AMTA Paper Archive
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Highly Efficient Near-Field to Far-Field Transform for Polar Near-Field Scanned Data
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  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. . 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.
A New Valid Angle Equation for PNF Measurements
A new far-field valid angle equation for rectilinear planar near-field measurements is presented. The new valid angle equation was derived by viewing the planar near-field to far-field transformation process as generating a set of pseudo plane waves by a synthetic phased array and subjecting the antenna-under-test to the radiation from this synthetic array. The synthetic phased array does not physically exist; rather, the array is formed during the post-processing of the planar near-field measurements. As part of this discussion, we present results from a numerical model, illustrating the total electric field present in the test zone due to the finite extent of the synthetic phased array. The new far-field valid angle equation accounts for the diffraction effects of the finite-sized synthetic array, and uses the industry accepted test-zone magnitude ripple of +/- 0.5 dB to limit the valid far-field angle for a fixed scan plane size. The resultant valid far-field angle computed with the new equation is compared against previously established and popularly accepted valid angle equations, such as the equations previously presented by Yaghjian, Maisto, and Joy [1, 2, 3]. Brief discussionsare offered on the measurement of low directivity antennas with a planar near-field measurement system, and on amplitude tapering of the near-field measurements to improve the quality of the pseudo plane wave. REFERENCES:  A. D. Yaghjian, "Upper-bound errors in far-field antenna parameters determined from planar near-field measurements, part 1: analysis," National Bureau of Standards (NBS), Boulder, Colorado, USA, vol. Technical Note 667, no. October 1975.  M. Maisto, R. Solimene and R. Pierri, "Valid angle criterion and radiation pattern estimation via singular value decomposition for planar scanning," IET Microwaves, Antennas & Propagation, vol. 13, no. 13, pp. 2342-2348, 2019.  E. B. Joy, C. A. Rose, A. H. Tonning, and EE6254 Students, “Test-zone Field Quality in Planar Near-field Measurements,” in Proceedings of the 17th Annual Meeting and Symposium of the Antenna Measurement Techniques Association, Williamsburg, 1995.
Full-Sphere Characterization of Low-Gain Antennas via Truncated Field Pattern Stitching
Our work proposes a novel method for obtaining full-sphere radiation patterns from truncated measurements. This is achieved by stitching partially overlapping truncated field patterns, which together cover the whole measurement sphere. Measuring an antenna in different orientations results in a misalignment between the measurements which is not perfectly known and needs to be accounted for in order to stitch the patterns together. Our method first makes use of an iterative procedure to compute spherical wave coefficients capable of accurately describing the truncated patterns. Recent investigation of properties of radiation patterns from iteratively obtained spherical wave coefficients under rotation and translation has shown that, while coordinate system manipulation introduces additional errors, these errors are contained predominantly in the region near the angle of truncation. They are thus negligible if a sufficient overlap between the truncated patterns exists. To align truncated patterns, a bounded minimization of the normalized mean squared error in the overlapping range between patterns is done, varying through a range of different translation and rotation vectors for one truncated pattern while keeping the other pattern fixed. Finally, the fixed and the optimally aligned patterns can be stitched together. The proposed method was validated on spherical wave coefficients (SWCs)-based models and EM simulation models for randomly chosen misalignment offsets. For the SWCs-based models, the normalized mean squared error (NMSE) after pattern stitching was found to be below -53 dB for all tested misalignment offsets. Similar results were observed in the case of EM simulation models as well, where the error was found to be below -52 dB for all tested misalignment offsets. In the final validation step, the method was tested on actual measurement results of two low-gain antennas. For each of the validation steps, potential sources of error are identified. The method demonstrates promising results in achieving full-sphere characterization of low-gain antennas in typical non-full-sphere measurement chambers.
Single-Cut Phaseless Near-Field Measurements using Specialized Probes
This paper introduces a single-cut near-field measurement technique using only-amplitude data. The technique is based on measuring the near-field amplitude of an antenna over a ring, i.e. phi=0 cut, and performing a far-field transformation to obtain the radiation pattern over the same ring. This avoids the need of a full near-field spherical measurement if one is interested in only a few cuts of the far-field pattern. The lack of phase information complicates the field transformation. A common approach to solve this issue is to perform two near-field scans a different antenna-probe distance. This has the drawback of doubling the measurement time with respect to a complex measurement and a translation stage is required, which may be infeasible in some antenna measurement facilities. The technique proposed on this paper can retrieve the phase without measuring the near-field in two rings. Instead, the field is measured in one ring using a specialized probe. This probe provides partial coherence information between measurement samples, which can be exploited in a non-convex minimization solver to retrieve the phase of the near field with high convergence guarantee. The specialized probe can be implemented by using two separate probes connected two a dual channel Software Defined Radio (SDR) unit, so that the relative phase between measurement samples is known. Theoretical background of the proposed technique will be disclosed on the paper, along with simulated and measured transformation examples to demonstrate the potential capabilities: -Very fast near-field measurements (only one ring is measured instead of the full sphere). -Only amplitude information is required (no need of maintain stable phase reference, suitable for OTA testing). -No double-scan is required to retrieve the phase (measurement time reduced by half, no need for translation stage). -High reliability: Partial coherence provides a significant amount of independent information to the phase retrieval algorithm.
Further Exploration of the Holographic PNF Filter
Abstract— A 2021 AMTA paper 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 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  S.T. McBride, P.N. Betjes, “Holographic PNF filtering based on known volumetric AUT bounds,” AMTA 2021, Daytona Beach, FL.
Experimental Validation of Non Invasive SAR Evaluation from Measurements and Numerically Assisted Post Processing
During the last few years, the increasing use of wireless equipment has raised the quantity of radiation energy to which human bodies are exposed. For this motivation, an evaluation of the Specific Absorption Rate (SAR) for persons is fundamental to determine the amount of radiation that human tissue absorbs and to comply with human safety regulations. Standard testing methodology consists of measurements with robot-based scalar/vector near-field probes and post-processing. The probe acquires the field level inside a phantom filled with liquid to ensure compliancy with certification standards. Although accurate, this technique could be extremely time-consuming, especially with the arrival of new frequency bands, new standards (5G, Wi-Fi 7), and the requirement to test different beams directions for beam-forming MIMO configuration. Another testing methodology, used especially for pre-assessment, consists of a full simulation of the radiator in the presence of the phantom, but this implies that the full wave model of the device is available, and this is rarely the case. To overcome the above-mentioned limitations, an alternative technique presented in this paper can be applied. This is based on a standalone measurement of the radiating device, that is post-processed with the method of the equivalent currents to generate NF source (in the form of a Huygens box). The SAR values inside the phantom are assessed using a non-invasive procedure with the assistance of a numerical simulation tool. Such method represents a fast procedure for pre-analysis of device prototypes, allowing to perform the conclusive testing only on the final device to verify the compliance with the regulations. The methodology is here experimentally validated on a dipole radiating in a presence of a phantom model by comparison of numerical simulated data and a reference measured data by a MVG ComoSAR V5 system.
EMF simulation of base station antennas in real sites using Spherical Wave Expansion and diffracted fields
Base station antennas for mobile communications (BTS) emit high levels of electromagnetic radiation in their vicinity. These antennas are usually located on the top of a building, and it is critical to determine those areas where the total power density surpasses the levels dictated by the regulators of the corresponding country. This estimation allows mobile operators to optimize the performance of the cellular network while keeping safe EM emission levels in occupational and public areas. The power density on a given region depends not only on the total radiated power but also the radiation pattern of the antenna and the influence of the environment. As a result, antenna measurements become useful to perform these calculations. This paper presents a simulation tool which computes EMF exposure values of BTS antennas considering the influence of the building roof. The tool uses analytical calculations to obtain a fast evaluation of the fields radiated by all the antennas of a given cell site. The calculations are performed considering the radiation of the antenna as a contribution of three different propagation phenomena: a free space direct radiation component, a reflected component due to the presence of the ground and a diffracted field due to the roof corners. Both direct and reflected rays are computed using the Spherical Wave Expansion (SWE) of the BS antenna assuming PEC boundary. The diffracted ray is computed using ITU 526-8 recommendation. The proposed software requires a measurement of the BTS antenna radiation pattern in anechoic chamber. Spherical near-field measurements are proposed to retrieve all antenna parameters needed for the calculations (SWE, efficiency, electrical steering configurations). Full details of all performed calculations will be disclosed on the paper, as well as some simulation examples with measurement data of real antennas to demonstrate its capability and computational efficiency.
Metal Mesh Selection Guide for Shielding Effectiveness and Optical Visibility
This paper presents a selection guide for metal mesh based on shielding effectiveness and optical visibility requirements. Concepts of mesh sizing, wire diameter, metal type, opening size, metal color, and mesh patterning are discussed. The guide provides a detailed explanation of factors that contribute to the shielding performance and optical transparency of various mesh options. Metal mesh has a range of applications in the microwave, antenna, and EMC industries as they are particularly suited for protecting chamber viewing windows. Shielding effectiveness performance is dependent on the mesh sizing, wire diameter, and opening size, where the dimensions of the apertures directly influence the suitability of the mesh for a given frequency range. Finer mesh yields superior shielding, but with limited optical visibility. For this reason, the necessity for finding an optimal tradeoff between shielding performance and optical properties arises, where the selection guide in this paper can be used to make an informed decision. For ideal optical properties, the mesh sizing is critical since it determines the visibility through the material. Mesh layering and alternative optically transparent shielding solutions like RF film are also compared. Although layering of metal mesh offers additional shielding, the layering is associated with a reduction in visibility due to the mesh density and patterning from the Moiré effect. Alternatives like RF film can offer highly transparent solutions, but with inferior shielding effectiveness than metal mesh. This paper provides a quantitative analysis of shielding effectiveness results based on mesh parameters and aperture dimensions. The appropriate frequency ranges for the various metal meshes are also calculated. Using the selection guide presented in this paper, the user is enabled to make an educated technical decision on the metal mesh best suited to satisfy the shielding effectiveness and optical visibility requirements for the application.
Emulation of LTE Link Scenarios Reproducibly Derived from Field-Operational Tests
Motivation and background: Wireless communications are key for connected and automated driving. Beyond automation levels that require the presence of a driver, tele-operated driving has been receiving more attention recently. For such applications, gapless wireless coverage and stable connectivity are required, enabling a reliable exchange of data like control information or high-definition maps even under poor radio wave propagation conditions. Therefore, extensive testing of the link stability of mobile wireless communication systems is necessary, especially in challenging scenarios that are susceptible to link failure. Objectives and methods: We propose the emulation of relevant corner-case scenarios for virtual-drive testing, consistently and reproducibly derived from field-operational tests on public roads. The available data rate of a LTE link near mobile cell edges is considered a relevant test metric, since the link is expected to be particularly susceptible to failure under such conditions. We performed field-operational tests on two different test tracks, in order to prove the reproducibility and consistency of the proposed case. We have emulated the scenarios in a wired setup under realistic conditions using a communication tester and an interference generator. Power-related key-performance indicators like RSRP and SINR as well as the achievable throughput were systematically studied under laboratory conditions. Results and conclusions: The region around cell edges could undoubtedly be identified as a challenging scenario for automotive LTE communications, leading to a reduction of the data throughput by a factor of 5, on average, compared to the maximum data rate during a test run. This effect could be consistently observed on both test tracks. The emulation of wireless link parameters in such corner-cases reproduced the physical parameters of the field-operational test results very well. Changes of the data rate could be associated with the channel quality indicator. Approaches to improve the emulation of the drive tests is in the focus of future work. However, given the simplicity of the test setup, it represents a sound basis for refined over-the-air tests.
Revision of IEEE Std. 1720-2012: Recommended Practice for Near-Field Antenna Measurements
Near-field (NF) measurements is considered as a very accurate and versatile antenna testing technique. It became widely used as a preferred measurement technology in antenna measurement systems about four decades ago. Today, hundreds of near-field antenna test facilities are installed worldwide. The IEEE Std 1720™ “Recommended Practice for Near Field Antenna Measurements” is specifically dedicated to near-field antenna measurements. It therefore complements the IEEE Std 149-1979™ “Standard Test Procedures for Antennas” which describes general antenna measurement procedures. The Std 1720™ was originally approved in 2012 as a completely new standard by the IEEE Standards Association Standards Board. It is highly relevant for users performing NF antenna measurements but also the design and evaluation of NF antenna measurement facilities. After 10 successful years, the standard expires this year and will no longer be an active standard under the IEEE. A revision is required to update the document with new developments and technologies that have matured since the first edition. This is the scope of project P1720 that was approved by IEEE-SA in 2019 to undertake “minor revision” of the current standard. A Working Group (WG) of the Antennas and Propagation Society Standards Committee (APS/SC) has been formed for this task. The WG is transversal, with users and experts of the near field measurement field and consist of approximately fifty dedicated volunteers from industry, academia, and government. This paper gives an update on the running activities and discusses the suggested changes to the standard.
Investigation of Inhomogeneity of Material Characteristics on the Hybrid-Absorber Performance
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
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.
Numerical considerations to improve the Reduced-Order Model Approach for Antenna Measurements
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.
Reduced Distance OTA Testing Methodologies for Automotive Applications
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  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  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 .
Design of a Thermal Testbed for Metrology of Active Antennas
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.
Using a Higher-Order Basis Function based Method of Moments Analysis for Designing Compact Antenna Test Ranges
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.
A Novel Reduced-Complexity Low-Profile Beam Steerable Risley Prism Antenna
Wide-angle beam steerable antennas are critical devices for 5G and next-generation Internet of Things (IoT). In general, beam steerable antennas are realized electronically by controlling the phase of an array of elements, or mechanically through moving parts. While electronic approaches typically offer fast beam switching and low system profile, the use of a substantial number of active components can considerably impact efficiency in the millimeter-wave range and increase cost. Meanwhile, novel mechanical steering concepts such as the Risley prism antenna (RPA) have become attractive because of their low electronic complexity, low cost, and better efficiency. An RPA typically contains three co-axially placed panels, including a stationary feed source generating planar illumination and two independently rotatable beam-deflective surfaces. Beam scan is realized by in-plane rotations of the components. Therefore, RPA avoids any distributed active components, and the profile of the antenna stays unchanged while scanning. These features make RPA a competitive candidate in scenarios that have moderate requirements on steering speed, e.g., for satellite-tracking ground terminals. In this work, we propose a novel RPA configuration that realizes wide-angle 2D beam steering with merely two flat panels coaxially placed in parallel, including a rotatable feed and a rotatable transmitarray. The feed radiates a pre-defined gradient-phase and the transmitarray provides another gradient phase shift. The combination of the two gradients becomes a new gradient at the exiting aperture. In-plane rotations of the feed and the transmitarray changes the value and direction of the aperture phase gradient, and eventually scans the beam. This configuration uses fewer components than a conventional RPA, and can significantly reduce system complexity, weight, profile, and loss. Based on this concept, we present the principle, design, and verification of a K-band circularly polarized (CP) RPA at 19 GHz. A sequentially rotated truncated-corner patch array feed generates the first CP phase gradient. A CP transmitarray using S-ring element unit cells is placed on top of the feed to provide the second gradient phase shift. The total thickness of the RPA is less than a wavelength at 19 GHz. Simulated and measured results showing beam scans beyond 50°in elevation will be presented.
A Trade Study on Quasi Far Field Accuracies and Measurements
Recent papers have addressed making far field measurements at much less than the traditional far field distance, particularly for 5G MIMO test articles. These papers have focused on main beam measurements only, such as Total Radiated Power (TRP) and have stated that other normal antenna pattern metrics, such side lobe level measurements are not appropriate for this shortened distance. These papers have addressed fixed error levels acceptable for this quasi far field technique. This paper will present a sliding scale of main beam error versus measurement distance that can provide a more precise evaluation for the practitioner on selecting this technique. In addition, the paper will present a trade study in terms of chamber size, measurement durations and measurement methods between the quasi far field, compact range, and spherical near-field approaches. This trade study will cover three representative test articles in the C, Ka, and V frequency bands for 5G applications. A case study for a particular test article will provide an evaluation example.
Calibration and Cross-Polarization Measurement Standard Requirements for Focus Beam Material Characterization Systems
Novel metamaterial and metasurface realizations provide unique control of the wave dispersion but present many challenges for accurate constitutive parameter extraction. Practical material measurement approaches for characterizing complex materials, such as biaxial anisotropic and gyrotropic media, rely on either waveguide or focus beam techniques with trade-offs in sample size and bandwidth. Multi-static focus beam setups offer many advantages for complex material measurements by enabling wider sampling bandwidths, measurement degrees of freedom and larger sample sizes. However, rotational misalignment of the principal crystal and measurement coordinates of the biaxial anisotropic media sample results in cross-polarized scattered field contributions. Likewise, interrogating gyrotropic or general bianisotropic media with a dual-polarized focus beam measurement setup produces cross-polarized scattering matrix components. These non-diagonalized S-parameters for the complex sample under test must be measured to successfully extract the constitutive parameters. A time-domain gated response isolation calibration scheme is one common approach to establish the measurement reference plane and minimize fixture uncertainties for samples with limited cross-polarization scattering. This paper extends the time-domain gated response isolation methodology for free-space focus beam systems to account for cross-polarization terms present in both the sample under test as well as the measurement setup. The featured analysis leverages a 4x4 S-parameter matrix notation to capture the polarimetric scattering at each cascaded stage. An equivalent line standard procedure is developed where four unique, linearly independent calibration standard measurements are shown to account for all unknown terms. Finally, a sensitivity analysis of the calibration standards is performed via numerical simulations to show the potential trade-off and limitations of the cross-polarization extended time-domain response isolation calibration scheme performance.
A Technique of Holographic Projection from Far Field Pattern to an Unconstrained Planar Surface
Holographic back projection to a plane from spherical pattern data offers more details than from planar data because the image is derived from data over an entire hemisphere. A previous study introduced a back projection method where the far field pattern can be projected to a plane which is orthogonal to the radial direction, with thetwo transverse axes parallel to the local θ and ∅ vectors. This limits the hologram to only certain specific planes which may not match the desired surfaces. To overcome the shortcomings, we apply a more general back projection method in this paper which allows projection to an arbitrary plane. This is achieved through a translation and a series of rotation operations of the far field pattern. Specifically, as a first step, the phase center of the far field pattern is moved to coincide with the center of the projection plane through a translation matrix. Next the far field pattern is rotated such that its x, y and z directions are aligned with the desired projection area. Then the plane wave spectrum is computed based onthe new far field pattern. The hologram can be obtained by applying inverse Fourier transform to the plane wave spectrum. The phase shiftterm exp(jγd) in the conventional back projection technique is no longer needed after the pre-process of the far field pattern because it is included in the translation operation. This is a much more general back projection algorithm which provides the holographic projection onto an arbitrary plane. The method can be especially useful for cases when the desired projection areahas an arbitrary orientation with an offset from the origin.
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