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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.
The characterization of antenna radiation patterns in the millimeter wave band are particularly challenging. This is due to the fact that a misalignment of just a few millimeters between the probe and the antenna can generate substantial measurement errors. This paper describes a strategy to reduce measurement errors by introducing a highly precise measurement system using a 6-axis small robotic arm to characterize the performance of a phased array antenna operating at 60 GHz. The position accuracy of the robotic arm itself is approximately 20 m and a maximum far field distance of approximately 380 mm can be achieved. The robot is programmed to perform a spherical trajectory around the array with stops every 0.5⁰ along the path to gather the measured gain. It operates continuously by communicating with a computer, which triggers the network analyzer at preprogrammed locations. The system is tested initially using two horn antennas as the antenna under test (AUT), and the results are presented.
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.
Over-the-air (OTA) performance evaluation requires large investments in anechoic environments. The question of minimizing the test distance is hence critical, and even more in this time where millimeter-wave technologies are about to be largely deployed in 5G devices. A recent publication has identified that direct far-field measurements can be accurately carried out at a much shorter range length than the well-known Fraunhofer distance. This paper introduces a further validation of this reduced distance, by employing an innovative method to simulate spherical measurements with arbitrary DUT, test probes and range lengths. The studies carried out confirm the relevance of this shorter distance, not only for the evaluation of the peak equivalent istropic radiated power (EIRP) or sensitivity (EIS), but also for the total radiated power (TRP) or sensitivity (TIS). In addition, it is demonstrated that the usual assumption that the TRP or TIS measurement is almost independent from the range length is flawed. Two main reasons relating to the test antenna are established which create this dependence: (i) OTA test probes have a finite resolution, and (ii) the probe and instrumentation typically captures the magnitude of two components of the E-field, which are not straightforwardly related to the power density in the near-field.
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.
Wireless industry through 3GPP has standardized 5G in both FR1 (sub 6 GHz) and FR2 (24.25-52.6 GHz) frequency ranges. While FR1 will be using frequencies already in place for LTE-4G technology, FR2 is dealing with mmWave frequencies. Due to the high free space path loss (FSPL), 5G at mmWave would impose the use of directive antennas on both ends of the communication link, the User Equipment (UE) and the Base Station (BS). A black box approach (i.e. the location of the antenna within the device is unknown) has been agreed to be used for Over The Air (OTA) measurements. The physical center of the device must be aligned with the center of the measurement setup. Hence, the test antennas will likely be offset with respect to the center of the coordinate system. The measurement distance will be for most systems sufficient to minimize the amplitude error while will introduce a phase deviation between the actual spherical wave and the desired plane wave which may cause an effective phase shaping of the radiated beam of the small phased array under test. In this paper we will analyze the impact of the phase curvature on the beam antenna pattern and spherical coverage for the different testing environments. Specifically, simulation of a 5G terminal device with multiple beams will be considered and realistic spherical near field measurement at different finite distances will be emulated also taking into account different measurement antennas (probes).
The Plane Wave Generator (PWG) is an array of elements generating an approximately plane wave over a finite volume in the test area called Quiet Zone (QZ). The plane wave condition can be achieved in close proximity to the array with suitably optimized complex coefficients. The PWG thus achieve far-field testing conditions in a manner similar to the Compact Antenna Test Range (CATR) but with a reduced distance to the QZ [1-2]. As a complete system the PWG has the advantage of reduced physical size compared to the a CATR with equivalent testing capabilities, in particular at lower frequencies. In [3-4], the concept of a high performance, dual polarized PWG supporting up to 1:10 bandwidth was presented. A prototype of a dual polarized PWG has been designed, manufactured and tested in the 600MHz to 6GHz frequency range. This paper presents the initial verification of the prototype PWG. The testing is performed using a representative analog beam forming network with narrow bandwidth. The QZ uniformity of the PWG is verified by spherical near-field measurements and back-propagation. The peak gain of a low directivity antenna is measured at different distances in the QZ and compared to reference measurements in a spherical near-field system. The aim of the comparison is to access the measurement accuracy of the PWG.
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.
In vehicle communications, so as Vehicle-to-X (V2X), field trials are challenging due to high mobility scenarios and dynamic network conditions. It is complex to interpret measurements, to isolate performance from different components in an integrated system. Consequently, it is desirable to test under repeatable laboratory conditions in the early stages of the development cycle, where designers can quickly validate performance and make rapid modifications to prototype hardware and software cost-effectively. Virtual Drive Test (VDT) has attracted great interest from industry and academia. The objective of VDT is to recreate an approximation of the real-world communication conditions in a controlled laboratory environment. VDT is appealing, since testing can be performed in an automated, controllable and repeatable manner, which can considerably reduce testing time and costs, and meanwhile accelerate actual infrastructure deployment. In this paper we present a new VDT technique which allows to evaluate the V2X communications performances taking into account the measured characteristics of transmit and receive antennas installed on vehicles. The proposed VDT technique is a multistage process where radiation characteristics of the vehicle mounted antennas are first measured in free-space conditions in a controlled and repeatable laboratory environment. The Spherical Wave Expansion (SWE) is then applied to the acquired data in order obtain the Spherical Wave Coefficients (SWC) of the measured devices. From the SWC, the transmission formula (or coupling equation) normally involved for probe correction purposes in spherical near field measurements, is then applied in order to evaluate the coupling between two vehicles. The transmission formula has been properly adapted in order to consider variable distances between the vehicles and arbitrary vehicle orientation so that a generic road path can be easily emulated. In the proposed formulation also variable ground conditions can be considered allowing for a more realistic emulation of the final environment. The proposed technique is presented taking into account measurements of a representative scaled automotive scenario.
This paper presents a new type of P-band first-order dual-port probe for spherical near-field antenna measurements. The probe is based on the well-known shorted annular patch antenna but some extensions are introduced for the probe application. This probe is mechanically simple which facilitates its manufacturing and operation. In addition, it has high performance for impedance bandwidth, pattern, directivity, and gain.
The major drawback of Spherical Near-Field (SNF) measurements is the comparatively long measurement time, since the scanning of a whole sphere enclosing an Antenna Under Test (AUT) is required to calculate the Spherical Mode Coefficients (SMCs) required for the computation of the far field. Since the SMCs prove to be sparse under certain conditions, efforts have been made to apply compressed-sensing techniques to reduce the measurement time by acquiring a smaller number of sampling points. These approaches have been successfully tested in simulation using classically acquired measured data. This decouples the measurements from practical problems, such as basis mismatch due to the finite precision of the mechanical positioner and environment effects. In this paper, results from a sparse data acquisition performed with a physical system are reported. To decouple the error introduced by the approach itself from the error introduced by non-idealities in the measurement system, an AUT is measured using both traditional near-field sampling and compressed near-field sampling. The classically acquired data is used both as reference and as source to simulate a synthetic compressed measurement. The effects introduced by real considerations are calculated by comparison between the synthetic compressed measurement and the acquired one, while the error of both is evaluated by comparison to the reference measurement. The results further demonstrate the viability of this method to accelerate SNF measurements and pave the way for further research.
The Three-antenna polarization measurement technique is used to determine the axial ratio, tilt angle and sense of polarization of three antennas from measurements on each of three antenna pairs. The three antennas are generally nominally linearly polarized and the measurement data consists of the change in amplitude from the initial antenna orientation where they are co-polarized to the orientation where one of the antennas is rotated about its axis to the null amplitude position. The sign of the phase change is also noted and the phase change at the null position is known from theoretical calculations to be either plus or minus 90 degrees. The correct sign is determined from the sign of the phase change. For antennas with axial ratios in the range of 50 to 80 dB that will be used as near-field probes or as feeds for reflector antennas, it is imperative to measure the polarization parameters as accurately as possible. The primary source of uncertainty in the measurement is due to scattered signals in the measurement range that arise from multiple reflections between the two antennas and from the absorber on the chamber walls. For antennas with very large axial ratios, the scattered signals can be larger than the true measurement signal. These scattered signals can change the sign of the phase and produce large errors in the amplitude at the null. If the separation distance between the antennas is adjusted after rotating to the null to produce a maximum amplitude, the scattered signal is in phase with the true measurement signal. If the distance is adjusted for the minimum at the null, the scattered and true signals are out of phase. Measurements at these two positions will produce the best measurement of the phase sign and the true amplitude. But if measurements are being performed at a number of frequencies, the maximum and minimum amplitude positions will be different for each frequency, and this will complicate automated multifrequency measurements. New improvements have been developed in the details of the measurements that greatly improve the determination of the phase sign and the amplitude at the null for multiple frequency measurements and these will be described and illustrated in the following paper. With these improvements, the estimated uncertainty of a 60 dB axial ratio is on the order of 1.8 dB. A new technique has also been developed to improve the source correction of the pattern data for probes with large axial ratios that guarantees that the on-axis polarization of the pattern data will be identical to the results of the Three-antenna measurement. The probe correction processing will then produce the highest accuracy results for the polarization of the AUT.
A robotic near-field antenna measurement system allowing for acquisition over non-canonical measurement surfaces is presented. The robot consists of a six-axis robotic arm and a seventh axis rotary positioner and the created acquisition surface is parametrically reconfigurable. The near-field to far-field transformation required is also described. The success of the technique is demonstrated through measured results, compared to canonical measurement data.
This paper introduces the new Portable Laser Guided Robotic Metrology (PLGRM) system at the National Aeronautics and Space Administration's (NASA) Glenn Research Center. Previous work used industrial robots in fixed facilities to characterize antennas and required fixtures that do not lend themselves to portable applications. NASA's PLGRM system is designed for in-situ antenna measurements at a remote site. The system consists of a collaborative robot arm mounted on a vertical lift and a laser tracker, each on a mobile base. Together, they enable scanning a surface larger than the robot's reach. To accomplish this, the robot first collects all points within its reach, then the system is moved and the laser tracker is used to relocate the robot before additional points are captured. The PLGRM implementation will be discussed including how safety and planning are combined to effectively characterize antennas. Software defined triggering is a feature, for flexible integration of vector network analyzers and antenna controllers. Lastly, data will be shown to demonstrate system functionality and accuracy.
In this paper, we will discuss the impact of mounting structures on the installed performance of phased arrays. In particular, performance data for the Conformal, Lightweight Antennas for Aeronautical Communications Tech-nology (CLAS-ACT) antenna will be presented. Performance data from a series of mounting configurations will show that null depth and location is particularly susceptible to change while the main beam steering angle remains relatively stable. In addition, the Portable Laser Guided Robotic Metrology (PLGRM) system will be discussed as a suitable instrument for measuring antenna patterns in complex or difficult locations that are challenging for traditional ranges. The PLGRM system was recently developed at the National Aeronautics and Space Administration's (NASA) Glenn Research Center (GRC) and deployed to measure in situ antenna patterns.
A dual linear polarized 3D printed magneto-electric phased array antenna for various 5G New Radio (NR) frequency bands is proposed and its beam steering performance is investigated. The magneto-electric radiating element exhibits a well-defined stable pattern quality, low variation in the impedance over a wider bandwidth and high port to port isolation in a dual polarization configuration. The analog beamforming network (BFN) of the array is also designed. The fabricated board will be combined with the 3D printed array aperture for experimental verification of the scan performance.
This paper presents a technique for reduction the acquisition time in the measurement of antennas in spherical near field systems using multifrequency on-the-fly acquisitions. When these acquisitions are performed, a shift in the pattern is performed. This shift appears in polarization when the scan is in roll axis, or in pointing direction when the scan is azimuth or elevation axis. In any case, this shift has to be corrected in order to compensate the different trigger time for each frequency point. This paper presents a technique for compensating this rotation in spherical near field system, by rotation of the radiation pattern in the scan axis through an easy calculation of the that shift.
The aim of the paper is to address a relevant issue in the Near-Field (NF) measurements: the reduction of the measurement time. Generally speaking, for a given hardware, two main directions can be pursued. The first requires the adoption of an optimal field sampling strategy that reduces the number of sampling points, and the length of the scanning path, without impairing accuracy. The second strategy adopts an optimized control system able to exploit at the best the available hardware (scanning system and measurement instrument). Indeed, the latency of the instrument defines the maximum probe velocity during the field acquisition. Accordingly, unlike the conventional continuous scanning, an optimized controller can speed up the scanning by moving the probe along the measurement trajectory with a variable velocity, accelerating and decelerating between two consecutive sampling points, to increase the average speed. However, the use of an optimized controller is fruitful only when the optimized sampling scheme allows large distances between two consecutive sampling locations, to increase as much as possible the maximum probe speed. In this paper, by suitably using both the above strategies, it is proposed a fast NF system, implemented on a microcontroller Arduino Due, an extremely cheap and off the shelf hardware, that is able to handle the scanner and realize the synergy between the optimized sampling and the optimized control strategy. The simulation and experimental results show a dramatic reduction of the measurement time (up to one order of magnitude) with a high tracking precision (also in accordance with the proposed methodology), and of the costs with respect to standard solutions.
Full probe compensation techniques for Spherical Near Field (SNF) measurements have recently been proposed [1-5]. With such techniques, even antennas with more than decade bandwidth are suitable probes in most systems. The abolition of otherwise frequent probe changes during multi-service campaigns is a highly desirable feature for modern measurement applications such as automotive. In this paper, a standard dual-ridge horn with 15:1 bandwidth is investigated experimentally as probe in a SNF automotive range. The accuracy of the probe compensation technique is validated by comparison to standard single probe measurement.
Spherical near-field measurements are regarded as the most accurate technique for the characterization of an Antenna Under Tests (AUT) radiation. The AUT's far-field radiation characteristics can be calculated from the Spherical Mode Coefficients (SMC), or spherical wave coefficients, determined from near-field data. The disadvantage of this technique is that, for the calculation of the SMC, a whole sphere containing the AUT must be Nyquist-sampled, thus directly implying a longer measurement time when only a few cuts are of interest. Due to antennas being spatially band-limited, they can be described with a finite number of SMC. Besides, the vector containing the SMC can be proved sparse under certain circumstances, e.g., if the AUT's radiation pattern presents information redundancy, such as an electrical symmetry with respect to coordinate system of the measurement. In this paper, a novel sampling strategy is proposed and is combined with compressed-sensing techniques, such as basis pursuit solvers, to retrieve the sparse SMC. The retrieved sparse SMC are then used to obtain the AUT's far-field radiation. The resulting far-field pattern is compared for both simulated and measured data. The reduced number of points needed for the presented sampling scheme is compared with classical equiangular sampling, together with the estimated acquisition time. The proposed sampling scheme improves the acquisition time with a reasonable error.