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Anechoic ranges require constant temperature and humidity, proper lighting to be able to work inside the range and closed-circuit television (CCTV) cameras to monitor the system while the measurement is being done. In addition, anechoic chambers require fire detection and suppression. Traditionally these penetrations are minimized and placed in non-critical areas. But the true effect of them has not been fully investigated. In this paper, antenna measurements as simulated in an indoor far field range. The approach to model the measurement is like the one the author presented in [1] and [2]. Thus, a range antenna (or near- field probe) and an antenna under test (AUT) are placed in free space and the AUT is rotated at discrete angles as it was done in [1]. Then a second model includes CCTV cameras, HVAC vents, light fixtures and both air sampling tubes and fire suppression nozzles and placed around. The simulation with these disruptions is repeated at the given discrete angles. The model does not include the absorber on the range. The model assumes a perfect absorber and the results of the simulated antenna measurement are compared to an ideal case with no disruptions. The results, while being approximations, provide a worst-case error for those disruptions of the RF-absorber layout. The results can be used to estimate the potential uncertainty on the measurement caused by the different systems that must be part of the anechoic enclosure. The technique is applied here to indoor far field measurements, and for near-field systems. Results show that for your typical roll over azimuth positioner, the effects of the penetrations on the ceiling are very small with differences in the -35 to -40 dB levels.
This paper presents research validating the forward scattered wave in bistatic radar geometries. In contrast to monostatic radar, where transmitter and receiver are collocated, bistatic radar incorporates receivers positioned in different locations from the transmitter. In the case of forward scattering, the receiver is positioned in the far field behind the reflecting target in line of sight of the transmitter. The paper describes forward scattering physics and presents forward scatter radar cross section measurements conducted at the Naval Air Warfare Center Weapons Division (NAWCWD) Radar Reflectivity Laboratory (RRL), which are validated with computational electromagnetic predictions.
A new method is developed for accurate transmission measurements through a surface using a far-field Gaussian weighting approach for use in anechoic chambers. A trivial approach to measuring transmission characteristics would be to mount a sample-under-test between two antennas and simply measure the boresight transmission using a network analyzer. This approach is enhanced by instead measuring the transmitted fields over a hemispherical field-of-view and then weighting the measured far-fields to synthesize a Gaussian illumination of the sample. This approach can be utilized to effectively illuminate the sample-under-test with an arbitrary excitation with a high degree of customization in post-processing (e.g., beam polarization, direction, waist size, and location). The approach is validated by characterizing transmission through a copper clad substrate and a frequency selective surface (FSS) from 4 to 40 GHz. Additionally, a sample of Rogers 5870 is measured from 8 to 40 GHz. For each sample-under-test, S21 measurements are compared for the boresight and Gaussian weighted methods, showing greater agreement with theoretical or simulated values for the Gaussian case.
Hypersonic platforms develop skin temperatures exceeding 1500°C. Materials applied to these skins often require specific electric and magnetic properties which must be validated at temperature, requiring tailored furnaces, refractory metals, specialized ceramics, and procedures to protect RF hardware and personnel. Free space measurement configurations maximize the distance between high temperature components and measurement hardware, thus limiting errors and damage due to radiant heat. Free space material property inversions assume planewave illumination on an infinite sample, which, in practice, is approximated by a finite sample with attenuated fields at its boundaries. Far field generation using Gaussian shaping dielectric lenses, Hogg horn reflectometers, and lens covered antennas have all been previously employed. Given the need to maximize sample- to-hardware distance, an alternative room temperature measurement approach has been developed for the 8-18 GHz band as a baseline architecture for testing up to 1500°C and beyond. This baseline system utilizes two opposing ellipsoidal reflectors with a shared focus to create a localized sample plane. First published in 1971 for use with high temperature dielectric measurements, the simplicity of the elliptical shape, frequency scalability, low-cost material, and straightforward fabrication, makes this approach a good candidate for high temperature sample illumination. The baseline system extends the initial work to include magnetic materials and a discussion of primary error sources, including incident field performance versus feed displacement, reflector conductivity, feed-reflector interactions, and off-normal illumination. System performance is demonstrated through complex permittivity and permeability estimates for Neoprene, reticulated absorber, one-pound density Polystyrene, and commercial magnetic absorber. Planned modifications to the baseline system for high temperature applications are also presented.
This paper explores the use of the commercially available software Altair Feko® to model the on-axis directivity and gain of a commercial off-the-shelf (COTS) X-band (8 GHz – 12 GHz) standard gain horn (SGH) with and without waveguide adapter, specifically the SGH820 by the Microwave Vision Group (MVG). SGH820 antenna was simulated using full wave (Method of Moments (MoM) and Multilevel Fast Multipole Method (MLFMM)) solvers that are available in Feko. Reflection coefficient, boresight gain, and directivity parameters are computed with and without waveguide adapter. Both MoM and MLFMM solvers in Feko show good agreement and are shown to be within ±0.008 dB of each other for boresight gain and directivity calculations. Computational resource comparisons for simulations of the SGH820 antenna with an adapter using the MoM and MLFMM solvers are presented. The results show a significant reduction in computational time when using the MLFMM solver compared to the MoM solver without any degradation in accuracy.
Plane Wave Generators (PWGs) utilize arrays of radiating elements to approximate plane wavefronts, thereby creating localized far-field-like conditions within a Quiet Zone (QZ). Their compact form factor makes them especially advantageous at low frequencies, such as in the VHF and UHF bands, where traditional Compact Antenna Test Ranges (CATRs) become impractically large. This paper presents results from a comprehensive validation campaign of a 19-element PWG demonstrator, conducted as part of a broader development program aimed at realizing a full-scale system for VHF/UHF testing. The campaign, executed at Pulsaart by AGC, involved both element-level and array-level assessments using a spherical near-field multi-probe system. Key objectives included validating QZ synthesis, calibrating array excitations via digital twin modeling and field expansion methods, and quantifying realized excitation errors. The findings confirm the robustness of the PWG design, the effectiveness of the calibration process, and the minimal impact of mutual coupling and active impedance variations on performance.
In this work, an effective procedure to compensate for 3-D mispositioning errors of the probe, occurring when characterizing a long antenna through a non-redundant (NR) near to far-field (NTFF) transformation with helicoidal scan, is developed. The pro- posed technique involves two steps. The former allows the correction of the mispositioning errors, caused by the deviation of each sampling point from the nominal measurement cylindrical surface, using a phase correction technique called Cylindrical Wave correction. The latter restores the samples at the sampling points required by the NR representation along the scan helix from the previous ones affected by 2-D mispositioning errors, via an iterative scheme. Finally, the compensated near-field samples are effectively interpolated via an optimal sampling interpolation (OSI) formula to accurately recover the input data required to perform the traditional cylindrical NTFF transformation. The OSI representation is here developed by assuming a long antenna under test as enclosed in a cylinder terminated by two half spheres (rounded cylinder), in order to make the representation effectively non-redundant. Numerical results, assessing the effectiveness of the proposed technique, are reported.
In this work, an efficient two-step algorithm to compensate for 3-D probe positioning errors, which occur in a near-field–far-field transformation (NF–FFT) using a minimum number of spherical NF measurements, is developed and numerically assessed. Firstly, a so called spherical wave correction is exploited to correct the phase shifts caused by the deviations from the nominal spherical surface. Then, an iterative technique is employed to recover the NF samples at the exact sampling points from those, altered by 2-D mispositioning errors, attained at the previous step. Once the correctly positioned samples have been retrieved in such a way, an optimal sampling interpolation formula is used to accurately determine the massive input NF data for the classical spherical NF–FFT. Numerical tests will be shown to prove the capacity of the devised method to correct even severe 3-D positioning errors.
This paper presents a numerical investigation into the cylindrical mode filtering method and the application of the Compressed Sensing (CS) for far-field single-cut antenna pattern measurements. For measuring a single cut antenna pattern in a far- or quasi-far-field distance, cylindrical mode filtering using an intentional offset effectively removes multipath reflections from the test environment. The CS algorithm enhances this method by enabling sampling on an irregularly spaced grid. This investigation uses numerically simulated data to examine the cylindrical mode filtering method, addressing questions about the mechanism of modal separation and sparsification facilitated by the coordinate translation of the pattern to the rotation center. It also discusses potential limitations of the method, including aspects not previously covered in the literature. We then assess the efficacy of modal recovery via CS compared to FFT and pseudo-inverse methods for both non-sparse and sparse modal data. For non- sparse data, the L1 minimization used by CS can accurately compute the antenna modes but does not offer advantages in terms of reducing the number of samples needed. When the modal data is sparse, the CS algorithm not only allows for irregular sampling but also reduces the number of samples below the Nyquist rate. Additionally, the study evaluates the algorithm’s robustness against added noise and compares its performance with traditional dense data acquisition schemes. The findings provide greater insights into the cylindrical mode filtering method and validate the effectiveness of the CS algorithm.
The size and optics of a Compact Antenna Test Range (CATR) determine its quiet zone and the lowest frequency at which it will meet nominal quiet zone specifications. If a reflector is not large enough to present a sufficient multi-wavelength surface at a given frequency, a plane wave is not generated. Operating compact ranges at lower and lower frequencies is a continuing desire in the measurement community. The normal solution for both instances is to increase the reflector size. This leads to larger test chambers, hence increasing cost. Collecting spherical near- field (SNF) data in a CATR within its normal operating frequency band is well known. However, this leads to collecting more data than required to obtain principal plane cuts in the CATR. This paper presents a study and empirical data on using low-frequency range antennas that operate down to one half the nominal CATR low frequency using SNF techniques to measure test articles at these frequencies with relative accuracy. The paper includes simulations of the quiet zone performance at low frequencies.
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 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.
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.
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.
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
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.
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.
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 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.
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.