AMTA Paper Archive
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Generalized Test-Zone Field Compensation
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.
Experimental validation of Reference Chip Antennas for 5G Measurement Facilities at mm-Wave
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.
A Simple High-Perfomance P-Band First-Order Dual-Port Probe for Spherical Near-Field Antenna Measurements based on the Shorted Annular Patch Antenna
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.
Practical Considerations in Compressed Spherical Near-Field Measurements
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.
Recent Changes to the IEEE std 1502 Recommended Practice for Radar Cross-Section Test Procedures
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 . The new standard will reflect the recent improvements in numerical tools, measurement technology and uncertainty estimates in the past decade.
Using Standard Wideband Antennas as Probe in Spherical Near Field Measurements with Full Probe Correction: Experimental Validation
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.
A Compressed Sampling for Spherical Near-Field Measurements
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.
Aircraft Radome Characterization via Multiphysics Simulation
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.
Geometric Effects on Radar Echoes from a Corner Reflector
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.
Measurement of Active Reflection Coefficient for Co-located MIMO Radar Using Dual Directional Couplers
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.
Spot-Probe Reflectometer Measurements of Geological Core Slab Samples
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.
Evaluation of the Monostatic-Bistatic theorem applied to the radar signature of aerial platforms in low frequency
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  and acoustics  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.
Conex Antenna, Radar, and Measurement Equipment Lab
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.
A Novel S-band Two-Layer Dielectric Rod Antenna with High Gain and Very Low Cross-polarization
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.
A Novel Method for Suppression of Individual Problematic Sidelobes in Phased Array Antennas
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.
Complex waveforms generation by SVO
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.
Optimized Compact Antenna Test Range with Short Focal Length for Measuring Large L/Ku-Band Active Antennas
A new Compact Antenna Test Range (CATR) has been built, as a turnkey facility, with a cubic quiet zone (QZ) of 4.8m x 4.8m x 4.8m in the frequency range 0.9-18 GHz. The CATR has been installed in a new building with an isolated and stable foundation. The dimensions of a traditional CATR for such QZ size becomes impractical and requires a very large chamber. A new, diagonally fed, short focal length reflector has been developed to minimize the chamber size to fit the dimensions of 22 m x 14.5 m x 14.5 m.
Top-fed P-band Dual Circular Polarization Patch Antenna Design
This paper discusses about the design, fabrication and testing of a compact P-band (370 MHz) dual circular polarization (CP) patch antenna. The antenna is intended for reflectometry applications by measuring both direct and ground reflected 370 MHz signals transmitted from a satellite or airborne source. This design adopts quadrature-phase hybrid feeding network for achieving excellent polarization purity and supporting simultaneously LHCP and RHCP measurements. Another novel design aspect is placing the feeding network on top of the patch so that the antenna can be mounted directly on a ground plane. Therefore, the resonant modes inside the patch is excited from the top instead of from ground plane as in conventional designs. High dielectric material (ECCOSTOCK®HiK) with a dielectric constant of 9 and loss tangent of 0.002 was used as the substrate to reduce the antenna size. The final antenna has a dimension of 5.9" x 5.9" x 1.3" (excluding ground plane) and weight of 1620 gram. The measured performance on a 1-foot diameter circular ground plane showed 4.5 dBic gain and 23 dB co-polarization to cross-polarization isolation at the center frequency for both LHCP and RHCP. The 1-dB gain bandwidth is approximately 3.7%.
Antenna Modeling on Complex Platforms Through Constrained Equivalent Aperture Distributions
Accurate in situ antenna manifolds are desired for performance evaluation of radio frequency systems, including communication, navigation, and radar among others. In situ antenna measurements are the most accurate way to obtain antenna manifolds on the platform of interest, but are often impractical or impossible to obtain. Instead, combinations of simulations and measurements are used to estimate antenna manifolds on platforms. First, the gain and phase of the target antenna are measured on a simple ground plane, over the frequencies and field of view of interest. The measurements are then imported into computational electromagnetics codes to simulate platform scattering from the platform of interest. However, the representation of the measured data is not unique, which leads to inaccuracies and/or a large run time in computational electromagnetics codes. This paper presents a new method to represent measured antenna data in electromagnetics codes through aperture current distributions of simulated cross-slots and monopoles. A weighted sum of far-fields from the simulated equivalent elements approximates the measured antenna far-fields, with weights determined by the minimization of the L2-norm difference between measured far-fields and equivalent element far-fields. However, the least square solution may place large, unrealizable currents on the aperture. A constraint is introduced to limit the amount of current on the aperture, by minimizing the length of the solution vector. In this paper, the details of the suggested method will be presented. We will also illustrate the accuracy of the method through example simulations, where good agreement is achieved between truth data and equivalent antennas on complex platforms.
Effective Polarization Filtering Techniques for Ground Penetrating Radar Applications
The effect of different decomposition techniques on the imaging and detection accuracy for polarimet-ric surface penetrating data is studied. We derive the general expressions for coherent polarimetric decomposition using the Stokes parameters and model based polarimetric decomposition using the Yamaguchi technique. These techniques are applied to multi-frequency (0.4-4.8GHz) full polarimetric near-field radar measurements of scattering from surface laid calibration objects and shallow buried landmine types and show in detail how the decomposition results provide effective surface and sub-surface clutter reduction and guide the interpretation of scattering from subsurface objects. Data processing methods assume cross-polar symmetry and a novel bistatic calibration procedure was developed to enforce this condition. The Yamaguchi polarimetric decomposition provides significant clutter reduction and image contrast with some loss in signal power; while Stokes parameters also provide imagery localising targets, complementary information on the scattering mechanism is also obtained. Finally a third novel polarimetric filter was formulated based on differential interferometric polarimetric decomposition. The three combined techniques contribute to a significant improvement of subsurface radar performance and detection image contrast.
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