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Cosme Culotta-López, Zhong Chen, Thomas M Gemmer, Dirk Heberling, October 2019

Electromagnetic Compatibility (EMC) radiated emissions measurements above 1 GHz are performed in a nominal free space environment as required by international standards, typically in an anechoic chamber. In an EMC chamber, the test zone consists of a circular region defined by a turn table, where an equipment under test is rotated and measured. The test zone is commonly referred to as quiet zone (QZ). Due to the non-ideal nature of absorbers, multiple reflections in the chamber affect the quality of the QZ. The constructive and destructive interferences from the reflections form standing waves in the QZ. The maximum value of the standing wave is used as the figure of merit for validation of testing facilities. Site Voltage Standing Wave Ratio (sVSWR) as specified in CISPR 16-1-4 is broadly used for the validation of test sites above 1 GHz. This method requires the measurement of six positions along a linear 40 cm transmission path at various locations in the QZ, with a frequency step of no greater than 50 MHz using an omnidirectional-like antenna (e.g. a dipole). Concerns have been raised that this method delivers an overly optimistic result due to both spatial and frequency domain undersampling. In this work, an alternative method to sVSWR for the validation of EMC chambers based on Spherical Mode Coefficients (SMC) is proposed. Two 90 •-rotated measurement cuts of an omnidirectional-like antenna are acquired around the periphery of the circular QZ. The measured situation and cut is replicated by applying translation and rotation of spherical waves to the known SMCs of the used omnidirectional-like antenna and transforming using the spherical wave expansion. The generated and measured cut are compared and the characteristics of the chamber are extracted. The major advantage of this method is the relatively high measurement speed and reliability.

A reflectarray antenna working at 28 GHz is proposed to replace the reflector antenna of a Compact Antenna Test Range (CATR) system. As a first approach, the quiet zone obtained using a far-field collimated reflectarray is analysed. Due to the size of this area is not large enough, the generalized Intersection Approach is employed to carry out an optimization of the near-field for both phase and amplitude in order to maximize the size of the quiet zone at one plane. Simulations are compared for the near-field before and after the optimization process, showing an important enhancement of the size of the quiet zone, especially in the main cuts. From the obtained phase distribution a design is carried out. The unit cell chosen is based on a two-layer stacked patch, having good agreement between optimization and design results. Finally, the bandwidth response of the designed reflectarray is analysed, in order to assess its performances as probe in a CATR system.

A transmitarray antenna is proposed as a multi-focusing antenna in the near-field region with capability for focus scanning and/or simultaneous and independent focus spots generation at 28 GHz. The transmitarray optics is defined for a centred configuration and the elements are designed to focus the radiated near-field at a given point. Then, a number of feeds is placed along arcs in the principal planes and the near-field generated by the transmitarray when its illuminated by each one is obtained, demonstrating the capability to generate multiple independent near-field spots. The focusing performance is improved for the centered feed through a Phase-Only synthesis technique based on the generalized Intersection Approach in near-field. Finally, the spots produced by the whole cluster are calculated, demonstrating the overall improvement and validating the designing process. This configuration can be applied in near-field systems as radar for surface inspection, measurement systems or wireless power transfer among others.

Seckin Sahin, Niru K Nahar, Kubilay Sertel, October 2019

We present a novel, non-contact characterization technique for simultaneous characterization of conventional antenna parameters, including the antenna port input impedance, antenna gain and its radiation pattern, without requiring a network analyzer connection to the antenna port. The test antenna and the network analyzer are considered as a 2-port open-air fixture whose network representation corresponds to the desired antenna parameters. The unknown network parameters of the 2-port open-air fixture are determined via a novel calibration process using 4 offset-short termination standards. The error parameters determined by the calibration are then related to the test antenna port impedance and its gain as a function of frequency. Furthermore, the radiation pattern of the test antenna can also be characterized using measured reflection coefficient at the network analyzer port for two offset-short terminations of the test antenna port, while rotating the test antenna over the desired angular range. This novel technique is particularly attractive for installed-antenna applications where an active connection to the test antenna port is either difficult or undesirable, such as on-chip antennas and implanted antennas, to name a few. To demonstrate the efficacy our new method, we present the measured impedance, gain and radiation pattern of a diagonal-horn antenna operating over 360-450 GHz, and a lens-integrated planar butterfly antenna for the 220-325GHz band.

Antenna measurement errors occur due to reflections and diffractions within the measuring chamber. In order to extract and correct the undesired signals, a technique based on test-zone field compensation and spherical wave expansion is applied to Compact Antenna Test Range (CATR) and Spherical Near-Field (SNF) measurements of a base transceiver station antenna. The required spherical test-zone field is acquired by simulating the corresponding measurement environment with the multi-level fast multipole method. Due to the numerical complexity of the problem, only the parts of the chamber with a significant influence on the measurement results are modeled. Comparing the determined directivities after applying the correction method, an exact overlap is achieved between the SNF and CATR solution.

A Giacomini, L Scialacqua, F Saccardi, L J Foged, E Szpindor, W Zhang, M Oliveira, P O Iversen, J M Baracco, October 2019

In this paper, the experimental validation of a micro-probe fed reference antenna targeting the upcoming 5G applications (24.25-29.5GHz band) is presented. The main purpose of these reference antennas is to serve as "gold standards" and to perform gain calibration of 5G test facilities through the substitution method. The outline of these antennas is based on a square array of four printed patches enclosed in a circular cavity. The RF input interface is a stripline-to-coplanar waveguide transition and allows for feeding the device with a micro-probe. Performance obtained by high-fidelity modeling is reported in the paper and correlated to experimental data. Interaction and unwanted coupling with the test equipment are discussed. The use of echo-reduction techniques and spatial filtering is investigated to mitigate these effects.

M Brandt-Møller, M Fröhner, O Breinbjerg, October 2019

This paper presents a new type of P-band first-order dual-port probe for spherical near-field antenna measurements. The probe is based on the well-known shorted annular patch antenna but some extensions are introduced for the probe application. This probe is mechanically simple which facilitates its manufacturing and operation. In addition, it has high performance for impedance bandwidth, pattern, directivity, and gain.

Cosme Culotta-López, Brett Walkenhorst, Quang Ton, Dirk Heberling, October 2019

The major drawback of Spherical Near-Field (SNF) measurements is the comparatively long measurement time, since the scanning of a whole sphere enclosing an Antenna Under Test (AUT) is required to calculate the Spherical Mode Coefficients (SMCs) required for the computation of the far field. Since the SMCs prove to be sparse under certain conditions, efforts have been made to apply compressed-sensing techniques to reduce the measurement time by acquiring a smaller number of sampling points. These approaches have been successfully tested in simulation using classically acquired measured data. This decouples the measurements from practical problems, such as basis mismatch due to the finite precision of the mechanical positioner and environment effects. In this paper, results from a sparse data acquisition performed with a physical system are reported. To decouple the error introduced by the approach itself from the error introduced by non-idealities in the measurement system, an AUT is measured using both traditional near-field sampling and compressed near-field sampling. The classically acquired data is used both as reference and as source to simulate a synthetic compressed measurement. The effects introduced by real considerations are calculated by comparison between the synthetic compressed measurement and the acquired one, while the error of both is evaluated by comparison to the reference measurement. The results further demonstrate the viability of this method to accelerate SNF measurements and pave the way for further research.

Eric Mokole, Vince Rodriguez, Jeff Fordham, L J Foged, ,, October 2019

Radar scattering is typically represented as the RCS of the test object. The term RCS evolved from the basic metric for radar scattering: the ratio of the power scattered from an object in units of power per solid angle (steradians) normalized to the plane-wave illumination in units of power per unit area. The RCS is thus given in units of area (or effective cross-sectional area of the target, hence the name). Note that the RCS of the test object is a property of the test object alone; it is neither a function of the radar system nor the distance between the radar and the test object, if the object is in the far field. Because the RCS of a target can have large amplitude variation in frequency and angle, it is expressed in units of decibels with respect to a square meter and is abbreviated as dBsm (sometimes DBSM or dBm2). In terms of this definition, the RCS of a radar target is a scalar ratio of powers. If the effects of polarization and phase are included, the scattering can be expressed as a complex polarimetric scattering (CPS) matrix. The measurement of the RCS of a test object requires the test object to be illuminated by an electromagnetic plane wave and the resultant scattered signal to be observed in the far field. After calibration, this process yields the RCS of the test object in units of area, or the full scattering matrix as a set of complex scattering coefficients. This paper describes the planned upgrades to the old IEEE Std 1502™-2007 IEEE Recommended Practice for Radar Cross-Section Test Procedures [1]. The new standard will reflect the recent improvements in numerical tools, measurement technology and uncertainty estimates in the past decade.

F Saccardi, A Giacomini, L J Foged, L M Tancioni, S Khlif, Martin Kuhn, ,, November 2018

Full probe compensation techniques for Spherical Near Field (SNF) measurements have recently been proposed [1-5]. With such techniques, even antennas with more than decade bandwidth are suitable probes in most systems. The abolition of otherwise frequent probe changes during multi-service campaigns is a highly desirable feature for modern measurement applications such as automotive. In this paper, a standard dual-ridge horn with 15:1 bandwidth is investigated experimentally as probe in a SNF automotive range. The accuracy of the probe compensation technique is validated by comparison to standard single probe measurement.

Cosme Culotta-López, Dirk Heberling, Arya Bangun, Arash Behboodi, Rudolf Mathar, November 2018

Spherical near-field measurements are regarded as the most accurate technique for the characterization of an Antenna Under Tests (AUT) radiation. The AUT's far-field radiation characteristics can be calculated from the Spherical Mode Coefficients (SMC), or spherical wave coefficients, determined from near-field data. The disadvantage of this technique is that, for the calculation of the SMC, a whole sphere containing the AUT must be Nyquist-sampled, thus directly implying a longer measurement time when only a few cuts are of interest. Due to antennas being spatially band-limited, they can be described with a finite number of SMC. Besides, the vector containing the SMC can be proved sparse under certain circumstances, e.g., if the AUT's radiation pattern presents information redundancy, such as an electrical symmetry with respect to coordinate system of the measurement. In this paper, a novel sampling strategy is proposed and is combined with compressed-sensing techniques, such as basis pursuit solvers, to retrieve the sparse SMC. The retrieved sparse SMC are then used to obtain the AUT's far-field radiation. The resulting far-field pattern is compared for both simulated and measured data. The reduced number of points needed for the presented sampling scheme is compared with classical equiangular sampling, together with the estimated acquisition time. The proposed sampling scheme improves the acquisition time with a reasonable error.

Eamon Whalen, Gopinath Gampala, Katelyn Hunter, Sarthak Mishra, C J Reddy, November 2018

Altair Engineering Inc. Troy, MI USA-https://www.altairhyperworks.com Figure 1. The electromagnetic, aerodynamic, and structural performance of a nose cone radome can be characterized by computational simulation, allowing for early design concept validation and reducing the dependence on physical testing. Abstract-Radomes protect antennas from structural damage due to wind, precipitation, and bird strikes. In aerospace applications, radomes often double as a nose cone and thus have a significant impact on the aerodynamics of the aircraft. While radomes should be designed not to affect the performance of the underlying antennas, they also must satisfy structural and aerodynamic requirements. In this paper, we demonstrate a multiphysics approach to analysis of airborne radomes not only for electromagnetic (EM) performance, but also for structural, aerodynamic, and bird strike performances, as depicted in figure 1. We consider a radome constructed using composite fiberglass plies and a foam core, and coated with an anti-static coating, paint, and primer. A slotted waveguide array is designed at X-band to represent a weather radar antenna. The transmission loss of the radome walls is analyzed using a planar Green's function approach. An asymptotic technique, Ray-Launching Geometric Optics (RL-GO), is used to accurately simulate the nose cone radome and compute transmission loss, boresight error, and sidelobe performance. In addition to EM analysis, Computational Fluid Dynamics (CFD) analysis is used to predict pressures resulting from high air speeds, which are then mapped to an implicit structural solution to assess structural integrity using the Finite Element Method (FEM). We also demonstrate damage prediction due to a "bird strike" impact using an explicit structural FEM solver. The multiphysics simulation techniques demonstrated in this paper will allow for early design validation and reduce the number of measurement iterations required before a radome is certified for installation.

Radar data on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz have been collected and studied. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, or combinations of these. A two-dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd-or even-bounce nature. Use of the corner reflector as a calibration target is discussed.

N Colon-Diaz, D Janning, T Corigliano, L Wang, J Aberle, November 2018

This paper presents a way to determine mutual coupling effects through analysis of the active voltage standing wave ratio (VSWR) to predict the presence of large reverse power levels in co-located multiple input multiple output (MIMO) radars in transmit mode. The methodology consists of measuring the forward and reverse waves on a dual directional coupler (DDC) to directly obtain the active reflection coefficient on a co-located MIMO radar system. The active VSWR of each individual antenna is computed from measurements of the active reflection coefficient. These results are compared against analytical methodologies.

Jose Oliverio Alvarez, Development, John W Schultz, November 2018

Rock core specimens collected during surveys for oil drilling have, in a standard form, a 4" diameter. Cores are cut in half or in 1/3-2/3 sections to provide core slab. We developed a measurement procedure based on spot probe illumination to characterize geological and/or geochemical properties of core slab specimens via their complex permittivity for frequencies between 2.5 GHz and 20 GHz. Conventional reflectometer methods are based on illumination of a thin slab of air-or metal-backed material. However, in this case only the front surface is flat and the back surface is semicircular. A measurement method was developed based on time-domain gating to separate the back-surface reflection from that of the front. Material inversion is then based on the amplitude and phase of the reflection just from the front surface. This paper presents details of the calibration for this reflectometer measurement method, along with example measurements of core slab materials. Two different inversion methods are applied to these measured data. The first is a more conventional frequency-by-frequency method for inverting complex permittivity from the amplitude and phase of the reflection. The second method applies a physical model, the Debye relaxation model, to the data. This model-based approach minimizes the errors from edge diffraction from the small sample size.

In this paper, we explore the capabilities of the Monostatic-Bistatic Theorem (MBT) applied to Radar Cross Section (RCS) in low frequency. Originally, the validity of this theorem has been shown in high frequency for targets whose RCS is produced by elementary interactions (specular reflection in particular). We are interested in aerial platforms and in particular some Low Observable targets that have relatively "pure" geometries limiting the presence of complex interactions. Several variants of the MBT from the field of electromagnetism [1][2][3] and acoustics [4] are used. Their performances are compared from data obtained from a MoM method that is recognized to produce accurate scattering data. To highlight the discrepancies produced by the different variants, we use both a metric to compare the quality of the bistatic holograms obtained and also radar imaging which allows locating the areas of the target where the echoes are not correctly restored.

Gregory Kiesel, Daniel Dykes, Eric Brown, Edward Garcia Iii, Ryan Buchanan, Tim Conn, November 2018

The Conex Antenna, Radar, and Measurement Equipment Lab (CARAMEL) is a ten-element VHF antenna array that operates from 30 MHz-120 MHz with an attached lab space. This array was developed for use in low frequency Radar Cross Section (RCS) measurements. The antenna elements support both vertical and horizontal polarizations. The antenna was designed using a genetic algorithm, employing the fragmented aperture technique; measured and modeled data will be presented. The attached lab space is air conditioned and provisioned for rack mounted equipment. The structure uses a modified 20' Conex shipping container where an entire sidewall has been replaced with a reinforced composite radome for the antennas. The overall mechanical frame design included a Finite Element Analysis to ensure structural integrity. The system is intended for long-term standalone use as an outdoor measurement radar system but can be moved using standard shipping container methods. The structure was shipped using a standard cargo carrier from Atlanta, Georgia to White Sands, New Mexico.

Alessio Mancini, Jorge L Salazar-Cerreño, November 2018

In this paper, the concept of a new S-band dual-polarized dielectric rod antenna is discussed. The antenna is composed of two concentric dielectric cylinders. The inner dielectric presents high dielectric constant, while the outer has a lower dielectric constant. Given this configuration, the resulting antenna provides high gain, narrow beamwidth, large bandwidth, and very low cross-polarization. In addition, the antenna is lower size in the transversal dimensions, and is predicted to be lighter than other antennas that provide equivalent performance, especially at low frequencies (S-band). An antenna with such an architecture can be 3D-printed, and therefore, the cost for the fabrication are considerable low. Numerical results of the antenna performance are presented and discussed.

Pat Woollen, John Mccann, Moussa Harouna, Eric R Wandel, Jeff Snow, November 2018

Shipboard phased array radar antennas typically have high gain, low sidelobe specifications, and testing after initial production, overhaul or repair often reveals sidelobes that fail specifications, requiring rework. Further, some systems only allow phase adjustments as a means to fine tune the pattern. To correct sidelobe failures in these systems, the phase distribution of the array is first mapped using near-field scanning techniques, then specific element phases are adjusted, such as by using phase shifters. The standard method of determining phase changes has been based on trying to achieve a nominal phase profile; however, this method does not allow targeting specific problematic sidelobes. The authors have developed a novel method, dubbed "Whack-a-Lobe", which targets suppression of specific sidelobes while minimizing other impacts to the pattern. Recognizing that far-field sidelobes are a summation of complex vectors of the individual elements in the direction of the sidelobe, the authors have developed a cross product technique that identifies elemental vectors orthogonal to a far-field sidelobe vector such that only a minimal phase change to these elemental vectors is needed to reduce the sidelobe level. This technique is targeted, deterministic, and reduces tuning cycles, labor hours and antenna test chamber time.

Amedeo Capozzoli, Claudio Curcio, Angelo Liseno, November 2018

We deal with an approach, based on the Singular Value Optimization (SVO) technique, for the synthesis of array-based generators of near-field complex waveforms. The approach selects the grid wherein enforcing the design specifications and the radiator locations to control the ill-conditioning associated to the determination of the array excitation coefficients. Following the SVO optimization, the array coefficients are determined by a Singular Value Decomposition (SVD). Numerical results are provided for the synthesis of the near-field scattered by a perfectly conducting sphere illuminated by an elementary dipole.

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