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We present the synthesis, prototyping and measurement of a transmitarray antenna for the generation of circularly-polarized (CP) orbital angular momentum (OAM) beams. A novel ”S-ring” transmitarray element is designed to sustain CP operation with only three metal patterned layers. The unit cell provides arbitrary CP phase compensation by merely changing the rotation angle of the element, while the transmission magnitude remains greater than 0.9. In previous work, transmitarrays that support circular polarization were designed based on the assumption of normal plane wave incidence; thus, the feed source of the transmitarray must be placed far from the transmitarray. In this work, an aperture phase synthesis methodology that accounts for the spherical phase of the feed is presented such that the feed can be placed near the aperture at about F=D = 1. A proof-of-concept prototype transmitarray antenna with a thickness of 0.3 cm operating at 19 GHz is constructed and measured for far-field performance. The measurements agree well with predictions obtained by full-wave simulations and demonstrate that the proposed transmitarray antenna can be a unique apparatus that generates OAM CP cone-shaped patterns.
A probe-compensated near-field-far-field (NF-FF) transformation with planar spiral scan, particularly suitable for flat antennas under test (AUTs), is proposed in this communication. It relies on the nonredundant sampling representations of electromagnetic fields and has been achieved by properly applying the unified theory of spiral scannings for nonvolumetric antennas, when such a kind of AUT is considered as enclosed in a dish with diameter equal to its maximum dimension, thus better shaping its geometry. An efficient two-dimensional optimal sampling interpolation (OSI) algorithm is then developed to recover the NF data required by the standard NF-FF transformation with plane-rectangular scan from those collected along the spiral. Since the number of NF data and spiral turns is related to the area of the modeling surface, the here proposed NF-FF transformation technique allows one to further reduce the measurement time with respect to those based on the modelings for quasi-planar AUTs, which instead involve, in such a case, a residual volumetric redundancy. Some numerical simulations, assessing the accuracy of the OSI algorithm and of the so developed NF-FF transformation, are shown.
We present a fast source reconstruction method suitable for antenna diagnostic applications of radiating structures on electrically large platforms. The method is based on a novel implementation of a recent reformulation of the inverse electromagnetic scattering problem, and is solved using a Higher Order Method of Moments (MoM) discretization. The novel implementation achieves asymptotically better scaling the previously possible, and in particular the memory use is substantially lower than was previously possible. Results from two example cases are presented where the new method is compared to the current commercial state-of-the-art solver in DIATOOL 1.1, and significant improvements are observed in terms of computation times and memory requirements.
The main goal of this work is to present a real-time system to evaluate the impact of multi-sines in Multiple Signal Classification (MUSIC) algorithm. The MUSIC algorithm is applied in several localization applications, where the accuracy of this algorithm is required. For this reason, a specific real-time system was developed to characterize the impact of multi-sine parameters in the MUSIC algorithm in order to improve system efficiency. From several experiments, the impact multi-signal number of tones and space between tones is analyzed.
Electrical properties of materials are requisite to analyze and design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurand is the free-space scattering parameters of an MUT (material under test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing the MUT without prior machining and physical contact in high frequency ranges. This paper proposes a free-space two-tier one-port calibration method using three planar offset shorts with the respective offset of ???????? for the measurement of the full scattering parameters of a reciprocal planar MUT from two successive oneport calibrations. Measurement results of a glass plate of 4.775 mm thickness are shown in W-band (75-110 GHz).
We present a new method for measuring thin, polymer sheets using a slotted rectangular coaxial transmission line (RCoax). This method allows a sheet of material to be inserted into the R-Coax slot, greatly simplifying the measurement procedure over traditional waveguide methods. The permittivity inversion is performed with the aid of computational simulations of the RCoax conducted across a range of expected dielectric properties. In particular, the slotted R-Coax device was optimized to enhance signal strength but has no simple analytical solutions for inversion. This new measurement technique is demonstrated on several thicknesses of commercial polyethylene terephthalate (PET) films, with a maximum thickness of 10 mils (0.254 mm). Due to the coaxial geometry, this technique does not have an intrinsic lower frequency cutoff and has an upper frequency cutoff near 3 GHz from over-modeing within the transmission line, though this frequency range could be extended by shrinking the fixture. However, the signal strength and calibration stability limit the useful range of permittivity measurement to 0.5-3 GHz for 10 mil thick specimens (and a range of ~1 GHz-3 GHz for 0.5 mil thick specimens). Repeatability for the real part of the permittivity ranged between 2-5% and loss tangents of ~0.006 were measured. Thus, this paper demonstrates the R-Coax measurement technique as a potential QA tool for microwave frequency electrical properties of thin polymer films.
In this paper, a method is presented that allows for the experimental verification of far-field conditions in a direct far-field measurement setup. The method is based on a relativedistance sweep (i.e., increasing the distance by linearly translating one antenna) and on the Friis equation. The presented method is only valid for one specified direction and is therefore well suited to assess whether or not far-field conditions are achieved when performing an absolute measurement, such as a maximum gain or effective-isotropic-radiated-power (EIRP) measurement. It is shown that antenna measurement uncertainties due to the finite antenna separation, scattering, positional inaccuracies, drift and noise on the order of hundredths of dBs around 30 GHz for a separation on the order of 1 m can be obtained. Using this method, it is also experimentally shown that whether or not farfield conditions are met depends not on one but on both antennas in a two-antenna measurement setup. This implies that, strictly speaking, the far-field distance cannot be determined by solely considering the largest antenna in a two-antenna measurement setup.
An origami-based Tightly Coupled Dipole Array (TCDA) is proposed for small satellite applications. The array is formed by a two-layered structure using rigid and flexible substrates to enable accordion-like folding. The proposed TCDA operates across 0.4-2.4 GHz with VSWR < 3 at broadside and across 0.6-2.4 GHz with VSWR < 3 when scanning down to 45 in the E-, D-, and H-plane. An 8 prototype was fabricated using Kapton Polyimide and FR4 and tested to verify the bandwidth and gain of the origami array. The fabricated prototype was demonstrated to be packable, low-profile, and lightweight (only 1.1kg). Notably, when packed, the array has a one-dimensional size reduction of 75%. As will be discussed, the packing compression is made possible by eliminating vertical PCB boards and incorporating the balun feeds within the dipole layer. To our knowledge, this is one of the first foldable, low profile, and low-scanning ultra-wideband arrays in the literature.
An active S-band dual-polarized multifunction phased array radar (MPAR), the Advanced Technology Demonstrator (ATD), has recently been developed for weather sensing and aircraft surveillance. The ATD is an active electronically scanned array (AESA) with 4864 transmit/receive (T/R) modules and was installed in a spherical radome. Simulations and a novel phased array measurement technique have been explored to assess the impact of high reflectivity from a wet radome during rain that can potentially induce voltages exceeding the transmit amplifier breakdown voltage. The measurement technique uses array elements radiating one at a time to illuminate the radome, and uses superposition to quantify the received signal power in a reference antenna on the face of the array. It is shown that when the radome surface is wet and highly reflective, certain electronic steering angles sum to a large reflected signal focused on the array face. This measurement technique can be used prior to high-power phased array radar operation to monitor the magnitude of reflections and help avoid element transmit amplifier failures.
This paper presents comparison of planar near field (PNF), cylindrical near field (CNF) and compact antenna test range (CATR) measurements for Standard Gain Horn (SGH) at K (18-25 GHz) and Ku-band (10-15 GHz) and metamaterial based high gain flat panel antenna at Ku-band. The effect of azimuth step size, number of cylindrical modes and radial distance error on CNF measurement accuracy are presented. The advantage of CNF for wide/large scan angles is discussed and measured results for metamaterial antenna at high scan angles are compared with those of CATR. Measurement time comparison between PNF and CNF is presented. One of the limitations of CNF compared to PNF is angular coverage in the elevation plane and this aspect is tried to be addressed supported by measured results.
The equivalence principle allows for the replacement of a radiating structure with a Huygens’s box resulting from a near-field scan obtained from either simulations or measurements. The near-field on the surface enclosing the structure is replaced by a set of equivalent electric and magnetic dipoles that can be used as sources for numerical simulations. In this manner, problems of high complexity can be handled, using less unknowns, by separately addressing the radiating structure and the surrounding diffracting objects. Furthermore, sometimes the antenna model is not available, and it must be considered as a black box. However, important challenges arise when the Huygens’s box approaches a surrounding structure. Indeed, the near-field resulting from discretized equivalent dipoles presents a singularity at each dipole’s position. Therefore, the obtained fields with the equivalent dipoles are not valid in the close vicinity of the Huygens’s box. A novel method is proposed to handle the aforementioned singularity. It is based on the interpolation of the electromagnetic field between the non-singular region and the surface of the box. The field on the non-singular region is obtained using equivalence principle whereas the near field scan data is used to retrieve the field on the box surface.
With the event of integrated and multi-standard wireless links, phaseless antenna measurements are attracting more and more interest in research. Especially in the context of connected and automated driving, antennas, frontends, and digital signal processing units merge into telematic units and require new methods for performance evaluation in the installed state. The measurement of the phase diagram and the exact absolute positioning of electrically large antennas, i.e., antennas interacting with the car body, present challenges for safety-relevant applications and reliable test methods. This paper describes a way to determine the position of automotive antennas in the installed state with sub-wavelength precision from phaseless measurements. Realistic LTE uplink signals were used as test signals as they would be transmitted by an active device in a real-world scenario. The localization algorithm is based on orthogonal power measurements of the transmitted signal on a cylinder surface and a non-linear optimization. By comparison with a conventional localization based on spherical far-field data, an accuracy of the approach of less than 1 cm was achieved, which is less than λ/16 at the considered frequency of 1870 MHz.
This contribution presents a way to manufacture antennae, which allows to both effectively simplify the production and reduce associated costs as well as the weight. Amongst other examples for aperture antennae this is shown for a configuration given by the slotted waveguide antenna toolkit presented in [1]. In simplified terms the procedure consists of shaping the HF-transparent rigid foam to the size of the antenna’s cavity, attaching the connector(s) and electroplating it with copper. The manufacturing steps are shown in detail, which is followed by a characterization including the weight as well as the antenna performance such as S11 and the antenna pattern for horizontal polarization. These results validate the applicability of the presented method and open windows of opportunities especially in contexts in which intricate cavities and weight pose critical issues.
Diagnostic techniques are crucial in antenna development and testing to enhance the Device Under Test (DUT) performances and identify the cause of possible failures in the qualification process. Among different approaches [1]-[8], it has been demonstrated that the equivalent currents method (EQC) [8]-[9], implemented in [10], is one of the most efficient for investigations in various application areas [11]-[13]. Indeed, the generality of the 3D reconstruction surface enclosing the DUT is a key feature, it ensures that this technique is unique and highly suitable for diagnostics, respect to traditional methods based on plane wave expansion. To handle electrically large problems, the EQC method has been initially based on a Fast Multipole Method (FMM) [14]. The recent advent of 5G technologies has led to an increasing need in terms of antenna electrical dimensions. Therefore, a novel technique based on a Nested Skeletonization Scheme (NSS) has been implemented to guarantee a further reduction of memory requirements and computational time. The new capability has been demonstrated in the past for a patch array antenna [15]. In this paper, the diagnostic capabilities of the EQC approach are applied to an early prototype of an electrically large array antenna for 5G antenna measurements applications [16].
An antenna measurement system at the Institute of High Frequency Technology at RWTH Aachen University is being established containing a six-axis robot arm allowing the realization of numerous measurement geometries. Room scattering is one of the most crucial uncertainty terms in every antenna measurement which becomes even more interesting in a dynamic scattering situation. To determine the scatteringinduced interference caused by the robot, a motion-capable model is developed and firstly simulated using the multi-level fast multipole method between 8GHz and 12GHz to qualitatively assess the surface currents. Secondly, asymptotic simulations are carried out using physical optics for the most important robot positions at 60GHz which is in the frequency range where the system is operated. For example, differences in the same simulation points of up to 20dB are shown for different robot positions. Based on the simulation results, the measurement sequences can be optimized by selecting a trajectory which reduces the scattering effects. In addition, the strongest scattering sources of the robot are identified in order to cover these parts by absorbers. Therefore, the knowledge gained from the simulations can be applied to the measurement system to improve the performance of antenna measurements.
The validity and viability of using frequency domain mode filtering to qualify an EMC chamber above 1 GHz has been demonstrated in a previous study [1]. The novel approach overcomes the difficulties with under sampling encountered in the traditional spatial sampling method adopted by CISPR 16-1-4, and it also has the distinct advantage over the time domain method adopted by ANSI C63.25.1 in that broadband and low ring-down antennas are not required. In this study, we further examine one of the assumptions made in the previous study to translate the quasi-far-field pattern to the rotation center. The approximate method is compared to a more rigorous method by using a quasi-far-field to far-field transformation first before applying the phase translation and subsequent mode filtering. In this paper, we further validate the method by conducting an intercomparison study based on measurements conducted in a 3 m anechoic chamber to show the correlations of the mode filtering method to the CISPR and the time domain (TD) site VSWR methods. We demonstrate how the proposed method improves the test repeatability, lowers measurement uncertainties, and increases measurement efficiencies.
Instrumentation radar metrology waveform techniques that simultaneously transmit two orthogonal sequences of orthogonal electromagnetic polarizations are explored for applicability toward both static and dynamic RCS signature and ultra-wideband imaging measurements using simultaneous H-pol and V-pol (SHV) waveforms. Static, pulsed measurements with independent transmit polarizations are modulated and radiated; reflections from a depolarizing target are measured where the return signals are coherently combined. Each transmit polarization is independently modulated using a diverse phase sequence, which leaves a unique “fingerprint” by which the orthogonal polarization separation is achieved. Using only the coherent combination and associated transmit and receive RF channel characterizations, the original measurements are reconstructed. Simulations serve as a baseline for measured results, from generating pure SHV waveforms and then providing simultaneous full scattering matrix (FSM) measurements, in order to achieve greater purity of FSM signatures, while reducing measurement times by a factor of two.
This work presents the concept and design of an antenna element, aimed at 5G time division duplex (TDD)-based digital massive MIMO applications, operating in millimeter waves (mm-waves). The proposed radiating structure is based on printed slot antennas, fed by a substrate integrated coaxial line (SICL) for significantly reducing mutual coupling among the array elements. Furthermore, the slot has its own cavity for creating a broadsidedirection beam, without increasing mutual coupling. Numerical results demonstrate 1.31 GHz bandwidth at 26 GHz, 6.4 dBi gain and beamwidth of 70° and 80° in the main orthogonal planes. A two-element array is reported as a proof-of-concept and the mutual coupling between its elements has been kept lower than 32 dB from 25 to 27 GHz, illustrating its potential for scalability to high-order massive antenna TDD arrays.
A compact ultra-wideband (UWB) antenna operating from 300 MHz to 6 GHz was developed for operating with a Lunar Heat Flow Radiometer (LHR) system is presented. The antenna was required to fit within 36 cm x 36 cm x 10 cm volume with an emphasis of small antenna height so that it can be mounted under rovers. This paper presents an innovated design which combine a dielectric-loaded TEM horn mode from 2 GHz to 6 GHz and bowtie dipole mode 300 MHz to 2 GHz. The simulation results show a minimum realized gain of 2.2 dBi at 300 MHz and the gain monotonically increases to approximately 15 dBi at 6 GHz and maintains approximately constant gain and patterns from 2 to 6 GHz
A Spherical Passive/Active Radar Calibration System (SPARCS) has been designed as an advanced, airborne, radar calibration device (CD). SPARCS is currently under development as an autonomous, battery-powered, high-endurance, flying platform. This self-contained, multi-function radar calibration and diagnostic system functions as 1) a Passive Spherical Reflector, 2) an Active RF Repeater, 3) a Synthetic Target Generator, and 4) an UWB RF Sensor and Data Recorder of the radar under test or the localized RF environment. This innovative CD exploits major advances in commercial technology during the past decade associated with autonomous airborne drones and miniaturized digital RF systems on chips (RFSoCs), and other miniature electronics. Emphasis has been placed on a recoverable, reusable CD that enables precision calibrations over extensive open-air test volumes used for dynamic aircraft RCS measurement, test and verification, or time space position information (TSPI) test range tracking radars. This paper highlights early efforts to parameterize and develop SPARCS, including advances in autonomous navigation and flight time, electric ducted fan performance, radar screens for thruster inlet and outlet ports, active calibration functionality, and improving calibration uncertainty. SPARCS will provide an unprecedented capability for radar instrumentation calibration, target emulation, environmental assessment and in situ, real-time calibration.