Single Antenna Dual Circularly-Polarized Chipless RFID Tag Reading Methodology
RFID technology can be classified as active, passive, or chipless based on tag design. While active and passive tags rely on electronics to modulate and return the irradiating signal, chipless tags rely on geometry to produce a distinct signal which is viewed in the time-, frequency-, or spatial-domain. Within the field of chipless RFID, frequency-domain tags are the most popular and different design approaches with different polarization schemes have emerged. Primarily, these tag design approaches can be categorized as linearly polarized (LP), orientation insensitive (OI), and cross-polarizing. This diversity in tag designs leads to a variety of requirements for reader antennas and also leads to current reader antennas being non-universal (i.e., reader antennas can only be used for specific types of tags rather than all tags). LP and cross-polarizing tags require that the reader antennas have their polarization be perfectly aligned with that of the tag, as a small tag rotation with respect to the reader can greatly affect the response. Cross-polarizing tags additionally require either a dual-polarized reader antenna or a bistatic measurement setup. While specialized chipless RFID reader antennas and bistatic reading schemes have been developed, there are still limitations with these approaches, such as requiring tag/reader polarization alignment, hardware complexity, mutual coupling, and other related issues in bistatic setups. Tag interrogation with circular polarization (CP), however, accommodates the polarization diversity of different tag designs, while also relaxing the tag/reader relative alignment requirements.
This work proposes a novel chipless RFID tag reading methodology that utilizes a single existing dual CP X-band (8.2-12.4 GHz) septum polarizer antenna as a universal (i.e., all types of tags) frequency-domain reader antenna that can generate and receive both right-hand and left-hand CP, as well as LP (through mathematical manipulation). This antenna has been optimized for this application and its specifications are provided. Additionally, through post processing the rotation offset of LP tags can be determined, a capability which can then be used for rotation sensing. To demonstrate the tag reading methodology and the rotation determination capabilities of the method, simulation and measurement results are presented for LP and OI tags.
A Validated Model for Non-Line-of-Sight V2X Communications
As vehicle autonomy and active safety features become more advanced and ubiquitous, it becomes clear that vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication, together under the umbrella of vehicle-to everything (V2X), can help to enable these autonomous and active safety features by providing useful sensing and localized network capabilities. One problem facing the development of such communication networks is the difficulty involved in accurately predicting the key performance indicators (KPIs) associated with it - e.g., packet error rate (PER) and received signal strength indicator (RSSI) - for an arbitrary setup of antennas and occluders.
In this paper we will present a model for V2X communications using a commercial simulation software, Altair's FEKO and WinProp suites, for predicting electromagnetic field intensity at microwave frequencies under different scenarios of antenna placement and occluder setup. We will also validate this model against field data collected using Cellular V2X (C-V2X) radios.
We considered three distinct non-line-of-sight (NLOS) scenarios involving a pair of vehicles operating a CV2X inter-vehicle communication channel:
1. A stationary vehicle located directly behind a shipping container and a moving vehicle performing loops around it.
2. So-called urban canyon with tall metal walls on either side of the moving vehicle for a portion of its loop around the stationary vehicle.
3. Fully-covered tunnel, where both vehicles are moving, one leading the other around a loop and through a tunnel.
We will discuss the simulations of these three scenarios in light of real-world data taken from field tests of identical scenarios, for the dual purposes of validating the simulation model against real-word data, and developing a model to predict the PER and RSSI at any theoretical receiver location. Together, these will allow us to perform a simulation for any arbitrary potential (NLOS) scenario and get an estimate of the channel PER and RSSI between any two spatial points, thereby help in developing standards for V2X communications, optimizing antenna placement for vehicles and infrastructure, and better understanding of these V2X systems overall.
Measuring G/T with a Spherical Near-Field Antenna Measurement System via the CW-Ambient Technique
In modern systems where RF front ends are tightly integrated, the parameters of passive aperture gain and active electronics noise figure become difficult to obtain, and, in many cases, impossible to measure directly. Instead, the parameter referred to as Gain over Noise Temperature, or G/T, becomes the performance metric of interest. Recently, the antenna measurement community has seen an increased demand to use near-field measurement systems for determining G/T values. Papers presented at AMTA over the past few years have shown that it is possible to determine G/T values using measurements taken in planar near-field antenna ranges. The CW-Ambient technique was one of the techniques proposed for computing G/T values by utilizing planar near-field measurements [1,2].
In this paper, we show how the CW-Ambient technique can also be applied to calculate G/T values in spherical near-field antenna measurement systems. This paper provides a brief summary of the CW-Ambient technique, and then presents the procedure and equations required for computing G/T using a spherical near-field system.
To validate the recommended procedure, we compare predicted and measured G/T values for a separable unit under test (UUT). Since the passive aperture for this UUT is separable from the back-end active electronics, we measure the aperture gain of the UUT and the noise figure of the back-end electronics individually, and then compute the composite G/T value for this assembly. We then compare these composite values against G/T measurements from a spherical near-field antenna measurement system. We summarize these comparisons and provide conclusions regarding the validity of using a spherical near-field system to measure G/T.
Reduced Azimuthal Sampling for Spherical Near-Field Measurements
This paper investigates on the use of under-sampling over the azimuthal dimension to reduce measurement time on spherical near-field scanning. This means that the number of angular phi samples is reduced, which allows to reduce the number of positioner steps, obtaining measurement time savings virtually proportional to the number of samples reduction. Of course this under-sampling introduces an error, which can be interpreted as an aliasing term over the retrieved Spherical Wave Expansion of the Antenna Under Test (AUT). The axial symmetry of the vast majority of antennas allows the application of significant under-sampling ratios with little aliasing errors. However, this information is not a priori known due to the lack of a reference AUT radiation pattern, or in the case of malfunctioning antennas with degraded symmetry.
Here we proposed a measurement procedure for the exploitation of the AUT axial symmetry. The procedure consists on an iterative AUT measurement with increasing number of azimuthal cuts. As the number of cuts increases, the aliasing error decreases, thus obtaining the final radiation pattern with a lower uncertainty. We will introduce an aliasing error estimator, which estimates the error caused by the under-sampling without any a priori knowledge of the AUT. This estimator can be used as a stopping criterion of the iterative measurement procedure when the desired accuracy is achieved.
The proposed technique will be demonstrated using different antennas, showing considerable reductions in measurement time with low errors in the transformed far-field pattern, and with the guarantee that the error is below a given threshold thanks to the derived estimator.
Examining and Optimizing Compact Antenna Test Ranges for 5GNR OTA Massive MIMO Multi-User Test Applications
Direct far-field (DFF) testing has become the standard test methodology for sub-6 GHz over the air (OTA) testing of the physical layer of radio access networks with the far-field multi-probe anechoic chamber (FF-MPAC) being widely utilized for the test and verification of massive multiple input multiple output (Massive MIMO) antennas when operating in the presence of several users. The utilization of mm-wave bands within the 5th generation new radio (5G NR) specification has necessitated that since the user equipment should, preferably, be placed in the far-field of the base transceiver station (BTS) antenna, excessively large FF-MPAC test ranges are required or, the user equipment is paced at range-lengths shorter than that suggested by the classical Rayleigh criteria or, a modified compact antenna test range geometry must be developed and utilized.
This paper presents a novel design for a new compact antenna test range (CATR) design that uses a parabolic toroid as the main reflector. The folded optics utilized within this design possesses superior pseudo-plane wave scanning capabilities than those available from equivalent, classical, point source offset parabolic reflector CATR designs. This wide-angle scanning capability is a crucial feature for successful over-the-air testing and measurement of mm-wave 5G NR Massive Multiple Input Multiple Output (MIMO) antenna systems within multi-user applications providing 60 degrees of azimuth scan and 15 degrees of elevation scan to the incoming plane wave at the AUT. CATR quiet-zone results are presented, compared and contrasted with more classical designs before results of the effects of the CATR test channel on a number of commonly encountered communication system figures of merit in wide scanning cases are presented.
Evaluation of Integrated Antenna Performance through Combined Use of Measurements and Full-Wave Simulation
With trending topics such as car-to-car communication or automotive radar, the evaluation of antenna performance, including the impact of the larger structure in which it is integrated, is a topic of rapidly growing interest. Implementing large anechoic chambers, supporting e.g. full-vehicle testing, is definitely an approach, which can serve as a reference. Nevertheless, the cost and complexity associated to such antenna test ranges are quite prohibitive. Some measurements eventually get close to impossible, as they involve unreasonable testing times and efforts, due, for instance, to the size to wavelength ratio of the DUT.
This paper introduces an economic alternative, based on the convergence of measurements and full-wave simulation. The described method relies on a three-step approach: (i) measure the phasor electric field radiated by a smaller-size antenna module over a surface enclosing the test sample; (ii) use an algorithm to calculate equivalent electric and magnetic currents over a surface closely surrounding the DUT; (iii) inject these currents, as a Huygens source, into a full-wave solver, where the entire supporting structure is then taken into account.
The relevance of the technique is demonstrated with a complex example, where a multilayer 8x8 antenna array frontend module operating at Ka-band is both evaluated numerically and experimentally. Two sets of simulations are created, where either the detailed numerical antenna model or the equivalent current surface is integrated under a radome at the nose of an aircraft. Key radiation parameters (peak gain, side lobe levels, etc.) are compared, showing the relevance of the approach. Its limitations and uncertainties are also discussed.
Application and Improvement of Fast Antenna Characterization via Sparse Spherical Harmonic Expansion
The characterization of 3D radiation pattern of antennas using spherical measurements techniques is nowadays a common testing procedure. However, the well-established technique using Spherical Harmonics (SH) can be time consuming since the required number of field samples is proportional to the square of the electrical length of the antenna. The main solution is to reduce as much as possible the number of field samples to reconstruct the antenna pattern. This can be achieved by either exploiting prior knowledge about the antenna or leveraging general properties common to the fields radiated by antennas field. Our approach uses the sparsity of the spherical harmonic expansion of the field radiated by antennas and only requires the maximum electrical dimension of the antenna and its frequency to set the truncation order of the harmonic series. It nonetheless requires the proper tuning of specific parameters to ensure that the antenna radiation patterns, reconstructed from a small number of sampling points, is accurate enough.
First, the minimum number of field samples to ensure a proper interpolation must be set, the reconstruction accuracy (for undersampled data) is linked to the number of significant modes involved in the expansion: roughly speaking, the sparser, the better. Second, the data fidelity term and more specifically the error tolerance, common to all regularization scheme such as the sparse one at hand, must be carefully chosen. Finally, an efficient post-processing procedure, based on the rotation of the antenna, is proposed for improving the sparsity of its SH spectrum, and thus enables extracting as much information as possible from a given fast measurement data set.
All these points are validated numerically and experimentally on spherical near and far-field data.
Spherical Test-Zone Field Measurements of a Compact Antenna Test Range
A method to solve the far-field distance problem is the usage of reflectors in order to transform the spherical wave from the feed antenna into a plane wave which ultimately leads to the same far-field condition but in a more compact way. Therefore, these facilities are called compact antenna test ranges (CATRs). However, the finite size of the reflector(s), despite edge treatment, is the main cause of erroneous signals impinging into the test zone in which an antenna under test (AUT) is characterized. Especially at lower frequencies, every further structure inside the test chamber, although covered with absorbers, is an additional source of scattered signals.
One method of correcting stray signals and to improve the AUT measurement accuracy is to compensate the non-ideal test-zone field (TZF) via spherical wave expansion (SWE). For this technique, a complete description of the TZF, i.e. measurements on a closed surface around the test zone, is required. The most convenient approach is to use a spherical scanning surface. In ranges with a roll-over-azimuth positioning system, the spherical scanning can be realized by an additional measurement arm. Using the chamber positioning system, thus, strongly reduces the required additional hardware and makes spherical scanning of the test zone a practical approach. With the knowledge of the incident unwanted field components, AUT measurements carried out in the corresponding test zone are correctable.
Spherical near-field measurements of the test zone created by the CATR installed at the Institute of High Frequency Technology, RWTH Aachen University are performed at a frequency of 2.4 GHz using a simple scanning arm. Reliability of the results is ensured by comparing the measurements to full-wave simulations of the CATR. The radiation pattern of a base transceiver station antenna serves as a test case and is subsequently corrected for the erroneous signals using the SWE.
The Cost of Accuracy - Mechanical Systems
Accuracy in a measurement campaign is dependent on many factors. Some of these factors are in the physical components used, the requirements of the electromagnetics involved and the procedural requirements of the campaign. This paper will focus on how the mechanical accuracy of the equipment can impact total cost. The current stage in the life cycle of the AUT (design, production, repair) also impacts total cost. The affordability of the accuracy in terms of more costly equipment, calibration processes and operator and test range time may be the determining factor. Throughput needs may limit the accuracy that can be obtained. The accuracies required for each metric must then be evaluated against the accuracy of an available test range(s) or the renovation of an existing range or construction of a new range to meet the accuracy requirements. Two case studies included in this paper are: 1) the improvement of the positioning accuracy of a rotator via custom hardware and calibration for a severe global positioning accuracy specification and 2) the improvement of the planarity of an X-Y scanner system for use at increasing frequencies.
A Low-cost and In-field Antenna Characterizing Method Based on Statistics Measurement
With the development of the Internet of Things (IoT) technology, the antenna becomes increasingly integrated and miniaturized. Vector Network Analyzer (VNA) is a standard instrument for characterizing antennas. However, IoT devices are small and scattered installed, which makes it costly to carry out in-field characterizing on IoT devices. Integrating an antenna characterizing system into IoT devices can release the difficulty significantly, but characterizing antennas usually requires a high-performance Analog to Digital Conversion system, which is expensive and power-consuming.
However, the antennas are not required to be tested frequently, which means this is not a real-time application. Therefore, this paper proposes a method that can realize time-domain reflectometry based VNA with just a comparator or differential receiver. In this system, impulses are sent into the antenna, and the frequency response is captured by analyzing the time-domain reflection. Analog to Probability Conversion (APC), Probability Density Modulation (PDM), and Equivalent time sampling (ETS) concepts are used to reduce the real-time performance requirements of the ADC system, so that the cost and power consumption can be tolerable on a tiny IoT device. With this technology, the antennas could be characterized in-field and remotely, making the maintenance easier. This technology is implemented with Xilinx ZYNQ Ultrascale+ series FPGA. Two antennas are tested, and the experimental results show that the system can successfully measure the radiofrequency. The sampling rate is set to 89.6Ghz, and a micro-volt level voltage resolution is achieved. Since the essence of technology is a trade-off between time and performance, the sampling rate and resolution can be further increased theoretically according to specific applications.
Increasing 4-D Imaging Radar Calibration Accuracy Using Compact Antenna Test Range
Advanced driver assistance systems (ADAS), such as blind spot warning and braking assistants, have been in use for years to improve road security. ADAS are currently further promoted through the autonomous driving trend. Due to their cost / performance trade-off, the automotive industry perceives 4-D high-resolution radar sensors, as one of the backbones of autonomous driving. With human safety being at stake, the topic of calibration of these sensors is obviously of the utmost importance.
Performing an accurate calibration requires a test condition where the target is in the far-field of the radar under test (RUT). Due to the requirements for angular resolutions, 77 / 79 GHz radars with 15 cm radiation aperture or more are quite common. Applying Fraunhofer formula then results into a necessary measurement range length of 11.5m. Because of the high cost of ownership of an adequate anechoic range, radar manufacturers usually limit their measurements to the strict minimum and try to simplify the calibration process. A typical approach is to go for a diagonal calibration where the target is always at boresight for each beam-formed pattern of the RUT. This technique however delivers a sub-optimal compensation of the RUT biases. In particular, it creates high peak-to-side-lobe ratios (PSLR), where energetic echoes are observable in directions of side lobes of each beam.
This paper introduces a new system for radar measurements, made of a short-size focal length offset-fed compact antenna test range (CATR), interfaced with an analog echo generator. With a chamber size of only 0.9 m x 2 m x 1.6 m, the setup has been designed to test apertures up to 30 cm size. The quality of the quiet zone achieved is discussed in the paper, as well as various uncertainty contributions relating to radar measurements. Tests are presented which involve a latest generation 4-D imaging radar on chip (RoC). Results obtained in the CATR are compared to a reference 7 m far-field range. Diagonal and full angular calibrations of the RoC are carried out and analyzed, demonstrating an improvement of 10 dB PSLR when the target is swept over the complete azimuth region.
Near-field testing with a 8.9x1.6 m2 planar scanner at Christiaan Huygens Laboratory (CHL)
A near-field scanner has been upgraded, maintaining mechanical hardware more than 65 years old and extending it with suitable computer control to enable an 8.9x1.6m^2 scanplane. Already in 1957 X-band phase accuracies within 3 degrees were reported (ref.1). The facility is computer controlled, with servo's to enable position and polarisation control and a Rohde and Schwartz network analyser in the loop. It is positioned in an area near the main workshop and runs proprietary software for control, acquisition and transformation. An old satellite antenna has been aligned as Antenna Under Test (AUT) and measured near 12 GHz. It was measured before as reported in (ref.2). The antenna is an engineering model of an antenna used on the OTS satellite in mid 80's. It has a few properties which are worthwhile to use for inspection, to enable to get insight into scanner properties and transformation results. Deviation between electrical and mechanical axis, low cross polarisation, orthogonal channels and specific input impedance can be mentioned as points to verify and to control with verification measurements exploiting symmetries and flip-tests, rather than ticking off in an 18-term error budget usually adopted. Direct gain measurements have been established. The probe can be selected, either an open-ended waveguide or a circular waveguide with annular corrugation as probe for instance. It involves related discussion of probe correction.
The first results show acceptable information for the facility, with initial comparison to previous results for pattern and absolute gain. It has allowed to survey alignment, assess scanner control properties and assess microwave component properties - with interest into direct gain measurements.
A short historical description for the facility (ref.1) and antenna precedes a main discussion of the followed procedures and obtained results for the AUT with related discussion.
RFID in Packaging Surveillance: Impact of Simulation tools in design, coverage planning and placement of Smart readers along the supply chain.
Internet of things (IoT) has impacted global supply chains in terms of improving performance and increasing productivity. Modern IoT devices such as Radio Frequency Identification (RFID) tags play a pivotal role in packaging surveillance and monitoring of products as it moves along the supply chain. Additionally, recently these tags are equipped with sensing elements that convert the traceability-centric supply chain to value-centric by enhancing visibility of the nature of product as it moves along the supply chain. To utilize the full potential of such IoT technologies, an intelligent network planning is a pre-requisite that ensures good and reliable communication between the RFID tags, the reader, and the cloud. Traditionally, optimal positioning of these RFID devices to obtain a complete coverage is a difficult task that requires careful planning and physical experimentation which involves investing significant time and financial resources. To avoid such limitations, computer aided simulation of positioning the devices allows engineers to explore different scenarios to ensure complete coverage within a given area within a short time frame. The design and coverage planning of various entities of the RFID infrastructure is pivotal in realizing value-centric approach towards supply chain management.
In this paper, two different case studies are presented that utilizes a grid-based optimization approach for coverage planning of passive and active RFID tags. For this purpose, first, a high-frequency solver FEKO is used for designing the RFID reader and tag antenna. Second, for the coverage analysis, a wave propagation tool WinProp is used for optimizing the position of the RFID readers and tags. The antennas are designed to operate at a UHF RFID frequency (915 MHz) and a semi-industrial warehouse setting is used for network planning. The details of the design and simulation of the individual entities of the RFID infrastructure along with two different scenarios for coverage analysis of active (battery-powered) RFID tags and passive (battery-less) RFID tags are presented in the paper. These simulations are the first step towards realizing the value-centric supply chain management approach.
Examination of EMC Chamber Qualification Methodology for Applications above 1 GHz Using Frequency Domain Mode Filtering
Anechoic chambers used for Electromagnetic Compatibility (EMC) measurements above 1 GHz are qualified based on the Site Voltage Standing Wave Ratio (SVSWR) method as per the international standard CISPR 16-1-4. The SVSWR measurements consist of a series of scalar measurements using a dipole-like antenna placed along several linear transmission paths that are located at the edge of the quiet zone (QZ). The measurement process is conceptually similar to measuring VSWR using a slotted line and a moving probe. A full set of tests is time consuming because of the number of positions, antenna heights, polarizations and frequencies that are generally required. To reduce the test burden, the SVSWR method intentionally under-samples the measurement by requiring only 6 measurement points along each 40 cm long linear path to characterize the standing wave. As a result, the test results are generally overly optimistic. At microwave frequencies (note the upper frequency limit is 18 GHz), this under-sampling becomes far more pronounced. In this paper, we explore the effectiveness of using Cylindrical Mode Coefficients (CMC) based frequency domain mode filtering techniques to obtain the VSWR. Here, we place the test antenna on the outer edge of the turntable to obtain a full rotational pattern cut of amplitude and phase data. The antenna is then mathematically translated to the rotation center, whereupon a band-pass filter that tightly encloses the test antenna mode spectrum is applied. The difference between the mode filtered antenna pattern and the original perturbed pattern is attributed to chamber reflections. The measurement is comparatively easy to implement with no special positioning equipment needed. In this paper we present measured results taken from two horizontal polarization measurements (where the antennas were oriented 90 degrees from each other), and one vertical polarization measurement. For an EMC chamber test at a fixed height, an entire measurement campaign reduces to taking three vector pattern cuts. In contrast to the conventional technique, the proposed novel method does not suffer from positional under-sampling, so it is well-placed to be applied at microwave frequencies and above.
Combining Measurements and Simulations for Antenna Coupling Analysis
In numerical simulation of antenna problems, accuracy of antenna representations is essential to ensure the reliability of results. Integration of measured Near Field (NF) representation of antenna in Computational Electromagnetic (CEM) solvers opens new perspectives to solve this problem.
Moreover, it is possible to replace the simulated model of the antenna by a measured model, which represents the real antenna. No additional information about mechanical and/or electrical design of the antenna is required by the numerical solvers. Indeed, the measured NF model in terms of equivalent currents already provides a complete and detailed representation of the antenna itself. The applicability of this approach has been already studied for complex and/or large scenarios, antenna placement, scattering problems and EMC applications.
Another interesting use of the combination between measurement and simulation is to enhance the evaluation of the antenna coupling. Previous investigations have been carried out on an H/V polarized array of three identical cavity-backed cross-dipole antennas. In this study only the radiation pattern of the central element of the array was measured (in a stand-alone configuration). Its representation in terms of equivalent currents was integrated in the simulation, for the calculation of the coupling with other elements. For each element two feeding ports, H/V polarization, have been investigated. In particular, measured patterns at five frequency points were used to determine the antenna coupling over the whole frequency band by simulation. A good agreement was found between the measured mutual S parameters on the real array and results obtained by the combination between measurement and simulations. This investigation demonstrated the validity of this approach.
In this paper a continuation of the previous study will be performed, exploring the following topics:
Enhancement of the representation of the NF source by inclusion of placement boundary condition.
Use of measured NF source models to represent another element of the array, not only the central one. The calculation of the antenna coupling will be determined for these new configurations.
Definition, Implementation, and Evaluation of a Novel Spiral-Sampling Technique
Building on the theory of spiral near-field acquisitions, the authors present a novel spiral acquisition implemented in a spherical near-field (SNF) chamber for a large automotive application. This new spiral permits the relaxation of certain restrictions associated with the standard spiral. Specifically, it allows us to eliminate extra or redundant rings beyond the poles, allows for greater control of the angular velocity ratio (i.e. gear ratio) between the theta and phi physical positioning axes, and does not require that the theta axis retrace between acquisitions.
In this paper, we describe the new spiral?s motivations, implementation, advantages, and measurement results. We first discuss the new spiral sampling, its mathematical definition, and its comparison to a standard spiral. Next, we describe the practical considerations and implementation of the coordinated motion between theta and phi for spiral sampling over a spherical surface. Next, we present the results showing good pattern agreement between conventional SNF and the new spiral method. We also discuss the reductions in near-field acquisition time and total test time that were achieved using the new spiral.
Application of Kernel Density Estimation to Achieve Automated Near Real-Time Antenna Pattern Data Processing and Analysis in an Anechoic Chamber
The Benefield Anechoic Facility (BAF) at Edwards Air Force Base is the world's largest known anechoic chamber. Due to its unmatched size and complement of test equipment, the BAF hosts far-field pattern measurements at all azimuth angles and multiple simultaneous elevations of installed antennas on large aircraft across a frequency range of 0.1 - 18 GHz. Antenna tests at the BAF rapidly produce large quantities of data, which often require immediate analysis to allow system owners to make relevant improvements. Historically, the BAF had accomplished quality assurance manually. Analysis was accomplished post-test by customers and the BAF team. Today, the BAF team has developed scripts that use kernel density estimation and basic machine learning to automatically check incoming data for errors and highlight unusual results for review. During a 2019 test of over sixty installed antennas on a B-1B bomber, the BAF team used these scripts to produce calibrated, quality-assured antenna patterns in near real-time. Rapid processing brings deficiencies to the customer's attention fast enough to allow corrections to be applied and re-tested during the same test event ? highly significant and valuable as aircraft and BAF schedule times are limited and may be a one-time opportunity to gather required data.
This paper will explore the algorithm used to evaluate antenna patterns, as well as the expected characteristics of patterns that enable the selection of relevant data. Development and application of this algorithm found that using kernel density estimation to calculate the number of maxima in a pattern's distribution of gain values, then performing this recursively over only the main lobe, can identify problems such as incorrect switching, mismatched transmission lines, and multipath. Algorithm optimization was achieved using generated data, then verified by applying the algorithm to previous test data. For the B-1B, the script searched for data that deviated from an expected pattern with clean main and side lobes, minimal frequency dependency, and a low-power noise distribution at all azimuth angles outside the lobes. Finally, this paper will discuss the results of using this algorithm during a live test, and future improvements and applications for this data processing technique.
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.
Amplitude and Phase Uncertainty Analysis due to Cable Flexing in Robot-Based Measurement Systems
The 18-term NIST error model is a common tool for analyzing potential sources of error in antenna measurements. One of the error terms to be considered describes the phase errors occurring in a measurement system. However, this quantity plays a rather negligible role for conventional ranges, such as roll-over-azimuth positioning systems. In particular, the contribution caused by flexing cables is normally insignificant. This results from the fixed installation of the cables or the decoupling of the movement at important points using rotary joints.
Current developments in the field of antenna measurement technology focus, among other things, on performing measurements using industrial robot arms. These are characterized by their high flexibility regarding the various measurement sequences, such as planar, cylindrical or spherical measurements. However, it is to be expected that the high freedom of positioning possibilities will introduce additional phase uncertainties, since the RF cables in the cable carrier chain of the robot arm itself are often not decoupled. Instead, a single cable is used for each signal path, which follows the movements of the robot.
The robot-based measurement system at the Institute of High Frequency Technology at RWTH Aachen University has been designed for frequencies above 60 GHz, where phase stability is a challenging task. Depending on the setup, it may even be required to pass Intermediate Frequency (IF) signals on the same cable as the Local Oscillator (LO) signals. This results in different test cases for the phase deviations depending on the frequency range of the IF (279 MHz) and LO (typically 10 GHz to 18 GHz) signals. Additional factors such as the measurement path of the robot or the position of the linear axis must also be taken into account. Therefore, a thorough analysis of the phase uncertainties caused due to flexing cables is of outstanding importance for robot-based measurement systems.
Adaptive Sampling for Compressed Spherical Near-Field Measurements
One of the main disadvantages of Spherical Near-Field (SNF) measurements is their acquisition time. This is due to the need of sampling a whole sphere around the Antenna Under Test (AUT) to perform the Near-Field-to-Far-Field Transformation (NFFFT). A step of the NFFFT is to decompose the measured signal in each one of the spherical waves it consists of, thus retrieving the Spherical Mode Coefficients (SMCs) associated to the AUT. Under typical measurement conditions, the SMCs of most physical AUTs prove sparse, i.e., most of their terms are zero or neglectable. Using this assumption, the system of linear equations with the SMCs as variables can be solved with fewer equations, that is, fewer measurement samples. This is done by applying an l1-minimization solver, following classical methodology from the field of compressed sensing. However, the location of the measurement points that generate non-redundant equations is not trivial. In typical compressed-sensing applications, a random sampling matrix is taken. Since a random matrix is inefficient for the acquisition with mechanical roll-over-azimuth positioner systems, a recent approach is to take an equidistant distribution of points on elevation and to calculate their corresponding pair on azimuth that delivers the minimum coherence of the sampling matrix. However, the number of sampling points M required for a successful reconstruction depends on the sparsity level of the SMCs of the unknown AUT, making its choice critical and based on a pessimistic approach.
A method for the adaptive choice of M is suggested. After the acquisition of a starting set of M_0 measurement points, chosen using phase transition diagrams, the SMCs are estimated online with few iterations of an l1-minimization algorithm. Afterwards, further points are acquired, and the SMCs are estimated again using them. Following the evolution and the decrease of the variation between estimates, it is possible to truncate the measurement at a point where a successful reconstruction is guaranteed. The method for the construction of a minimum-coherent sampling matrix for adaptive acquisition and the truncation criteria for a specific accuracy are discussed with a focus onimplementation, and supported with numerical experiments, performed with measurementdata.