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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.
In a previous paper a new referenceless measurement setup based on a reference antenna was used for characterizing the radiation of antennas in the planar scanner [1]. The method is based on using a low-cost receiver to retrieve the amplitude and phase of the signal. This paper explores the limitations of the method for different geometries and implements a multiprobe electromagnetic compatibility measurement system. Once the amplitude and phase are recovered, diagnostic techniques can be applied and also near-field to near or far-field transformations to calculate the field at distances defined by standards. The results demonstrate the good accuracy of the method in comparison with traditional electromagnetic compatibility laboratories.
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 [3], 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.
A reliable technique for antenna array characterization and calibration is demonstrated for two state-of-the-art antenna measurement systems, a near-field system and a compact antenna test range system. Both systems are known to reduce the measurement distance between device under test and the probe antenna in comparison to classical far-field systems, which need to provide at least the Fraunhofer distance as minimum range length. Equivalent magnetic surface currents are derived from measurements, which represent the electric field on the applied Huygens surface. The calculated equivalent magnetic currents are utilized for characterizing two completely different antenna arrays in the millimeter-wave region. Magnitude and phase calibration opportunities of antenna arrays are discussed, as well as the accuracy provided by the proposed calibration technique.
Intrinsic magnetic and dielectric properties of magneto-dielectric composites are typically determined at microwave frequencies with both transmission and reflection data. An iterative method, such as root-finding, is often used to extract the properties in a frequency-by-frequency basis. In some situations, materials may be manufactured on a metal substrate that prevents transmission data from being obtained. This happens when the materials are too fragile or too strongly bonded to the substrate for removal and must be characterized with the metal substrate in place. This paper compares two different free-space extraction algorithms, developed for the simultaneous extraction of complex permittivity and permeability from metal-backed reflection. One of the algorithms relies on reflection measurements of the same material with two known thicknesses. The second method takes advantage of wide bandwidth measurements to fit the reflection to analytical models (e.g. Debye). The accuracy of these methods are evaluated and the stability criteria for the techniques will be discussed, as well as the tolerance of the techniques to various measurement errors.
A wideband, four element array is designed to create suitable radiation patterns for angle of arrival estimation over a field of view of 0 • to 80 • in elevation and 360 • in azimuth, using the Cramer-Rao Lower Bound (CRLB) as the figure of merit. The antenna elements are truncated monocones over a circular ground plane and operate over 100 MHz to 6 GHz. A study of the antenna geometry was performed to meet size constraints while minimizing the reflection losses at the input for frequencies up to 1 GHz. A method is presented to find the ideal loading impedance required for each frequency using multiport S-parameters derived from field simulations. The loading improves the maximum return loss from 0.4 dB to 6 dB. The study reveals a trade-off between minimal reflection losses and direction finding (DF) performance evaluated using the CRLB over the operating frequency. For the best investigated geometry using top loading the maximum root mean square error of the azimuth DF estimate remains below 13 • .
We demonstrate simultaneous amplitude and phase measurements of a radio-frequency (RF) field through the use of a Rydberg atom-based sensor embedded inside a waveguiding structure. This measurement uses the Rydberg atom-based sensor in a mixer configuration, which requires the presence of a local oscillator (LO) RF field. The waveguiding structure supplies the LO field. The combined waveguide and Rydberg atom system is used to measure phase and amplitude in the near-field of a horn antenna to extract the far-field pattern.
The IEEE Standards Association Standards Board (IEEE-SASB) approved the IEEE Std 1720™ "Recommended Practice for Near-Field Antenna Measurements" in 2012 [1]. More than forty dedicated people from industry, academia and other institutions contributed to the creation of this new document. The main motivation for a new standard dedicated to near-field measurements was to complement the existing IEEE Std 149-1979™ "Test Procedures for Antennas" [2]. According to the IEEE-SA policies, the existing standard IEEE Std 1720-2012™ is approaching expiration in 2022. A working group of the APS Standard Committee has been formed to review the current document. Most of the current standard is still relevant and useful for individuals designing and evaluating near-field antenna measurement facilities and other people involved in antenna measurements. However, the standard needs update and renewal in areas in which new developments and technologies have matured. This paper gives an overview of the current standards and discusses the suggested potential changes.
The IEEE Standard 149, Standard Test Procedures for Antennas, has not been revised since 1979. Over the years the Standard was reaffirmed, that is, its validity was re-established by the IEEE APS Standards Committee, without any changes. Recently however, the IEEE Standards Association stopped the practice of reaffirming standards. This change in policy by the IEEE has been the "medicine" that this Standard needed. A working group was organized and a project authorization request (PAR) was approved by IEEE for the document to be updated. In this paper, the expected changes to the document are described and commented. The main change is to convert the Standard to a recommended practice document. Additionally, some new techniques to measure antennas, such as the use of reverberation chambers for efficiency measurements and more information on compact ranges, is discussed. Other topics inserted are more guidance on indoor ranges and an updated section on instrumentation. Most importantly, a discussion on uncertainty is included. The result will be a very useful document for those designing and evaluating antenna test facilities, and those performing the antenna measurements.
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.
The Compact Antenna Test Range (CATR) was initially conceived as an efficient way of testing electrically large antennas at very much reduced, fixed, range lengths than would otherwise be the case. However, when testing lower gain, physically smaller antennas, the measurements can become susceptible to inhomogeneities within the CATR QZ including phenomena associated with edge diffraction effects, feed spill-over, chamber multipath etc. Whilst it has been demonstrated experimentally that many of these measurement artefacts may be effectively mitigated using standard and modern more sophisticated post-processing techniques. This paper supports those findings through simulation of the direct and indirect far field ranges and by careful examination of the data processing chain. Results are presented, the relative success of the various techniques examined and the utility of this is set, and expounded, in the context of modern, i.e. 5G, communications systems.
The generalized three-antenna method is a standard method for measuring on-axis gain and polarization of an antenna without a priori knowledge. The cornerstone of the method is the use of the extrapolation technique and the key relationship in the extrapolation technique is Wacker's equation. This equation expresses the received signal as a function of the separation distance between any two antennas. The derivation of Wacker's equation is not readily available in the literature. In this paper, we provide a streamlined derivation of Wacker's equation and address some of the common misconceptions associated with it.
Antenna systems commonly used in space applications, are often exposed to extreme environmental conditions and to significant temperature variation. Thermal stress may induce structural deformations of the radiators or affect the RF performance of the active front-ends. These are some of the reasons that pushed the testing technology to characterize the radiating proprieties of Antennas Under Test (AUT) in realistic thermal conditions. Testing facilities available for these purposes are nowadays typically limited in terms of temperature range, measurable radiation pattern and size of the AUT. This paper describes the multi-physics design considerations (i.e. thermal, structural and RF) for the development of a novel facility to evaluate AUT radiation pattern characteristics in thermal conditions, from L to Q band, as an add-on feature to the ESA-ESTEC Hybrid European RF and Antenna Test Zone (HERTZ), located in Noordwijk (The Netherlands). The goal is to extend such a testing to AUTs up to 2.4m diameter in envelope over an extreme temperature range (+/-120°C), allowing a free movement of the AUT and taking advantage of Spherical Near-Field (SNF) measurement techniques.
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.
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.
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).
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.
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.