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Measurements of modern multi-service antenna systems require ever increasing bandwidths of the measurement equipment. The main bandwidth limiting factor of traditional Spherical Near Field (SNF) systems is mainly the probe as it should radiate only first-order azimuthal spherical modes to apply the first-order Probe Compensation (PC). Even though full PC techniques are becoming standard, enabling the use wideband antennas with more than 10:1 bandwidth as probes, high-purity first-order probes are still required in many applications, because of the simplification of data processing and calibration. Conventional dual-polarized first-order probes are based on Ortho-Mode Junctions (OMJ) with externally balanced feeding. The OMJ is fully symmetrical using two pairs of excitation pins fed by high precision 3dB, 0º/180º hybrid couplers to achieve good matching and maximize the cx-polar performance. Unfortunately, realistic couplers provide some excitation errors which are one of the main contributors of the generation of unwanted higher order spherical modes. Even a small unbalancing in amplitude or phase of the coupler will excite higher order modes at frequencies where these modes are allowed to propagate. Beside the possibility to compensate the effect of the higher-order modes in post-processing (e.g. full PC), the propagation of the spurious spherical modes can be controlled directly on the probe with improved designs of the feeding mechanism or considering ad-hoc designed hybrid couplers. In this paper, two innovative and high performance SNF probes will be presented. Both probes are based on an inverted quad-ridge waveguide technology. An advanced feeding mechanism allows the first probe to provide a high modal purity in the 617-960 MHz band, rejecting errors introduced by the external coupler. In the second one, a highly accurate coupler has been designed to minimize the higher order modes on a large bandwidth, 1427-4200 MHz. The two probes have been designed as part of the upgrade of a gantry arm system used to test modern base station antennas. The same measurement system has been used to calibrate the two probes and to verify the expected performance both in terms of radiation pattern and spherical modal content. The achieved measurement results will be shown in this paper.
The Field Strength Metrology Project at the National Institute of Standards and Technology (NIST) in Boulder, CO has restarted field probe calibration services from 10 kHz to 40 GHz, after a renovation of ouranechoic chamber. WhileNISThas long served as the nation’s link to the SI for radiated field measurements, in 2014, the anechoic chamber used for generating standard electromagnetic fields from 0.5 – 40 GHz was renovated. The positioning system was upgraded with a new rail, motion control, and a robotic arm. New absorber was installed in the main section of the chamber. During the renovation, Field Strength services were unavailable. In order to resume operation in the chamber, several tests were done to validate the chamber. We show the results of a thorough comparison of three facilities, the anechoic chamber, a TEM cell and a GTEM cell. Measurements of electric field probes in the new chamber were also compared with past measurements in the chamber before renovation. The Electromagnetic Field Strength special test services are now operational.
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.
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.
The mission of the Naval Surface Warfare Center, Indian Head, Maryland, EOD Department, is to utilize the latest available technology in the advancement of Explosive Ordnance Disposal (EOD) equipment and techniques. This mission includes the test and evaluation of current and developmental systems, which will be discussed in this paper. EOD exploits multiple physical phenomena in its task of ordnance detection, including chemical and electromagnetic. Electromagnetics include RF fields, light (including laser, infrared and ultraviolet), and nuclear radiation. For each phenomena, there may be several different technologies used to provide multi-mode detection capability. This study focuses on the electromagnetic subset of detection RADAR, and specifically Ground Penetrating Radar (GPR), which is distinguished by its earth surface domain and generally downward field of view. The paper will give a very brief overview of GPR theory and equipment, its use in EOD, and then will focus on the RF test and measurement of electromagnetic fields generated by GPR systems and antennas. An RF antenna/system test plan will be detailed, along with the design and development of antenna gain and radiation pattern measurement techniques. The measured data from GPR technology will be graphically displayed, analyzed and compared in terms of the potential for GPR effectiveness.
Abstract. In order to characterize the Boeing 9-77 compact range, the empty chamber background was measured as a function of frequency, polarization, and the azimuth angle of the upper turn-table (UTT). The results exhibited a diffraction pattern with enlarged hot-spots on a 4-fold symmetry [1]. A 2-D FFT on the diffraction pattern yielded a mapping on the relative arrangement of the very weak absorber tips on the UTT [2]. Here, we take a closer look at the scattering geometry of the UTT as illuminated by the residual field above and beyond the quiet zone (QZ). The different responses in VV and HH are discussed. The enhanced diffraction due to a “blazed grating” condition is identified and analyzed. Some interesting physics are discussed. An extended long object usually gives rise to a strong reflection (glint) when viewed near its surface normal. To take advantage of this phenomenon, a discrete Fourier transform (DFT) on RCS measurements taken within a small angular range would yield a spectrum of incident wave distribution along that object [3]. Some results along the horizontal direction have been reported [4]. As a complementary, we present and discuss the results in the vertical direction. References [1]. P. S. P. Wei, A. W. Reed, and C. N. Ericksen, “Radar cross section measurements amid interfering backgrounds,” Proc. 22nd AMTA, pp. 99-104 (2000). [2]. P. S. P. Wei, “Scattering of the residual field above and beyond the quiet zone of a compact range,” Proc. 35th AMTA, Columbus, OH (2013). [3]. P. S. P. Wei, “Measurements on long and rigid objects for radar field probes" Proc. 34th AMTA, pp. 195-200 (2012); Also in ACES Journal 28, 1228-1235 (2013). [4]. P. S. P. Wei, “Measurements on extended objects for radar field probes," Proc. 41st AMTA, pp. 199-204 (2019); Also presented at ICECOM-2019 (23rd International Conf. on Applied Electromagnetics & Communications), paper S_16_3, Dubrovnik, Croatia, Sept. 30, 2019.
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
“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.”
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.
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.
When using the gain substitution method with a pyramidal standard gain horn (SGH), it is common practice to use the on-axis NRL gain curves derived by Schelkunoff and Slayton [1]. Due to approximations in this formulation, Slayton assessed an uncertainty of ±0.3 dB for typical SGHs operating above 2.6 GHz. Since this uncertainty term is often one of the largest terms in the range measurement uncertainty budget for AUT gain, it is highly desirable to reduce it. Many studies in the past have attempted to improve upon Slayton’s expressions for SGH gain, but none have achieved widespread use. With the advent of high-performance computing (HPC), antenna simulations with computational electromagnetic (CEM) full-wave solvers are now capable of solving complex, electrically large models with high accuracy. This paper investigates the use of several commercially available solvers, including HFSS, CST, and FEKO to model the on-axis directivity and gain of a commercial off-the-shelf (COTS) X-band SGH. Relevant modeling techniques are described in detail and are shown to employ best practices as well as conformance with the IEEE 1597.1 and 1597.2 “Standards for Validation of CEM Modeling and Simulations” and “Recommended Practice for Validation of CEM Computer Modeling and Simulations,” respectively. The challenges and trade-offs of each CEM solving technique used, as well as their limitations, are discussed. Simulation errors are quantified via the IEEE standards, and other practical limitations of SGH manufacturability and measurement are discussed. Finally, the results from the CEM simulations are compared with the NRL gain curve and measured on-axis directivity and gain of the COTS SGH. Based on the compiled results of multiple simulations and measurements, this simulation methodology could be applied to other models of COTS SGH antennas to provide more accurate on-axis gain predictions. [1] W. T. Slayton, “Design and calibration of microwave antenna gain standards,” US Naval Res. Lab., Washington, DC, Rep. 4433, Nov. 1954.
The novel extension of the synthetic aperture radar (SAR) technique to the terahertz (THz) spectrum has emerging short-range applications, especially in an indoor environment. One of the key applications is the generation of a high-resolution indoor environment map in emergency scenarios such as a burning or smoky building, where optical technology might not provide any relevant information. The THz SAR map enables precise localization, classification, and material characterization of concerning objects which can assist in identifying the danger from electrical cables located in the walls and ceilings, and the structural integrity and failure of the walls/ceilings. Hence, the investigation of through-wall sensing at the THz spectrum is of vital importance. This paper addresses the through-wall sensing at the THz spectrum by employing the SAR technique. A miniature version of the wall using gypsum plasterboards is constructed, where the plasterboards are mounted on a frame. Two types of frames are considered, where one frame is of wood and the other is of metal. Additionally, electrical cables are placed between the plasterboards. This miniature version is quite similar to a practical environment. Besides, some of the considered components of the wall are in a burned state. For through-wall sensing, a vector network analyzer (VNA) based testbed is implemented and measurements are recorded in both transmission and reflection modes for three frequency spectrums, which are 75-100 GHz, 220-330 GHz, and 325-500 GHz. At the THz spectrum, the penetration capabilities are always of concern. Therefore, foremost, penetration losses among different components of the wall are investigated with transmission measurements. Further, to evaluate the sensing capabilities behind the wall, transmission measurements are recorded by considering the whole structure of the wall. Besides, relative attenuation among different frequency spectrums is presented. The addressed evaluation is also of significant interest in the area of wireless communication such as 6G and security. Lastly, in reflection mode, a 2D SAR trajectory is implemented and a 3D image of the wall is reconstructed. It is analyzed for identification and precise localization of the cables and frame-blocks. The identified components are further processed for burned state detection.
Highly integrated radar systems are becoming widely used, such as in automotive radars. To cope with the challenges of signal attenuation at millimeter-waves, antennas are placed directly on the circuit board of the chip or even integrated in the chip package. However, this complicates or even prevents conventional passive antenna measurements, as additional connectors or detached antennas will have severe impact on the antenna’s performance. We propose a measurement setup, that does not require physical connection to the antenna ports nor a phase reference from the transceiver chip. It allows for veritable measurements of a radar-module’s transmit antennas, while it is fully operational including beam-forming and beam-steering. The setup is built up in the Compact Antenna Test Range at the Institute of High Frequency Technology at RWTH Aachen University with a state-of-the-art 85 GHz spectrum analyzer. The propagation direction inside the chamber is reversed, so that the actively transmitting DUT can be mounted on top of the roll-over-azimuth positioner. The method is fully incoherent and therefore only suitable for phaseless measurements. It is tested with different antenna types and arrays mounted to an in-house designed evaluation board, based on the TI AWR automotive radar chipset, which operates at frequencies from 76 to 81 GHz. The results are compared against standard spherical near-field measurements, used as reference. Benchmarking the setup’s performance, dynamic range and uncertainty analysis are carried out with respect to the used spectrum analyzer mode, which allows for generic sweep-mode operation, pulse- and chirp-analysis with up to 5 GHz real time bandwidth. The transceiver’s power drift needs to be considered as well as timing constraints given by the chosen chirp mode and duty cycle. Contemplating the challenges of the proposed method, it can serve an emerging market for carborne radar systems, which cannot be appropriately measured by network-analyzer-based setups only.
Direct far-field (DFF) testing has become the baseline test methodology for sub-6 GHz over the air (OTA) testing of the physical layer of radio access networks (RAN). However, the proliferation of mm-wave massive multiple input multiple output (Massive MIMO) antennas for 5G New Radio (NR) rollout and the use of complex waveforms for communication system testing and primary Figure of Merit (FoM) determination has necessitated the adoption of the Compact Antenna Test Range (CATR) as the preferred test solution. The CATR was initially conceived as comprising an efficient way of testing electrically large antennas at very much reduced, fixed, range lengths [1]. However, early workers quickly recognized that the reflector edge treatment and chamber wall illumination are significant factors determining the quality and purity of the collimated pseudo plane-wave with this becoming especially important at mm-wave frequencies [3]. Using modern powerful digital computational simulation techniques [2] in combination with genetic optimization, the edge treatment can be evolved for a specific CATR application as part of the design process for a range of reflector edge treatments [3, 4]. This paper extends the authors previous work to present a novel approach for the reflector edge treatments than have hitherto been considered within the design and genetic optimization procedure, while also taking into account both wall illumination and direct quiet-zone illumination. Resulting quiet-zone performances are compared and contrasted. [1] C.G. Parini, S.F. Gregson, J. McCormick, D. Janse van Rensburg “Theory and Practice of Modern Antenna Range Measurements”, IET Press, 2014, ISBN 978-1-84919-560-7. [2] S.F. Gregson, C.G. Parini, “Examination of the Effect of Common CATR Quiet Zone Specifications on Antenna Pattern Measurement Uncertainties”, Loughborough Conference on Antennas and Propagation, Loughborough, November 2017. [3] M. Dirix, S.F. Gregson, “Optimisation of the Serration Outline Shape of a Single Offset-Fed Compact Antenna Test Range Reflector Using A Genetic Evolution of the Superformula”, EuCAP virtual conference, March 22-26 2021. [4] M. Dirix, S. Gregson and R. Dubrovka, “Genetic Evolution of the Reflector Edge Treatment of a Single Offset-Fed Compact Antenna Test Range for 5G New Radio Applications,” in AMTA Annual Meeting and Symposium, Daytona Beach, Florida, 2021
In recent years, many activities have been carried out within the European Association on Antennas and Propagation (EurAAP) and the working group (WG) on measurements in particular. This group constitutes an important framework for collaboration to advance research and development of antenna measurements. The activities are divided in different tasks comprising measurements and comparison of reference antennas, contributions in the revision of IEEE standards on antenna measurements, self-evaluation measurements for facilities and new and emerging technologies for antenna OTA measurements. Special attention is dedicated to the activity on international comparison campaigns and evaluation that span more than 10 years of dedicated work by the WG. This task constitutes a crucial foundation for facilities to document and validate measurement accuracy among participants and provide an important prerequisite for certification of facilities and inputs to standards and research on measurement uncertainties. As regular inter-comparisons are a precious tool for traceability and quality maintenance, the campaigns have become a useful instrument for facilities to obtain and/or maintain an ISO17025 accreditation. International intercomparison campaigns within the WG span the frequency range from UHF-V band using different antennas: a mm-VAST antenna, a set of MIMO PIFA antennas, a SH800 ultra-wideband horn, a BTS1800 base station antenna, a SR40-A offset reflector and a set of chip reference antennas. This paper gives an overview and status on current campaigns within the working group, focusing on the useful criteria for comparing and evaluating large amount of measured antenna data. Updated results on running campaigns and proposed future initiatives will be discussed including an interesting synergy between measurement and simulation modelling tools.
NASA mission requirements have driven an increased interest in active phased arrays antennas for space-based user communication terminals. Recent advancements in 5G technology have driven down the cost of phased array development and manufacturing all while providing a technology solution that covers many existing Ka-Band satellite communication spectrum bands. While these developments have provided ample opportunities to leverage new chips and arrays for use in space, there is also a need to evaluate these antennas in a relevant environment. Active arrays, as designed for use in 5G, require thermal management to avoid damage to the array as well as to maintain performance. Measured performance under various thermal conditions is essential both for understanding array performance and ensuring operation is within required tolerances. To address these measurement needs, the SmallSat Ka-band Operations User Terminal (SKOUT) at the NASA Glenn Research Center (GRC) developed a test environment that combines traditional antenna and communication system metrology with a conduction cooling/heating thermal control system approximating the space thermal environment. This paper will address the metrology system design and performance specifications as well as test article setup and operation. Tests that are typically performed at a single operating temperature can be performed over the typical temperature range of a Low Earth Orbit (LEO) mission. These tests include error vector magnitude (EVM), gain to noise temperature (G/T), antenna patterns, and non-linear characterization (e.g. P_in/P_out, intermodulation distortion products). The paper will cover the specific configuration for each test and provide results from recent test campaigns. The results illustrate the importance of higher fidelity environmental testing when evaluating the performance of active antennas.
Among the near-field - far-field (NF-FF) transformations, that with spherical scan is the most appealing due to its feature to allow the whole radiation pattern reconstruction of the antenna under test (AUT). To considerably save measurement time, spherical NF-FF transformations for AUTs with one or two predominant dimensions, requiring a minimum number of NF data, have been developed by using the non-redundant sampling representations of the electromagnetic fields and adopting suitable AUT modellings. Another effective possibility to save the measurement time is to make faster the scan by collecting the NF data through continuous and synchronized movements of the probe and AUT. To this end, non-redundant NF-FF transformations with spherical spiral scan have been recently proposed by exploiting the unified theory of spiral scannings for volumetric and non-volumetric AUTs. However, the characterization of heavy and large AUTs (such as, e.g., a vehicle) in a NF spherical facility from measurements collected over the full scanning sphere can become infeasible. To ensure a mechanically stable support and guarantee a high repeatability of the measurements, a more viable way is to place the AUT on a turning metallic ground plane and characterize the considered AUT from the NF data acquired over a hemisphere. In recent works, instead to set to zero the NF data required by the classical NF-FF transformation, which would be acquired over the lower hemisphere, it has been proposed to synthesize them by properly applying the image theory, thus avoiding that the truncation error affects the FF reconstructions. This work aims to propose an efficient spherical spiral NF-FF transformation for volumetric AUTs, using a minimum number of spiral data, which, due to the presence of an infinite perfectly conducting ground plane, are collected over a proper spiral wrapping the upper hemisphere. Once the voltage NF data which would be acquired over the spiral wrapping the lower hemisphere will be properly synthesized, then an efficient 2-D optimal sampling interpolation scheme will allow the recovering of the NF data required by the classical spherical NF-FF transformation. Numerical tests will show the accuracy of the developed non-redundant spherical spiral NF-FF transformation.
A free-space material measurement using the scattering parameters of a planar MUT (material under test) placed between Tx and Rx antennas is suitable for non-destructively testing the MUT without physical contact and precise machining in a high-frequency range. Improving the measurement uncertainty of the material parameter of an MUT extracted from a free-space material measurement requires accurate and precise measurement of the scattering parameters of the MUT, which is highly dependent on the characteristics of impedance standard (or reflect standard) and method used in calibrating the material measurement system, including a VNA (vector network analyzer). A free-space material measurement system usually needs at least three independent reflect standards (e.g., short, open, and load for coaxial case) for the calibration at both sides of an MUT in free space. Unfortunately, it is not easy to implement the reflect standards in free space. Recently, a planar offset short has been proposed as a free-space calculable reflect standard. The magnitude of its reflection coefficient is unity, and the phase is the linear function of the offset of the short and the signal frequency. This paper reviews the recently developed one-/two-port calibration methods using a planar offset short for a free-space material measurement in a millimeter-wave frequency range. An adapter characterization scheme, which is widely utilized to measure the scattering parameters of a non-insertable device (e.g., SMA to 3.5 mm adapter) by using a two-tier one-port calibration, may be applied to a free-space one-port calibration. On the other hand, an unknown thru calibration method, widely used to measure the scattering parameters of non-insertable devices whose connectors could not mate together (e.g., a coaxial to waveguide adapter), may be applied to calibrate a free-space two-port measurement system. These works use three planar offset shorts as free-space reflect standard, which gives the phase difference of 120° between the reflection coefficients of the planar shorts at the center frequency of the operating frequency band of a waveguide. A review of the two calibration methods and measurement results in the W-band (75-110 GHz) will be presented.
Electromagnetic materials characterization at UHF and VHF frequencies is typically done with laboratory fixtures such as the coaxial airline or rectangular waveguide. These conventional methods require material specimens to be cut or machined to precision tolerances for insertion within the transmission line fixture. Measurement accuracy dictates there should be little or no air gaps between the specimen and the transmission line walls. Transmission line methods also require significant handling and multi-step calibration procedures to characterize a material specimen. This paper describes a new handheld measurement device that overcomes these limitations with a simple calibration and non-destructive measurement procedures. This new method applies an open-ended stripline sensor tuned to maximize measurement sensitivity in the 100 to 1000 MHz range. The sensor footprint is approximately 100 mm square and utilizes an integrated one-port vector network analyzer. It operates by measuring the amplitude and phase of the reflection coefficient when placed adjacent to a material specimen. While traditional transmission line methods employ analytical expressions to relate scattering parameters to intrinsic properties, The open-ended stripline sensor geometry and its interaction with the material cannot be easily modeled with an analytical approximation. Instead, it is modeled with a full-wave Finite Difference Time Domain (FDTD) code to develop the relationship between measured reflection and complex permittivity. This inversion method precomputes a translation table by iteratively modeling the measurement fixture across a range of complex permittivities and specimen thicknesses. From this inversion database, interpolation is then used to calculate the frequency dependent complex permittivity or sheet impedance of a given specimen. This paper provides details about the calibration and use of this new device as well as the material property inversion algorithm. Measurement examples of low-loss and lossy materials as well as resistive sheets are also presented and compared to more conventional transmission line results, and are discussed in relationship to measurement uncertainties.
Using a Higher-Order Basis Function based Method of Moments Analysis for Designing Compact Antenna Test Ranges Abstract:- Full wave electromagnetic simulation of a Compact Antenna Test Range (CATR) is not trivial given its electrical size. Typically, the reflector geometry is simulated using asymptotic methods using an assumed feed pattern, while RF absorber and its effects are ignored. A boundary element method of moments (MoM) implementation, using higher-order basis functions is a good numerical technique for analyzing these ranges since the equations are only solved at the interfaces between different homogeneous regions. There is therefore no need to discretize and solve equations for the fields in the large empty volume portion of the CATR, unlike when using Finite-Difference Time-Domain (FDTD) or Finite Element Methods (FEM). Using higher-order basis functions allows for the mesh size of the discretized CATR geometry to be as large as two wavelengths, reducing the number of unknowns while enabling fast, efficient solutions. In this paper, a commercial software package that uses MoM with Higher-Order Basis Functions is used to model a CATR that incorporates a blended rolled edge reflector. The results for the reflector and feed model are compared with asymptotic analysis results to show agreement. A realistic feed horn, support structure and RF absorber is then introduced to the model and its performance is also included to predict field distribution in the CATR test zone. Using this field solution the Poynting vector is calculated to visualize the flow of energy in the range and from these results proper RF absorber layout can be designed to ensure optimum test zone performance. It is also shown how feed structure absorber treatment impacts CATR test zone performance.