Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The near-to-far-field transformation (NTFFT) technique adopting the plane-rectangular (PR) scanning is the most simple one from the analytical and computational points of view and can be suitably employed when characterizing antennas which exhibit pencil beam radiation patterns. In recent years, NTFFTs using the nonconventional planar wide-mesh scanning (PWMS) have been developed. They allow a remarkable measurement time saving with respect to that adopting the classical PR scanning, since their raster grid is characterized by meshes which become larger and larger as their distance from the scanning plane center increases. These NTFFTs have been obtained by applying the non-redundant sampling representations of the electromagnetic fields to the voltage detected by the scanning probe and adopting suitable AUT modellings for volumetric and quasi-planar AUTs. The evaluation of the AUT far field is then got by applying the PR NTFFT, whose input data are accurately recovered through optimal sampling interpolation expansions from the collected PWMS samples. In both the conventional and non-conventional scannings, the sampling points are reached through an x-y scanner. However, the finite resolution of the probe positioners and/or their imprecise control can prevent to exactly collect the near-field samples at the prescribed sampling points and imperfections in the mechanical rails driving the motion of the probe can cause a deviation from the considered measurement plane. Accordingly, 3-D positioning errors, which can be revealed via laser interferometric techniques, affect the acquisition. The aim of this work is to develop an effective NTFFT with PWMS from 3-D probe positioning error affected near-field measurements. To this end, the so named k-correction (Joy and Wilson, AMTA Proceedings 1982) will be used to compensate the positioning error related to the deviation from the considered measurement plane. Then, an iterative procedure (D’Agostino et al., International Journal of Electronics and Communications 2020) will be applied to retrieve the near-field samples at the points specified by the non-redundant sampling representation from those obtained at the previous step and affected by 2-D positioning errors. Numerical tests will show the capability of the procedure to fully compensate the 3-D positioning errors affecting the acquisition of the PWMS samples.
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.
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.
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.
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.
The characterization of antennas is a time-consuming task. Its acceleration leads often to large and sensitive numerical problems. Therefore, special care must be taken of the choice of the parameters, the optimization, and the stability of the employed resolution methods. Based on Huygens’ principle, the radiation operator can be defined from an equivalent surface enclosing the Antenna Under Test (AUT). The discretization of this operator leads to the so-called radiation matrix. An expansion basis of the fields radiated from the equivalent to the measurement surface is constructed by the Singular Value Decomposition (SVD) of that matrix. The Reduced-Order Model (ROM) is the compressed representation of this basis obtained by truncating the SVD. The truncation order, T, is computed by inspection of the singular value distribution and is strongly linked to the number of degrees of freedom of the radiated fields. Several practical and technical aspects are studied in this article to provide a systematic, efficient and reliable procedure for the characterization of the radiated fields using the ROM. Analytical criteria are used to define the dimensions of the radiation matrix enabling a stable determination of the compressed basis. The truncation order, T, is the key-point of this method as it determines the size of this basis. Therefore, its variation is studied with respect to the discretization step and the geometry of both equivalent and measurement surface. Finally, the Randomized SVD (RSVD) is used in order to significantly reduce the computation time with negligible impact on the accuracy. To illustrate our procedure, it is applied to various scenarios and experimental results of spherical measurements. Estimations of the time savings by using the RSVD are also provided.
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].
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.