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AMTA Paper Archive

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.

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.

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].

Background and clutter removal algorithm for RCS extraction in semi-anechoic chamber
Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023

The purpose of radar cross-section (RCS) measurement is to determine the amount of scattering that occurs when the radar signal illuminates the target. It is generally performed to prove a design concept. RCS measurement chamber requires a good signal-to-noise ratio during the measurement. When the measurement is performed in a non-controlled environment, coherent background subtraction associated with time gating is commonly used to improve the quality of the RCS data. Although these techniques are usually effective, residual clutter and background level still need to be removed to accurately characterize the target’s RCS in highly cluttered environments, such as semi-anechoic chambers. In this paper, a four-step post-processing technique is presented. In addition to the vector background subtraction and timegating techniques implemented in our previous work, a statistical algorithm called Principal Component Analysis (PCA) is applied to the ISAR image of the target. It is an extension of the PCA technique to RCS measurement. It is shown that residual background and clutter can be reduced by the statistical filtering method through eigenvalue decomposition of the RCS data. The technique is presented and evaluated through measurement of the RCS of a dihedral corner reflector at the X-band in the semi-anechoic chamber of the Directed Energy Research Center.

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.

An In-Situ Probe for Continuous Dielectric Permittivity Monitoring
John Schultz, October 2023

This paper describes a new materials measurement method that includes a sensor embedded within a ground-plane to continuously measure complex permittivity of an adjacent material. The sensor works with a 1-port vector network analyzer (VNA) to collect amplitude and phase of the sensor reflection signal, which is then converted to intrinsic dielectric properties or sheet impedance. The complexity of the fields near this sensor makes a conventional analytical method to relate reflection data to dielectric permittivity impractical. Instead, this sensor uses a computational electromagnetic (CEM) inversion method based on finite difference time domain (FDTD) simulations to derive real and imaginary dielectric properties from the amplitude and phase of the measured reflection. This paper describes the sensor design and inversion method. Additionally the sensor is demonstrated on several material types including i) sheet materials that may be manufactured in an in-line process and ii) concrete, which is a material whose properties change as it cures.

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.

Range-Doppler imaging method based on FFT-MUSIC for FMCW radar
Sangdong Kim, Bong-seok Kim, Jonghun Lee, Tarun Chawla, Greg Skidmore, Ram Narayanan, October 2023

This paper proposes a range-Doppler imaging method based on FFT-MUSIC method for FMCW radar systems. With the growing significance of vehicle and human motion recognition in automotive radar, the accuracy of conventional deep learning network-based recognition methods is reduced because it depends only on distance, speed, and angle information provided by conventional radars. Therefore, various types of imaging radar methods have recently been proposed. Among them, the range- Doppler imaging algorithm is widely used. This algorithm can simultaneously analyze both distance and velocity characteristics of a vehicle or person. However, conventional range-Doppler imaging based on the FFT algorithm has limited resolution, which cannot obtain detailed information on the target. Although the FFT algorithm is widely used in many applications, its lowresolution characteristics can limit its ability to provide detailed information. In particular, improving velocity resolution often requires the extraction of a significant amount of data. To address this issue, a range-Doppler imaging method based on FFT-MUSIC is proposed in this paper. This technique has been simulated using Remcom’s WaveFarer® software package. The proposed algorithm is effectively able to distinguish between two moving vehicles in several cases in which the ranges and velocities are too close to be resolved by conventional FFT methods. We can observe that the proposed algorithm enhances the velocity resolution by approximately twice as much as the conventional algorithm. Additionally, in indoor environments, the proposed algorithm provides a detailed representation of the indoor multipath, outperforming conventional algorithms. The high-resolution radar imaging offered by the proposed method will enable improved target recognition and thus enhance overall performance in practical applications.

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.

A Squat Cylinder-Dihedral Dual Calibration Device for Compact Ranges at UHF
Hirsch Chizever, Laura Suzuki, October 2023

The use of squat cylinders as both primary and secondary calibration targets is commonplace within the radar cross section (RCS) measurement community. Secondary calibrations have become a best practice activity for ranges seeking or maintaining certification. The calibration process, often referred to by the measurement community as a “Dual-Cal,” uses two squat cylinders of similar but unequal dimensions that provide range operators with a broadband calibration vector and a measurement uncertainty metric important to range certification. Despite their popularity, the need to ensure resonance scattering occurs below the desired measurement band results in physically large cylinders at UHF. In addition, the need to access the test zone for separate cylinder measurements may add substantial time to the calibration process and require specialized equipment, especially for large ranges. In response to these issues, a 22.5-degree right dihedral has been inserted into a squat cylinder form factor, creating a primary and secondary calibration target within one body, each separated in azimuth by 180 degrees. This two-target calibration device removes the need to access the target zone twice and mitigates errors associated with separate mounting schemes. The cylinder aspect, now truncated by the imposition of a dihedral, has 50% extended lower frequency coverage at UHF due to oblique edge scattering at vertical polarization. At horizontal polarization, the dihedral interruption of the cylinder creeping wave reduces its contribution for ka<4. The dihedral aspect provides a full polarimetric calibration, resulting in co-equal frequency responses for each polarization in the high frequency limit. The design parameters of the squat cylinder-dihedral device, its computed full-wave frequency response, and relevant scattering features are discussed.

Constrained FoV Radiated Power as a Figure of Merit of Phased Arrays
Alejandro Antón Ruiz, Samar Hosseinzadegan, John Kvarnstrand, Klas Arvidsson, Andrés Alayón Glazunov, October 2023

In this paper, we propose quantifying the radiated power of phased arrays or, in general, directive antennas, by the Constrained-View Radiated Power (CVRP). The constrained view shall be interpreted here as the Field-of-View (FoV) of an antenna that defines a region in space where focusing the radiated power is highly desired. In the limiting cases, we have that CVRP equals the Total Radiated Power (TRP) when the FoV covers the whole sphere, while, if the FoV reduces to a single point in space, the CVRP equals the Equivalent Isotropic Radiated Power (EIRP). We further present an analysis based on measured radiation patterns of a 16-element, linearly polarized, millimeter- Wave (mmWave), planar phased array antenna operating at 28 GHz. We compare the results to two ideal planar array antennas with the same number of Huygens and cosine elements. The evaluated figure of merit is computed for different scanning angles, as well as for different malfunctions of antenna elements, both for the real and simulated arrays. The results show that the introduced figure of merit could be potentially used for the detection of malfunctioning elements in antenna arrays as well as to characterize the impact of scan loss. Furthermore, CVRP is useful to straightforwardly and significantly characterize the performance of a directive antenna in terms of the power radiated towards a specific region in space.

Modified Thru-Reflect-Match Polarimetric Calibration Technique for Focused Beam Systems
Jeffrey Massman, Michael Havrilla, October 2023

This paper extends the time-domain gated response isolation scheme for full polarimetric calibration with a modified Thru-Reflect-Match procedure for network analyzer selfcalibration where precise knowledge of the metrology standards is not required. Cross-polarization contributions from the measurement setup are neglected to simplify the procedure. A simulated cascade analysis is included to demonstrate the relative scattering parameter error of the sample under test when the measurement setup cross-polarization level is neglected. The featured calibration analysis leverages a 4x4 scattering parameter matrix notation to capture the polarimetric scattering at each cascaded stage and develops a 16-term error correction factor model to account for cross-polarization scattering contributions from the measurement sample. Finally, a wire-grid polarizer is used as a modified Match standard where a series of interrogations at multiples orientations, in combination with Thru and Reflect measurements, enables cross-polarized scattering channels to be characterized. This polarimetric self-calibration approach uses physically realizable metrology standards and accounts for all error terms for precision focus beam system measurements.

The Small Resonant Sphere for Validating Radar Cross Section Measurement Accuracy
Donald Hilliard, Michael Emire, Long To, October 2023

This paper presents research results conducted at the Naval Air Warfare Center Weapons Division (NAWCWD) Radar Reflectivity Laboratory (RRL) to characterize RCS measurement quality of a compact range anechoic chamber using a small resonant sphere as a test probe measured over a 3.17-octave bandwidth, which covers the first half of the resonance region. Specifically, tests were performed on 1-inch and 12-inch diameter spheres over 2-18 GHz, which is a very prevalent test spectrum for RRL customers. The spheres were tested at the quiet zone center and the 1-inch was rotationally scanned over a 1- meter radial arc within the test zone. Spectral and spatial analysis was performed using techniques developed by Dr. Dean L. Mensa [1].

Measurements on extended long objects for radar field probes
Pax Wei, October 2023

In a compact range when the antenna is used for both transmitting and receiving in a monostatic fashion, the wave packet senses everything within its view. An extended long object usually gives rise to a bright reflection (glint) when viewed near its surface normal. To take advantage of this phenomenon, a discrete Fourier transform (DFT) on RCS measurements would yield a spectrum of incident wave distribution along that object, provided the scattering property is uniform along its length. Compared with traditional field-probes which translate a sphere across the test zone in horizontal and vertical directions, this new method extends out from the usual quiet zone, and is faster and less interfering to the field being probed. Inspired by this idea, the progression to practical innovation is discussed.

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.
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