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Near Field

Recommendations for RF Absorber Treatment of Ranges Having a Movable Gantry or Multiple Probes
Vince Rodriguez, Mark Ingerson, October 2023

Absorber treatment for an anechoic range is designed to attenuate the potential reflections from the walls, ceiling, and floor and to keep a certain level below the direct path between the range antenna (or probe) and the quiet zone (or minimum radiated sphere for spherical near-field ranges). There are, however, some antenna measurement systems where the range changes or moves as the data is acquired. In some cases, the probe moves around the antenna-under-test (AUT) along a section of circle supported by an arch or a gantry. In other ranges, the multiple probes are switched on and off; these probes are supported by an arch. Because the direction of the range moves with respect to the walls, ceiling, and floor, it is a bit more complex to arrive to an optimal absorber layout, as well as locating the preferred placements for the instrument rack, door, and vents in the range. In this paper, a higher-order-basis-function method of moments approach is used to model a gantry-supported probe as it moves around the location of the AUT. The power density at the walls as the probe moves is analyzed to arrive to an optimal absorber layout that will provide adequate levels of reflections for measuring an antenna. The paper looks at a gantry that moves from +135° to -135° with the AUT rotating 180° and for a gantry that moves from 0° to +135° with the AUT rotating 360°. The latter will require a smaller range with one of the walls closer to the location of the antenna under test. A series of recommendations based on the electrical size of the absorber at different areas of the range are provided.

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.

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.

Predication of Planar Near-Field Measurements Based on Full-Wave Three-Dimensional CEM Measurement Simulation
Rostyslav Dubrovka, Robert Jones, Clive Parini, Stuart Gregson, October 2023

In this paper, the full-wave computational electromagnetic simulation of the production test, measurement, and calibration of a 5G, 24 elements, C-band, active, planar array antenna together with a representative open-ended rectangular waveguide probe with, and without, absorber collar were evaluated using a large computing cluster and a proprietary full-wave solver. In this way, various components within the measurement could be carefully and precisely examined providing a framework for further measurement optimization. Particular attention has been paid to the presence of the standing waves in the simulated near-field measurement. This is a crucial feature of most practical measurements, but is omitted from the vast majority of simulations due to the computational effort required to evaluate it, and which is absent from the standard near-field theory. Here, the presence and impact of this phenomenon has been carefully examined with a range of intensive simulations being harnessed to quantify their impact, as well as enabling various methods for their minimization to be explored in a convenient and highly controlled fashion.

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.

Enhanced Simulation-Augmented OTA Technique Applied to Absorbed Power Density Evaluation
Benoit Derat, Thorsten Liebig, David Schaefer, Winfried Simon, October 2023

This paper proposes a fast human exposure Absorbed Power Density assessment approach, based on a combination of over-the-air radiative field measurements and fullwave electromagnetic simulations. This so-called augmented OTA technique relies on the computation of an equivalent source or digital twin, which reproduces the radiation properties of the device under test. At short separation distances, the interaction between the human model and the device is however not negligible. A novel solution to model the influence of multiple reflections is introduced, where the inside of the equivalent source box is filled with a perfect electric conductor, thereby creating a reflective digital twin model. Simulation results demonstrate the relevance of this approach for enabling accurate absorbed power density evaluations.

A Simple Non-Linear Planar Near-Field Antenna Measurement System
Jason Jerauld, Tarron Teeslink, Felix Yuen, Nathan Landy, Tom Driscoll, October 2023

We describe a planar near-field instrument capable of measuring the non-linear response of an electronically steered antenna (ESA) up to the third harmonic while requiring only a single scan with a single probe. The system performs phase-coherent measurements of the aperture near-field at the fundamental frequency, second harmonic, and third harmonic simultaneously, which are then transformed to the far-field. When system losses are appropriately accounted for, these far-fields are accurate representations of the harmonic patterns relative to the fundamental. A broadband dual-polarized probe combined with a specially-configured network analyzer is used to capture all frequencies and both polarizations within a single scan. Using a ultra-broadband probe introduces some limitations to the measurement, but offers a significant increase in measurement speed. In this paper we disclose various architecture and design aspects of the instrument, discuss its advantages and limitations, and compare non-linear PNF measurements with non-linear array simulations and direct far-field measurements.

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.

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].

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.

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.

Near Field Measurement and Analysis in Frequency Ranges of 20 GHz to 90 GHz
Jungkuy Park, Sol Choi, October 2023

In General, theoretical RF attenuation in free space is characterized according to the Friis equation in far field range The equation says that the free space propagation of electromagnetic waves is inversely proportional to the square of distance from source It holds only in far field range. We investigate a propagation characteristic of millimeter wave in all ranges of field. The study provides measurement results of free space insertion loss from 20GHz to 90GHz of frequency ranges, where the separation between transmitting and receiving antennas is increased from 1mm to 1400mm with 1mm step. The measurement distances cover all range including the reactive and Fresnel ranges as near field, and the far field too. The measuring values are fitted in the free space path loss factor (λ/4πr)2. There are discrepancies between theoretical and measuring values in near field ranges. We added an extra terms to the formula in order to resolve the difference in near field. The results calculated by new formula are shown in good agreement at Fresnel range and also at some parts of the reactive range. The new formula having the extra terms can be also proposed for antenna gain measurements in the near separation between antennas in the context of results according to this study.

Introducing LORENTZ: A novel Low-temperature Near-field Terahertz Chamber for instrument characterisation.
Paul Moseley, Luis Rolo, Andrey Baryshev, Tobias Vos, Alena Belitskaya, Daniele Ronso da Costa Lima, Peter de Maagt, Paul Hartogh, October 2023

The Low-temperature Near-field Terahertz Chamber (LORENTZ) is a novel facility recently installed and commissioned at ESTEC, ESA. This facility has the unique ability to characterize the antenna performance off full submillimeter instruments in operational environments down to 80k. We provide an overview on the various design and commissioning steps that were required to ensure all parts of the test facility would operate reliability in such challenging conditions. We also present how the facility performed during the first full measurement run of flight hardware and a roadmap for future developments.

Evaluation of Near-Field to Far-Field Transformation Accuracy Based on Reference Radiation Models
Arun Bhatt, Afroditi Kyrligkitsi, Thomas Gemmer, Adam Tankielun, Hendrik Bartko, Benoit Derat, Thomas Dallmann, October 2023

Measuring the radiation behavior of antennas in an anechoic environment with far-field (FF) conditions requires large measurement sites resulting in high costs. To overcome this problem, near-field (NF) measurement techniques and nearfield to far-field (NFFF) transformation are used to derive the FF of an antenna under test. Thus, evaluating the accuracy and reliability of the NFFF transformation is highly important. The first step for this evaluation process is creating a valid and accurate reference data library consisting of NF and FF data. NF data is used as input to the NFFF transformation, whereas FF data is used as a reference for comparison to the FF obtained after the transformation. This paper explores analytical studies for different antenna models, namely a pyramidal horn antenna and an open-ended waveguide. Simulations of the models are carried out in a commercial full-wave electromagnetic software, considering the various input parameters influencing the FF results. The NF and FF data from the parameterized simulation model is used to evaluate the NFFF transformation methods. After optimization of the simulation parameters, the required accuracy levels of −30 dB and −40 dB for two different accuracy metrics defined throughout the paper are achieved.

Revision Progress: IEEE Std 1720 Recommended Practice for Near-Field Antenna Measurements
Lars Jacob Foged, Justin Dobbins, Vince Rodriguez, Jeff Fordham, Vikass Monebhurrun, October 2023

The IEEE Std 1720™, "Recommended Practice for Near-Field Antenna Measurements," serves as a dedicated guideline for conducting near-field (NF) antenna measurements [1]. It serves as a valuable companion to IEEE Std 149-2021™, "IEEE Recommended Practice for Antenna Measurements," which outlines general procedures for antenna measurements [2]. IEEE Std 1720 was originally approved in 2012 as a completely new standard by the IEEE Standards Association Standards Board. It holds significant importance for users engaged in NF antenna measurements and contributes to the design and evaluation of NF antenna measurement facilities. With its tenyear term coming to an end in 2022, the standard will no longer remain active. Nonetheless, a "minor revision" of the existing standard is in progress and is expected to be completed in 2023. The objective of this paper is to provide insights into the ongoing activities surrounding the revision and to explore the proposed changes. It aims to facilitate a discussion on the modifications to and their implications for modern NF antenna measurements.

Accurate Evaluation of Antenna Measurement Range Performance with the SWE Transmission Formula
Francesco Saccardi, Andrea Giacomini, Lars Foged, October 2023

The spherical wave expansion-based transmission formula allows to accurately evaluate the coupling (or S21 parameter) between a transmitting and a receiving antenna. Its use as tool for probe corrected spherical near-field to far-field transformation is well accepted and documented. On the other hand, its direct use in the evaluation of antenna measurement performance has been exploited only in recent years. In this paper we will show how measurement performances predicted with the transmission formula compare with actual measurements. Taking as examples relatively complex antenna measurement systems like spherical near-field, plane wave generators and CATR, we will focus on the prediction of the accuracy of the measured radiation patterns, also including the approximation of reflections from the test environments, and on the evaluation of link budgets.
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