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Highly Efficient Near-Field to Far-Field Transform for Polar Near-Field Scanned Data
Stuart Gregson, Clive Parini, October 2022

The plane-polar approach for near-field antenna measurements has attracted a great deal of interest in the open literature during the past four decades [1, 2, 3, 4, 5, 6, 7]. The measurement system is formed from the intersection of a linear translation stage and a rotation stage with the combination of the axes enabling the scanning probe to trace out a radial vector in two-dimensions facilitating the acquisition of samples across the surface of a planar disk, typically being tabulated on a set of concentric rings. In its classical form, the probe moves in a fixed radial direction and the AUT rotates axially. However, with the ever more prevalent utilization of industrial multi-axis robots and uninhabited air vehicles (UAV), i.e. drones, being harnessed for the task of mechanical probe positioning, such systems offer the possibility of acquisitions being taken across non-planar surfaces. In this paper an accelerated, rigorous, near-field to far-field transform for data that was sampled using a polar acquisition scheme that is based on a Fourier-Bessel expansion [4] is developed and presented that can be employed in the above circumstances. This highly efficient, robust, transform enables near-field data acquired on planar, and non-planar, surfaces to be transformed to the far-field providing the acquisition surface is rotationally symmetric about some fixed point in the x,y-plane with z being purely a function of the radial displacement. The utility of the non-planar acquisition interval stemming from the ability to minimize truncation effects without needing to increase the measurement size. The transform efficiency stems from the utilization of the fast Fourier transform (FFT) algorithm with the rigor and robustness deriving from the avoidance of recourse to approximation, e.g. piecewise polynomial interpolation cf. [7]. Numerical results are presented and used to verify the accuracy and efficiency of the novel transformation, as well as to confirm convergence of the requisite Bessel series expansion and sampling theorem.

Optimization of In-Vehicle Connectivity through Simulation-Augmented Antenna Measurements
Benoit Derat, Mert Celik, Winfried Simon, David Schaefer, Adrian Fleidl, Konstantin Schorp, October 2022

As radiocommunications and internet-based services have become ubiquitous, customer expectations for infotainment capabilities and reliability in vehicles have largely increased. As such, the optimization of the distribution and orientation of antennas within the car is required to deliver the adequate connectivity performance. Yet, making direct measurements of electromagnetic field distributions radiated by structure-integrated radiofrequency transceivers is extremely tedious, if not practically and economically impossible. Recent papers introduced the approach of simulation-augmented measurements, appearing as a relevant solution to that problem. This method relies on a three-step approach: (i) measure the phasor electric field radiated by the standalone or part-integrated antenna module around the test sample; (ii) use an algorithm to calculate equivalent electric and magnetic currents over a surface closely encompassing the device under test (DUT); (iii) inject these currents as a Huygens source into a full-wave solver, where the complete scattering and absorbing environment is then taken into account. This paper presents the concrete application of this approach to the evaluation of the electric field inside a vehicle, based on separate measurements of WiFi and Bluetooth antennas. These measurements are performed using a spherical near-field system, with either the standalone antennas as DUT or the antennas embedded into the physical middle console of a car. The equivalent sources generated from experimental data are then imported into the virtual car model, and interior electromagnetic fields are computed using the Finite-Difference Time-Domain technique. The assessment is realized for various conditions without and with driver and passengers. The results are analyzed and limitations, as well as uncertainties of the technique are discussed.

Design and Measurements of a Small End-fed Normal Mode Helical Antenna with Integrated Microstrip Structure
Sri Lekha Srimat Kilambi, Herbert Aumann, Mauricio Pereira da Cunha, October 2022

Compact omnidirectional antennas are highly sought for a multitude of present-day wireless applications such as smart car keys, radio frequency identification (RFID) tags, tire pressure monitoring system, hand-held communication devices, and high-temperature harsh-environment wireless sensors. This paper discusses the performance and the unique challenges in measuring the radiation performance of a compact (~1/25th to 1/10th of a wavelength) helical and microstrip combined structure operating as a normal mode helical antenna (NMHA) around 300MHz. The helical wire structure (27-turn, 37mm high and 6.2 mm wide) is connected to the end of a 50 Ωmicrostrip line fabricated on 1.5 mm thick FR4 substrate. The microstrip line provides a ground plane to the helical structure, serving as an integral part of the radiating element. A tiny 1:1 balun transformer was used to partially decouple the integrated NMHA from the external sheath of the coaxial cable connected to a vector network analyzer, thus allowing proper NMHA impedance measurement. The NMHA S-parameters were simulated on two different platforms, ANSYS-HFSS and WIPL-D Pro, and compared to the frequency of the measured structures, with all simulations and measurements agreeing within 3.5%. Varying the length of the ground plane associated with the microstrip line from 13 mm to 76 mm resulted in the decrease of the measured NMHA operational frequency by 3.2%. The measured impedance of the fabricated NMHA (including the balun) was close to 50 Ω for the 51 mm long line without the need of additional matching circuit. The measured transmission loss for two identical antennas (each 26 cm3) placed about 1 m apart was 22 dB. This performance is comparable or better than the coupling between much larger antennas currently used in harsh environment power plant applications, such as suspended plate antennas (42,500 cm3) or planar inverted F-antennas (11,800 cm3) operating around the same frequency. In addition, the proposed NMHA structure can be implemented using substrates and wires capable of operation at temperature above 300 °C, which constitutes an appealing solution for high-temperature harsh-environment applications such as those found in industrial machinery, metallurgic industry, power plant boilers, and turbine engines.

Further Exploration of the Holographic PNF Filter
Scott McBride, October 2022

Abstract— A 2021 AMTA paper[1] introduced a 3D holographic filtering algorithm optimized for the planar near-field (PNF) geometry. This filter has been shown to have an excellent combination of AUT-signal preservation, stray-signal rejection, and processing speed. It requires only the sampling of a conventional PNF measurement, along with a specified 3D boundary surrounding all of the AUT’s possible radiating sources. The 2021 paper[1] suggested some topics for further investigation, specifically the optimal Z spacing through the 3D hologram and the X- and Y-widths of the blanking window’s tapered extension, and those are investigated here. This paper also explores the combination of filtering and probe correction, since the measured convolution of probe and AUT spatial distributions will be wider than that of the AUT by itself. Finally, additional comparisons are made to the more traditional spherical-mode-truncation approach with different synthesized constellations of stray-signal radiators. Keywords: modal filtering, spatial filtering, holographic filtering, stray signals, planar near field [1] S.T. McBride, P.N. Betjes, “Holographic PNF filtering based on known volumetric AUT bounds,” AMTA 2021, Daytona Beach, FL.

Compact Ultra-wideband Ground Penetrating Radar Antenna Design
Cole Harlow, Chi-Chih Chen, October 2022

A compact ultra-wideband (UWB) ground penetrating radar (GPR) antenna has been developed for extraterrestrial subsurface sensing from 130 MHz to 2000 MHz continuously. The antenna was designed as a payload of a new cube rover developed by Astrobotic for lunar exploration. The maximum payload dimensions are 58cm x 20cm x 18cm (LxWxH). The lower frequency bound of 130 MHz allows for deeper penetration depth and the upper frequency bound of 2000 MHz provides a large bandwidth for achieving a high depth resolution. Designing such compact UWB GPR is challenging for many reasons: small antenna volume relative largest wavelength, wide operating frequency range, minimization of clutter from antenna and surface reflections, and maximization of the transmission of radar signals through the air-soil interface. The proposed antenna design adopts the dielectric loaded horn-fed bowtie dipole design. The antenna operates primarily from the dielectric-loaded horn section at frequencies above 400 MHz where the ULTEM 1010 material is used to load the horn thereby increasing its electrical size and controlling the radiation patterns. Below 400 MHz, this antenna functions as a folded dipole where the entire 3-D conducting arms and the conducting top plate contribute to the antenna operations. Special RF choke designs are also developed to suppress undesired cavity modes which are excited in the cavity behind the feed position. In addition, a wideband microstrip balun circuit board was designed and integrated directly on to the antenna arm for connecting the antenna’s balanced 120 ohm port to a 50 ohm coaxial connector without being affected by high-G vibrations and shocks during typical rocket launches. An antenna prototype was fabricated, and its antenna performance was measured with a good agreement with simulation predictions. This paper will describe the antenna specification, operating principles, as well as measurement and simulation performances.

Millimeter-Wave S-Parameter Measurements with a Vector Field Analyzer in Antenna Measurement Systems
Niyati Sanandiya, Bruce Williams, Steve Nichols, October 2022

Antennas are used in virtually all wireless, communications and radar systems. As key elements in these applications, antennas play a crucial role in determining the system’s overall performance. This makes accurate antenna characterization essential for any wireless application. Traditionally, electrically large antenna ranges are not equipped to perform return loss measurements and thus a separate benchtop vector network analyzer (VNA) setup is required for measuring reflection coefficient or VSWR of an antenna under test (AUT). In this paper, we demonstrate the two-port S-parameter measurement capability of the NSI-MI Vector Field Analyzer™ (VFA) and how it can be used to integrate return loss measurements in an antenna range. The VFA’s ability to perform multi-channel vector (amplitude and phase) electrical measurements and its long cable support using remote mixer interface (RMI) modules makes it well suited for antenna characterization, especially in electrically large measurement systems. For this experiment, we selected three known millimeter-wave components as devices-under-test (DUT) and measured the S-parameter matrix for each. These WR-10 band measurements were made using the VFA with Virginia Diodes VNAX frequency extension modules. Results are compared with Keysight’s N5225A performance network analyzer (PNA) using the same set of frequency extension modules for verification. Millimeter-wave S-parameter measurements taken on VFA and PNA setups for all DUTs are compared based on three factors: Repeatability, reproducibility, and measurement comparison. The variations between successive measurements are presented in graphical form to compare repeatability of both instrument setups. Reproducibility results are compared to show the difference between independent repeat measurements taken on both instrument setups. Error distribution comparison is presented for reproducibility test data to compare the measurement variations contributed by random sources for both instrument setups. Measurement comparison result shows the total difference between independent VFA and PNA measurements taken for each DUT. Keywords — Millimeter wave measurements, Scattering parameters, Calibration, Network Analyzers, Antenna measurements

Electromagnetic Field Transformations of Near-Field Data Without Global Reference for Magnitude and Phase
Alexander Paulus, Jonas Kornprobst, Thomas Eibert, October 2022

Over the past decades, near-field (NF) measurements have been established as a reliable alternative to direct far-field (FF) or compact-range measurements for the verification of radiation properties of antennas. Quantities of interest typically include the FF characteristic obtained by means of an NF FF transformation (NFFFT), which is a computational post-processing step applied to the NF data. Common NFFFTs work with time-harmonic data and require the acquisition of magnitudes and phases of the NF samples with respect to a common reference signal. In other words, classical NFFFTs require the observed magnitude and the observed phase data to be drift-free during the time span of the complete measurement. With increasing measurement frequency and exposed or complex measurement setups in uncontrollable environments, e.g., as encountered in outdoor NF antenna measurements with unmanned aerial vehicles (UAVs), phase stability quickly becomes a limiting factor. Therefore, nonlinear phaseless NFFFTs have been developed that do not require any phase information, which, however, heavily rely on accurate and globally consistent magnitude information and are notorious for their unreliable behavior. Recently, a linearized and reliable NFFFT operating on locally consistent, i.e., relative, phase information has been reported. By using local phase differences within the NF data, the method becomes immune to phase drifts of the reference signal. However, in real-world measurements in uncontrolled environments, drifts in the measured power or magnitude may occur as well, e.g., caused by temperature variations during UAV flights, which render common phaseless NFFFTs useless. In particular, this prevents the use of the otherwise reliable linearized transformation. We investigate NFFFTs requiring a variable degree of global synchronization. In particular, a linearized transformation utilizing only relative, i.e., locally synchronized, information, both in magnitude and phase, is presented. It is shown that the transformation yields similarly accurate results as transformations employing global data, while being immune to magnitude and phase drifts. Furthermore, we compare the overall benignancy of a complete set of retrieval problems: completely phaseless, magnitudeless and mixed relative/global magnitudes and phases. Transformation results for simulation data illustrate the accuracy and suitability of the transformations with relative data, even for electrically large problems.

Comparison of Angle-Dependent Scattering of Convoluted and Straight Microwave Absorbers
Willi Hofmann, Andreas Schwind, Christian Bornkessel, Matthias Hein, October 2022

Motivation and background: The increasing sophistication of wireless communication systems necessitates accurately designed test environments such as anechoic chambers. The minimum achievable level of noise and interference in such test environments is essentially determined by the reflectivity of the absorbers installed, emphasizing the importance of characterizing their scattering behavior under realistic test conditions. In order to improve the modeling of absorber-lined anechoic chambers e.g., based on ray-tracing methods, a profound understanding of the relationships between the geometrical (e.g., pyramidal or convoluted shapes) and material properties (complex-valued dielectric permittivity) and the frequency- and angle-dependent reflectivity of the absorbers is needed. Objectives and methods: The angle-dependent scattering off convoluted microwave absorbers at normal and oblique incidence was investigated at frequencies between 2 GHz and 18 GHz. Based on measured permittivity values, a unit-cell model was constructed to compute the angle-dependent reflectivity of absorbers of different shapes. To verify the model, the scattering off such absorbers was measured in a bi-static setup at different angles-of-incidence up to 60 degrees, and compared to the numerical results. In addition to the convoluted absorber geometry, pyramidal and wedge-shaped absorbers were studied, in order to analyze the influence of the absorber geometry on the reflectivity while maintaining the same material properties. Results and conclusions: The numerical results of the convoluted absorbers agreed well with the measured reflectivity, thus validating the numerical model. The results revealed an increase of the reflectivity at angles-of-incidence above 45 degrees, in accordance with expectation. Compared to the convoluted geometry, the pyramidal and wedge absorber shapes showed reflectivity values about 10 dB lower, for frequencies at which the electrical size of the absorbers exceeded unity. Together with the results of previous studies, these findings provide important ingredients for a comprehensive database of the angle- and frequency-dependent absorber reflectivity, from which a consistent ray-tracing modelling of anechoic test environments can be derived. This research has been funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under the grants HE3642/14-1 and BO4990/1-1 (Electromagnetic modeling of microwave absorbers - EMMA; Project-No. 418894892).

Using a VNA Based Spherical Near Field Antenna Measurement System for Active Antenna System Performance Verifications
Chang-Lun Liao,, You-Hua Lin, Ike Lin, Bo-Cheng You, Chang-Fa Yang, De-Xian Song, Wen-Jiao Liao, Yuan-Chang Hou, Tswen-Jiann Huang, October 2022

Nowadays, 5G new radio and commercial networks have been widely developed and deployed by many communication service providers around the world. State-of-the-art techniques such as massive multiple input multiple output systems, 3D beamforming technologies, etc. are utilized to enhance spectral efficiencies and system capacities within cellular coverage areas. Additionally, 5G base stations with active antenna systems (AAS) are comprised of the passive antenna array, transceiver frontend and base band units, all integrated into one module, so that the traditional antenna RF ports are replaced with ethernet-based interfaces. Consequently, different from conventional 4G antenna system verification processes, to ensure the optimal cellular signal coverages of the 5G AAS base stations, new measurement methods to verify the radiation properties at RF carrier frequencies of the AAS need to be employed. The radiation pattern test method of the AAS by using a vector network analyzer (VNA) based spherical near field antenna measurement system through over-the-air (OTA) to obtain the magnitude and phase distributions of the electromagnetic fields from the antenna under test (AUT) with single-tone transmitting will be presented in this paper. In order to verify the above mentioned concepts, a signal generator is used to provide the single-tone source into a commercial passive base station antenna. Also, two received channel connections to the VNA are included, where a reference antenna placed adjacent to the back of the AUT for phase recovery is added together with the existing probe of the spherical near field antenna measurement system for reconstructing the near-field amplitude and phase of the fields during scanning. Thus, the near-field to far-field transformations and back projections can be performed for active antenna performance verifications. Radiation patterns obtained by the above OTA near field measurement method demonstrate good agreements with those from conventional near field tests at 3.5 GHz for 5G FR1 AAS performance verifications.

Investigation of Inhomogeneity of Material Characteristics on the Hybrid-Absorber Performance
Amin Enayati, October 2022

With the growing applications of wireless systems in different aspects of everyday life, from the consumer-electronic devices to internet-of-thing applications to wireless health-monitoring systems, there is an expanding need for reliable measurement of the radiating performance of these devices. The electromagnetic compatibility (EMC) tests, including immunity and interference tests, are normally done in shielded rooms the walls of which are covered by the so-called hybrid absorbers. These absorbers are made of a magnetic lossy layer giving absorption at low frequencies and a dielectric lossy geometrical absorber which is mainly responsible for the electromagnetic absorption at higher frequency range. The heart of hybrid-absorber design, which can be reduced to a wide-band matching problem, is to match the dielectric lossy part to the magnetic lossy layer. The magnetic layer which is mostly made up of a ferrite material, is a relatively thin layer, i. e. less than 1 cm thick, which is supposedly homogeneous. The dielectric geometrical absorber part has a relatively large thickness, e.g. 30 inches, and incorporates geometries like a pyramid to make a tapering against the wave which is going to be absorbed by the absorber. Because of the relatively large thickness of the dielectric lossy part and the production-process techniques used to make these parts, there are inhomogeneities in this part. In the current paper, the impact of the inhomogeneity of the dielectric part on the matching performance of the hybrid absorber is investigated in details. In the 1st stage, a 30-ich absorber is chosen and sliced in 15 different layers the permittivity is of each measured separately. Using these permittivity values, a complex model is made and simulated to get an expected reflectivity value on the ferrite layer. In the 3rd step, the absorbers of the same production batch are chosen to be measure in real-scale measurement setup and the reflectivity values are measured. Finally, the measurement and simulation results are compared and the impact of inhomogeneity of the dielectric absorber on the hybrid-absorber performance in concluded.

Numerical and Experimental Validation of Near-Field Power Density Reconstruction from Radiative Field Measurements
Benoit Derat, Martin Wittmann, Mert Celik, Walid El Hajj, Davide Colombi, October 2022

For millimeter-wave wireless devices used in the close proximity to the human head or body, the compliance evaluation to regulatory exposure limits is determined from near-field free-space power density measurements. For mobile phones and other portable equipment, the standard assessment technique involves the characterization of the power density on planes, as close as 2 mm from each facet of the device under test (DUT), as well as on anthropomorphic surfaces. These measurements are typically realized by means of a pseudo-vector diode-detected probe, acquiring the electric field magnitude and polarization ellipse at multiple locations over the scanning area. Phaseless techniques have been employed to deduce the required phase and magnetic field information for the calculation the Poynting vector. Although accurate, this technique presents some limitations: prohibitive test times; the inability to distinguish between various frequency contributions of the electric field due to the detection process; or the necessity to implement specific test modes in the DUT to fix the radiated beam in a given state. A recent paper proposed an alternative method which overcomes these listed limitations. The approach relies on the use of spherical antenna / over-the-air (OTA) phasor electric field acquisitions in an anechoic chamber environment, performed in the radiative field region and combined with near-field processing through equivalent currents reconstruction. This paper proposes an extensive validation of this method based on simulations and measurements of reference antennas, as defined in the IEC 63195. The reference measurements are realized with a standard-compliant 6-axis robot assessment system. The uncertainty contribution coming from probing at distances where reactive field components cannot be captured is investigated, demonstrating a negligible influence on reconstructed field distributions down to a third of the wavelength from the reference antenna, for which a theoretical interpretation is provided. It is also shown that characterizing the detailed changes in the reactive field is not necessary to obtain an accurate peak spatial-average power density value, which is the relevant metric for compliance assessment. A systematic analysis of the error affecting this specific quantity is also provided.

Extension of Single-Cut NFFFT to Multi-Cut Fresnel-Field FFT Depending on Antenna Height
Masanobu Hirose, Satoru Kurokawa, October 2022

We propose a new Fresnel-field to far-field transformation to measure the absolute gain patterns of an antenna on a strip region between some elevation angles when the long axis of the antenna is placed in the horizontal plane. The measurements are done on multi circles of the same radius on the multi-cut planes (parallel to each other) at the Fresnel distance. The number of the multi circles depends on the antenna height and the radius of the circles. The number reduces to one, that is, the single-cut near-field to far-field transformation if the measurement radius is larger than the far-field distance determined by the height of the antenna. In our method, the number of the circles or the cut planes is proportional to the square root of the antenna height, whereas the conventional cylindrical scanning needs the number proportional to the antenna height because the height interval is a constant less than the half wavelength. Therefore, the measurement time by our method can be much less than the one by the cylindrical scanning. The proposed transformation is an extension of a single-cut near-field to far-field transformation combined with the Fresnel approximation along the z (height) direction. In our method, we can obtain the absolute gain pattern in the stirp region within the elevation angles spanned by the cut planes where the measurements are done. Whereas the elevation angles are limited by the angles where the Fresnel approximation holds, the azimuth angle range is only limited by the measured one and can be 360 degrees. In the presentation at the Symposium, we will show the simulation results and demonstrate the measurement results for a standard horn antenna at 70 GHz band using the new type of a photonic sensor. Our method has a possibility to extend the measurements on the circles to arbitrary curves on the multi-cut planes. This means that our method is most suitable to the measurement system using a robotic arm and a RoF (Radio on optical Fiber) technique.

Antenna Diagnostics of a Large Deployable Reflector Antenna
Andreas Ericsson, Oscar Borries, Martin Haulund Gaede, Peter Meincke, Erik Jørgensen, Cecilia Cappellin, Erio Gandini, October 2022

Large deployable reflectors are critical for future Earth observation missions, science missions and in telecommunication, where an enhanced footprint and increased resolution are required and ensured by electrically very large reflector antennas. To accurately correlate simulations and measurements of such large and complicated antenna structures is a crucial step in improving the technology readiness level of these innovative antenna designs. A useful tool in this process is equivalent current reconstruction methods for antenna diagnostics, to allow comparisons between expected and realized performance. By finding the equivalent currents in the extreme near-field region that radiate a given/measured electromagnetic field, the user can accurately characterize the electromagnetic behaviour of the antenna under test. In this work, we present an antenna diagnostics investigation of an electrically large reflector antenna from the European Large Deployable Reflector project [1]. The antenna consists of a 5.1 m diameter deployable offset reflector in lightweight mesh technology. The antenna is an offset parabolic reflector with f/D equal to one and it has been measured at 10.65 GHz and 18.7 GHz. At such electrical sizes, an equivalent current investigation has previously been out-of-scope for the computational solvers in the market. In a recent ESA study, an accelerated equivalent current reconstruction solver based on [2] has been carefully implemented and then applied [3] to perform source reconstruction of the full reflector antenna based on measured and simulated data. Comparing the two sets of reconstructed currents gives the possibility to highlight potential deviations and pinpoint problematic aspects of the antenna design. [1] C. Cappellin, M. Lori, A. Geise, C. Hunscher, and L. Datashvili, “Predicted and Measured Antenna Patterns of the European Large Deployable Reflector,” Proceedings of EuCAP, 2022. [2] J. Kornprobst, R. A. M. Mauermayer, E. Kılıç and T. F. Eibert, "An Inverse Equivalent Surface Current Solver with Zero-Field Enforcement by Left-Hand Side Calderón Projection," Proceedings of EuCAP, 2019. [3] O. Borries, M. H. Gaede, P. Meincke, A. Ericsson, E. Jørgensen, D. Schobert, and E. Gandini, “A Fast Source Reconstruction Method for Radiating Structures on Large Scattering Platforms,” Proceedings of AMTA 2021.

Reduced Distance OTA Testing Methodologies for Automotive Applications
Alessandro Scannavini, Francesca Mioc, Francesco Saccardi, Kim Rutkowski, Lars Foged, October 2022

With the growing of vehicular communication technologies, the need for performing radiated measurements accounting for the full vehicle is becoming increasingly important. In modern cars, antennas are an integral part of the vehicle which is too complex to be represented by a simple ground plane during the tests. 5GAA test report [1] unifies measurement procedures for vehicle mounted antennas for both passive (at the antenna level) and Over-the-Air (OTA - at the modem level) measurements. Both Far-Field (FF) and Near-Field (NF) measurement systems are considered for testing of the full vehicle. In NF systems the radiated signals are measured on a closed surface at reduced distance, and a NF-to-FF transformation is applied. Since the transformation requires phase coherence between the transmitter and the receiver, NF systems are suited for passive tests. On the other hand, FF ranges are more suited for OTA tests, but requires large measurements distances, and hence expensive testing environments. OTA testing at reduced distances offers several advantage including the possibility to consider smaller and cost-effective anechoic chambers and the reduction of the system path losses (improved dynamic range). Moreover, the use of multi-probe systems dramatically reduces the testing time. The possibility of performing automotive OTA tests in spherical multiprobe systems at a reduced distance will be investigated in this paper. Simulations of a realistic vehicle with antennas installed in different locations will be considered to assess the uncertainty introduced by the reduced measurement distance. Figure of merits like different partial radiated powers [1] will be considered. Experimental automotive OTA measurements of a monopole-like antenna, installed on the roof of a vehicle will also be presented. These measurements have been performed at LTE bands in a spherical multiprobe system with a 4m radius. The measurements will be analysed comparing direct OTA measurement with a two-stage method, where passive antenna measurements and a few sampled OTA measurement points are combined. The outcome of the simulated analysis and experimental tests will be used to preliminary assess the uncertainty of automotive OTA measurements at reduced distance considering metrics relevant to automotive technologies [1].

Additional Insights into Chamber Effects in the Gain Extrapolation Data using Empirical Mode Decomposition
Zhong Chen, Yibo Wang, October 2022

Gain extrapolation data as a function of antenna distance is influenced by reflections in a chamber, manifested as ripples riding on otherwise smooth response data. A previous study introduced a method to analyze and remove the ripples by transforming the antenna response data to k-space via Fourier transform, and applying a filter in the k-space. The Fourier analysis assumes the antenna response is stationary as a function of its position. Because of the large travel distance in the gain extrapolation measurement, the stationary assumption cannot be guaranteed, especially for the influence from chamber reflections. As a result, in k-space, the reflected signals are smeared, making it difficult to identify and filter out the reflections. In this study, we apply the Empirical Mode Decomposition (EMD) and Hilbert-Huang Transform (HHT) to analyze the nonstationary antenna response data. The EMD is especially suited to analyze non-stationary empirical data. It decomposes the measurement data into intrinsic mode functions, from which localized spectrum at each antenna position can be derived using HHT. The position based spectrum can provide additional insights into the reflection sources, such as how the spectral contents of the reflections vary as the antenna travels along the track. Unlike the Fourier method, EMD can operate on real valued data (e.g., the magnitude of the antenna response), so there is no need to obtain the vector response data. As a result of the EMD process, “clean” data with minimal chamber influence is obtained, which can be used to compute the far field gain. It provides yet another powerful post processing algorithm to reduce chamber effects in the gain extrapolation method.

Three-Antenna Polarization Measurements Again
Ronald Wittmann, Michael Francis, David Novotny, Allen Newell, October 2022

This paper is an extension of a 2020 AMTA paper [1] in which we simulated (with added noise) an older, and seemingly forgotten, technique for processing three-antenna polarization data that employs well known signal processing methods. In this paper we analyze actual measured data. Topics to be explored include: (1) Time-domain gating to mitigate antenna-antenna multiple reflection and room-reflection error signals, (2) Bi-directional S-parameter measurements, (3) a scheme [2] (based on the geometric mean of the S-parameters S_pq and S_qp) to mitigate calibration drift errors, (4) Fourier filtering. We plan to demonstrate a robust method that is effective and easy to implement. [1] R. C. Wittmann and M. H. Francis, "Three Antenna Polarization Measurement Revisited," Proc. AMTA, Virtual, pp. 3–6, Nov. 2–5, 2020. [2] D. R. Novotny, A. J. Yuffa, R. C. Wittmann, M. H. Francis, and J. A. Gordon, "Some advantages of using bi-directional s-parameters in near-field measurements," Proc. AMTA, Williamsburg, VA, pp. 327–332, Nov. 4–9, 2018.

Arbitrary Rigid Translation of the Spherical Vector Wave Functions
Kyriakos Kaslis, Samel Arslanagic, Olav Breinbjerg, October 2022

Spherical Near-Field Antenna measurements are based on the formulation of characterizing antennas based on the spherical vector wave modes that they can transmit. Given that one antenna (called Probe) has already been characterized, it can be used to characterize an other one (called Antenna Under Test (AUT)) using the spherical transmission formula [1, Chapter 3.2.2]. The spherical transmission formula relates the signal received by the Probe to the signal transmitted by the AUT (or vice versa), given their relative distance in space and the coordinate system with respect to which the Probe has been characterized. Since practical antennas cannot coincide in space, a translation in space is always necessary. The common practice in Spherical Near- Field Antenna measurements is to translate along the z-axis of the measurement coordinate system (see [1, Appendix A3] or [2, Chapter 7.4]). This is adequate for most practical applications in near-field antenna measurements, since a translation along the z-axis, in combination with proper rotations, can align any two coordinate systems in space. However, practical implementations of rigid translations along an arbitrary direction do not seem to have been considered by the antenna measurements community, even though the theoretical developments have been in place for decades, for example [3]. Potential applications include the simulation of probe transverse errors in Spherical Near-Field Antenna measurements, spherical scattering measurements and near-field calculation in a spherical coordinate system. In this work we review, implement and validate the necessary algorithms that translate the modal solutions of the spherical vector wave equation along an arbitrary direction. The challenges of such implementation are analyzed and it is shown that efficient computer algorithms exist for the accurate computation of the translation coefficients. A few potential applications are also presented. [1] J. E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Peregrinus, Ltd., London 1988
 [2] C. Parini et al, “Theory and Practice of Modern Antenna Range Measurements”, The Institute of Engineering and Technology, London, 2014 [3] S. Stein, “Addition Theorems for Spherical Wave Functions”, Quarterly of Applied Mathematics, Vol. 19, pp. 15-24, 1961

Ultra-thin EBG backed flexible antenna for 24 GHz ISM band WBAN
Mubasher Ali, Irfan Ullah, John Batchelor, Nathan Gomes, October 2022

“A novel, ultra-thin, electromagnetic bandgap (EBG) backed antenna is presented for 24 GHz ISM band wearable applications. The via-less EBG unit cell shows both Artificial Magnetic Conductor (AMC) and EBG Characteristics. With dimensions of 0.254λ0× 0.254λ0, it is easy to fabricate at a millimeter-scale. The antenna has bow-tie slots, designed with an overall dimension of 0.91λ0 × 0.84λ0 × 0.01λ0, backed by a 5 × 5 element 0.01λ0 thick EBG/AMC structure; it is manufactured on a flexible Rogers 5880 substrate (thickness = 0.127 mm, = 2.2, tanδ = 0.0009). The proposed antenna is the thinnest (0.02λ0) EBG-backed antenna when compared to available K-band EBG-backed antennas. The performance of the EBG-backed antenna in terms of reflection coefficient and free-space radiation patterns is investigated in scenarios with and without structural bending. It is shown that the integration of the EBG enhances the antenna’s front-lobe gain by 2.63 dBi, decreases back-lobe radiation by 12.2 dB, and decreases 93% the specific absorption rate (SAR (1 g)) from > 28 W/kg to <1.93 W/kg, significantly reducing potential harm to the human body. Furthermore, the EBG-backed antenna was analyzed under a tough on-body and structural deformation measurement setup and the results show the performance of the EBG-backed antenna is highly insensitive to body proximity, and that its performance is preserved when bent along either axis. Therefore, Proposed EBG backed antenna structure demonstrates suitability for K band conformally mounted WBAN applications.”

Numerical modeling of deployable crossed-dipole antennas
Zachary Myrtle, Ashanthi Maxworth, October 2022

In this work, we present simulation results of a half-wavelength and full wavelength crossed-dipole antenna, which will be deployed on a 1U cube satellite. The crossed dipole antennas have been used in low Earth orbit small satellites such as the Enhanced Polar Outflow Probe (ePOP, also known as Swarm E). The advantage of the crossed-dipole antenna is that, given the two orthogonal dipoles, this configuration can measure a signal in-phase and quadrature forms, giving accurate information on the polarization. These antennas have not been implemented on cube satellites so far, therefore, this work would lead to new findings in space science. We modeled crossed dipole configurations: half-wavelength and full wavelength. The half wavelength configuration comprised of two orthogonal dipoles each with a length of half a wavelength, and in the full wavelength configuration, each dipole was one full wavelength long. The numerical simulations were done using both Ansys HFSS and Altair FEKO, which are widely used industrial software platforms. Given that the numerical techniques are highly sensitive to mesh sizes, and the technique itself, we used both of these software in our study. In addition, we implemented the analytical equations on MATLAB from Mathworks for comparison. We were able to obtain interesting results during this process. The analytical results obtained with MATLAB accurately represented the expected results for both the full-wavelength and half-wavelength configurations. Those also agreed with vector analysis and antenna array dynamics. For the full wavelength crossed-dipole configuration, the results from HFSS and FEKO matched the analytical results obtained from MATLAB. As for the half-wavelength crossed-dipole configuration, the numerical results obtained from HFSS and FEKO agreed with each other but did not agree with the analytical results from MATLAB. It is important to perform a detailed study on the discrepancy between the numerical and analytical results for the half-wavelength crossed-dipole configuration. Given that two industry standard software platforms (HFSS and FEKO) which are heavily being used by antenna designing engineers produced similar results that did not agree with the analytical results, it is important to discuss this further in a future publication.

Crosstalk is Good: Antenna Design to Enable Polarimetric Compressive Sensing
Jeffrey Massman, Julie Jackson, John Becker, October 2022

The dropped-channel polarimetric synthetic aperture radar (PolSAR) compressed sensing (CS) model [1,2] is able to recover an unmeasured polarimetric channel by utilizing antenna crosstalk and compressed sensing techniques. For successful recovery of a dropped channel, a sufficient amount of crosstalk is required to mix the information from the dropped channel into the measured channels. Recently, Monte Carlo simulations were conducted on the dropped-channel PolSAR CS model, and a range of crosstalk values of -9 dB to -3 dB was found to produce low recovery error for a variety of SAR image point spread functions and scene sparsity levels [3]. However, dual-polarized antennas are typically designed to have very high channel isolation, with crosstalk much less than the – dB minimum desirable value. To lend credibility to the dropped-channel PolSAR CS model, a new antenna is needed that can provide such high amounts of crosstalk without sacrificing gain, bandwidth, and radiation pattern. In this paper, we design a new, high crosstalk, dual-polarized patch antenna, using Ansys/HFSS to optimize pin placement and patch size for the desired gain, center frequency, and crosstalk values. The designed antenna is constructed, and S-parameters, gain, and radiation patterns are measured. The measured crosstalk values are then tested in the dropped-channel PolSAR CS model over a few deterministic scenes, demonstrating sufficient expected performance of the physical antenna for sparse scene recovery. 1. J. A. Jackson and F. A. Lee-Elkin, “System, Method, and Apparatus for Recovering Polarization Radar Data," United States of America Patent US11 194 104B1, Dec., 202 2. J. A. Jackson and F. A. Lee-Elkin, “Exploiting Channel Crosstalk for Polarimetric SAR Compressive Sensing," IEEE Transactions on Aerospace and Electronic Systems, vol. 56, no. 1, pp. 475-485, Feb. 2020. 3. J. Becker, Theory and Design of a Highly Compressed Dropped-Channel Polarimetric Synthetic Aperture Radar, PhD Dissertation, Air Force Institute of Technology, June 2022.
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