AMTA Paper Archive
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Radome Enhanced Antennas
Hybrid radome-antenna designs can enable novel applications and unique benefits that would be difficult to achieve with standalone radomes and antennas. Examples of such designs are provided which use simple antennas and novel radomes to reduce antenna size and weight, to generate and steer antenna beams without use of complex phased arrays and beam forming networks, and to enable precise direction finding with only two antenna elements.
Flat Lens Antenna Technology for Free Space Material Measurements
Free space material measurements at VHF and UHF bands require antennas that are necessarily large and heavy to accommodate the long wavelengths in these bands. Large antennas make measurement less practical and more expensive. This paper presents a new flat lens antenna technology, which enables significant reductions in size and weight compared to conventional wide bandwidth horn antennas. These new antennas utilize artificial dielectric loading combined with lossy materials to give directivities similar to much larger and heavier horns. This paper also presents the direct application of these antennas for free space dielectric material characterization. Example measurements of dielectric specimens are shown with a pair of 200 MHz to 4 GHz antennas.
Measuring Water-Cut with Dielectric-Filled Ridged Waveguides
A microwave water-cut meter for production fluids applications was designed and a basic test performed. The meter uses a vector network analyzer to measure the reflection (S11) and transmission (S21) spectra of the material under test (MUT), such as production fluids, oil spills, rock cores or soil. The initial design concept consisted of a pair of waveguides whose ends face each other and are placed on the inner surface of the pipe/core holder. The waveguides have a diameter similar to the main pipe and are filled with specific low loss materials with dielectric constant similar to that of the fluid in the pipe. Based on the initial design, a refined water-cut meter design was optimized, via numerical simulations, built and tested. To maximize bandwidth, the improved design adds ridges to the original cylindrical waveguide and optimizes the feed details to maintain an impedance match to the feed connectors. Results show that the ridges in the waveguide significantly improve transmission compared to just the waveguide alone. Initial experimental results show the measuring system is sensitive to the water content of production fluids.
One-port Calibration of Free-space Material Measurement System Using Planar Offset Short
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).
New Method for Determining Permittivity of Thin Polymer Sheets
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.
3D Printed Anisotropic Metamaterial Substrates for Antenna Applications
In this paper, a design of a linear-to-circular polarization converter based on a 3D printed anisotropic metamaterial substrate (AMS) is presented. The AMS is a stack of thin sheets of acrylonitrile butadiene styrene (ABS) material, with air gaps in between, placed over a ground plane. This produces a metamaterial structure composed of periodic anisotropic unit cells, enabling the conversion of a linearly polarised (LP) incident wave to a circular polarised (CP) reflected wave. Results demonstrate that the proposed 3D printed AMS provides good angular stability. Using the AMS as a substrate, a CP antenna application is proposed operating within the L1 GPS reducing the complexity of designing/feeding and fabricating of the primary antenna.
Wide Incident Angle Digital Coding Metasurface Applied to Reconfigurable Intelligent Surfaces
This paper presents a wide incident angle metasurface unit cell element applied to a reconfigurable intelligent surface (RIS) for beamforming and beam-steering applications in the 26 GHz frequency band from the fifth generation of mobile communications (5G) frequency range 2 (FR2). Each metasurface unit cell is based on a printed frequency selective surface (FSS) loaded with a varactor diode. The FSS-based structure is based on a circular loop at the top and a slot-based ground plane at the bottom resulting in a 0.25x0.25λ0 total area. The complete unit cell element encompasses four conducting layers, in which the first two ones form the FSS. RF chokes are printed at the middle layer to isolate the DC circuit, and the bias lines are routed at the fourth layer. The unit cell has been conceived using the full-wave electromagnetic solver ANSYS HFSS. Its numerical results demonstrate a reflection phase shift up to 180º and reflection magnitude higher than 0.4 at the 26 GHz frequency band for incident wave angle from 0 to 50º. The proposed reconfigurable intelligent surface might be applied to future wireless communication systems, planar antenna reflectors, and vortex beam generation.
Effect of Surface Roughness on Material Characterization using 3D Printed Waveguides at W-Band
In this work, a simulation-based study is presented exploring the effect of surface roughness of 3D printed plastics on the accuracy of material parameter extraction. A homogenous sample material is placed inside the W-band waveguide and the S-Parameters are simulated. Two different methods for estimating the dielectric properties of the sample using the simulated S-Parameters are presented (i) NRW (Nicolson-Ross-Weir) technique-based estimation method, and (ii) Feko optimization-based estimation method. An error analysis study is presented to understand the percentage of error due to the surface roughness of the 3D printed plastics. For N7 grade surface roughness, NRW predicts 14% error in material parameters due to surface roughness, whereas Feko optimization method predicts 10% error compared to estimation without any surface roughness. Process outlined in the paper can be used to estimate effect of surface roughness of waveguides on material property measurements at mm wavebands such as W-band.
Causal Models of Frequency Dependent R-cards Suitable for Time Domain Simulations
Resistive materials are often employed in antenna or absorber design for radio frequency (RF) applications. Causal material models are needed when modeling wideband RF systems using time-domain numerical models (e.g. FDTD). To this end, the frequency-dependence, from 10’s of MHz to 10’s of GHz, of spatially patterned and un-patterned resistive-cards (R-cards) were measured using free space and specialty materials measurements fixtures. Specifically, the complex sheet-impedance of two R-card specimens were measured at VHF frequencies using either an 8.5-inch slotted rectangular-coax (R-coax) or a recently developed resistive material mapping probe (RMMP). At GHz frequencies measurements were conducted using a standard 2’ focused beam lens system. The multi-band complex-impedance data were fit using a set of causal sheet material models. Typically, the fit errors are in the 1-3% range for causal models of measured data over two-plus decades of bandwidth.
Microwave Material Characterization using Epsilon Near Zero (ENZ) Tunnel Structures
Over the years many methods have been developed and used for measuring permittivity and permeability of materials. The most widely used methods are: 1) free-space techniques; 2) cavity perturbation techniques; and 3) transmission line of waveguide methods. Each technique has its own advantages and limitations. The free-space methods are employed when the material is available in a big sheet form. These measurements are less accurate because of unwanted reflections from surrounding objects, difficulty in launching a plane wave in a limited space, and unwanted diffraction from the edges of the sample. The resonant cavity measurement or cavity perturbation techniques are more accurate. Recently "epsilon-near-zero (ENZ) metamaterials have received much attention for several interesting phenomena like super-coupling, transparency and cloaking devices and pattern reshaping at microwave and optical frequencies. The rapid growth and excitement of ENZ materials was due to their ability to achieve very long wavelength in zero permittivity material, allowing propagation in a static-like manner. This paper presents the evaluation of complex dielectric permittivity and magnetic permeability of materials using planar ENZ tunnel structure with substrate integrated waveguide technology. The changes in resonance frequency and quality factor are related to the dielectric permittivity and magnetic permeability properties of the sample through Cavity Perturbation Technique. ENZ tunnel structure has very high sensitivity, which yields more accurate results when compared to other techniques, such as perturbation of conventional cavities. Design, optimization, and simulation of the ENZ tunnel structure at microwave frequencies is presented. Simulations are performed on various dielectric and magnetic samples using the cavity perturbation technique of the ENZ tunnel structure and validated with measured data.
SOLR Calibration Using Planar Offset Short in Free-space Material Measurement
Electrical material parameters such as permittivity and permeability are a prerequisite to analysis and to design of EM devices and systems.For the measurements of the EM material parameters, coaxial/waveguide methods and cavity method are used in low frequency range whereas free-space method is suitable in high frequency range. In free-space method, one of the non-destructive methods without prior machining of a MUT (Material Under Test), TRL (Thru-Reflect-Line) calibration method is used when the free-space measurement system has a linear slide to precisely adjust the separation distance between transmit (Tx) and receive (Rx) antennas, and GRL (Gated-Reflect-Line) calibration method in the case that the separation distance between the two antennas is fixed. As one of the well-known calibration methods, SOLR (Short-Open-Load-Reciprocal) calibration method assumes seven unknowns in the free-space material measurement configuration, i.e., the port-#1-side directivity, match, and tracking (E_DF, E_SF, E_RF) between the VNA test port #1 and the MUT, the port-#2-side directivity, match, and tracking (E_DR, E_SR, E_RR) between the VNA test port #2 and the MUT, and the quotient ?/…. This calibration, first performs two one-port calibrations at the port-#1-side and port-#2-side to determine (E_DF, E_SF, E_RF) and (E_DR, E_SR, E_RR) using three reflection standards, and then performs one transmission measurement using a reciprocal two-port standard (in this case, thru) for determining ?/…. Calibration of a free-space material measurement system by using SOLR method requires three free-space reflection standards. Recently, a planar offset short is proposed as a free-space reflection standard because its reflection property has the magnitude of unity and the phase proportional to the offset of the offset short. This paper proposes the SOLR method using three planar offset shorts with the respective offset of (0, ?/6, 2?/6) for calibrating a free-space measurement system. Effects due to the thickness of the MUT are compensated by a de-embedding process. The thinner the thickness is, the better results this method can get. The proposed method does not require a linear slide to precisely adjust the separation distance between Tx and Rx antennas of the measurement system. Theoretical details and measurement results will be presented at the symposium.
Increasing the Material Diversity in the Austin RCS Benchmark Suite Using Thin Plates
The Austin RCS Benchmark Suite has recently been introduced to enable quantitative and objective comparison of computational systems for solving electromagnetic scattering problems, particularly, those relevant to aerospace applications. In the last year, five sets of problems were added to it: dielectric almonds (problem set III-B), mixed material almonds (III-C, III-D), perfectly electrically conducting (PEC) aircraft models (IV-A), and dielectric aircraft models (IV-B). For each problem set, a range of lengths and frequencies of interests are identified, interesting features are highlighted, and datasets containing reference results (from measurements, analytical methods, or numerical methods) are shared online. Although data from several radar cross section (RCS) measurement campaigns of non-metallic targets are available in the literature, these lack the information necessary to precisely model the materials, target geometries, and measurement setups, to quantify uncertainties in the data, and to identify appropriate directions for improving computational methods' performance. This limits their utility for benchmarking computational systems. This article presents an expansion of the Suite to include problems with more complex materials and reference results from a measurement campaign that attempted to ameliorate the deficiencies of existing datasets. Specifically, a set of thin-plate problems are added to the Austin RCS Benchmark Suite to increase material diversity. These consist of problem sets II-B: thin perfect-electrically conducting (PEC) plates, II-C: thin dielectric plates, II-D: thin magnetic radar absorbing material (MagRAM) plates, and II-E: thin MagRAM-coated PEC plates. Reference RCS data that enables validation, RCS measurement and material property uncertainty quantification, and benchmarking are also provided by conducting a simulation-supported measurement campaign in a compact range. To facilitate reproducibility, a popular low-loss dielectric material and a commercially available MagRAM were chosen for these problems: The dielectric material for problem set II-C is PolymaxTM polylactic acid (PLA). For problem sets II-D/E, the ARC Technologies' DD-13490 material is used. Thin plates were manufactured and their RCS were measured at Lockheed Martin's Rye Canyon Facility. The monostatic RCS measurement results and supporting simulation results are available online. Performance data for simulations as well as RCS measurement results with accompanying uncertainty will be presented for problem sets II-B/C/D/E at the conference.
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.
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