AMTA Paper Archive
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Estimation of the Realistic Ground Effect in Free-Space Automotive Measurements
Testing of automotive antennas are commonly performed in large Spherical Near Field (SNF) ranges [1-3] able to host the entire vehicle to test the effect of the antenna coupling with the structure . The impact of a realistic ground, such as asphalts or soil, on the radiation performance of the vehicle mounted antennas is often a desired information. As long as the free-space response of the vehicle is available, such information can be obtained with fairly good accuracy considering post-processing techniques based on the Image Theory (IT). Automotive systems with absorber material on the floor  are thus ideal for estimating such effects because the free-space signature of the vehicle is directly measured and because the radiation pattern is usually available on more than just a hemisphere. In this paper an IT-based technique which allows for the estimation of a realistic ground is proposed and validated with simulations where the measurement setup of a typical multi-probe free-space automotive system is emulated. The impact of the truncation of the scanning area is analyzed in detail showing how advanced post-processing techniques [4-6] can be involved to mitigate the truncation errors and thus obtain a better estimation of the realistic ground effect.
Validation of Additive Manufacturing for Broadband Choked Horns at X/Ku Band by Extensive Antenna Measurements
Additive manufacturing has become a popular alternative to traditional CAM techniques, as it has reached a suitable maturity and accuracy for microwave applications. The main advantage of the additive technologies is that the manufacturing can be performed directly from the 3D CAD model, available from the numerical simulation of the antenna, without significant modifications. This is a highly desirable feature, in particular for time and cost critical applications such as prototyping and manufacturing of small quantities of antennas. Different 3D-printing/additive manufacturing technologies are available in industry today. The purpose of the paper is an investigation on the accuracy and repeatability of the Selective Laser Melting (SLM) manufacturing technique applied to the construction of a batch of 15 broad band fully metallic chocked horns, operating at X/Ku band, manufactured in parallel. Manufacturing accuracy and repeatability has been evaluated using RF parameters as performance indicators comparing measured data and high accuracy simulations. The radiation patterns have been correlated to the numerical reference using the Equivalent Noise Level, while manufacturing repeatability is quantified on input matching by defining an interference level. These indicators have also been compared to state-of-the-art values commonly found for traditional manufacturing.
Reference Chip Antenna for 5G Measurement Facilities at mm-Wave
In this paper, we present a chip antenna in the 27GHz band, targeting 5G measurements. This antenna can be used as reference in mm-wave measurement systems, such as the MVG µ-Lab, feeding the antenna under test through a micro-probe station. The reference antenna is employed to calibrate in gain through the substitution method. The antenna shown in this paper is an array of four patches, fed through a strip-line beam forming network. A transition strip-line to coplanar waveguide allows the antenna be fed by the micro-probe.
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%.
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.
Enhanced PNF Probe Positioning in a Thermally-Uncontrolled Environment using Stable AUT Monuments
The need for thermal stability in a test chamber is a well-established requirement to maintain the accuracy and repeatability sought for high frequency planar near-field (PNF) scanner measurements. When whole chamber thermal control is impractical or unreliable, there are few established methods for maintaining necessary precision over a wide temperature range. Often the antenna under test (AUT) itself will require a closed-loop thermal control system for maintaining stable performance due to combined effects from transmission heat dissipation and the environment. In this paper, we propose a new approach for near-field system design that leverages this AUT stability, while relaxing the requirement of strict whole chamber thermal control. Fixed reference monuments strategically placed around the AUT aperture perimeter, when measured periodically with a sensing probe on the scanner, allow for the modeling and correction of the scanner positioning errors. This process takes advantage of the assumed stability of the reference monuments and attributes all apparent monument position changes to distortions in the scanner structure. When this monument measurement process is coupled with a scanner structure that can tolerate wide thermal variations, using expansion joints and kinematic connections, a robust structural error correction model can be generated using a bilinear mapping function. Application of such a structure correction technique can achieve probe positioning performance similar to scanners that require tightly controlled environments. Preliminary results as well as a discussion on potential design variations are presented.
Resurfacing the NASA Langley Experimental Test Range Reflector
An ambitious resurfacing campaign was launched in late 2017 to correct for large reflector surface distortions present at the NASA LaRC Experiment Test Range (ETR) in support of performing Europa Clipper flight High Gain Antenna (HGA) measurements at X-and Ka-band frequencies. The effort was successful as the worst case peak-to-peak amplitude ripple was reduced from 4.0-dB to 1.5-dB across the 4.1-meter quiet zone.
A Novel GO-PSO Algorithm for Designing 3D- Printed Optimized Pixelized Inhomogenous and Shaped-Profiled Lens Antennas
In this paper, a novel algorithm for designing 3D-printed shaped inhomogeneous dielectric lens antennas is provided. The synthesis approach is based on a novel combination of Geometrical Optics (GO) and the Particle Swarm Optimization (PSO) method. The GO method can trace rays through inhomogeneous media and calculate the amplitude, phase, and polarization of the electric field. The algorithm is used to design an inhomogeneous lens antenna to produce an electronically scanned revolving conical beam to replace a mechanically scanned parabolic reflector antenna for spaceborne weather radar satellite antenna applications. Two breadboard model on-axis fed lens designs are presented and measured results given to validate the approach. A representative optimum off-axis design is presented which produces the revolving conically scanned beam. Imposition of a Body-of-Revolution restriction allows the optimization to be performed at a single offset feed location. The complex inhomogeneous engineered materials that results from optimization are printed using new 3D printers.
A New Dielectric Analyzer for Rapid Measurement of Microwave Substrates up to 6 GHz
This paper presents a new measurement method based on the parallel plate capacitor concept, which determines complex permittivity of dielectric sheets and films with thicknesses up to about 3.5 mm. Unlike the conventional devices, this new method uses a greatly simplified calibration procedure and is capable of measuring at frequencies from 10 MHz to 2 GHz, and in some cases up to 6 GHz. It solves the parasitic impedance limitations in conventional capacitor methods by explicitly modeling the fixture with a full-wave computational electromagnetic code. Specifically, a finite difference time domain (FDTD) code was used to not only design the fixture, but to create a database-based inversion algorithm. The inversion algorithm converts measured fixture reflection (S11) into dielectric properties of the specimen under test. This paper provides details of the fixture design and inversion method. Finally, example measurements are shown to demonstrate the utility of the method on typical microwave substrates.
Measurement of RF Absorber at Large Angles of Incidence using Spectral Domain Transformations
Pyramidal RF absorber, widely used in indoor antenna ranges, is designed to minimize reflectivity by creating an impedance transform from free space to the impedance of the absorber material. The pyramidal shape provides this transition quite well at normal incidence. It has been shown in  that pyramidal RF absorber performs very well up to angles of incidence of about 45 degrees off-normal, but at wider angles of incidence, the performance degrades significantly. Unfortunately, it is very difficult to perform RF absorber measurements at large oblique incidence angles. In such measurements, the reflected path and the direct path between the antennas are very close in length, making it difficult to use time-domain gating techniques to eliminate the direct coupling. In this paper, a novel approach for performing oblique RF absorber measurements is introduced based on spectral domain transformations. Preliminary measurements using this technique have been compared to RF simulations. Results appear to indicate that this approach is a valid way to perform RF absorber reflectivity measurements at highly oblique incidence angles.
Measurements of Non-Metallic Targets for the Austin RCS Benchmark Suite
A simulation-supported measurement campaign was conducted to collect monostatic radar cross section (RCS) data as part of a larger effort to establish the Austin RCS Benchmark Suite, a publicly available benchmark suite for quantifying the performance of RCS simulations. In order to demonstrate the impact of materials on RCS simulation and measurement, various mixed-material targets were built and measured. The results are reported for three targets: (i) Solid Resin Almond: an almond-shaped low-loss homogeneous target with the characteristic length of ~10-in. (ii) Open Tail-Coated Almond: the surface of the solid resin almond's tail portion was coated with a highly conductive silver, effectively forming a resin-filled open cavity with metallic walls. (iii) Closed Tail-Coated Almond: the resin almond was manufactured in two pieces, the tail piece was coated completely with silver coating (creating a closed metallic surface), and the two pieces were joined. The measured material properties of the resin are reported; the RCS measurement setup, data collection, and post processing are detailed; and the uncertainty in measured data is quantified with the help of simulations.
GSS (Gated-Short-Short) Calibration for Free-space Material Measurements in millimeter-Wave Frequency Band
Electrical properties of materials are requisite to design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurands are the free-space scattering parameters of MUT (Material Under Test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing MUT without prior machining and physical contact in high frequencies. In this paper, GSS (Gated-Short-Short) calibration method using a planar offset short is proposed for calibrating a free-space material measurement system and the measurement result is shown in W-band (75-110 GHz).
Personal Near-field System
In 1987 the author built the world's first Personal Near-field antenna measurement System (PNS). This led to the formation of Nearfield Systems Inc. (NSI) a company that became a major manufacturer of commercial near-field antenna measurement systems. After leaving NSI in 2015 several new personal antenna measurement tools were built including a modern updated PNS. The new PNS consists of a portable XY scanner, a hand held microwave analyzer and a laptop computer running custom software. The PNS was then further generalized into a modular electromagnetic field imaging tool called "Radio Camera". The Radio Camera measures electromagnetic fields as a n-dimensional function of swept independent parameters. The multidimensional data sets are processed with geometric and spectral transformations and then visualized. This paper provides an overview of the new PNS and Radio Camera, discusses operational considerations, and compares it with the technology of the original 1987 PNS. Today it is practical for companies, schools and individuals to build low-cost personal antenna measurement systems that are fully capable of meeting modern industry measurement standards. These systems can be further enhanced to explore and visualize electromagnetic fields in new and interesting ways.
Topology for Maintaining Symmetry in Hybrid LPDA-Broadband-Dipole Antennas
Topologies for hybrid LPDA-broadband-dipole antennas (hybrid antennas) are systematically presented and evaluated regarding their ability to provide symmetric response as defined and required in recent standards. The symmetry property of the hybrid antenna is fundamentally related to the intrinsic infinite balun, the choke structure, and the matching transformer for the broadband dipole, if one is employed. In general, hybrid antennas incorporating matching transformers are more symmetric if the transformer is effectively center-tapped. More specifically, in a hybrid antenna employing an impedance matching transformer derived from an equal-delay hybrid, the sum port can be advantageously connected via a low-impedance load to the center of a symmetric choke arrangement. A specific topology for a hybrid LPDA-broadband-dipole antenna is given here which employs a 1:4 impedance transforming balun between the LPDA and broadband dipole but at the same time provides symmetry such that the antenna satisfies the requirements given in recent standards. Thus, the advantages of the impedance transforming balun are realized, but the symmetry of the antenna is maintained. Finally, it is shown that a hybrid antenna satisfies the symmetry requirements if a 180 • rotation about the bore sight axis is equivalent to a 180 • electrical phase shift in the source and that this behavior is obtained with a combination of 2-fold rotational symmetry in the radiating structure and electrical symmetry in the intrinsic balun structure.
Multiphysics Analysis of RF Pyramidal Absorbers
RF absorbers dissipate the incident electromagnetic wave by converting the RF energy into heat. In many applications, the absorbers can be subjected to high power incident fields. It is imperative to characterize and analyze the thermal behaviors for these high power applications. In this paper, a multi-physics (of EM and thermal) study has been conducted. The absorbers are first simulated in the ANSYS HFSS for electromagnetics. The absorbers are placed under plane wave incident field as well as from a pyramidal horn antenna in the near field. The output of the HFSS model is then imported to the thermal and computational fluid dynamics (CFD) tool, ANSYS ICEPAK. In order to obtain accurate thermal properties of the material, an experimental setup was designed. The simulation results are validated against measured data. Several effects are shown to affect the absorber internal temperatures for the same incident field level at the front of the absorbers, such as the antenna test distance to the absorber, the shape of the pyramid, and the measurement frequency. These simulation data provide greater insights into the heat dissipation and temperature distribution inside the absorbers.
Reduced Aperture Flanged Rectangular Waveguide Probe for Measurement of Conductor Backed Uniaxial Materials
An algorithm is developed for the non-destructive extraction of constitutive parameters from uniaxial anisotropic materials backed by a conductive layer. A method of moments-based approach is used in conjunction with a previously-determined Green function. A dominant-mode analysis is done for rapid comparison of the derived forward model with that of commercially-available software. Finally, laboratory measurements are taken to compare this approach to that of a destructive, high-precision method.
Feasibility of Coaxial Resonators for Permittivity Measurements of Pressurized Gases
This paper investigates numerically the feasibility of using quarter wavelength coaxial resonators for permittivity measurements of pressurized gases, as found in the subsurface. The non-short-circuited end of the resonator is facing the inside of a pressure cell and is filled with pressure resistant, low-loss dielectric material. Results show that as pressure increases, the corresponding increase in dielectric constant can be detected through a shift in the resonant frequency of |S11| and confirmed by a change in the phase of S11.
Testing mmWave Phased Arrays for the 5G New Radio
As the wireless industry continues the move to 5G, the development and subsequent testing of mmWave radios for both base stations and user equipment still face numerous hurdles. The need to test most conformance and performance metrics through the antenna array at these frequencies poses significant challenges and has resulted in excessively large measurement uncertainty estimates to the point where the resulting metrics themselves may be useless. A large contribution to this measurement uncertainty is the impact of the over-the-air (OTA) test range used, driving the industry towards expensive compact range reflector systems in order to overcome the path loss considerations associated with direct far-field measurements. However, this approach necessitates the use of a combined axis measurement system, which implies the need for considerable support structure to hold the device under test and manipulate it in two orthogonal axes. This paper explores some of the limitations and considerations involved in the use of traditional "RF transparent" support materials for mmWave device testing.
Measurements on extended objects for radar field probes
An extended long object usually gives rise to a bright reflection (a glint) when viewed near its surface normal. To take advantage of this phenomenon and as a new concept, a discrete Fourier transform (DFT) on the RCS measurements, taken within a small angular range of broadside, would yield a spectrum of incident wave distribution along that object; provided that the scattering is uniform per unit length, such as from a long cylinder [1, 2]. In this report, we examine the DFT spectra obtained from three horizontal long objects of different lengths (each of 60, 20, and 8 feet). Aside from the end effects, the DFT spectra looked similar and promising as an alternative to the conventional field probes by translating a sphere across a horizontal path. Keywords: RCS measurements, compact range, field probes, extended long objects 1. The Boeing 9-77 compact range The Boeing 9-77 indoor compact range was constructed in 1988 based on the largest Harris model 1640. Figure 1 is a schematic view of the chamber, which is of the Cassigranian configuration with dual-reflectors. The relative position of the main reflector and the upper turntable (UTT) are as shown. The inside dimensions of the chamber are 216-ft long, by 80-ft high, and 110-ft wide. For convenience, we define a set of Cartesian coordinates (x: pointing out of the paper, y: pointing up, z: pointing down-range), with the origin at the center of the quiet zone (QZ). The QZ was designed as an ellipsoidal volume of length 50-ft along z, height 28-ft along y, and width 40-ft along x. The back wall is located at z = 75 ft, whereas the center of the roll-edged main reflector (tilted at 25 o from vertical) is at z =-110 ft. It is estimated that the design approach controls the energy by focusing 98% of it inside the QZ for target measurements. The residual field spreading out from the main reflector was attenuated by various absorbers arranged in arrays and covering the chamber walls.-, Tel. (425) 392-0175 2. Anechoic chamber In order to provide a quiet environment for RCS measurements, the inside surfaces of an anechoic chamber are typically shielded by various pyramidal and wedged-shaped absorbers, which afford good attenuation at near-normal incidence for frequencies higher than ~2 GHz. At low frequencies and oblique angles , however, Figure 1. A schematic view of the Boeing 9-77 compact range with dimensions as noted. insufficient attenuation of the radar waves by the absorbers may give rise to appreciable backgrounds. Figure 2 shows a panorama view inside the compact range, as viewed from the lower rear toward the main reflector and the UTT. With the exception of the UTT, all other absorbers are non-moving or stationary. A ring of lights on the floor shows the rim around the lower turntable (LTT), prior to the installation of absorbers. In order to minimize the target-wall interactions, the surfaces facing the QZ from the ceiling, floor, and two sidewalls are covered with the Rantec EHP-26 type of special pyramidal absorbers.
Extraction of Magneto-Dielectric Properties from Metal-Backed Free-Space Reflectivity
Intrinsic magnetic and dielectric properties of magneto-dielectric composites are typically determined at microwave frequencies with both transmission and reflection data. An iterative method, such as root-finding, is often used to extract the properties in a frequency-by-frequency basis. In some situations, materials may be manufactured on a metal substrate that prevents transmission data from being obtained. This happens when the materials are too fragile or too strongly bonded to the substrate for removal and must be characterized with the metal substrate in place. This paper compares two different free-space extraction algorithms, developed for the simultaneous extraction of complex permittivity and permeability from metal-backed reflection. One of the algorithms relies on reflection measurements of the same material with two known thicknesses. The second method takes advantage of wide bandwidth measurements to fit the reflection to analytical models (e.g. Debye). The accuracy of these methods are evaluated and the stability criteria for the techniques will be discussed, as well as the tolerance of the techniques to various measurement errors.
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