AMTA Paper Archive


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AMTA Paper Archive

Equivalent Multipole Source Models for the TE/TM-R Spherical Wavefunctions
James McLean, October 2022

We examine equivalent multipole sources for the TE/TM-R spherical wavefunctions (SWFs) used in spherical near-field measurements and analysis. The impetus is the understanding of compact sources for wireless power transfer. However, these equivalent multipole sources have other applications including the design of near field probes, especially those with higher orders than typical probes. Equivalent multipole sources for low-order (n<=3) SWFs have been given by Rusch et al. (1986), but with no derivation. A much earlier but less-known work replete with a postulated general mathematical expression for the singular multipole sources has been given by Kennaugh (1959). The multipole source models for the lowest-order SWFs (n<=2, m<=1) given in these two references are identical. This includes the six fundamental electric and magnetic dipoles as well as the TM02-R, TE02-R, TM12 e/o-R, and TE12 e/o-RSWFs. However, careful comparison of the multipole sources given by Rusch et al. and Kennaugh reveals a discrepancy for some of the higher-order SWFs. It is known that the equivalent sources are not unique; that is, there can be more than one equivalent multipole source for a particular SWF. Nevertheless, it appears that some of the multipole source models given by Rusch et al. are incorrect. Specifically, there is a difference in the equivalent multipole source given for the TM22 e-Rspherical wavefunctions given by Rusch et al. and the one given by Kennaugh. Likewise, there is a difference in the multipole sources given for the TE22 e-Rspherical wavefunctions. A SWF decomposition reveals that the source given by Rusch et al. actually produces TM22 e-R with an admixture of a TM44 e-R and TM04-R SWFs, while that given by Kennaugh produces a pure TM22 e SWF. This is shown here using two features in FEKO: (1) construction of an ideal multipole source from electric/magnetic dipoles, and (2) a spherical wavefunction expansion of the fields produced by such a multipole source. More generally, in this paper, corrected multipole sources for the low-order spherical wavefunctions are given along with a discussion of the derivation of these corrected multipole sources.

Co-Site Interference Analysis on Aerospace and Naval Platforms using Advanced Simulation Tools
V B Murthy D, CJ Reddy, October 2022

Modern Aerospace, Naval and Defense platforms are overwhelmed with radiofrequency (RF) signals competing for spectrum. RF co-site interference has become a major problem due to the RF interference from jamming, radio stations nearby, or even from civilian communications such as mobile phones. It can be a problem on military platforms like surface warships, land vehicles, and aircraft where many different RF transmit and receive antennas must share a relatively small space. This can be a communications nightmare in which separate RF systems inadvertently step on each other's signals, causing an RF communications fratricide problem that can also be compounded by intentional or accidental RF jamming. It has become more important than ever to address these issues that arise due to RF co-site interference. In this paper, we present advanced simulation tools for antenna placement, antenna coupling and cosite interference on electrically large naval and air platforms using Altair Feko and Wrap softwares. S-parameter coupling matrix of various antennas (both in-band and out-of-band) are computed using either full-wave solutions such as MoM, MLFMM or using asymptotic methods such as PO, RL-GO and UTD. Alternatively Coupling Loss Matrix, defined as the power ratio between powers at the terminals of the transmitting and receiving antennas can be computed using equivalent sources. S-Parameter matrix or Coupling Loss values are then used to study the parameters of co-site/collocation interference such as inter-modulation products, adjacent channel interference and harmonic interference. Furthermore, we will also discuss the options to mitigate collocation interference by adding appropriate filters.

Look Through Hygroscopic Indoor Materials at Frequencies from 750 GHz to 1.1 THz
Fawad Sheikh, Aman Batra, Andreas Prokscha, Dien Lessy, Thomas Kaiser, October 2022

This paper reports the look through losses witnessed for four hygroscopic indoor material groups, namely, wood, paper, brick, and leather employing the VNA-based Swissto12 MCK terahertz transmission waveguide measurements system. This study focuses on materials encountered widely in the interior of indoor environments. The hygroscopic nature of the chosen materials is studied by measuring the look through losses (i.e., penetration losses) for the dry materials followed by the wet ones in the 0.75–1.1 THz frequency range. The moisture or water content may significantly influence the terahertz wave propagation depending on the free and/or bound water percentage. In addition, this acquired knowledge facilitates the characterization as well as localization of these materials precisely and hence, demands thorough investigations. The chosen material samples along with their frequency-dependent material parameters, thicknesses, and roughness are modeled in CST, which gives a further probe into the interesting hygroscopic effects on penetration losses witnessed for the chosen material groups. Utilizing well-known models such as Bruggeman and Landau-Lifshitz-Looyenga, a 1–60 percent moisture content range is employed in the CST simulations. This paper, however, is the first-ever to investigate the characterization of propagation in hygroscopic indoor materials at THz frequencies. Preliminary measurement results exhibit that the look through losses unexpectedly decline for the chosen material groups in the wet state. These unusual effects on look through losses signify that the bound water molecules as compared to free water content manifest less influence on the THz wave attenuation. All details about the measurement setup and material samples along with both measurement as well as simulation parameters are revealed in the full paper to be presented at the upcoming AMTA symposium.

A Novel Reduced-Complexity Low-Profile Beam Steerable Risley Prism Antenna
Junbo Wang, Yahya Rahmat-Samii, October 2022

Wide-angle beam steerable antennas are critical devices for 5G and next-generation Internet of Things (IoT). In general, beam steerable antennas are realized electronically by controlling the phase of an array of elements, or mechanically through moving parts. While electronic approaches typically offer fast beam switching and low system profile, the use of a substantial number of active components can considerably impact efficiency in the millimeter-wave range and increase cost. Meanwhile, novel mechanical steering concepts such as the Risley prism antenna (RPA) have become attractive because of their low electronic complexity, low cost, and better efficiency. An RPA typically contains three co-axially placed panels, including a stationary feed source generating planar illumination and two independently rotatable beam-deflective surfaces. Beam scan is realized by in-plane rotations of the components. Therefore, RPA avoids any distributed active components, and the profile of the antenna stays unchanged while scanning. These features make RPA a competitive candidate in scenarios that have moderate requirements on steering speed, e.g., for satellite-tracking ground terminals. In this work, we propose a novel RPA configuration that realizes wide-angle 2D beam steering with merely two flat panels coaxially placed in parallel, including a rotatable feed and a rotatable transmitarray. The feed radiates a pre-defined gradient-phase and the transmitarray provides another gradient phase shift. The combination of the two gradients becomes a new gradient at the exiting aperture. In-plane rotations of the feed and the transmitarray changes the value and direction of the aperture phase gradient, and eventually scans the beam. This configuration uses fewer components than a conventional RPA, and can significantly reduce system complexity, weight, profile, and loss. Based on this concept, we present the principle, design, and verification of a K-band circularly polarized (CP) RPA at 19 GHz. A sequentially rotated truncated-corner patch array feed generates the first CP phase gradient. A CP transmitarray using S-ring element unit cells is placed on top of the feed to provide the second gradient phase shift. The total thickness of the RPA is less than a wavelength at 19 GHz. Simulated and measured results showing beam scans beyond 50°in elevation will be presented.

Phaseless Spherical Near-Field Antenna Measurements Using an Arbitrary Oriented Translation Axis
Adrien Guth, Dirk Heberling, October 2022

Spherical Near-Field (SNF) antenna measurements are one of the most accurate antenna characterization methods. However, despite the high accuracy, they have several drawbacks. One of them is the requirement/challenge to acquire not only the amplitude but also the phase of the near-field measurements in order to reconstruct the far-field of the antenna. However, in comparison to amplitude, phase measurements require expensive equipment and are more prone to inaccuracies, particularly at higher frequencies. Moreover, in specific scenarios such as over the air measurements, a reference phase is unavailable or inaccessible. To overcome this, phaseless approaches are of interest and different methods such as using different probes or the two-spheres technique are investigated. The latter has gained more interest in recent research by introducing phase retrieval algorithms such as Wirtinger-Flow, PhaseLift or Gerchberg-Saxton to SNF. We consider an SNF setup with a roll-over-azimuth positioner for the Antenna Under Test (AUT) and a probe mounting with 90° rotation in azimuth. Measuring two spheres with different radii necessitates translating the measurement coordinate system on the AUT-probe axis. This can be done either via an offset of the probe mounting or via a linear axis, whereby the latter is the more flexible solution in terms of measurement automation. In our approach, the considered measurement setup is part of a hybrid test range, consisting of an SNF setup and a Compact Antenna Test Range (CATR). The AUT positioner, part of both CATR and SNF, is mounted on an additional linear axis. Since the linear axis has been designed to measure at different positions in the quiet zone of the CATR, it is not aligned with the near-field probe. Thus, when measuring two spheres, the probe misalignment must be compensated in post-processing by means of an additional rotation of the probe’s spherical mode coefficients. This method allows using arbitrary oriented linear axes to vary the measurement radius and hence increases the flexibility in choosing different radii for the two-sphere technique, which is a critical parameter in the phaseless reconstruction of the far-field.

Radiation pattern measurements using an active radar module
Anna Granich, Roland Moch, Amar Al-Bassam, Dirk Heberling, October 2022

In the automotive sector, driver assistance systems are playing an increasingly important role in automated driving, with radar sensors being a critical component for environmental perception. The implementation of safety-relevant functions places ever higher demands on sensor technologies in order to provide high-quality and reliable data. For radar sensors the radiation pattern of the antennas is a crucial factor for the performance of the overall system. As the technology moves towards highly integrated systems, the antennas are integrated directly on the circuit board or even the radar chip package. This complicates or even eliminates the possibility for classical antenna measurements, as there is no access to the antenna feed line. Here the integrated receiver and transmitter module of the radar system are used to measure the two-way radiation pattern with a reflector. However, this leads to a lot of unknown factors and influences that differ from classical antenna measurements. Within this study the formal built up radar measurement setup for the robot-based antenna measurement system of the Institute of High Frequency Technology of RWTH Aachen University is used for accurate two-way pattern measurements based on the sampled raw data of the radar system. A modular frequency modulated continuous wave radar setup with high configurability is used to build up measurements on well-known antennas. The flexibility of the modular radar allows for measurements on different antenna types as scalar feed horn and travelling wave antennas. Different parameters like the choice of the reflector, the measurement distance and repeatability of the measurements are examined on their influence on the measured two-way-patterns of these antennas. The radiation patterns are resolved over the frequency bandwidth of the chirps using the intermediate frequency signal of the radar sensor to investigate the influences on their frequency dependence. The possibility on measuring the co and cross polarization components of the patterns is studied.

A Trade Study on Quasi Far Field Accuracies and Measurements
Marion Baggett, October 2022

Recent papers have addressed making far field measurements at much less than the traditional far field distance, particularly for 5G MIMO test articles. These papers have focused on main beam measurements only, such as Total Radiated Power (TRP) and have stated that other normal antenna pattern metrics, such side lobe level measurements are not appropriate for this shortened distance. These papers have addressed fixed error levels acceptable for this quasi far field technique. This paper will present a sliding scale of main beam error versus measurement distance that can provide a more precise evaluation for the practitioner on selecting this technique. In addition, the paper will present a trade study in terms of chamber size, measurement durations and measurement methods between the quasi far field, compact range, and spherical near-field approaches. This trade study will cover three representative test articles in the C, Ka, and V frequency bands for 5G applications. A case study for a particular test article will provide an evaluation example.

Maximum Determinant Sampling Using Spline-Based Trajectories in a Robot-Based mm-Wave Antenna Test Range
Roland Moch, Dirk Heberling, October 2022

Robot-based measurement systems offer a high number of degrees of freedom for the configuration of the targeted antenna measurement. Especially in comparison to the conventional planar, cylindrical or spherical measurement chambers, complex measurement sequences can be exploited. In addition, the actual measurement path is not decisive for an antenna measurement since the data is acquired only at discrete sampling positions. Spline-based measurement trajectories provide a way to obtain the measurement data at the intended positions without specifying a fixed path. Instead, it is up to the robot controller itself to calculate and optimize the path between the various spline support points, for example regarding the greatest path speed. However, further optimization potential can be accessed by reducing the number of path-defining spline support points and instead recording the measurement data on the resulting path. This sampling data is thus not on a regular or uniform sampling grid, but on the path optimized by the robot controller, both in terms of position and orientation. Accordingly, a pointwise probe correction is essential so that the actual position and orientation of the probe can be considered in the calculation of the spherical mode coefficients. In order to evaluate the radiation pattern transformed into the far field of the antenna under test as well as the required measurement time, the spline-based measurements are compared to a conventional spherical measurement as usually performed with roll-over-azimuth positioners. The results show that spline-based motion sequences enable faster antenna measurements, since the actual trajectory is no longer predetermined, but can be optimized by the robot controller itself for the underlying measurement setup.

Calibration and Cross-Polarization Measurement Standard Requirements for Focus Beam Material Characterization Systems
Jeffrey Massman, Michael Havrilla, October 2022

Novel metamaterial and metasurface realizations provide unique control of the wave dispersion but present many challenges for accurate constitutive parameter extraction. Practical material measurement approaches for characterizing complex materials, such as biaxial anisotropic and gyrotropic media, rely on either waveguide or focus beam techniques with trade-offs in sample size and bandwidth. Multi-static focus beam setups offer many advantages for complex material measurements by enabling wider sampling bandwidths, measurement degrees of freedom and larger sample sizes. However, rotational misalignment of the principal crystal and measurement coordinates of the biaxial anisotropic media sample results in cross-polarized scattered field contributions. Likewise, interrogating gyrotropic or general bianisotropic media with a dual-polarized focus beam measurement setup produces cross-polarized scattering matrix components. These non-diagonalized S-parameters for the complex sample under test must be measured to successfully extract the constitutive parameters. A time-domain gated response isolation calibration scheme is one common approach to establish the measurement reference plane and minimize fixture uncertainties for samples with limited cross-polarization scattering. This paper extends the time-domain gated response isolation methodology for free-space focus beam systems to account for cross-polarization terms present in both the sample under test as well as the measurement setup. The featured analysis leverages a 4x4 S-parameter matrix notation to capture the polarimetric scattering at each cascaded stage. An equivalent line standard procedure is developed where four unique, linearly independent calibration standard measurements are shown to account for all unknown terms. Finally, a sensitivity analysis of the calibration standards is performed via numerical simulations to show the potential trade-off and limitations of the cross-polarization extended time-domain response isolation calibration scheme performance.

A Technique of Holographic Projection from Far Field Pattern to an Unconstrained Planar Surface
Yibo Wang, Zhong Chen, October 2022

Holographic back projection to a plane from spherical pattern data offers more details than from planar data because the image is derived from data over an entire hemisphere. A previous study introduced a back projection method where the far field pattern can be projected to a plane which is orthogonal to the radial direction, with thetwo transverse axes parallel to the local θ and ∅ vectors. This limits the hologram to only certain specific planes which may not match the desired surfaces. To overcome the shortcomings, we apply a more general back projection method in this paper which allows projection to an arbitrary plane. This is achieved through a translation and a series of rotation operations of the far field pattern. Specifically, as a first step, the phase center of the far field pattern is moved to coincide with the center of the projection plane through a translation matrix. Next the far field pattern is rotated such that its x, y and z directions are aligned with the desired projection area. Then the plane wave spectrum is computed based onthe new far field pattern. The hologram can be obtained by applying inverse Fourier transform to the plane wave spectrum. The phase shiftterm exp(jγd) in the conventional back projection technique is no longer needed after the pre-process of the far field pattern because it is included in the translation operation. This is a much more general back projection algorithm which provides the holographic projection onto an arbitrary plane. The method can be especially useful for cases when the desired projection areahas an arbitrary orientation with an offset from the origin.

Antenna gain determination by spherical near-field substitution method without full-sphere measurement of reference gain antenna
Sergiy Pivnenko, October 2022

In spherical near-field (SNF) antenna measurements, gain of an antenna under test (AUT) is usually determined by substitution method. In this method, a reference antenna with known gain, typically a standard gain horn (SGH), is measured and processed in the same way as the AUT, that is a full-sphere measurement is done for the SGH and this is followed by the near-to-far-field transformation (NFFT). The AUT gain is then determined comparing the calculated levels of the AUT and SGH far-field signals and using the known SGH gain value. It is always emphasized that in the SNF gain determination by substitution, both antennas must be processed in the same way, and thus measuring the full-sphere near-field data for the SGH is unavoidable. Since typical SGH is not a large antenna, its far-field distancedoes not exceed few meters, which is a usual measurement distance in many SNF setups. The SGH in these cases is measured in the far field and the NFFT does not change the measured SGH pattern shape. It is interesting to find out, in which conditions it is possible to skip the NFFT for the SGH, and thus also its full-spheremeasurement, and use the directly measured SGH data. The SGH measurement can in this case be reduced to a single direction, similar to what is done in the traditional far-field substitution method. In this paper the above question is clarified in detail, paying special attention to probe correction issues as well as to additional measurement uncertainties which may arise due to the explained simplification of measurement procedure.

Determination of the Number of Valid Scan Pairs in a Multielement Waveguide Simulator
Collin Wallish, Dejan Filipovic, October 2022

Modern design of phased array apertures typically begins with unit-cell design relying on an extensive use of full-wave simulations. The waveguide simulator is an equivalent, experimental simulator that allows for measurement of the active impedance of radiating elements at a prescribed pair {frequency, scan angle} in an infinite array environment. Therefore, the waveguide simulator offers a means of real-world verification of a unit-cell design before proceeding with finite array design or realization. In its simplest form, the waveguide simulator is constructed through placing an element within the walls of a rectangular waveguide. The natural imaging of the waveguide walls acts to emulates an infinite array environment. The multielement waveguide simulator described first by Gustincic (J. J. Gustincic, IEEE Trans. Antennas Propagat., vol. 20, no. 5, pp. 589–595, 1972), allows for experimental determination of the active reflection coefficient of an embedded array element for a discrete set of scan conditions corresponding to a given waveguide mode excitation. Though the theory behind the waveguide simulator is well documented in the literature, there is scant discussion of the effect of configuration on the number of and distribution of valid scan pairs. The number of valid scan pairs is proportional to the number of modes that are excited within a waveguide, which is set by the element spacing and size of the waveguide. It is desirable, for a given number of elements constituting a waveguide simulator, to maximize the number of valid scan pairs and simultaneously the information content that can be obtained from a single experimental setup. The number of valid modes for a square waveguide with half-wavelength element spacing is found to reduce to the well known Gauss circle problem. For the general case in which the element spacings are arbitrary and the waveguide is rectangular the problem is reduced to Hardy’s generalization of the Gauss circle problem (G. H. Hardy, Proceedings of the Royal Society of London. vol. 107, (744), pp. 623-635, 1925). The herto unrecognized connection to an existing and widely developed mathematical theory gives insight into the fundamental sampling limitations of the waveguide simulator.

Methodology and Practical Considerations for the Implementation of the Three-Antenna Method in a Spherical Near-Field Range
Bennett Gibson-Dunne, Jill Smithson, Ken Oueng, Greg Brzezina, Adrian Momciu, October 2022

The three-antenna method is a way of calculating antenna gain without the need for a gain standard. Unlike the comparison or direct methods, the three-antenna method calculates antenna gain solely from measured data and does not require the gain of any of the antennas to be known in advance. As a result, it’s the most favored method in applications where accuracy is of chief concern, like in calibration measurements. However, implementing this method presents additional challenges related to the equipment required, test procedures, and analysis of the resulting data. In this paper, these challenges are addressed with a new methodology used to create a custom script and user interface within the NSI2000 software environment. The script itself is described with the aid of flow charts and then the validation process involving two test campaigns, using both calibrated and non-calibrated standard gain antennas, is given. Following these efforts, the three-antenna method was successfully implemented for the first time in a facility that traditionally only used the gain comparison method. The lessons learned from this project could also prove valuable in understanding the practical considerations concerning the implementation and use of the three-antenna method in any other near-field test range.

5G Base-station Network Optimization in Urban Wireless Scenario using Machine Learning
Jaehoon Kim, October 2022

As the 5G system becomes today’s main wireless communication service, a MIMO(Multiple-In Multiple-Output) configuration has been considered as an essential feature to provide an unprecedented high date-rate transfer for the wireless service users. Therefore, it is a big concern to design best diversity antennas for a mobile station and base station which are supposed to operate in mm-Wave frequency bands. In addition to the diversity antenna design, optimally deploying the 5G radio frequency system consisting of the MIMO configuration is another big concern to the 5G wireless service providers, because the millimeter Wave (mm-Wave) is expected to lose its transmitting power more abruptly than the previous wireless services. In this paper, the deployment parameters related to the base-station antennas are studiedfor better 5G networkperformance by applying machine learning algorithm. At first, MIMO antennas based on a printed dipole pair are designed both for a mobile platform and a base-station platform by taking into account the MIMO performance factors: envelope correlation coefficient (ECC) and mean effective gain (MEG) at one of the 5G frequency bands (26.5~29.5 GHz). Secondly, the designed antennas are deployed into a urban wireless communication scenario mainly composed of mobile stations, base stations, and buildings. In the urban scenario, the 5G system performance are estimated in terms of received power, signal-to-noise-and-interference ratio, and maximum data rate. Finally, the 5G base-station system is studied to aim at the better system performance by using a machine learning technique which especially suggests optimum antenna parameters for the base-station deployment.

A 77 GHz Microstrip Comb Line Antenna Array for Automotive RADAR application
Neha Pazare, Vivek Kamble, October 2022

In this paper, a 77 GHz microstrip comb-line antenna array for an automotive RADAR application with a low sidelobe level is proposed. The microstrip technology is used for the antenna due to its low fabrication cost, small size, and easy integration with other microwave circuitry. At very high frequencies such as millimeter waves, the gain of a single element patch antenna is not enough to withstand the RADAR application requirements, hence an array of antennae is beneficial. A The Phased array antenna configuration is needed to have a high gain and low sidelobe level of -20 dB and a beam steering mechanism. The design procedure used here is the implementation of a single comb antenna, that is further realized into a 1 x 10 uniform linear array of a comb line array. It has a gain of 14.81 dB and a sidelobe level of -15 dB. The radiation in the comb antenna is primarily due to the open sides with the lengths of the comb serving as transmission lines. The adjacent combs are placed at the distance of λ in order to co-phase the antenna elements at the desired frequency. Additionally, with an aim of reducing the sidelobe level, Taylor amplitude distribution is used, and the tapered array is designed. This methodology helped to achieve a sidelobe level of -20 dB. The gain of an overall array is increased to 20 dB by realizing the array of 4 x 10. Another requirement of the Automotive Radar is beam steering to accurately detect the target. Butler matrix is a beamforming network chosen to feed the phased array antenna. The proposed antenna array is simulated in Ansys HFSS with Rogers RO 3003 substrate of the thickness of 1.27 mm and has an overall dimension of 9 x 14.96 mm2. The goal of the design of this antenna is to acquire an appropriate radiation pattern with a low side lobe level better than -20dB and achieve beam steering using the Butler matrix to have a phased array configuration. Index Terms— RADAR, Antenna array, Comb line array, Butler Matrix, Phased array.

RCS Compact Range Focal Plane Array Antenna Feed Design Concept
William Carter, Jerry Jost, Gabriel M. Rebeiz, October 2022

Diagnostic and verification testing of Low Observable (LO) platforms and components requires an Ultra-Wideband (UWB) Inverse Synthetic Aperture Radar (ISAR) imaging capability. A Compact Range (CR) is a test instrument that, when fitted with an instrumentation radar and target positioner, can efficiently produce ISAR images and other Radar Cross Section (RCS) data products required for LO research, design and production programs. Key limiting factors for the instantaneous radar imaging bandwidth of a CR is the feed antenna, where the criteria of a good feed is frequency bandwidth and illumination pattern shape. Maintaining a relatively constant reflector illumination characteristic typically requires several feeds with constant patterns functioning over smaller operating bandwidths, to be mechanically sequenced in the measurements. These feed limitations increase operational costs and complexity for LO measurements, driving a need for improved illumination sources providing constant reflector illumination for UWB collections. Focal Plane Arrays (FPAs) can be utilized to resolve these issues while increasing instantaneous bandwidth and measurement quality while reducing operational costs. This paper presents a procedure for defining complex weights of an FPA aperture to optimize radiation pattern matching to the reflector. A simulated plane wave arriving from the CR quiet-zone impinges on a model of the reflector. The FPA is placed in a region near the focal point and contained within the beam waist envelope, and the FPA weights are computed using a Computed Electromagnetic (CEM) techniques. The computational complexity of CEM simulations of electrically large CRs are usually prohibitive, however this method exploits the large focal lengths of CRs to sparsely model reflectors, and produces a tractable solution even at millimeter wavelengths. Practical aspects of FPA designs are presented and discussed as applied to the large outdoor CR at the US Army, Electronic Proving Ground (EPG), Fort Huachuca, Arizona.

Initial Development of Low-cost Custom Spherical Measurement Range
Songyi Yen, Ljubodrag Boskovic, Dejan Filipovic, October 2022

Measurements of antenna prototypes are a critical component of the development cycle for antennas, arrays, and other radiating structures. Benchtop tests to characterize the circuit performance of such devices are generally available to engineers and scientists, but the ability to capture the space (radiating) characteristics is often lesser available. This is not only due to the need for a vector network analyzer but also the necessity for mechanical infrastructure to sample the fields in a scan volume around the antenna (i.e., antenna range). Moreover, the software capable of any data manipulation is needed to obtain the far-fields either directly or from the sampled near-fields. Herein, we describe the initial exploratory development of a low-cost, bench-top, custom spherical range. The system consists of a phi-stage turntable where the antenna under test (AUT) is mounted, a theta-stage swing arm that sweeps the probe antenna in an arc about the center of rotation, and a polarization stage turntable on the probe antenna side. An adjustable scan radius of 30-40 cm is built into the theta-stage. The bulk of the range is fabricated using standard fused deposition modeling (FDM) 3D printing and inexpensive commercial off-the-shelf (COTS) components are used for the motors and controllers to keep cost of the system (excluding the network analyzer and RF cables) to around 500 US dollars, in accordance with the restrictions for an advanced antennas course class project. Development, fabrication, and assembly took place over the course of approximately a month. The drawbacks of the utilized materials, however, primarily manifest in the oscillations of the theta-stage due to the low-infill ratio (~10%) of the 3D printed plastic in conjunction with the weight of the probe antenna. Additionally, a basic spherical near-field to far-field transform code is developed. The measurement results of a wideband horn antenna are performed to validate the range performance and will be shown and discussed at the conference. Thoughts on future development and potential are also shared.

Evaluating the RF Performance of a 3D Printed Millimeter-Wave Helical Antenna for Operations in Harsh Conditions
Ljubodrag Boskovic, Mohamed Elmansouri, Dejan Filipovic, October 2022

Antennas on airborne platforms are subject to harsh environmental conditions such as rain and hail, as well as a wide range of pressure and temperature conditions. In this work, we propose a low-cost and easy to fabricate combined pressure and temperature test-bench for emulating the antenna performance while airborne. The specific antenna under test (AUT) is a millimeter wave 3d printed helix antenna enclosed in a specifically designed radome and fed with a 2.4mm connector. The entire system is subjected to airspeeds in excess of 200 m/s and operational altitude from the sea level through several kilometers. In the intended application, there are two possible pressure conditions that are considered, specifically open and closed. The open scenario assumes that the interior of the antenna is exposed to the ambient pressure level. In the closed case, the antenna has internal pressure that acts on the radome from inside at the higher altitudes in addition to the outside wind load due to the airspeed. Also, at higher altitudes, the temperature can drop to < -30˚C whereas at low altitudes it can be as high as 50˚C or more. Therefore, the structure needs to maintain its high-quality performance over the 80˚C temperature gradient. Note that both pressure and temperature can affect the antenna RF performance due to the drift of tolerances. To ensure proper operation, it is necessary to test the antenna when it is subjected to the above-discussed conditions, and for that purpose, the cost-effective combined pressure and temperature test bench is engineered for environmental tests of airborne antennas. Test-beds are mainly made of commercial off-the-shelf parts and in-house-made frames with all components integrated into one assembly. The system is developed for antennas having diameters and lengths of 125mm and 200mm, respectively, and occupies a relatively small volume. Experimental results that include live monitoring of VSWR during the variation in temperature from -20˚C to 50˚C and pressure from vacuum of half atmosphere to 20 PSI will be presented. This work is funded by the Office of Naval Research (ONR) grant # N00014-21-1-2641

Estimating Shale Maturity from Ultra-Fast Microwave Heating
Jose Alvarez, David Jacobi, Poorna Srinivasan, October 2022

Currently to determine the basic parameters of shales, namely Maturity (which indicates production potential), geochemical analysis need to be performed. These analyses may take days to weeks, depending on laboratory availability. Moreover, by the time the samples get to the laboratory, they could be damaged or poorly preserved, thus creating a significant source of uncertainty in the measurements. Pyrolysis is a method that introduces a sample of rock, of known mass, into a sealed oven that is programmed to heat the sample according to a prescribed rate of increasing temperatures that terminates at 650°C. During the initial heating, upon reaching a threshold in temperature somewhere around 300-350°C, a significant amount of hydrogen is recorded which is called the “S1” peak. With further increases in temperature beyond 350°C, another threshhold is reached at 550-600°C, where yet more hydrogen is evolved from the rock and the peak recorded at that stage is called “S2”. A microwave heating and testing for geological applications was tested with different shale samples. The system consists of a dual-mode microwave cavity, where heating and measuring is performed simultaneously with two different microwave sources. The cavity has a diameter of 104.92 mm and a height of 85 mm. A small shale sample of 9.8 mm diameter by 15 mm height is introduced in a quartz vial with an inner diameter of 9.8 mm and 120 mm. Depending on the electrical losses, the sample could be heated up to 1200°C. Initial complex permittivity measurements show that the imaginary part exhibits relaxation processes at specific temperatures. These temperatures coincide with the expected temperatures of the S1 and S2 peaks of the pyrolysis method, from which we can compute the vitrinite reflectance of the sample, which is an indicator of Maturity. Thus, allowing for quick estimates of maturity, which allows for real time decisions on the development of unconventional resources.

A Loss Tangent Measurement Surface for Free Space Focused Beam Characterization of Low-Loss Dielectrics
Christopher Howard, Kenneth Allen, Bill Hunt, October 2022

The precise characterization of the complex permittivity, particularly loss tangent, in low-loss dielectric samples at microwave frequencies usually employs resonant cavity methods, where the quality factor of some resonance is determined by a precisely dimensioned sample of the material placed inside the cavity. In order to characterize materials over a broad set of frequencies, a separate measurement fixture and sample is required for each frequency, a tedious and expensive endeavor. In response, one may turn to a single broadband measurement system, such as the focused beam system, but simple transmission and reflection measurements suffer from poor loss tangent sensitivity. In this work, a hybrid approach is investigated whereby a highly resonant periodic metallic array adjoined to a dielectric sample is measured in a broadband focused beam system. A frequency selective surface (FSS) is designed to be placed against a planar dielectric sample to create a transmission or reflection response that is sensitive to the loss tangent of the material under test. This sandwiched structure is illuminated by the focused beam system to approximate plane-wave-like incidence, and scattering parameters measured. It is shown that the magnitude of response at the resonant frequency is linearly dependent on the loss tangent of the material under test for a certain range of loss tangents, and sources of error that limit sensitivity at lower loss tangents are explored. The effect of various FSS design parameters on loss tangent sensitivity is investigated, including sample thickness, FSS substrate thickness and complex permittivity, and FSS element pattern. Techniques for extracting complex permittivity from the scattering parameters of the focused beam measurement are presented, along with measured permittivity data from the FSS against a variety of well-known materials.







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