa, Marc Andrew Valdez, Benoıt Derat, October 2021

An antenna’s practical far-field distance can be estimated from the upper bound on the ratio of its gain to quality factor. This upper bound is an infinite series that can be truncated based on the desired accuracy. We investigate the convergence properties of this bounding series. We find that the number of terms required for convergence depends on the antenna’s electrical radius in a way similar to the Wiscombe criterion used in Mie scattering theory. For typical experimental accuracy requirements, such convergence can significantly reduce the effective far-field distance.

Measuring Component Performance in an Integrated Antenna-Receiver System
Roy C Monzello, October 2021

In this article, a method is presented which describes how to measure the separate performance parameters of an antenna-receiver system after they have been integrated into one system. The integrated receiver may perform different than the cascaded prediction of the pieces that make up the system due to component interaction. This article develops a method that allows the integrated performance of the individual components (an antenna and a receiver for this discussion) to be measured without disassembly. Using the described method, parameters such as, antenna gain, receiver gain, and receiver effective input noise temperature (correspondingly, receiver noise figure) can be measured. Once the receiver effective input noise temperature is measured, then it is possible to determine the remaining parameters. In the past, the difficulty has been separating out the two noise temperature terms (sky noise and receiver effective input noise). The presented method develops multiple equations which essentially separates out the two terms. Once the two terms have been separated, solving for the others is now possible.

Synthesis of Van Atta Array Retrodirective Patterns Using Conventional Array Characterization
Songyi Yen and Dejan Filipovic, October 2021

Van Atta Arrays are antennas with uniquely configured beamforming networks (BFNs) that allow for innate retrodirection of incident signals. While useful for a range of applications, their characterization has typically necessitated the use of radar crosssection (RCS) ranges. Our work proposes an alternate method that uses conventional array characterization, specifically element patterns and scattering matrix measurements, to synthesize both bistatic and monostatic RCS patterns for Van Atta arrays. This method is demonstrated theoretically and experimentally first with a cross-polarized dipole array followed by a counterwound octafilar helix antenna array. The benefits of the proposed synthesis method include fast design studies and trades of the Van Atta BFN enabling retrodirective operation. Among other things, this allows for broader access to experimental research on this topic. The significance of the structural radar cross-section is also discussed.

Synthesis of a Phased Array with Planar Near-Field Techniques Based on Far-Field Measurements of a Sub-Array in a CATR
Bernd Gabler, Diego Lorente, L.G.T. van de Coevering, October 2021

Phased array antennas are often built from sub-arrays with identical or symmetrical layout. At an early project stage, performance verification measurements of the sub-array are valuable to proof the single module design. However, the characteristics of the final antenna are questionable without further processing. This work presents a concept that is based on far-field measurements of a sub-array in a Compact Antenna Test Range (CATR) in conjunction with planar near-field (PNF) processing to synthesize the entire phased array antenna characteristics. The procedure is explained with an example of a dual linear polarized L-band planar phased array antenna for an airborne synthetic aperture radar application. It is shown that the measured sub-array can be complemented by the synthesized twin to evaluate the characteristics of a final antenna that is not yet available in this form. The resulting performance of the synthesized entire phased array is presented and compared with simulations. The presented post-processing method would be beneficial to characterizing radiation patterns of large phased arrays by measuring only sub-arrays in a limited test-zone with any measurement principle.

On the Uncertainty Sources of Drone-Based Outdoor Far-Field Antenna Measurements
Cosme Culotta-L´opez, Stuart Gregson, Andrian Buchi, Carlo Rizzo,Diana Trifon, Snorre Skeidsvoll, Ines Barbary, Joakim Espeland, October 2021

Unmanned Aerial Systems (UAS), colloquially known as drones, offer unparalleled flexibility and portability for outdoor and in situ antenna measurements, which is especially convenient to assess the performance of systems in their realworld conditions of application. As with any new or emerging measurement technology, it is crucial that the various sources of error must be identified and then estimated. This is especially true here where the sources of error differ from those that are generally encountered with classical antenna measurement systems. This is due to the larger number of mechanical degrees of freedom, and to the potentially less repeatable and controllable environmental conditions. In this paper, the impact of some of these various error terms is estimated as part of an ongoing measurement validation campaign. A mechanically and electrically time invariant reference antenna was characterized at ESAESTEC’s measurement facilities which served here as an independent reference laboratory. The reference results were compared and contrasted with measurements performed outdoors at Quad- SAT’s premises using QuadSAT’s UAS for Antenna Performance Evaluation (UAS-APE). While a direct comparison between the measurement results from ESA-ESTEC and QuadSAT delivers information about the various uncertainties within a UAS-APE system in comparison to classical measurement facilities’ and the validity of such a system for antenna testing, other tests aim at providing an estimation of the impact of each error source on the overall uncertainty budget, thus paving the way towards a standardized uncertainty budget for outdoor UAS-based sites.

Reduced-Order Model for Antenna Pattern Characterization from a Small Number of Samples
Nicolas Mezieres, Benjamin Fuchs, Michael Mattes, October 2021

The characterization of the radiation performances is a necessary step in the conception of any wireless system. These systems require always more demanding radiation performances that calls for time consuming characterizations. This duration can be reduced by the decrease of the number of field samples. By enclosing the antenna in a Huygens’ surface, we can build a radiation matrix that maps equivalent surface currents to the radiated field. A singular value decomposition of this matrix enables to build a compressed representation of the antenna measurement and more specifically a reduced basis of the radiated fields. By harnessing the outer dimensions of the antenna, the number of field samples can be reduced as compared to spherical wave expansion techniques. This number is shown to be connected to the area of the convex equivalent surface enclosing the AUT, as hinted by previous analytical works for canonical enclosing surfaces. The whole antenna characterization procedure is validated by simulations and experiments.

Genetic Evolution of the Reflector Edge Treatment of a Single Offset-Fed Compact Antenna Test Range for 5G New Radio Applications
M. Dirix, S.F. Gregson, R. R. Dubrovka, October 2021

While the size of the parabolic reflector in general determines the usable area of the quiet zone of a compact antenna test range (CATR) inside which a pseudo plane-wave condition is produced, the reflector edge treatment also plays a significant role in terms of overall quality and electromagnetic field distribution & uniformity, and especially so at mm-wave frequencies. Using modern powerful digital computational simulation technology in combination with genetic optimization, the edge treatment can be evolved for a specific CATR application as part of the design process. This is crucial as it attempts to maximize the performance of a given solution while ensuring efficient use of the available space which correspondingly provides an economical implementation. This is particularly important in 5G production test applications where, in many instances, multiple systems are required to be collocated within a given host building and in which case, the savings become multiplicative. In this paper the novel design methodology is introduced for the genetic optimization (GO) of blended rolled edge single offset reflector CATRs. Several edge blends and treatments are considered with the genetically optimized design parameter. For each variation the quiet-zone performances are compared and contrasted.

Electromagnetic Interference Measurements at the MIT Lincoln Laboratory RF Systems Test Facility
Cara Yang Kataria, Alan J. Fenn, Adam J. Chapman, and Peter T. Hurst, October 2021

Robust and repeatable electromagnetic interference and compliance (EMI/C) measurements require specialized test equipment and adherence to a rigorous set of procedures corresponding to the necessary standard. In this work, we describe the EMI/C testing capabilities at the RF Systems Test Facility at MIT Lincoln Laboratory and share the findings from work done in accordance to MIL-STD-461G. Both conducted and radiated emissions were measured on an example RF test artifact in the large near-field anechoic chamber at the facility. CE102, CE106, and RE102 test setups and results are discussed.

Bi-static RCS variations of pedal and wheel movements on bicycles between 1 and 10 GHz
Andreas Schwind, Willi Hofmann, Ralf Stephan, and Matthias A. Hein, October 2021

One benefit of cooperative automated and connected driving lies in the fusion of multiple mobile wireless sensor and data transmission nodes, covering complementary technologies like radar, cellular and ad-hoc communications, and alike. Current developments indicate enormous potential to increase the environmental awareness through joint communication and radar sensing. In this respect, future channel models require knowledge of bi-static reflectivities of road users over a range of illumination and observation angles, both in the nearfield and in the far-field. To establish reference data and model such angle-dependent RCS variations, this paper deals with realistic pedal and wheel rotations of a bicycle based on electromagnetic simulations. In the simulation setup, idealized far-field conditions with plane-wave illumination and observation were assumed, while the angles covered the entire azimuth with 201 variations of the pedal and wheel positions. The fluctuation of the RCS is analyzed and discussed in terms of its probability density and cumulative distribution functions. Depending on the angular constellation, the range of the fluctuation varied between 1 dB and 14 dB, while the specular reflection and forward-scattering showed almost no fluctuation.

On the Challenge of Over-The-Air Measurements of High-Power Massive MIMO Radio Base Stations
Adam Tankielun, Gerd Saala, Sebastian Schmitz, Hendrik Bartko, Benoit Derat, Amin Enayati, October 2021

Using beam-steering technologies, 5G massive MIMO base stations are capable to radiate typical equivalent isotropic radiated powers as high as 80 dBm (100 kW). Such levels create challenges in Over-The-Air (OTA) testing, both for the RF test system hardware, and the anechoic chamber / absorber layout designs. In this paper, a calculation tool is introduced which allows evaluations of the Poynting vector at ”mid range” distances, from given base station models. This code is used to deduce conservative power density distribution estimates and identify possible critical exposure areas in the test facility. General design criteria for the chamber, absorber layout and choice of material are derived. The specific case of a plane-wave synthesis OTA test site is investigated, where an experimental setup is used to demonstrate the power tolerance of the solution and its compatibility with base station testing requirements.

Consideration of the Feeding Networks for Measurement of mmWave/Sub-THz SoP/SoC/SoD Antennas in 5G and 6G
Jae-Yeong Lee, Jaehyun Choi, Junho Park, Youngno Youn, Bumhyun Kim, Sungmin Cho, Kangseop Lee, Ho-Jin Song, and Wonbin Hong, October 2021

This paper presents a reliable design and measurement methodology of using various feeding networks for mmWave/Sub-THz SoP/SoC/SoD antennas in 5G and 6G communication. In order to achieve reliable and precison testing results, the electrical, mechanical, and thermal consideration have been precendently investigated and discussed through various examples of feeding network based on lots of the advanced materials and fabrication process. First, for a realization of the minimized discrepancy between simulation and measurement without any calibration kit and resistive films for 50-Ω termination load, two examples have been presented. In other words, a symmetrical power divider with back-to-back transition structures and a leaky wave antenna design topology featuring high attenuation constant have been demonstrated. Finally, despite challenging fabrication condition resulting in performance degradation, a low-loss transition structure in mmWave SoD antenna and its design methodology is also presented and discussed.

Base Station Specific Absorption Rate Assessment Based on a Combination of Over-The-Air Measurements and Full-Wave Electromagnetic Simulations
Benoit Derat, Mert Celik, Davide Colombi, Bo Xu, Christer Tornevik, David Schaefer, Winfried Simon, October 2021

Radio Base Stations (RBS) must comply with applicable radio frequency electromagnetic field exposure regulations. Although compliance evaluation is typically carried out using field strength acquisitions or computations, Specific Absorption Rate (SAR) measurement is the reference method for low-power RBS, such as those used for indoor coverage. As classical robotbased probing is extremely time-consuming, especially when the whole-body SAR in a large phantom is to be assessed, faster alternative techniques are of high interest. Such solutions are becoming even more crucial, as the number of test modes is multiplying with modern communication technologies. This paper introduces an alternative, based on the convergence of Over- The-Air (OTA) measurements, equivalent current reconstruction and full-wave electromagnetic simulation. A first set of results demonstrates the relevance of this combination, by comparing actual dosimetric measurements to OTA-based reconstructed SAR values in a flat body mannequin, for a commercial lowpower RBS. A test system is realized which enables OTA electric field phase evaluations for a self-powered device under test, using digitally modulated signals. This proof of concept establishes the applicability of the technique to actual regulatory testing conditions.

Simultaneous Measurement of Analog Phased Array Elements Using Orthogonal Coding
Michael D. Foegelle, October 2021

Evaluation and calibration of individual elements of a phased array is a time-consuming process that involves not only the radiation pattern and RF circuitry of each element, but the interaction of each element with all of the other elements within the array. Iterating through each element in order to test them one at a time is extremely time consuming, and in some cases, depending on the design of the array, this approach may not work reliably at all. In cases where the impedance of the “off” elements differs from their impedance when actively transmitting or receiving, they can distort the resulting single element pattern due to mutual coupling. Even in the case where the elements themselves are well behaved, the driving circuitry can exhibit non-linearities due to the differences in signal levels or device heating present when all elements are active vs. only a single element. Thus, it would be ideal to be able to extract individual element performance from the combined pattern of the array with all elements active. This paper will investigate the use of orthogonal coding applied to each element of the array through the onboard gain/phase control circuitry using different modulation coding schemes in order to extract the average performance of each element from the measured total result.

Radiation and Scattering Pattern Characteristics of Chamfered-Tip Open-Ended Rectangular Waveguide Probes for Planar Near-Field Antenna Measurement Applications
Elbert H. Ko, Domenic J. Belgiovane, October 2021

The radiation and scattering pattern characteristics of open-ended rectangular waveguide with a chamfered tip are examined. Despite common and widespread use as a probe antenna for planar near-field antenna measurements, a methodical investigation of the chamfered-tip design and resultant performance has not been published. A computational electromagnetics (CEM) model for an open-ended rectangular waveguide probe with a parameterized chamfered tip has been constructed and results for both radiation and scattering patterns are presented. A comparison of results includes a probe without a chamfer and a probe typical of that available from commercial suppliers. It is shown that, for a series of standard waveguide size probes sharing a common thickness for the waveguide wall and chamfered tip, the radiation pattern is relatively insensitive to the chamfer tip designs studied until frequency increases into W-band (WR-10). The scattering pattern characteristics for the same series of standard waveguide size probes show a reduction in on-axis (boresight) monostatic radar cross section (RCS) for chamfered tip waveguides compared to blunt-ended waveguides and that this reduction increases for increasing frequency.

Measurements and Simulations of a 2.4 GHz Circular Waveguide Antenna for a Portable Radar Kit
Alan J. Fenn, October 2021

A custom radar kit that integrates with a portable computer (laptop) for assembly and operation by students and researchers has been developed at MIT Lincoln Laboratory. The assembled radar kit uses two low-cost cylindrical metal cans that serve as the antennas, one for transmitting and one for receiving radar signals. The antennas operate as linearly polarized openended circular waveguides (10.5 cm diameter) fed with a thinwire monopole probe. Over the 2.4 to 2.5 GHz band, the measured reflection coefficient is less than −10 dB, the peak realized gain is greater than 7 dBi, and the half-power beamwidth is approximately 70 degrees in both the E- and Hplanes. FEKO method of moments simulations of the antenna are compared with the measured data and good agreement is demonstrated.

Modular Horn Antenna for VHF Reference Field Strength Applications
A. Akar, B. Neubauer, R. Geise, October 2021

Antennae in critical applications such as in-flight navigation, e.g. the instrument landing system (ILS), have to be calibrated on a regular basis. This allows for an error-free operation by verifying the absolute field strength as well as the spatial field distribution. Hence, it remains indispensable to calibrate the receiving antenna used by flight inspection services in absolute terms. The Calibration itself can only be achieved by measurements within a well known field distribution, ideally in situ, hence in the measurement environment of the targeted system. In this contribution a modular pyramidal horn antenna capable of providing reference field strengths within the frequency range of 75 MHz - 114 MHz is presented. The aperture’s field strength can be calculated analytically as well as measured with a high degree of accuracy. For the frequency range at hand, the size of the reference antenna ends up in a challenging scale of a truck. Construction details and manufacturing aspects of the light weight, modular and easy to assemble horn antenna are presented. Near field measurement results are shown, compared with simulations and discussed with respect to one another.







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