AMTA Paper Archive
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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.
A Low-Cost Multicopter Based Near-Field Antenna Measurement System Employing Software Defined Radio and 6-D Laser Metrology
A near-field antenna measurement system is presented that consists of components that are rather unusual compared to conventional antenna measurement setups. Instead of a vector network analyzer (VNA), a dual-channel wideband software defined radio (SDR) is used to measure the signals at the ports of a dual-polarized probe antenna. Instead of an exact multi-axis positioner for the antenna under test (AUT) or the probe antenna, a multicopter moves the probe along a predefined trajectory on a surface around the AUT. Instead of using expensive laser interferometry equipment, the position and orientation of the probe antenna are determined by a 6-D tracking system that was originally developed for virtual reality (VR) applications. Still, the first measurement results show the usability of the low-cost system for antenna measurements in the frequency range of mobile communication services.
EMC Measurement System Based on Software Defined Radio and Diagnostic Techniques
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 . 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.
Near-Field Techniques for Millimeter-Wave Antenna Array Calibration
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.
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.
Measuring and Processing Near-Field Data on Non-Standard Grids at NIST
This paper demonstrates the capability of the NIST CROMMA antenna measurement facility to perform near-field measurements by collecting data at "arbitrary" positions near the test antenna. We have devised several measurement campaigns involving non-standard near-field measurement grids, including (1) a regular (equispaced in and ) spherical grid with random probe-position displacements and (2) a spiral grid on the surface of a sphere. Simulations have been used to demonstrate the robustness and accuracy of NIST processing software. Near-field measurements have been performed at 72 GHz on a horn antenna. We compare radiation patterns obtained using the standard regular spherical grid with those obtained with the nonstandard grids (1) and (2).
Top-loaded 60:1 Wideband Direction-Finding Sensor
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 • .
Waveguide-integrated Rydberg Atom-based RF Field Detector for Near-field Antenna Measurements
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.
Use of OTA System Performance Metrics in the Design & Optimization of CATRs for 5G Testing
Delivering on the promise of 5G measurements requires the adoption of new RF system technologies that encompass both the mobile user equipment and the active base station. Keeping pace with the impact of new wireless system test parameters such as: Data throughput, Error Vector Magnitude, Symbol Error Rate, and technologies such as mm-wave Massive MIMO, OFDM, and QAM presents significant challenges to antenna test community. For the most part, the market has attempted to react by adapting traditional test equipment to the wireless market however 5G testing presents an ever-greater challenge and demands the incorporation of simulation effects when designing and optimising an antenna test system, especially as these systems have increased in complexity with the adoption of the indirect far-field method and specifically the compact antenna test range (CATR). This paper discusses how 5G communication system parameters affect the design of the CATR and how newly developed simulation capabilities have been incorporated to optimize the CATR design for 5G test applications.
A Review of the Changes and Additions to the Antenna Measurement Standard IEEE Std 149
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.
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.
Accurate Calibration of Truncated Spherical Near Field Systems with Different Ground Floors using the Substitution Technique
The calibration of the antenna measurements system is a fundamental step which directly influences the accuracy of any power-related quantity of the device under test. In some types of systems, the calibration can be more challenging than in others, and the selection of a proper calibration method is critical. In this paper, the calibration of the truncated spherical near-field ranges typically used for automotive tests is investigated, considering both absorbing and conductive floors. The analyses are carried out in a 12:1 scaled multi-probe system, allowing access to the "true", full-sphere calibration which is used as reference. It will be demonstrated that the substitution (or transfer) method is an excellent calibration technique for these types of systems, if applied considering the efficiency of the reference antenna.
Small Antenna Testing in a Compact Antenna Test Range
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.
Comparative Investigation of Spatial Filtering Techniques for Ground Plane Removal in PEC-Based Automotive Measurements
Radiating performances of vehicle-installed antennas are typically performed in large spherical near-field systems able to accommodate the entire car. Due to the size and weight of the vehicle to be tested, such spherical systems are often nearly hemispherical, and the floor is conductive or covered with absorbers. The main advantage of the first is the ease of the accommodation of the vehicle under test. Conversely, the latter is more time consuming in the setup of the measurements because the absorbers need to be moved in order to be placed around the vehicle. On the other hand, the absorber-covered floors emulate a free-space environment which is a key enabling factor in performing accurate measurements at low frequencies (down to 70 MHz). Moreover, the availability of the free-space response allows easy emulation of the cars' behaviors over realistic automotive environments (e.g. roads, urban areas etc.) with commercially available tools. Such emulations are instead much more challenging when a conductive floor is considered. Furthermore, the raw measurements over conductive floors are a good approximation of realistic grounds (such as asphalts) only in a limited number of situations. For these reasons, when PEC-based automotive measurements are performed, it is often required to retrieve the free-space response, or equivalently, to remove the effect of the conductive ground. In this paper two spatial-filtering techniques (the spherical modal filtering and the equivalent currents) will be experimentally analyzed and compared to verify their effectiveness in removing the effect of the conductive floor. For this purpose, a scaled automotive PEC-based measurement setup has been implemented considering a small spherical multi-probe system and a 1:12 scaled car model. The two techniques will be analyzed considering two different heights of the scaled car model with respect to the conductive floor.
Impact of Sparse Measurements in Freehand Setup for Antenna Characterization
The aim of the paper is to assess the quality of the obtained results using a portable system to perform antenna diagnostics versus acquisition time. The system comprises a handheld probe antenna, a motion capture system to track its position and a laptop to process the acquired data. The probe antenna is arbitrarily moved in front of the antenna under test (AUT) aperture, acquiring its near-field (NF) while its position is measured. The obtained data is processed in real-time using the Sources Reconstruction Method (SRM) to compute an equivalent currents distribution on the aperture of the AUT. Furthermore, a near-field to far-field (NF-FF) transformation is performed to retrieve the far-field radiation pattern of the AUT from the computed equivalent currents distribution. Specifically, the system was evaluated at 32 GHz using a vector network analyzer to measure the NF radiated by the AUT. The obtained results show that a scan of only a few seconds can provide a fast diagnostic of the AUT.
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.
Spherical Near-Field Measurements of Satellite Antennas at Extreme Temperatures
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.
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.
Robotically Controlled Pattern Measurements of 60 GHz Phased Array Antenna
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.
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