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Robot-Based Antenna and Radar Measurement System at the RWTH Aachen University
High precision antenna measurements can be carried out in different ways, for example directly in the far-field, in a compact antenna test range (CATR) or in a near-field range. Far-field measurements have the disadvantage that they require a lot of space and are sensitive to environmental influences. Although these effects can be avoided with a CATR, these systems are demanding and expensive. For this reason, simplifying near-field measurement setups are often preferred, whereby three main methods have become established: planar near-field (PNF), cylindrical near-field (CNF) and especially spherical near-field (SNF) measurements. Current measurement setups are usually optimized for one of these techniques and are therefore only of limited use for the other methods. Recent developments have focused on robotic measurement systems, which are able to overcome the boundaries between the different measurement techniques. Due to the high number of degrees of freedom and an almost unlimited positioning and orientation possibility in space, these systems offer numerous advantages due to their flexibility. This allows the same measurement system to be used seamlessly to perform PNF, CNF and SNF measurements. Even for more demanding measurement procedures, for example based on compressed sensing, robotic systems create the possibility to efficiently approach the required sampling points. At the Institute of High Frequency Technology at RWTH Aachen University, such a robot-based measurement setup is currently being established. In addition to the six-axis robot arm, the system has two additional axes, specifically a linear axis on whose slide a rotary axis is mounted. In the currently used configuration, the antenna under test is mounted on the axis of rotation, while the probe antenna is installed at the robotic arm. This results in a total of eight degrees of freedom combined in a novel test range design, which distinguishes the measurement setup from existing ones. Further details on the measurement setup, the implementation of antenna and radar measurements and the operation of the entire system will be explained.
Standard Definitions of Terms for Antennas and Radio Wave Propagation
The standard definitions of terms for antennas and radio wave propagation are provided in IEEE Std. 145-2013 and IEEE Std. 211-2018, respectively. These documents define the accepted and harmonized terminology used in the fields of antennas and propagation. Arbitrary use and misuse of fundamental terms are still observed in technical papers and presentations at conferences.
Influence of the Phase Uncertainty in Spherical Wave Expansion in the Millimeter-Wave Range
Phase uncertainty in antenna measurements introduces significant errors to the amplitude of the transformed pattern in Spherical Wave Expansion (SWE). To get a better understanding of the impact of phase errors, the measured phase error of a Low Noise Amplifier (LNA) is synthesized as a random phase error and subsequently added to the measured antenna patterns of three different antennas during the SWE. The resulting erroneous patterns are compared with the measured reference patterns and the error magnitude and probability distribution are studied. It is proven that the introduced errors to the transformed far-field patterns can be substantial. Furthermore, the relation between the antenna type and the error level and distribution is elaborated. The error level is different for the three antennas and the error level distribution is dependent on the mode spectra of the antennas.
2D RCS Prediction from Multistatic Near-Field Measurements on a Plane by Single-Cut Near-Field Far-Field Transformation and Plane-Wave Synthesis
A near-field far-field transformation (NFFFT) technique with a plane-wave synthesis is presented for predicting two-dimensional (2D) radar cross sections (RCS) from multistatic near-field (NF) measurements. The NFFFT predicts the FF of the OUT illuminated by each single source, then the plane-wave synthesis predicts the FF of the OUT each illuminated by each plane-wave by synthesizing the FFs given in the NFFFT step. The both steps are performed in the similar computational procedure based on a single-cut NFFFT technique that has been proposed previously. The method is performed at low cost computation because the NF and source positions are required only on a single cut plane. The formulation and validation of the method is presented.
A Straightforward Dynamic Range Error Analysis
The significant measurement standards in the antenna measurement community all present suggested error analysis strategies and recommendations. However, many of the factors in these analyses are static in nature in that they do not vary with antenna pattern signal level or they deal with specific points in the pattern, such as realized gain, side lobe magnitude error or a derived metric such as on-axis cross polarization. In addition, many of the constituent factors of the error methods are the result of analyses or special purpose data collections that may not be available for periodic measurement. The objective of this paper is to use only a few significant factors to analyze the error bounds in both magnitude and phase for a given antenna pattern, for all levels of the pattern. Most of the standards metrics are errors of amplitude. However, interest is increasing in determining phase errors and, hence, this methodology includes phase error analysis for all factors.
Measurements of Non-Metallic Targets for the Austin RCS Benchmark Suite
A simulation-supported measurement campaign was conducted to collect monostatic radar cross section (RCS) data as part of a larger effort to establish the Austin RCS Benchmark Suite, a publicly available benchmark suite for quantifying the performance of RCS simulations. In order to demonstrate the impact of materials on RCS simulation and measurement, various mixed-material targets were built and measured. The results are reported for three targets: (i) Solid Resin Almond: an almond-shaped low-loss homogeneous target with the characteristic length of ~10-in. (ii) Open Tail-Coated Almond: the surface of the solid resin almond's tail portion was coated with a highly conductive silver, effectively forming a resin-filled open cavity with metallic walls. (iii) Closed Tail-Coated Almond: the resin almond was manufactured in two pieces, the tail piece was coated completely with silver coating (creating a closed metallic surface), and the two pieces were joined. The measured material properties of the resin are reported; the RCS measurement setup, data collection, and post processing are detailed; and the uncertainty in measured data is quantified with the help of simulations.
Personal Near-field System
In 1987 the author built the world's first Personal Near-field antenna measurement System (PNS). This led to the formation of Nearfield Systems Inc. (NSI) a company that became a major manufacturer of commercial near-field antenna measurement systems. After leaving NSI in 2015 several new personal antenna measurement tools were built including a modern updated PNS. The new PNS consists of a portable XY scanner, a hand held microwave analyzer and a laptop computer running custom software. The PNS was then further generalized into a modular electromagnetic field imaging tool called "Radio Camera". The Radio Camera measures electromagnetic fields as a n-dimensional function of swept independent parameters. The multidimensional data sets are processed with geometric and spectral transformations and then visualized. This paper provides an overview of the new PNS and Radio Camera, discusses operational considerations, and compares it with the technology of the original 1987 PNS. Today it is practical for companies, schools and individuals to build low-cost personal antenna measurement systems that are fully capable of meeting modern industry measurement standards. These systems can be further enhanced to explore and visualize electromagnetic fields in new and interesting ways.
Experimental Validation of a Non-Redundant NF-FF Transformation for Long AUTs Mounted in Offset Configuration in a Spherical NF Facility
The experimental validation of an accurate and fast near-field-far-field (NF-FF) transformation technique with spherical scan, suitable for long antennas under test (AUTs) mounted in offset configuration, is provided in this work. The main feature of such a NF-FF transformation is to require, unlike the traditional spherical (TS) one, an amount of NF samples, which is minimum and results to be practically the same in both cases of offset and onset mount-ings of the AUT. To this end, an optimal sampling interpolation formula , developed by properly exploiting the non-redundant sampling representations and modeling an offset mounted long AUT by a cylinder ended by two half-spheres, is employed to precisely recover the massive input NF data for the TS NF-FF transformation from the collected non-redundant samples. A considerable measurement time-saving can be so achieved. Experimental results assessing the validity and the practical feasibility of such a technique are shown.
De-embedding Radome Depolarization from Measurements with a Non-Ideal Circularly Polarized Antenna Source
An approximation method is developed to remove the source antenna's cross-polarization discrimination (XPD) contribution from the total measured XPD. This modeling is shown to correlate very well on a flat-panel test with a radome's predicted (ideal-source) XPD. Additionally, a mathematical formulation of the theoretical cross-polarization discrimination (XPD) bounds is presented to validate the proposed method. The measured axial ratio should not exceed these bounds. The measured result is within these bounds and thus this model serves as an additional validation step to both the proposed method and the measured results.
Indoor 3D Spherical Near Field RCS Measurement Facility: A new high resolution method for 3D RCS Imaging
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment . This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. 3D imaging is a suitable tool to accurately locate and characterize in 3D the main contributors to the RCS. However, this is a non-invertible Fourier synthesis problem because the number of unknowns is larger than the number of data. Conventional methods such as the Polar Format Algorithm (PFA), which consists of data reformatting including zero-padding followed by an inverse fast Fourier transform, provide results of limited quality. We propose a new high resolution method, named SPRITE (for SParse Radar Imaging TEchnique), which considerably increases the quality of the estimated RCS maps. This specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. In this paper, this algorithm is tested on measured data from a metallic target, called Mx-14. It is a fully metallic shape of a 2m long missile-like target. This object, composed of several elements is completely versatile, allowing any change in its size, the presence or not of the front and / or rear fins, and the presence or not of mechanical defects, … Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Fast Spherical Near-Field Measurements on Arbitrary Surfaces by Application of Pointwise Probe Correction to Compressed Sampling Schemes
The major disadvantage of Spherical Near-Field (SNF) measurements is their long acquisition time. To calculate the Antenna Under Test's (AUT) far-field radiation characteristics , a sphere containing the AUT must be sampled. Classically, equiangular sampling is chosen, being the resulting sphere heavily oversampled. Since the Spherical Mode Coefficients (SMCs) are usually sparse, an approach to reduce the measurement time of SNF measurements is to undersample the sphere and to reconstruct the SMCs using compressed-sensing techniques. Using a sampling matrix with a minimum mutual coherence for the given bases of the SMCs increases the probability of recovery. The SMCs are defined in the basis of the spherical harmonics or Wigner D-functions, which limits the geometries in which this technique can be applied. In this work, the application of pointwise probe correction for the description of non-spherical surfaces in the Wigner-D basis expansion is suggested. The chosen sampling points are radially projected onto the measurement surface and the new distance to each point is calculated. New equivalent probe response coefficients are calculated per measurement point according to their distance to the AUT. To compensate for different orientations other than the probe pointing to the AUT's minimum sphere's center, the probe's SMCs are rotated to reflect the real orientation of the probe at each point prior to the calculation of the probe response coefficients. Although more computationally demanding than classical probe correction, this technique allows measurements with different, potentially faster geometries and enables the application of compressed sensing to other, non-spherical conventional scanning systems.
Measurements on extended objects for radar field probes
An extended long object usually gives rise to a bright reflection (a glint) when viewed near its surface normal. To take advantage of this phenomenon and as a new concept, a discrete Fourier transform (DFT) on the RCS measurements, taken within a small angular range of broadside, would yield a spectrum of incident wave distribution along that object; provided that the scattering is uniform per unit length, such as from a long cylinder [1, 2]. In this report, we examine the DFT spectra obtained from three horizontal long objects of different lengths (each of 60, 20, and 8 feet). Aside from the end effects, the DFT spectra looked similar and promising as an alternative to the conventional field probes by translating a sphere across a horizontal path. Keywords: RCS measurements, compact range, field probes, extended long objects 1. The Boeing 9-77 compact range The Boeing 9-77 indoor compact range was constructed in 1988 based on the largest Harris model 1640. Figure 1 is a schematic view of the chamber, which is of the Cassigranian configuration with dual-reflectors. The relative position of the main reflector and the upper turntable (UTT) are as shown. The inside dimensions of the chamber are 216-ft long, by 80-ft high, and 110-ft wide. For convenience, we define a set of Cartesian coordinates (x: pointing out of the paper, y: pointing up, z: pointing down-range), with the origin at the center of the quiet zone (QZ). The QZ was designed as an ellipsoidal volume of length 50-ft along z, height 28-ft along y, and width 40-ft along x. The back wall is located at z = 75 ft, whereas the center of the roll-edged main reflector (tilted at 25 o from vertical) is at z =-110 ft. It is estimated that the design approach controls the energy by focusing 98% of it inside the QZ for target measurements. The residual field spreading out from the main reflector was attenuated by various absorbers arranged in arrays and covering the chamber walls.-, Tel. (425) 392-0175 2. Anechoic chamber In order to provide a quiet environment for RCS measurements, the inside surfaces of an anechoic chamber are typically shielded by various pyramidal and wedged-shaped absorbers, which afford good attenuation at near-normal incidence for frequencies higher than ~2 GHz. At low frequencies and oblique angles , however, Figure 1. A schematic view of the Boeing 9-77 compact range with dimensions as noted. insufficient attenuation of the radar waves by the absorbers may give rise to appreciable backgrounds. Figure 2 shows a panorama view inside the compact range, as viewed from the lower rear toward the main reflector and the UTT. With the exception of the UTT, all other absorbers are non-moving or stationary. A ring of lights on the floor shows the rim around the lower turntable (LTT), prior to the installation of absorbers. In order to minimize the target-wall interactions, the surfaces facing the QZ from the ceiling, floor, and two sidewalls are covered with the Rantec EHP-26 type of special pyramidal absorbers.
Testing mmWave Phased Arrays for the 5G New Radio
As the wireless industry continues the move to 5G, the development and subsequent testing of mmWave radios for both base stations and user equipment still face numerous hurdles. The need to test most conformance and performance metrics through the antenna array at these frequencies poses significant challenges and has resulted in excessively large measurement uncertainty estimates to the point where the resulting metrics themselves may be useless. A large contribution to this measurement uncertainty is the impact of the over-the-air (OTA) test range used, driving the industry towards expensive compact range reflector systems in order to overcome the path loss considerations associated with direct far-field measurements. However, this approach necessitates the use of a combined axis measurement system, which implies the need for considerable support structure to hold the device under test and manipulate it in two orthogonal axes. This paper explores some of the limitations and considerations involved in the use of traditional "RF transparent" support materials for mmWave device testing.
Measurement procedures and post-processing for fast antenna characterization
Three measurement procedures and associated post-processing for the fast characterization of antennas are presented. First, an approach for the fast diagnosis of antenna under test (AUT), ie. the identification of potential defaults with respect to an ideal antenna, is described. The technique leverages the knowledge of the ideal (expected) radiation pattern and uses a sparse recovery algorithm to locate the few potential defaults. Second, a scheme is proposed to interpolate the near field radiated by the AUT. It exploits the low complexity of the electromagnetic field and does not resort to any knowledge on the AUT. Third, an approach to speed up the measurement of the AUT far field radiation pattern is detailed. The only input is the maximum dimension of the AUT. The technique relies on the sparse expansion of antenna radiation patterns on spherical harmonic basis. For each of the three examples, experimental results will be shown for various complex radiating structures in different frequency bands.
Validation of Electromagnetic Compatibility Chambers with a Spherical Wave Expansion Approach
Electromagnetic Compatibility (EMC) radiated emissions measurements above 1 GHz are performed in a nominal free space environment as required by international standards, typically in an anechoic chamber. In an EMC chamber, the test zone consists of a circular region defined by a turn table, where an equipment under test is rotated and measured. The test zone is commonly referred to as quiet zone (QZ). Due to the non-ideal nature of absorbers, multiple reflections in the chamber affect the quality of the QZ. The constructive and destructive interferences from the reflections form standing waves in the QZ. The maximum value of the standing wave is used as the figure of merit for validation of testing facilities. Site Voltage Standing Wave Ratio (sVSWR) as specified in CISPR 16-1-4 is broadly used for the validation of test sites above 1 GHz. This method requires the measurement of six positions along a linear 40 cm transmission path at various locations in the QZ, with a frequency step of no greater than 50 MHz using an omnidirectional-like antenna (e.g. a dipole). Concerns have been raised that this method delivers an overly optimistic result due to both spatial and frequency domain undersampling. In this work, an alternative method to sVSWR for the validation of EMC chambers based on Spherical Mode Coefficients (SMC) is proposed. Two 90 •-rotated measurement cuts of an omnidirectional-like antenna are acquired around the periphery of the circular QZ. The measured situation and cut is replicated by applying translation and rotation of spherical waves to the known SMCs of the used omnidirectional-like antenna and transforming using the spherical wave expansion. The generated and measured cut are compared and the characteristics of the chamber are extracted. The major advantage of this method is the relatively high measurement speed and reliability.
Bandwidth response of a reflectarray antenna working as a Compact Antenna Test Range probe
A reflectarray antenna working at 28 GHz is proposed to replace the reflector antenna of a Compact Antenna Test Range (CATR) system. As a first approach, the quiet zone obtained using a far-field collimated reflectarray is analysed. Due to the size of this area is not large enough, the generalized Intersection Approach is employed to carry out an optimization of the near-field for both phase and amplitude in order to maximize the size of the quiet zone at one plane. Simulations are compared for the near-field before and after the optimization process, showing an important enhancement of the size of the quiet zone, especially in the main cuts. From the obtained phase distribution a design is carried out. The unit cell chosen is based on a two-layer stacked patch, having good agreement between optimization and design results. Finally, the bandwidth response of the designed reflectarray is analysed, in order to assess its performances as probe in a CATR system.
Near-field multi-focusing transmitarray optimization for multi-position feed
A transmitarray antenna is proposed as a multi-focusing antenna in the near-field region with capability for focus scanning and/or simultaneous and independent focus spots generation at 28 GHz. The transmitarray optics is defined for a centred configuration and the elements are designed to focus the radiated near-field at a given point. Then, a number of feeds is placed along arcs in the principal planes and the near-field generated by the transmitarray when its illuminated by each one is obtained, demonstrating the capability to generate multiple independent near-field spots. The focusing performance is improved for the centered feed through a Phase-Only synthesis technique based on the generalized Intersection Approach in near-field. Finally, the spots produced by the whole cluster are calculated, demonstrating the overall improvement and validating the designing process. This configuration can be applied in near-field systems as radar for surface inspection, measurement systems or wireless power transfer among others.
Non-contact Characterization of Antenna Impedance, Gain and Pattern through Open-Fixture Network Calibration
We present a novel, non-contact characterization technique for simultaneous characterization of conventional antenna parameters, including the antenna port input impedance, antenna gain and its radiation pattern, without requiring a network analyzer connection to the antenna port. The test antenna and the network analyzer are considered as a 2-port open-air fixture whose network representation corresponds to the desired antenna parameters. The unknown network parameters of the 2-port open-air fixture are determined via a novel calibration process using 4 offset-short termination standards. The error parameters determined by the calibration are then related to the test antenna port impedance and its gain as a function of frequency. Furthermore, the radiation pattern of the test antenna can also be characterized using measured reflection coefficient at the network analyzer port for two offset-short terminations of the test antenna port, while rotating the test antenna over the desired angular range. This novel technique is particularly attractive for installed-antenna applications where an active connection to the test antenna port is either difficult or undesirable, such as on-chip antennas and implanted antennas, to name a few. To demonstrate the efficacy our new method, we present the measured impedance, gain and radiation pattern of a diagonal-horn antenna operating over 360-450 GHz, and a lens-integrated planar butterfly antenna for the 220-325GHz band.
Generalized Test-Zone Field Compensation
Antenna measurement errors occur due to reflections and diffractions within the measuring chamber. In order to extract and correct the undesired signals, a technique based on test-zone field compensation and spherical wave expansion is applied to Compact Antenna Test Range (CATR) and Spherical Near-Field (SNF) measurements of a base transceiver station antenna. The required spherical test-zone field is acquired by simulating the corresponding measurement environment with the multi-level fast multipole method. Due to the numerical complexity of the problem, only the parts of the chamber with a significant influence on the measurement results are modeled. Comparing the determined directivities after applying the correction method, an exact overlap is achieved between the SNF and CATR solution.
Experimental validation of Reference Chip Antennas for 5G Measurement Facilities at mm-Wave
In this paper, the experimental validation of a micro-probe fed reference antenna targeting the upcoming 5G applications (24.25-29.5GHz band) is presented. The main purpose of these reference antennas is to serve as "gold standards" and to perform gain calibration of 5G test facilities through the substitution method. The outline of these antennas is based on a square array of four printed patches enclosed in a circular cavity. The RF input interface is a stripline-to-coplanar waveguide transition and allows for feeding the device with a micro-probe. Performance obtained by high-fidelity modeling is reported in the paper and correlated to experimental data. Interaction and unwanted coupling with the test equipment are discussed. The use of echo-reduction techniques and spatial filtering is investigated to mitigate these effects.
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