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Far Field

Design and performance comparison of 3D metal printed near field probe for K-Ka band
Ila Agnihotri, October 2023

Frequency band requirements for satcom applications in certain cases overlap two conventional microwave frequency bands. Characterizing antennas using near field techniques over such bands require two separate probes resulting in substantial increase in measurement time. This work is motivated by providing solution to such requirements of overlapping bands (K and Ka-band) and wideband operation over an octave frequency from 17-33 GHz. We propose a new WR-38 band and present the design and development of WR-38 waveguide probe realized using 3D metal printing. Impact of higher order modes on operational bandwidth of waveguide, 3D metal printing surface roughness and fabrication tolerances is investigated. Fabricated probe is characterized using planar near field (PNF) and measured results are presented. Performance comparison is done by characterizing SGH-1800 (18-26.5 GHz) and SGH-2200 (22-33 GHz) with 3D printed WR-38 probe and commercial WR-42 and WR-34 probes.

A New Closed Form Field Asymptotic Expansion Applied to Far-field Evaluation of Antenna Arrays at Short Range Lengths
Benoit Derat, October 2023

A new general formulation for the asymptotic expansion of electromagnetic fields radiated by an arbitrary antenna is introduced and demonstrated. The presented approach is based on an extended application of the method of stationary phase, updating a methodology proposed by Jones and Kline in 1956. Explicit formulas are derived up to the sixth order (sixth power of the inverse of the distance to a chosen antenna reference point), where coefficients of the spatial field expansion are obtained as linear combinations of even partial-derivatives of the plane-wave spectrum. Provided equations are verified by application to canonical cases of a Hertzian dipole and a 12 × 4 dipole array. An example how these findings could be leveraged in realistic use cases is delivered, using measured data from the antenna array of a 5G radio base station.

Performance Comparison of Traditionally Manufactured and Additively Manufactured Luneburg Lenses
Anna Stumme, Alexander Golding, Mark Dorsey, October 2023

Luneburg lenses are popular antenna apertures for applications requiring a wide field of view and beam steering capability as they are a lower cost alternative to phased array apertures with straightforward beam-switching. Traditionally, Luneburg lenses are fabricated by layering shells of different dielectric constants to approximate a theoretically continuous dielectric gradient. Recent developments in additive manufacturing (AM) for radio frequency (RF) applications have led to advances in AM Luneburg lenses. AM enables functional dielectric printing where the dielectric properties can vary spatially within a print. Lenses manufactured using AM can continuously vary the dielectric constant through the material based on the theoretical Luneburg lens dielectric gradient; these AM lenses more accurately match the dielectric gradient of the theoretical Luneburg lens. Additionally, the integration of transformational optics (TO) to AM Luneburg lenses allows for physical reshaping of the lens structure while maintaining the same electrical properties. TO allows reshaping of the focal surface, which enables lateral translations of the feed location to steer the beam instead of radial rotations. This lowers the complexity of the feed arrangement without loosing the beam steering capability. This paper presents the results and analysis of a simulation and measurement study including performance comparisons between three Luneburg lens fabrication methods: traditionally manufactured, AM, and TO-AM.

Antenna Coupling Evaluation Based on Accurate Measured Source Models and Simulations
Lucia Scialacqua, C. J. Reddy, Lars Foged, October 2023

When numerically simulating antenna problems, the accuracy of the antenna representation is crucial to improve the reliability of the results. Integrating the measured near-field (NF) model of the antenna into Computational Electromagnetic (CEM) tools opens new horizons in solving such problems. This approach has been studied for complex and/or large scenarios, antenna placement, scattering issues, and EMC applications [1- 3]. Another appealing use of merging measurements and simulations is the evaluation of antenna coupling [4-6]. Previous investigations regarded an array of three identical cavity-backed cross-dipole antennas [7-8]. In all the experiments the coupling between elements was evaluated only between an NF source and an antenna represented by its full-wave model and fed by ports. In this new study, following on the heels already presented in the publication [9] in which coupling between multiple simulated NF sources was illustrated using the commercial EM simulation tool Altair Feko [10], we want to show how antenna coupling between NF sources both coming from measurements can be evaluated in numerical simulations. The validation will be done combining two identical NF sources of MVG SMC2200 monocone antennas flush mounted on a rectangular plate. An additional demonstration will be shown on three NF sources of the same monocone on a rotorcraft model.

Planar Wide Mesh Scanning using Multi-Probe Systems
Fernando Rodriguez Varela, Manuel Sierra-Castañer, Francesco Saccardi, Lucia Scialacqua, Lars Foged, October 2023

The reduction of acquisition time in planar near field systems is a high interest topic when active arrays or multi beam antennas are measured. Different solutions have been provided in the last years: multi-probe measurements systems and the PlanarWide Mesh (PWM) methodology, which implements a non redundant sampling scheme that reduces the number of samples required for the far-field transformation, are two of the most well known techniques. This paper proposes the combination of both approaches to derive a multi-probe PWM grid which reduces the measurement times to the minimum. The method is based on treating the near-field to far-field transformation as an inverse source problem. The multi probe PWM is designed with a global optimization process which finds the best measurement locations of the probe array that guarantee a numerically stable inversion of the problem. A simulated measurement example with the VAST12 antenna is presented where the total number of samples is reduced by a factor of 100 using a 4×4 probe array

Electric-Field Pattern Measurements of Acoustically Driven Piezoelectric Field Emitters
Srinivas Prasad Mysore Nagaraja, Brook Feyissa, Tristan Wilson, Jack Bush, Darmindra Arumugam, October 2023

Piezoelectric transmitters operating at acoustical resonance have been shown to radiate effectively in the Very Low Frequency (3 kHz to 30 kHz) and Low Frequency (30 kHz to 300 kHz) regimes. Such transmitters make use of the inverse piezoelectric effect to couple electrical signals into mechanical vibrations, resulting in near field radiation. This new class of electrically small antennas, known as mechanical antennas or ‘mechtennas’ can provide several orders of magnitude higher efficiency than similarly sized electrically small conventional dipoles. Measuring the dipole-like near field pattern of such piezoelectric field emitters in the Very Low Frequency and Low Frequency range using conventional techniques is not possible. To address this limitation, a simple capacitor plate-based setup is presented that enables the measurement and plotting of the near field patterns of such transmitters. Design and simulation of the capacitor plates to model the fields along with electric field pattern measurements of a Y 36◦ cut Lithium Niobate transmitter having longitudinal mode resonance at 82 kHz are presented.

Novel Application of Compressed Sensing in Cylindrical Mode Filtering for Far-Field Antenna Measurements
Zhong Chen, Stuart Gregson, Yibo Wang, October 2023

Mode filtering has been shown to be very effective in suppressing spurious reflections in antenna measurements. Specifically, it has been well documented that in the quasi-far-field, the two polarizations are decoupled, making it possible to apply standard cylindrical near-field theory on the amplitude and phase data acquired from a single polarization measurement on a great circle cut [1]. The method was further extended to allow data collected from an unequally spaced angular abscissa by formulating the solution as a pseudo-inversion of the Fourier matrix [2]. This formulation, however, can be prone to spectral leakage because of nonorthogonality of the Fourier basis on an irregularly sampled grid, especially when the positions deviate significantly from the regular grid [2]. In this paper, we propose to use Compressed Sensing (CS) to compute the Cylindrical Mode Coefficients (CMCs), which improves the signal to noise ratio, allowing more accurate recovery of the prominent modes. The CS recovery is tenable because with the coordinate translation of the measurement pattern to the rotation center, the Maximum Radial Extent (MRE) of the antenna under test is greatly reduced, making CMCs quite sparse in the mode domain. The novel application of CS presented in this paper further expands the generality of the mode filtering method, which is now applicable to under-sampled data (at below the Nyquist rate) acquired on positions that grossly deviate from the equally-spaced regular grid.

The Demystification and Measurement of Receiving Efficiency
Ryan Cutshall, Justin Dobbins, October 2023

In the 2013 revision of the IEEE Standard for Definitions of Terms for Antennas [1], multiple new terms were added to describe active antenna systems. One such term is receiving efficiency, which was added to describe the behavior of either a passive receiving antenna or an active receiving antenna system. The definition of receiving efficiency contains other new terms such as isotropic noise response and isotropic noise response of a noiseless antenna. These new terms and definitions may cause some confusion for individuals responsible for antenna design and measurement. We attempt to demystify a few of the terms added to IEEE Std 145-2013, especially those terms that relate to receiving efficiency. In addition, we propose a measurement technique for measuring the receiving efficiency of an active receiving antenna system.

A Unique Spherical Near-Field Test System for Commercial Aircraft Radar Radome Testing
Kefeng Liu, Anbang Liu, Denis Lewis, October 2023

A novel test system has been developed using the Spherical Near-Field (SNF) test method to test commercial aircraft radar radomes fully complying to the RTCA-DO-213 Change 1A [1] test requirements. In contrast to either a compact range or a far-field outdoor range to test directly for far-field patterns, this test range employs a fixed scan area SNF test method [2] and transforms the near-field patterns to the required far-field patterns. This test system has the advantage of a more compact test site size than the other two types of test ranges; yet maintains a long enough test distance to minimize the radiated near-field coupling between the probes and the Antenna Under Test (AUT) to a negligible level. The test system also features a multi-axis AUT positioner that supports relative angular positions between the radome and the radar panel antenna to simulate both AZ/EL and EL/AZ gimbal motions as required by RTCA-DO-213A specifications. Additionally, a multi-probe SNF scan antenna system is employed to expediate SNF data acquisition. This compact, high precision SNF antenna test system also demonstrates the potential to eliminate the need for λ/4 shift in the test distance as required by RTCA-DO-213 Change 1A, resulting in a potential 50%-time savings in transmission efficiency testing using the near-field test method when the test distance is much greater than the required 10λ. Furthermore, it also demonstrates the potential to reduce the number of reference antenna pattern tests for transmission efficiency from 231 to 1, since the panel antenna is stationary during each of the 231 test configurations and will be of the same AUT patterns. Test data supporting the accuracy and efficiency of this test system is also documented.

NIST's Antenna Gain and Polarization Calibration Service Re-instatement
Joshua Gordon, Benjamin Moser, October 2023

After a five-year renovation of the National Institute of Standards and Technology (NIST) Boulder, CO, antenna measurement facility, the Antenna On-Axis Gain and Polarization Measurements Service SKU63100S was reinstated with the Bureau International des Poids et Mesures (BIPM). In addition to an overhaul of the antenna facility, the process of reinstatement involved a comprehensive measurement campaign of multiple international check-standard antennas over multiple frequency bands spanning 8 GHz to 110 GHz. Through the measurement campaign, equivalency with 16 National Metrology Institutes (NMIs) and continuity to several decades of antenna gain values was demonstrated. The renovation process, which included implementing new robotic antenna measurement systems, control software, and data processing tools is discussed. Equivalency results and uncertainties are presented and compared to checkstandard historical values.

Phase Measurement for 5G NR Modulated-Signal Using Rapid Spherical Near-Field System with Probe-Receiver Combined Array
Jong-Hyuk Lim, Jungkuy Park, Dong-Woo Kim, Soon-Soo Oh, October 2023

This paper proposes the measurement technique for the phase of 5G NR modulation signal using the fast spherical near-tofar field measurements utilizing the multi-probe combing the multi-receivers. The bandwidth of the 5G NR signal is 100 MHz at 28 GHz with 16-QAM (Quadrature Amplitude Modulation) or 64- QAM TDD (time division duplexing). The reference receiver is utilized since an absolute phase is changed every time. The relative phase at each receiver was recorded, and the medium value was calculated. It can be asserted that the middle value of phase could be similar to the exact value with a little error even for the 5G NR modulated signal.

A Novel Data Processing Technique for Calibrating Low Frequency Antennas with Long Ring Down Time in An Extrapolation Range
Yibo Wang, Zhong Chen, Dennis Lewis, Wayne Cooper, October 2023

Extrapolation method is regarded as one of the most accurate methods for obtaining the absolute far-field gain of an antenna. This paper will compare the efficacy of several data processing techniques for calibrating low frequency antennas with long ring down time. Traditionally, measurement data are preprocessed to remove ripples from multipath reflections before a curving fitting is applied. We will first investigate two traditional data processing techniques. The first technique is to apply time domain gating to the vector response vs. frequency data at each separation distance. Then the gated data as a function of distance is fitted to the polynomial equation. The second technique is spectrum domain filtering. The vector response as a function of distance is transformed to k domain at each frequency. A band pass filter is applied in k domain to keep only the direct antenna response. In this study, we propose a new approach - the magnitudes of the antenna response as a function of distance including the ripples is fitted to a more complete generalized antenna response equation with the antenna-to-antenna multiple reflection terms included. This paper will compare the three techniques using a set of measurement data on double-ridged waveguide horn antennas in a fully anechoic extrapolation range.

Compressive Sensing Applied to Planar Near-Field Based Array Antenna Diagnostics for Production Testing
Clive Parini, Stuart Gregson, October 2023

Compressive Sensing (CS) has been deployed in a variety of fields including wideband spectrum sensing, active user detection and antenna arrays. In massive MIMO arrays, CS has been applied to reduce the number of measurements required to verify the arrays excitation in a production environment. All follow the general approach of creating the sparsity needed for CS by subtracting the measured far-field or near-field of the test array from that of a 'gold standard' array measured under identical conditions. In a previous paper [1] the authors have shown that using a Far-Field Multi-Probe Anechoic Chamber (FF-MPAC) and an optimal sampling strategy CS can offer accurate reconstruction of array excitation with a mean square error (MSE) approaching -40dB using a sampling strategy of just 1.4% of the Nyquist rate. The approach assumed production standard arrays with failure rates up to around 2%. In this paper we extend the concept to using a planar near-field (NF) measurement offering a much more compact test facility that is more suited to the production environment for these antennas. In our initial work the reconstruction of array excitation with a mean square error (MSE) of -30dB was achieved for a 20 x 28 element array antenna at half wavelength spacing using just 1.5% (177 samples) of the samples needed for a conventional NF measurement (12,100 samples) employing back projection to the aperture. Critical to the performance is the realization that the CS samples need to be confined to the central region of the NF measurement plane which for a conventional NF to FF planar antenna pattern measurement would offer a massive truncation error. This paper addresses the optimal sampling strategy needed for this NF approach and presents a statistical performance analysis of the reconstruction accuracy.

Machine Learning Based Fourier Phase Retrieval for Planar Near-Field Antenna Measurements
Marc Dirix, Stuart Gregson, October 2023

The success and efficiency of many classical iterative plane-to-plane based phase retrieval algorithms is to a large extent dependent upon the fidelity of the initializing, i.e. guiding, phase estimation [1], [2]. This is especially so when using these techniques to recover the phase of active electronically scanned array antennas such as those employed within beam-steering mm-wave Massive MIMO antenna systems intended for 5G New Radio applications where the performance of the algorithm, and its ability to not become trapped within one of the (possibly many) local minima, is particularly dependent upon the quality of the initializing guess where access to a phase reference is not always convenient, or even possible. Many traditional phase recovery iterative Fourier methods employ simulation or passive measurement supported phase initialization [1], however this information is not always readily available, or in the measurement may require a destructive, invasive, examination of the device under test (DUT). In this work we address this issue by presenting a proof of concept which employs a machine learning based neural network [3] to estimate the initializing phase function based on the assessment of the measured amplitude only near-field pattern. Here, we show that there is sufficient information contained within the difference between the two near-field amplitude only scans to be able to determine the antenna beam steering characteristics. A simplified beam steering case with electronic scanning in one, or more, scanning axes is demonstrated and verifies the power of the novel method, as well as illustrating its inherent resilience to noise within the amplitude only measurements, and verification of the robustness of the approach thereby extending the range of measurement applications for which this class of iterative Fourier algorithms may be successfully deployed [4].

BIOMASS Calibration Transponder Antenna Measurements in ESA-ESTEC HERTZ Facility
Ines Barbary, Luis Rolo, Eric Van Der Houwen, Mauro Bandinelli, Davide Bianchi, Dean Rowsell, Mike Royle, October 2023

The BIOMASS Calibration Transponder Antenna (BCT) has been developed to track the BIOMASS satellite and to send calibration signals to it. It has been measured in the ESAESTEC HERTZ facility to ensure its performance before installation. As this anechoic chamber has not been designed to measure antennas at P-Band, its range of applications had to be extended. To this end, spherical nearfield measurements were carried out in order to minimize reflections and decrease measurement uncertainties. Using an average of several measurements, the very high requirements on gain accuracy, crosspolar values, and group delay could be met. However, certain effects in the phase patterns stemming from the chamber that affect the calculation of the phase centre have been observed. This work provides an account on the methods applied to extend the usability of the HERTZ facility, discusses their effectiveness, and infers some generalizations.

Using the Three-Antenna Gain Method to Improve Measurement Accuracy for VHF Satellite and Space Applications
Bennett Gibson-Dunne, Greg Brzezina, Ken Oueng, Adrian Momciu, October 2023

Antenna measurements in the VHF band are challenging because of the sensitivity to surroundings in both outdoor and indoor ranges. The large size of the antennas involved makes them difficult to manipulate and therefore more susceptible to damage. In addition, the gain tables for standard gain antennas at these low frequencies is often sparse, especially for older models. This paper proposes to use the three-antenna gain method to mitigate some of these problems by calculating the gains more accurately than other gain calculation methods or the original manufacturer’s datasheets. To this end, a new custom NSI2000 script was written and trialed with a trio of antennas commonly used to test new devices for satellite and space related applications. Using our newly refurbished large anechoic chamber with a nearfield system, gain data calculated in the 200 – 325 MHz frequency range shows notable differences relative to the datasheets. As compared to other methods of gain calculation, the results for the three-antenna method displayed smaller mean values and standard deviations – indicating a reduction in the influence of any single error on the overall outcome. The lessons learned from this experiment can help improve measurement accuracy at these frequencies.

Exploration of UAV-based testing and qualification of NGSO earth stations
Andrian Buchi, Ondrej Pokorny, Snorre Skeidsvol, Sigurd Petersen, October 2023

This paper presents a new test procedure to asses and validate key performance indicators for NGSO antennas, and serves to introduce said methodology to the antenna measurement community to foster a discussion on future evaluation procedures for modern day ground segments. Beyond introducing the proposed test methodology we also present results highlighting the actual accuracy of a UAV based measurement system enabling the proposed measurement procedure. The paper is intended to be viewed as an initial proposal for a qualification methodology.

Exploration of UAV-based testing and qualification of NGSO earth stations
Andrian Buchi, Ondrej Pokorny, Snorre Skeidsvol, Sigurd Petersen, October 2023

This paper presents a new test procedure to asses and validate key performance indicators for NGSO antennas, and serves to introduce said methodology to the antenna measurement community to foster a discussion on future evaluation procedures for modern day ground segments. Beyond introducing the proposed test methodology we also present results highlighting the actual accuracy of a UAV based measurement system enabling the proposed measurement procedure. The paper is intended to be viewed as an initial proposal for a qualification methodology.

Electrical Alignment Technique for Offset-Mounted and Arbitrarily Oriented AUTs in a Robot-Based mm-Wave Antenna Test System
Henrik Jansen, Roland Moch, Dirk Heberling, October 2023

One of the main advantages of a robot-based antenna measurement systems compared to traditional positioning systems like roll-over azimuth positioners are the additional degrees of freedom and, thus, the increased flexibility with respect to the sampling grid and the placement of the antenna under test (AUT). However, this flexibility also requires a precise alignment of probe antenna and AUT to obtain accurate measurement results. In this paper, an electrical alignment technique based on a six term error model is introduced. The misalignment errors are estimated from measurement of single θ-cuts of a reference AUT, using a least-squares optimization approach. The estimation results can be used subsequently to correctly align the probe antenna to the physical position of arbitrary AUTs, independent of the sampling grid. The technique is validated by measurements in the mm-wave frequency range. Results show that the proposed method allows a correction in the same order of magnitude as the repeatability of the robotic system, therefore contributing to an increased overall accuracy of the obtained measurement results.

Electrical Alignment Technique for Offset-Mounted and Arbitrarily Oriented AUTs in a Robot-Based mm-Wave Antenna Test System
Henrik Jansen, Roland Moch, Dirk Heberling, October 2023

One of the main advantages of a robot-based antenna measurement systems compared to traditional positioning systems like roll-over azimuth positioners are the additional degrees of freedom and, thus, the increased flexibility with respect to the sampling grid and the placement of the antenna under test (AUT). However, this flexibility also requires a precise alignment of probe antenna and AUT to obtain accurate measurement results. In this paper, an electrical alignment technique based on a six term error model is introduced. The misalignment errors are estimated from measurement of single θ-cuts of a reference AUT, using a least-squares optimization approach. The estimation results can be used subsequently to correctly align the probe antenna to the physical position of arbitrary AUTs, independent of the sampling grid. The technique is validated by measurements in the mm-wave frequency range. Results show that the proposed method allows a correction in the same order of magnitude as the repeatability of the robotic system, therefore contributing to an increased overall accuracy of the obtained measurement results.







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