AMTA Paper Archive
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AMTA Paper Archive
Use of UASs for Outdoor Diagnostics of Large Antennas
Unmanned aerial systems (UASs) enable the in situ diagnostic of antennas operated in outdoor environments. Additionally, their flexibility introduces the possibility of performing several diagnostic methods. In this overview work, the challenges of performing outdoor measurements with UASs are discussed and some of the possibilities they introduce are outlined. The main diagnostics tool when performing outdoor far-field measurements with UASs is the so-called raster scan. This is the two-dimensional scanning of a limited portion of the measurement sphere about the main lobe. From the information raster scans provide, it is possible to retrieve antenna parameters critical for the deployment of large antennas, such as the Side Lobe Level (SLL) in all directions, as well as the First Null Level. Additionally, assuming a fine scan, i.e., sufficient resolution, the interpolation of any 1D cut for diagnostics is possible. Once a problematic cut is interpolated and assessed, it can be measured using the UASs and increasing the measured angular range for further assessment. Assuming the measured large antennas are reflector antennas, the finding of a higher-than-expected SLL may point to a problem with the positioning of the feed. Measuring with UASs allows for an iterative measurement-and-adjustment process directly in situ, which guarantees that the antenna’s performance is within the boundaries required by either the application or regulations. Additionally, the flexibility of UASs provide further advantages, such as the assessment of the impact of environmental reflections in the radiation characteristics by flying along the radial component of the measurement sphere and assessing the measured ripple, using a method similar to the Voltage Standing Wave Ratio (VSWR) method used for the characterization of anechoic chambers. With this technique, the impact of the environment of candidates for an antenna deployment site can be assessed before the antennas are installed, thus supporting the choice process, and reducing the risk of malfunction. The discussion of the introduced techniques is supported by measurements, and future possibilities and advantages are studied.
Highly Efficient Near-Field to Far-Field Transform for Polar Near-Field Scanned Data
The plane-polar approach for near-field antenna measurements has attracted a great deal of interest in the open literature during the past four decades [1, 2, 3, 4, 5, 6, 7]. The measurement system is formed from the intersection of a linear translation stage and a rotation stage with the combination of the axes enabling the scanning probe to trace out a radial vector in two-dimensions facilitating the acquisition of samples across the surface of a planar disk, typically being tabulated on a set of concentric rings. In its classical form, the probe moves in a fixed radial direction and the AUT rotates axially. However, with the ever more prevalent utilization of industrial multi-axis robots and uninhabited air vehicles (UAV), i.e. drones, being harnessed for the task of mechanical probe positioning, such systems offer the possibility of acquisitions being taken across non-planar surfaces. In this paper an accelerated, rigorous, near-field to far-field transform for data that was sampled using a polar acquisition scheme that is based on a Fourier-Bessel expansion  is developed and presented that can be employed in the above circumstances. This highly efficient, robust, transform enables near-field data acquired on planar, and non-planar, surfaces to be transformed to the far-field providing the acquisition surface is rotationally symmetric about some fixed point in the x,y-plane with z being purely a function of the radial displacement. The utility of the non-planar acquisition interval stemming from the ability to minimize truncation effects without needing to increase the measurement size. The transform efficiency stems from the utilization of the fast Fourier transform (FFT) algorithm with the rigor and robustness deriving from the avoidance of recourse to approximation, e.g. piecewise polynomial interpolation cf. . Numerical results are presented and used to verify the accuracy and efficiency of the novel transformation, as well as to confirm convergence of the requisite Bessel series expansion and sampling theorem.
A New Valid Angle Equation for PNF Measurements
A new far-field valid angle equation for rectilinear planar near-field measurements is presented. The new valid angle equation was derived by viewing the planar near-field to far-field transformation process as generating a set of pseudo plane waves by a synthetic phased array and subjecting the antenna-under-test to the radiation from this synthetic array. The synthetic phased array does not physically exist; rather, the array is formed during the post-processing of the planar near-field measurements. As part of this discussion, we present results from a numerical model, illustrating the total electric field present in the test zone due to the finite extent of the synthetic phased array. The new far-field valid angle equation accounts for the diffraction effects of the finite-sized synthetic array, and uses the industry accepted test-zone magnitude ripple of +/- 0.5 dB to limit the valid far-field angle for a fixed scan plane size. The resultant valid far-field angle computed with the new equation is compared against previously established and popularly accepted valid angle equations, such as the equations previously presented by Yaghjian, Maisto, and Joy [1, 2, 3]. Brief discussionsare offered on the measurement of low directivity antennas with a planar near-field measurement system, and on amplitude tapering of the near-field measurements to improve the quality of the pseudo plane wave. REFERENCES:  A. D. Yaghjian, "Upper-bound errors in far-field antenna parameters determined from planar near-field measurements, part 1: analysis," National Bureau of Standards (NBS), Boulder, Colorado, USA, vol. Technical Note 667, no. October 1975.  M. Maisto, R. Solimene and R. Pierri, "Valid angle criterion and radiation pattern estimation via singular value decomposition for planar scanning," IET Microwaves, Antennas & Propagation, vol. 13, no. 13, pp. 2342-2348, 2019.  E. B. Joy, C. A. Rose, A. H. Tonning, and EE6254 Students, “Test-zone Field Quality in Planar Near-field Measurements,” in Proceedings of the 17th Annual Meeting and Symposium of the Antenna Measurement Techniques Association, Williamsburg, 1995.
Tunable Cellular Desensitization using SD Card Data Transfer
For some time now the CTIA W-IoT OTA Reverberation Chamber Ad-Hoc Group has been looking for an OTA test artifact that exhibited repeatable narrow band cellular desensitization so that it could be used to perform round-robin testing between labs and investigate the impact of different test methodologies (e.g. reverberation chamber vs. spherical antenna pattern measurement in an anechoic chamber) on intermediate channel desense testing. Since device manufacturers aren't eager to provide devices with known problems, some alternative was required. Attempts were made to modify a device by removing shielding cans, only to completely degrade device performance across the entire spectrum. Using an external signal generator feeding a coupler at the DUT was also considered, but cable effects and equipment variations would have impacted repeatability in the variety of anticipated test environments. Creating a custom device with an embedded interference source met with cost and other practical limitations that stalled progress along that avenue. A member of the working group related anecdotal evidence suggesting that SD card communication within the phone was known to be a problematic noise source that could cause cellular desensitization. Initial investigation centered on the use of an off-the-shelf SD card testing app to try to generate uniform traffic, but none of the evaluated tools had an option for continuous testing. Focus then turned to developing a custom app for the purpose, but later changes to the Android operating system have deprecated the use of external SD cards, removing standardized support and making the development of a custom application impractical. Another possible solution for long duration interference would be to just play a video from the SD card, but the variability of a typical compressed MPEG video, both in content and compression level, would likely cause surges of data transfer with pauses in between. What was needed was a solution to cause continuous data transfer with a constant signature (i.e. sending the same data constantly) to ensure stability and repeatability. Thus investigation turned to creating a customized video file in an uncompressed format to address these limitations. This paper will show the results of this effort.
Full-Sphere Characterization of Low-Gain Antennas via Truncated Field Pattern Stitching
Our work proposes a novel method for obtaining full-sphere radiation patterns from truncated measurements. This is achieved by stitching partially overlapping truncated field patterns, which together cover the whole measurement sphere. Measuring an antenna in different orientations results in a misalignment between the measurements which is not perfectly known and needs to be accounted for in order to stitch the patterns together. Our method first makes use of an iterative procedure to compute spherical wave coefficients capable of accurately describing the truncated patterns. Recent investigation of properties of radiation patterns from iteratively obtained spherical wave coefficients under rotation and translation has shown that, while coordinate system manipulation introduces additional errors, these errors are contained predominantly in the region near the angle of truncation. They are thus negligible if a sufficient overlap between the truncated patterns exists. To align truncated patterns, a bounded minimization of the normalized mean squared error in the overlapping range between patterns is done, varying through a range of different translation and rotation vectors for one truncated pattern while keeping the other pattern fixed. Finally, the fixed and the optimally aligned patterns can be stitched together. The proposed method was validated on spherical wave coefficients (SWCs)-based models and EM simulation models for randomly chosen misalignment offsets. For the SWCs-based models, the normalized mean squared error (NMSE) after pattern stitching was found to be below -53 dB for all tested misalignment offsets. Similar results were observed in the case of EM simulation models as well, where the error was found to be below -52 dB for all tested misalignment offsets. In the final validation step, the method was tested on actual measurement results of two low-gain antennas. For each of the validation steps, potential sources of error are identified. The method demonstrates promising results in achieving full-sphere characterization of low-gain antennas in typical non-full-sphere measurement chambers.
Optimization of In-Vehicle Connectivity through Simulation-Augmented Antenna Measurements
As radiocommunications and internet-based services have become ubiquitous, customer expectations for infotainment capabilities and reliability in vehicles have largely increased. As such, the optimization of the distribution and orientation of antennas within the car is required to deliver the adequate connectivity performance. Yet, making direct measurements of electromagnetic field distributions radiated by structure-integrated radiofrequency transceivers is extremely tedious, if not practically and economically impossible. Recent papers introduced the approach of simulation-augmented measurements, appearing as a relevant solution to that problem. This method relies on a three-step approach: (i) measure the phasor electric field radiated by the standalone or part-integrated antenna module around the test sample; (ii) use an algorithm to calculate equivalent electric and magnetic currents over a surface closely encompassing the device under test (DUT); (iii) inject these currents as a Huygens source into a full-wave solver, where the complete scattering and absorbing environment is then taken into account. This paper presents the concrete application of this approach to the evaluation of the electric field inside a vehicle, based on separate measurements of WiFi and Bluetooth antennas. These measurements are performed using a spherical near-field system, with either the standalone antennas as DUT or the antennas embedded into the physical middle console of a car. The equivalent sources generated from experimental data are then imported into the virtual car model, and interior electromagnetic fields are computed using the Finite-Difference Time-Domain technique. The assessment is realized for various conditions without and with driver and passengers. The results are analyzed and limitations, as well as uncertainties of the technique are discussed.
Huffman Radar Site: Far Field Calibration and Testing Range
This paper will describe the Huffman Radar Site (HRS), a unique in-situ remote radio frequency calibration and characterization capability located at the Air Force Research Laboratory Sensors Directorate, Wright Patterson Air Force Base (WPAFB), OH. HRS is a part of the OneRY Range complex which consists of Indoor and Outdoor Ranges used to conduct test, evaluation, integration, and demonstration of novel sensing systems and technologies. The Outdoor Range has diverse capabilities at several sites distributed across the local area. Within the Sensors Directorate complex there are three 100 foot antenna towers: the South Tower holds a dish-based S-Band Radar, the East Tower holds a large digital phased array radar, and the West Tower is reconfigurable as needed based on customer requirements. The Huffman Radar site is used to validate the proper functionality of systems on these towers, conduct experiment witness testing, and provide calibration signals for phased-array antennas. The site is primarily used as a Direct Illumination Far Field Range source standing approximately 2 miles away with direct line of sight to the South, East, and West towers. The capability includes full polarimetric transmit from 2.9 to 3.5 GHz and receive from 800 MHz to 6 GHz with future plans to expand the frequency range. This paper will include the design, link budget, hardware implementation, test, and validation of the site. Preliminary far-field antenna pattern data and calibration results for the S-Band Radar system and digital phased-array radar system will be presented. The discussion will include challenges and successes in standing up a multi-function outdoor remote testing capability.
Single-Cut Phaseless Near-Field Measurements using Specialized Probes
This paper introduces a single-cut near-field measurement technique using only-amplitude data. The technique is based on measuring the near-field amplitude of an antenna over a ring, i.e. phi=0 cut, and performing a far-field transformation to obtain the radiation pattern over the same ring. This avoids the need of a full near-field spherical measurement if one is interested in only a few cuts of the far-field pattern. The lack of phase information complicates the field transformation. A common approach to solve this issue is to perform two near-field scans a different antenna-probe distance. This has the drawback of doubling the measurement time with respect to a complex measurement and a translation stage is required, which may be infeasible in some antenna measurement facilities. The technique proposed on this paper can retrieve the phase without measuring the near-field in two rings. Instead, the field is measured in one ring using a specialized probe. This probe provides partial coherence information between measurement samples, which can be exploited in a non-convex minimization solver to retrieve the phase of the near field with high convergence guarantee. The specialized probe can be implemented by using two separate probes connected two a dual channel Software Defined Radio (SDR) unit, so that the relative phase between measurement samples is known. Theoretical background of the proposed technique will be disclosed on the paper, along with simulated and measured transformation examples to demonstrate the potential capabilities: -Very fast near-field measurements (only one ring is measured instead of the full sphere). -Only amplitude information is required (no need of maintain stable phase reference, suitable for OTA testing). -No double-scan is required to retrieve the phase (measurement time reduced by half, no need for translation stage). -High reliability: Partial coherence provides a significant amount of independent information to the phase retrieval algorithm.
ISAR Image Gating Using Backprojection and Smoothed Reweighted L1-optimization
Inverse Synthetic Aperture Radar (ISAR) image gating for RCS extraction using backprojection is compared with image gating using smoothed reweighted L1-optimization in this study. The RCS of an object is measured by placing the object placed on a turntable which is rotated in an angular range while sweeping the frequency in the desired frequency range. A common model with isotropic point scatterers fixed in the object coordinate system is used in the ISAR imaging process. This model is used to define a forward operator. The ISAR image can be formed by operating with the backpropagation operator (i.e. backprojection), the adjoint of the forward operator, on the measured RCS. This robust method to solve the inverse problem gives an image with a resolution limited by the frequency bandwidth and the angular range. The RCS for a scattering feature is commonly determined by using the forward operator on the point scatterers in the image that are determined to belong to the scattering feature in ISAR image gating. L1-optimization is a method that can be used to get images with higher resolution and hence better separation of the different scattering features than backprojection. L1-optimization is well suited for naturally sparse ISAR images. One method to mitigate that the scatterers are restricted to a fixed grid is to use smoothed reweighting . L1-optimizations are performed consecutively in a few steps where a smoothed version of the previous solution is used to determine a weighting matrix for the next step. Smoothed reweighted L1-optimization gives images with better separation of the scattering features in the ISAR image. Simulated and measured RCS data are used to compare image gating using backprojection with gating using smoothed reweighted L1-optimization. The main conclusion of this study is that the RCS can be extracted for scattering features, not resolved in backprojection images, using the smoothed reweighted L1-optimization.  D. Pinchera and M. D. Migliore, “Accurate reconstruction of the radiation of sparse sources from a small set of near-field measurements by means of a smooth-weighted norm for cluster-sparsity problems,” Electronics, vol. 10, no. 22, p. 2854, 2021.
Stepped-Frequency CW RCS measurement in Semi-Anechoic Chamber
The RCS of a target can be estimated using electromagnetic modeling if accurate geometries and material descriptions are available. An exact numerical calculation often requires prohibitive processing times. Moreover, numerical predictions with approximate techniques are difficult as it is challenging to consider all the physical phenomena. Therefore, a suitable RCS measurement facility adapted to the target size and specifications is required to estimate the RCS of a given target and to validate the numerical predictions. In general, the measurement of RCS takes place in anechoic chambers that simulate free-space and far-field conditions and where the unwanted reflections (walls, target mount, objects in the range, and the target interactions) are reduced. This paper presents a broadband measurement and validation of the RCS of a metallic trihedral corner reflector of 30 cm sides when fully anechoic conditions are not available, and consequently, some undesirable echoes are present in the measurements. Firstly, the measurement facility calibration and the target calibration are outlined. A single target reference approach is performed using a sphere as a reference, and its scattering response is shortly described. Then, the measurement of the target is performed. After these steps, a processing procedure is applied to isolate the target response from the background and the close responses due to unwanted reflections. The post-processing technique and the acquisition system are presented and discussed. The measurements are performed at X band as a function of the viewing angle for vertical transmit and receive polarization. To validate the technique, the RCS of the trihedral corner reflector is numerically simulated using the Integral Solver (I-Solver) of CST, with a Gaussian excitation, for vertical transmit and receive polarization. Measurements are compared with results obtained from CST software and show a good agreement with the numerical simulations. This setup will be used for RCS measurement of different complex targets and compared with measurements from other facilities to analyze and evaluate the RCS measurement uncertainty.
Antenna Far-Field Characterization From Small Phaseless Dataset
The characterization of antenna radiation pattern is a costly but a mandatory step for the development of any radiating system. Nowadays, these assessments commonly call for the estimation of the whole 3D radiated far field. Such evaluation is a time consuming task and many research works aim to minimize its impact on the whole prototyping process. Of course, the expected accuracy on the results has to be preserved in the meantime. To this end, two main approaches can be cited: reducing the field acquisition time or decreasing the cost of the measurement system itself. The first point can be achieved by more efficient sampling strategies (number of field samples as well as their spatial distribution) and the second one by magnitude-only, or phaseless, measurements. Unfortunately, phase retrieval problems are notoriously hard to solve and these numerical difficulties may be mitigated by considering large field samples, which goes against the reduction of the field acquisition time. We propose a phaseless measurement procedure in far field that amounts to solve a convex optimization problem to overcome the numerical difficulties arising in characterization of antenna radiation patterns from magnitude-only samples. More specifically, a regularization is applied to the spherical wave expansion of the radiation pattern to promote physical solutions. The proposed approach enables an accurate characterization of the far-field magnitude pattern from a small number of samples with respect to usual phaseless procedures. The proposed approach is confirmed by several experimental validations done at IETR that will be shown at the conference.
Compact Range Measurements of Digital Arrays Leading to Outdoor RADAR Experiments
Digital array technology has advanced over the past few years to where it is now possible to build a large, wideband, multi-element digital array that can produce multiple steerable beams from the same aperture. These types of systems often have the analog-to-digital and digital-to-analog converters (ADC/DAC) and radio-frequency (RF) front end mated directly to the antenna, requiring a new measurement technique as the antenna under test (AUT) cannot be connected to traditional RF measurement equipment. Several years ago the Air Force Research Laboratory (AFRL) Sensors Directorate developed a custom 32 element uniform linear array using commercial off-the-shelf (COTS) low-noise amplifiers (LNAs) and a multi-channel digital receiver. Custom software was developed to perform automated element and digital beam pattern measurements using the compact range position controller. Hand tuned calibration parameters were calculated to form the digital beams. This work demonstrated feasibility of digital array measurement within a compact range environment. Recently AFRL has developed a highly integrated digital array with 1024 elements, LNAs, high-power amplifiers (HPAs), attenuators, phase shifters, transmit/receive switches, polarization switches, and eight multi-channel digital receiver/exciters. Measurements were required to not only test the functionality of the digital array, but also to build a calibration table for each element across the full frequency range of the array. The hand-tuned calibration method developed for the 32 element array was automated in order to build all of the necessary calibration tables for the 1024 element array. Calibrated beam patterns were also collected to analyze beam shape and pointing angle metrics. Following compact range measurement, the 1024 element array was relocated to a 100ft tower for outdoor RADAR experimentation where the collected calibration tables were successfully applied on the system. This paper will discuss challenges and successes in measurement of digital arrays within the compact range environment.
Acceleration of Over-The-Air Measurements Under Extreme Temperature Conditions Through Optimization of Air Flow and Thermal Efficiency
5G communications have supported the deployment of millimeter-wave beam-steering technologies at an unprecedented commercial scale. Mobile phones operating in frequency ranges above 20 GHz integrate antenna arrays and radiofrequency front-ends which control magnitudes and phases of signals to enable adaptive beam-steering. As per the 3GPP Test Specifications, a large number of tests covering the various operational modes of such devices are required in order to establish that the performance is adequate for guaranteeing the integrity of the mobile network. The defined measurement methodology relies on Over-The-Air (OTA) evaluations in Compact Antenna Test Ranges (CATR). As mobile wireless user equipment are practically utilized in diverse environmental conditions, a part of the test plan imposes spherical OTA measurements at extreme temperature conditions ranging from -10 to +55° C. Such temperatures indeed influence the active electronics in the device under test (DUT) and hence the beam-forming performance. This paper presents an innovative realization of a CATR with embedded thermal chamber supporting the above-described test capabilities. Inventive steps are introduced to solve the complex engineering problem of allowing fast temperature ramps to go through the complete range in a few minutes, while allowing two-axis rotations of the DUT, and all of this with preventing damages from the anechoic chamber and positioner and limiting the radiofrequency impact of the thermal testing chain. Thermal and air flow simulation and measurement results are presented, establishing how the design allows sustaining high pressure with low air leakage. Measurements of the quality of quiet zone with and without thermal enclosure demonstrate the limited RF impact of the introduced materials encapsulating the DUT. The derived solution is applied to measurements of a commercial 5G mobile phone, illustrating the influence of the environment on the DUT operation and corroborating the need for such test scenarii.
Design and Measurements of a Small End-fed Normal Mode Helical Antenna with Integrated Microstrip Structure
Compact omnidirectional antennas are highly sought for a multitude of present-day wireless applications such as smart car keys, radio frequency identification (RFID) tags, tire pressure monitoring system, hand-held communication devices, and high-temperature harsh-environment wireless sensors. This paper discusses the performance and the unique challenges in measuring the radiation performance of a compact (~1/25th to 1/10th of a wavelength) helical and microstrip combined structure operating as a normal mode helical antenna (NMHA) around 300MHz. The helical wire structure (27-turn, 37mm high and 6.2 mm wide) is connected to the end of a 50 Ωmicrostrip line fabricated on 1.5 mm thick FR4 substrate. The microstrip line provides a ground plane to the helical structure, serving as an integral part of the radiating element. A tiny 1:1 balun transformer was used to partially decouple the integrated NMHA from the external sheath of the coaxial cable connected to a vector network analyzer, thus allowing proper NMHA impedance measurement. The NMHA S-parameters were simulated on two different platforms, ANSYS-HFSS and WIPL-D Pro, and compared to the frequency of the measured structures, with all simulations and measurements agreeing within 3.5%. Varying the length of the ground plane associated with the microstrip line from 13 mm to 76 mm resulted in the decrease of the measured NMHA operational frequency by 3.2%. The measured impedance of the fabricated NMHA (including the balun) was close to 50 Ω for the 51 mm long line without the need of additional matching circuit. The measured transmission loss for two identical antennas (each 26 cm3) placed about 1 m apart was 22 dB. This performance is comparable or better than the coupling between much larger antennas currently used in harsh environment power plant applications, such as suspended plate antennas (42,500 cm3) or planar inverted F-antennas (11,800 cm3) operating around the same frequency. In addition, the proposed NMHA structure can be implemented using substrates and wires capable of operation at temperature above 300 °C, which constitutes an appealing solution for high-temperature harsh-environment applications such as those found in industrial machinery, metallurgic industry, power plant boilers, and turbine engines.
Further Exploration of the Holographic PNF Filter
Abstract— A 2021 AMTA paper introduced a 3D holographic filtering algorithm optimized for the planar near-field (PNF) geometry. This filter has been shown to have an excellent combination of AUT-signal preservation, stray-signal rejection, and processing speed. It requires only the sampling of a conventional PNF measurement, along with a specified 3D boundary surrounding all of the AUT’s possible radiating sources. The 2021 paper suggested some topics for further investigation, specifically the optimal Z spacing through the 3D hologram and the X- and Y-widths of the blanking window’s tapered extension, and those are investigated here. This paper also explores the combination of filtering and probe correction, since the measured convolution of probe and AUT spatial distributions will be wider than that of the AUT by itself. Finally, additional comparisons are made to the more traditional spherical-mode-truncation approach with different synthesized constellations of stray-signal radiators. Keywords: modal filtering, spatial filtering, holographic filtering, stray signals, planar near field  S.T. McBride, P.N. Betjes, “Holographic PNF filtering based on known volumetric AUT bounds,” AMTA 2021, Daytona Beach, FL.
Compact Ultra-wideband Ground Penetrating Radar Antenna Design
A compact ultra-wideband (UWB) ground penetrating radar (GPR) antenna has been developed for extraterrestrial subsurface sensing from 130 MHz to 2000 MHz continuously. The antenna was designed as a payload of a new cube rover developed by Astrobotic for lunar exploration. The maximum payload dimensions are 58cm x 20cm x 18cm (LxWxH). The lower frequency bound of 130 MHz allows for deeper penetration depth and the upper frequency bound of 2000 MHz provides a large bandwidth for achieving a high depth resolution. Designing such compact UWB GPR is challenging for many reasons: small antenna volume relative largest wavelength, wide operating frequency range, minimization of clutter from antenna and surface reflections, and maximization of the transmission of radar signals through the air-soil interface. The proposed antenna design adopts the dielectric loaded horn-fed bowtie dipole design. The antenna operates primarily from the dielectric-loaded horn section at frequencies above 400 MHz where the ULTEM 1010 material is used to load the horn thereby increasing its electrical size and controlling the radiation patterns. Below 400 MHz, this antenna functions as a folded dipole where the entire 3-D conducting arms and the conducting top plate contribute to the antenna operations. Special RF choke designs are also developed to suppress undesired cavity modes which are excited in the cavity behind the feed position. In addition, a wideband microstrip balun circuit board was designed and integrated directly on to the antenna arm for connecting the antenna’s balanced 120 ohm port to a 50 ohm coaxial connector without being affected by high-G vibrations and shocks during typical rocket launches. An antenna prototype was fabricated, and its antenna performance was measured with a good agreement with simulation predictions. This paper will describe the antenna specification, operating principles, as well as measurement and simulation performances.
Experimental Validation of Non Invasive SAR Evaluation from Measurements and Numerically Assisted Post Processing
During the last few years, the increasing use of wireless equipment has raised the quantity of radiation energy to which human bodies are exposed. For this motivation, an evaluation of the Specific Absorption Rate (SAR) for persons is fundamental to determine the amount of radiation that human tissue absorbs and to comply with human safety regulations. Standard testing methodology consists of measurements with robot-based scalar/vector near-field probes and post-processing. The probe acquires the field level inside a phantom filled with liquid to ensure compliancy with certification standards. Although accurate, this technique could be extremely time-consuming, especially with the arrival of new frequency bands, new standards (5G, Wi-Fi 7), and the requirement to test different beams directions for beam-forming MIMO configuration. Another testing methodology, used especially for pre-assessment, consists of a full simulation of the radiator in the presence of the phantom, but this implies that the full wave model of the device is available, and this is rarely the case. To overcome the above-mentioned limitations, an alternative technique presented in this paper can be applied. This is based on a standalone measurement of the radiating device, that is post-processed with the method of the equivalent currents to generate NF source (in the form of a Huygens box). The SAR values inside the phantom are assessed using a non-invasive procedure with the assistance of a numerical simulation tool. Such method represents a fast procedure for pre-analysis of device prototypes, allowing to perform the conclusive testing only on the final device to verify the compliance with the regulations. The methodology is here experimentally validated on a dipole radiating in a presence of a phantom model by comparison of numerical simulated data and a reference measured data by a MVG ComoSAR V5 system.
Millimeter-Wave S-Parameter Measurements with a Vector Field Analyzer in Antenna Measurement Systems
Antennas are used in virtually all wireless, communications and radar systems. As key elements in these applications, antennas play a crucial role in determining the system’s overall performance. This makes accurate antenna characterization essential for any wireless application. Traditionally, electrically large antenna ranges are not equipped to perform return loss measurements and thus a separate benchtop vector network analyzer (VNA) setup is required for measuring reflection coefficient or VSWR of an antenna under test (AUT). In this paper, we demonstrate the two-port S-parameter measurement capability of the NSI-MI Vector Field Analyzer™ (VFA) and how it can be used to integrate return loss measurements in an antenna range. The VFA’s ability to perform multi-channel vector (amplitude and phase) electrical measurements and its long cable support using remote mixer interface (RMI) modules makes it well suited for antenna characterization, especially in electrically large measurement systems. For this experiment, we selected three known millimeter-wave components as devices-under-test (DUT) and measured the S-parameter matrix for each. These WR-10 band measurements were made using the VFA with Virginia Diodes VNAX frequency extension modules. Results are compared with Keysight’s N5225A performance network analyzer (PNA) using the same set of frequency extension modules for verification. Millimeter-wave S-parameter measurements taken on VFA and PNA setups for all DUTs are compared based on three factors: Repeatability, reproducibility, and measurement comparison. The variations between successive measurements are presented in graphical form to compare repeatability of both instrument setups. Reproducibility results are compared to show the difference between independent repeat measurements taken on both instrument setups. Error distribution comparison is presented for reproducibility test data to compare the measurement variations contributed by random sources for both instrument setups. Measurement comparison result shows the total difference between independent VFA and PNA measurements taken for each DUT. Keywords — Millimeter wave measurements, Scattering parameters, Calibration, Network Analyzers, Antenna measurements
A Benchmark Biconical Antenna for Standardized Antenna Measurement and Simulation: Contribution to IEEE P2816
The IEEE Antennas and Propagation Standards Committee (APS/SC), sponsored by the IEEE Antennas and Propagation Society (AP-S), is currently developing a recommended practice tailored for the modeling and simulation of antennas (IEEE P2816). Different numerical modeling techniques such as the finite element method (FEM), integral methods e.g. method of moments (MoM), the finite difference time domain (FDTD) method and the transmission line matrix (TLM) method are described. In addition, benchmark problems are also considered to ease the use of the recommended practice. For example, the biconical antenna is proposed as a benchmark problem for the numerical simulations using the above methods. Several international laboratories have already performed the numerical simulations of this biconical antenna. To confront the theoretical and numerical results with measurements, the same biconical antenna was proposed for fabrication and inter-laboratory measurement campaign. Herein, the fabrication and initial measurements of the prototype are discussed. An important difference between the theoretical, simulated and fabricated antenna is the feeding point. In theory, it is considered infinitely small whereas in the proposed numerical model, the gap in-between the two cones is assumed to be 0.3 mm. In practice, such a small gap cannot be enforced during the fabrication process. Since the off-the-shelf available SMA connector has a minimum diameter of 0.7 mm, the minimum diameters of the central and outer conductors of the 50W feed-line (Teflon filled) section were kept at 1 mm and 3.34 mm, respectively. In the actual fabrication, only a gap of 4 mm could be achieved in-between the two cones. An external quarter-wave skirt is further used as a balun for reasonable impedance matching. A Rohacell section is used to hold the arms of the antenna which are heavier. The realized prototype, therefore, differs significantly at the feeding point. The relevant simulation and experimental results are presented.
Electromagnetic Field Transformations of Near-Field Data Without Global Reference for Magnitude and Phase
Over the past decades, near-field (NF) measurements have been established as a reliable alternative to direct far-field (FF) or compact-range measurements for the verification of radiation properties of antennas. Quantities of interest typically include the FF characteristic obtained by means of an NF FF transformation (NFFFT), which is a computational post-processing step applied to the NF data. Common NFFFTs work with time-harmonic data and require the acquisition of magnitudes and phases of the NF samples with respect to a common reference signal. In other words, classical NFFFTs require the observed magnitude and the observed phase data to be drift-free during the time span of the complete measurement. With increasing measurement frequency and exposed or complex measurement setups in uncontrollable environments, e.g., as encountered in outdoor NF antenna measurements with unmanned aerial vehicles (UAVs), phase stability quickly becomes a limiting factor. Therefore, nonlinear phaseless NFFFTs have been developed that do not require any phase information, which, however, heavily rely on accurate and globally consistent magnitude information and are notorious for their unreliable behavior. Recently, a linearized and reliable NFFFT operating on locally consistent, i.e., relative, phase information has been reported. By using local phase differences within the NF data, the method becomes immune to phase drifts of the reference signal. However, in real-world measurements in uncontrolled environments, drifts in the measured power or magnitude may occur as well, e.g., caused by temperature variations during UAV flights, which render common phaseless NFFFTs useless. In particular, this prevents the use of the otherwise reliable linearized transformation. We investigate NFFFTs requiring a variable degree of global synchronization. In particular, a linearized transformation utilizing only relative, i.e., locally synchronized, information, both in magnitude and phase, is presented. It is shown that the transformation yields similarly accurate results as transformations employing global data, while being immune to magnitude and phase drifts. Furthermore, we compare the overall benignancy of a complete set of retrieval problems: completely phaseless, magnitudeless and mixed relative/global magnitudes and phases. Transformation results for simulation data illustrate the accuracy and suitability of the transformations with relative data, even for electrically large problems.
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