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The standard Near-Field antenna characterization allows to reconstruct the Far-Field pattern over the whole visible domain, even if, in many cases, the partial characterization of the Far-Field pattern just along some cuts can be sufficient, and becomes preferred if realized in shorter measurement time with respect to the standard case. A method for Partial Characterization has been proposed. The approach provides a general framework and defines the optimal distribution of the near-field samples required to reconstruct the Far-Field pattern along the cut of interest. The main features of the method are presented, and the performance is verified, experimentally, for two test cases.
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.
It is well-known that any structure in the proximity of a radiating antenna will affect its radiation pattern. This is one of the reasons that a vehicle-mounted antenna tends to be tested while mounted onto the actual vehicle. There is a current discussion regarding how the vehicle should be tested. Traditionally, metallic turntables are used, with the tested vehicle resting on this conductive half-space. Several new spherical near field (SNF) ranges elevate the vehicle over a floor treated with RF absorber to obtain a quasi-free-space pattern. Discussions regarding which method is better are on-going. One of the arguments in favor of the free-space SNF range approach is that, using computational methods, the equivalent radiating currents that radiate the measured near fields, be it over a spherical surface or a non-canonical surface, can be computed. The equivalent radiating currents computed on a triangular element mesh are then imported onto a quadrilateral element mesh on a higher order basis function method of moments (HOBF-MoM) package. Once imported into the HOBF-MoM these currents can be used as excitations to obtain the far field. Within the HOBF-MoMit it is possible to place these equivalent currents in the presence of a metallic (PEC) using symmetry. A new development that allows for the use of an arbitrary Green’s function hence it is possible to get the far field from the computed equivalent currents in the presence of a dielectric half-space. Thus, the theoretical radiation pattern of the vehicle mounted antenna can be computed when the vehicle is on concrete, dirt, or even salt water. In this paper the authors present the latest work performed using this approach to place free space measured antennas over a PEC or dielectric half-space. Results show the potential of this approach. The higher order basis functions allow for the modeling of large structures with reduced number of unknowns. Thus, the antenna under test can be then placed in proximity to not only various half-space materials, but also to towers, buildings, or spacecraft.
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.
A low-cost, custom radar cross-section (RCS) range is designed and built to measure the RCS of wideband linear and circular retrodirective arrays over a ground plane and good accuracy over 1-4 GHz is demonstrated. The only test equipment needed is a vector network analyzer (VNA). The 3.66 m × 1.22 m ground plane is constructed from a thin aluminum foil covering a frame constructed out of plywood and foam panels with wideband probe and wideband arrays inset into the ground plane. Excellent agreement with theoretical and simulated results is demonstrated. Additionally, the measurements validate a previously proposed method for the synthesis of the antenna component of the RCS using only the scattering parameters and embedded element patterns. Extension of this synthesis method into the measurements for the case when no beamforming network is available is demonstrated as well. Time-domain RCS measurements also agree well with theoretical and simulated data, which are used to illustrate the physics of linear Van Atta arrays.
At the Boeing 9-77 Range, we have used various dielectric strings and fishing ropes for target support. An advantage of a string-system is that when not at broadside to the incident wave, the strings would give rise to the least interference to the object being measured. A good example is a 60-ft long rod lifted from ground by the upper turntable (UTT) to the quiet-zone center, rotated horizontally and being measured. [1-2] For the long rod with abruptly terminated ends, there is often a ringing, called the Gibbs phenomenon, which modulates the responses at both ends. Yet, for a 40-ft long vertical metal cylinder supported by a rope through its center, it was curious that the ringing did not show up. By reviewing the metal-dielectric interference, we now realized that the dielectric rope must have contributed an opposite effect such that the ringing ceased. These results are described and discussed. [3-4]
Measuring radar cross-section (RCS) in high-noise environments remains a challenge. This paper presents an advanced signal processing framework that uses statistical dimensionality reduction to effectively separate the signal of interest from environmental noise. The proposed technique consists of two main steps. First, background subtraction and a gating technique are used to preprocess the measured data, separating and extracting the target’s reflectivity distribution from unwanted room contributions. Then, principal component analysis (PCA) is employed to analyze the target’s scattering image and localize its main scattering centers. To validate the proposed algorithm, a perfectly electrically conductive (PEC) scaled UAV model is manufactured and tested. The analysis of the experimental results demonstrates that the suggested technique effectively removes background and clutter, providing reliable RCS measurements in noisy environments.
Phaseless spherical near-field antenna measurements generally address the challenge of computing complex coefficients describing the antenna under test’s (AUT) radiation behavior from amplitude near-field measurements. The AUT’s far-field (FF) can then be obtained from those complex coefficients. Most of the techniques used in the literature result in a modified sampling method (e.g., two-spheres or sphere with two probes) and a phase retrieval algorithm (e.g., WirtingerFlow or PhaseLift). Sampling methods are chosen to increase the number of independent measurements to aid phase recovery. In our contribution, we introduce the approach of random masks, leaning on the concept of diffraction patterns, which is well-known and used in the phase retrieval theory. Random masks can be seen as intentional random perturbations occurring in the measurements either at the AUT, probe or in between; or as an extension to conventional measurements to increase the number of independent measurements. State-of-the-art phaseless sampling methods can also be interpreted as masks with limited randomness. A general mathematical model is presented, and different types of masks based on random distributions are investigated through simulations on a transformation with spherical wave expansion. Firstly, generic masks are considered to benchmark the achievable reconstruction error, and secondly, masks based on probes are examined.
This work presents a fast Planar Near-Field (PNF) technique for characterizing electrically large antennas from sparse acquisitions over localized angular domains employing a robotic arm in non-anechoic environments. The proposed technique has been designed for the effective characterization of complex antenna systems with multiple beams, such as Reflective Intelligent Surfaces (RIS) or Massive MIMO panels for millimeter wave (mmW) 5G communications although it is particularly suitable for any electrically large antenna with narrow and tilted beams. The near-field to far-field transformation is restricted to a reduced angular domain, and the non-redundant acquisition grid is computed by means of Singular Value Decomposition (SVD) techniques so that the number of unknowns of the inverse problem is highly reduced, significantly decreasing acquisition time. The proposed technique has been validated by means of a numerical example in the X-band and a measurement example in the Ka-bands yielding excellent results.
In this paper, we present an overview and comparison of various experimental techniques to identify the room scattering contribution to the overall measurement uncertainty in spherical near-field systems. Our primary objective is to determine the upper bound of the uncertainty due to room scattering in the automotive multi-probe system at the Pulsaart by AGC facility. This facility is designed for comprehensive vehicle testing across a broad frequency range from 64 MHz to 6 GHz. At the lower end of this frequency range, room scattering significantly impacts the overall antenna measurement uncertainty budget, making it crucial to quantify the upper bounds of this error. The considered experimental techniques to determine the room scattering contributions include measuring the same Antenna Under Test (AUT) at multiple translated positions within the chamber, employing advanced post-processing techniques to eliminate room scattering and identify its effects, and combining both approaches. This activity aims to define the room scattering contribution, particularly at lower frequencies, to the range’s overall antenna measurement uncertainty budget.
In this paper, a new approach for planar near-field (NF) measurements for lower band 5G applications is presented employing a customised Vivaldi antenna as a near-field probe. The paper includes a careful analysis of the impact that the absorber collar has on the overall measurement performance of the probe. A 5G, 24-elements, C-band, planar array antenna has been used as an antenna under test (AUT). Full-wave three-dimensional computational electromagnetic simulations (CEM) of the production test, measurement, and calibration of a given planar-near-field measurement setup with, and without, absorber collar, have been undertaken. Here, special attention has been paid to a thorough examination of the presence of scattering, and the standing waves in the simulated near-field measurement. The presence and impact of this phenomenon has been carefully inspected by intensive simulations and compared with results obtained for a standard open-ended waveguide probe (OEWG) probe, as well as with an alternative dielectric probe. The obtained results have demonstrated clear advantages when compared to the alternative solutions with superior results being obtained in terms of the scattering performance. Standing waves and ripple are found to be far less visible with the overall results after probe compensation being noticeably improved when compared with the more commonly used alternatives. We complete this study by verifying the suitability of the proposed Vivaldi probe for Spherical NF measurements by comparing its spherical mode coefficients with that of the ubiquitous OEWG. In conclusion, the Vivaldi probe spans several waveguide bands, and is suitable for planar, cylindrical, and spherical near-field testing applications.
The measurement of active devices or Active Antenna Systems (AAS) necessitates the assessment of transmitter and receiver characteristics, such as radiated power, sensitivity, and occasionally data throughput. The AAS transmit and receive properties can be fully characterized by evaluating two spatial power quantities. Equivalent Isotropic Radiated Power (EIRP) when the AAS operates as a transmitter, and Equivalent Isotropic Sensitivity (EIS) when it functions as a receiver. This testing often requires an Over-the-Air (OTA) measurement setup and is relatively simple to perform when the measurement distance is sufficient for both the probe and AAS to be in the far field. For physically and electrically large AAS’s, this assumption can be hard to satisfy. This paper explores techniques for assessing spatial-directional transmitted and received power-related performance metrics of active devices using spherical near field measurement techniques. This approach can be complemented by phase recovery techniques to enable accurate NFFF transformation. The presented method is validated by experiments on a suitable validation mockup.
We address the generation of complex near-field (NF) wavefronts through a two-step process involving the determination of an equivalent radiating panel and its practical implementation as an array. Our novel approach discretizes 2D radiating panels using an optimized, nonuniform 2D quadrature rule. The optimized quadrature nodes determine the array element locations, while the excitation coefficients are obtained using the Singular Value Decomposition (SVD). Numerical results demonstrate the effectiveness of the method in accurately generating NF waveforms.
The problem of calculating antenna phase centre from the phase pattern measured at finite measurement distance is investigated in detail and the corresponding geometrical problem is solved analytically. An accurate formula is derived for calculating the phase centre location from the data measured at finite distance, which still needs to satisfy the usual far field criterion. The calculated phase centre location is compared to that calculated from the far field results obtained from spherical near- field measurements of the same antenna. The importance of the potential error in the assumption of the phase centre location and its effect in antenna gain determination is further illustrated for typical test conditions, with simple criteria for the maximum allowed error for the determination of the phase centre location being proposed and discussed.
Over the past two decades, many measurement facilities have been involved in various international comparison campaigns led and supported by the European Association on Antennas and Propagation (EurAAP) working group (WG) on measurements (WG5). Its activities cover various areas of antenna measurements. These activities play an important role for the documentation and validation of laboratory proficiency and competence, helping to improve the antenna measurement procedures/protocols in facilities and standards such as ISO 17025, IEEE 149, IEEE 1720. The analysis and data elaboration have promoted discussions among the antenna measurement community experts and have led to modernization of comparison techniques. This paper highlights some selected EurAAP WG5 activities including, for example, international antenna measurement intercomparisons, self-assessment measurements of facilities, outreach collaborations and outcome disseminations (e.g., revisions of international standards for antenna measurements).
With the increasing need for higher quality automotive connectivity and location services, full-vehicle over- the-air testing has become a topic of growing interest. As the size and costs of ranges required for such tests is however prohibitive, many companies have looked into combining antenna measurement and electromagnetic compatibility tests into the same chamber environment. This paper provides an overview analysis of the complexity and constraints associated with such a choice. A numerical investigation of an entire test range is offered to derive conclusions on possible site limitations relating to unwanted reflections within a dual-purpose chamber.
In this paper, the Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) of Narrowband Internet of Things (NB-IoT) over Non-Terrestrial Networks (NTN) in the Anechoic Chamber (AC) are investigated. The following contributions are presented. Firstly, for the first time based on authors' best knowledge, this paper presents the Over-the-Air (OTA) TRP/TIS performance of NB-IoT NTN in the AC. Secondly, detailed analysis including test repeatability, measurement uncertainty, and test time etc. are given as well. The early exit algorithm for NB-IoT TIS based on Chi-Square distribution is investigated, and significant improvements have been found from the test results. Thirdly, various key NB-IoT NTN specific parameters, such as Round-Trip Time (RTT), Doppler shift, signal fading, and others have been discussed too. For TIS, we use 2 different RTT configurations (0 ms and 255 ms) to perform the tests, and give the corresponding analysis. Finally, some suggestions are being proposed for the future test methodology developing.
A previously presented prototype of a microwave oil/water ratio measurement system for geological applications has provided positive and applicable results in a commercial flow loop test. The prototype is a two-port circular waveguide traversed by a pipe. Both waveguides face each other on opposite sides of the pipe and are filled with dielectric material, which is molded to match the inside walls of the pipe. For maximizing transmission, avoiding higher order modes, the microwave waveguides in the system are designed with a diameter slightly smaller to that of the pipe. To expand the applications of the current system, a more thorough understanding of the waveguide propagation on a transversal pipe system and its limitations is needed. However, the complexity of the geometry makes the problem analytically intractable. Three dimensional simulations for transversal pipes geometries were performed in COMSOL 6.2 from 1 GHz to 4 GHz. Initial numerical results show waveguide mode propagation in the pipe at lower frequencies and pipe mode propagation at frequencies passed the optimum transmission frequency where the horizontally propagating waveguide/pipe mode wavelength are slightly smaller than the diameter of the waveguide/pipe intersection. Higher order modes can be seen in both waveguide and pipe at frequencies above 2.5 GHz.
This paper provides a brief overview of two new concepts of plane wave generators (PWGs) for applications in antenna and wireless device measurements. These PWGs aim to increase flexibility in emulating various multipath testing conditions within the same measurement environment, addressing the high cost and complexity of conventional systems. The first concept, utilizing a chamber antenna array (CAA) inside an overmoded waveguide (WG), leverages the reflecting walls of the metal rectangular WG in conjunction with a CAA to synthesize obliquely incident plane-wave fields at the device under test. This has been demonstrated for telecom sub-6 GHz applications (e.g., for total radiated power and pattern measurements), with ongoing studies on its evolution to Reconfigurable Intelligent Surface (RIS) based PWGs for millimeter-wave frequencies. We discuss the design and analysis foundations, highlighting key requirements and limitations to maximize flexibility. We use simulations with a customized modeling framework and measurements from our first prototype systems: an overloaded waveguide (WG) chamber with a 6 × 7 CAA at 915 MHz and a 16 × 16 RIS PWG at 28 GHz. WG-based PWG is evaluated using metrics such as uniformity, angular spread, and bandwidth. For the RIS PWG prototype, we demonstrate the practically achievable complex reflection coefficient values to evaluate the requirements for PWG applications.
We discuss an approach to represent the electromagnetic field radiated/scattered by oblong objects. The field representation exploits Vector Prolate Spheroidal Wave Functions (VPSWFs) which are computed by a stable and accurate numerical scheme and take into account the a priori information about the geometry of the radiator/scatterer. The presented numerical results highlight the satisfactory accuracy and the convenience of the approach against the classical representation by spherical harmonics.