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Far Field

On the Uncertainty Sources of Drone-Based Outdoor Far-Field Antenna Measurements
Cosme Culotta-L´opez, Stuart Gregson, Andrian Buchi, Carlo Rizzo,Diana Trifon, Snorre Skeidsvoll, Ines Barbary, Joakim Espeland, October 2021

Unmanned Aerial Systems (UAS), colloquially known as drones, offer unparalleled flexibility and portability for outdoor and in situ antenna measurements, which is especially convenient to assess the performance of systems in their realworld conditions of application. As with any new or emerging measurement technology, it is crucial that the various sources of error must be identified and then estimated. This is especially true here where the sources of error differ from those that are generally encountered with classical antenna measurement systems. This is due to the larger number of mechanical degrees of freedom, and to the potentially less repeatable and controllable environmental conditions. In this paper, the impact of some of these various error terms is estimated as part of an ongoing measurement validation campaign. A mechanically and electrically time invariant reference antenna was characterized at ESAESTEC’s measurement facilities which served here as an independent reference laboratory. The reference results were compared and contrasted with measurements performed outdoors at Quad- SAT’s premises using QuadSAT’s UAS for Antenna Performance Evaluation (UAS-APE). While a direct comparison between the measurement results from ESA-ESTEC and QuadSAT delivers information about the various uncertainties within a UAS-APE system in comparison to classical measurement facilities’ and the validity of such a system for antenna testing, other tests aim at providing an estimation of the impact of each error source on the overall uncertainty budget, thus paving the way towards a standardized uncertainty budget for outdoor UAS-based sites.

On the Challenge of Over-The-Air Measurements of High-Power Massive MIMO Radio Base Stations
Adam Tankielun, Gerd Saala, Sebastian Schmitz, Hendrik Bartko, Benoit Derat, Amin Enayati, October 2021

Using beam-steering technologies, 5G massive MIMO base stations are capable to radiate typical equivalent isotropic radiated powers as high as 80 dBm (100 kW). Such levels create challenges in Over-The-Air (OTA) testing, both for the RF test system hardware, and the anechoic chamber / absorber layout designs. In this paper, a calculation tool is introduced which allows evaluations of the Poynting vector at ”mid range” distances, from given base station models. This code is used to deduce conservative power density distribution estimates and identify possible critical exposure areas in the test facility. General design criteria for the chamber, absorber layout and choice of material are derived. The specific case of a plane-wave synthesis OTA test site is investigated, where an experimental setup is used to demonstrate the power tolerance of the solution and its compatibility with base station testing requirements.

Consideration of the Feeding Networks for Measurement of mmWave/Sub-THz SoP/SoC/SoD Antennas in 5G and 6G
Jae-Yeong Lee, Jaehyun Choi, Junho Park, Youngno Youn, Bumhyun Kim, Sungmin Cho, Kangseop Lee, Ho-Jin Song, and Wonbin Hong, October 2021

This paper presents a reliable design and measurement methodology of using various feeding networks for mmWave/Sub-THz SoP/SoC/SoD antennas in 5G and 6G communication. In order to achieve reliable and precison testing results, the electrical, mechanical, and thermal consideration have been precendently investigated and discussed through various examples of feeding network based on lots of the advanced materials and fabrication process. First, for a realization of the minimized discrepancy between simulation and measurement without any calibration kit and resistive films for 50-Ω termination load, two examples have been presented. In other words, a symmetrical power divider with back-to-back transition structures and a leaky wave antenna design topology featuring high attenuation constant have been demonstrated. Finally, despite challenging fabrication condition resulting in performance degradation, a low-loss transition structure in mmWave SoD antenna and its design methodology is also presented and discussed.

Simultaneous Measurement of Analog Phased Array Elements Using Orthogonal Coding
Michael D. Foegelle, October 2021

Evaluation and calibration of individual elements of a phased array is a time-consuming process that involves not only the radiation pattern and RF circuitry of each element, but the interaction of each element with all of the other elements within the array. Iterating through each element in order to test them one at a time is extremely time consuming, and in some cases, depending on the design of the array, this approach may not work reliably at all. In cases where the impedance of the “off” elements differs from their impedance when actively transmitting or receiving, they can distort the resulting single element pattern due to mutual coupling. Even in the case where the elements themselves are well behaved, the driving circuitry can exhibit non-linearities due to the differences in signal levels or device heating present when all elements are active vs. only a single element. Thus, it would be ideal to be able to extract individual element performance from the combined pattern of the array with all elements active. This paper will investigate the use of orthogonal coding applied to each element of the array through the onboard gain/phase control circuitry using different modulation coding schemes in order to extract the average performance of each element from the measured total result.

Measurements and Simulations of a 2.4 GHz Circular Waveguide Antenna for a Portable Radar Kit
Alan J. Fenn, October 2021

A custom radar kit that integrates with a portable computer (laptop) for assembly and operation by students and researchers has been developed at MIT Lincoln Laboratory. The assembled radar kit uses two low-cost cylindrical metal cans that serve as the antennas, one for transmitting and one for receiving radar signals. The antennas operate as linearly polarized openended circular waveguides (10.5 cm diameter) fed with a thinwire monopole probe. Over the 2.4 to 2.5 GHz band, the measured reflection coefficient is less than −10 dB, the peak realized gain is greater than 7 dBi, and the half-power beamwidth is approximately 70 degrees in both the E- and Hplanes. FEKO method of moments simulations of the antenna are compared with the measured data and good agreement is demonstrated.

Modular Horn Antenna for VHF Reference Field Strength Applications
A. Akar, B. Neubauer, R. Geise, October 2021

Antennae in critical applications such as in-flight navigation, e.g. the instrument landing system (ILS), have to be calibrated on a regular basis. This allows for an error-free operation by verifying the absolute field strength as well as the spatial field distribution. Hence, it remains indispensable to calibrate the receiving antenna used by flight inspection services in absolute terms. The Calibration itself can only be achieved by measurements within a well known field distribution, ideally in situ, hence in the measurement environment of the targeted system. In this contribution a modular pyramidal horn antenna capable of providing reference field strengths within the frequency range of 75 MHz - 114 MHz is presented. The aperture’s field strength can be calculated analytically as well as measured with a high degree of accuracy. For the frequency range at hand, the size of the reference antenna ends up in a challenging scale of a truck. Construction details and manufacturing aspects of the light weight, modular and easy to assemble horn antenna are presented. Near field measurement results are shown, compared with simulations and discussed with respect to one another.

Analysis of Far-field Condition of Broadband Quad Ridge Horn Antennas
Anoop Adhyapak, October 2021

The most common antennas used for antenna pattern or gain measurements are Standard Gain Horn Antennas, Circular Horn Antennas, Dual Ridge Horn Antennas or Quad Ridge Horn Antennas. In addition, the far-field criteria for the antennas is currently revised as per the latest draft of IEEE 149 standard, based on the largest dimension, D, of the antenna and wavelength, of interest. Conventionally, the largest aperture dimension of the antenna is considered as the dimension, D. One could question, if considering the same aperture dimension to compute the far-field distances over entire frequency range is accurate. It could lead to longer test range distances at higher frequencies for broadband horn antennas, which in turn will lead to much larger chamber sizes. Thus, it is imperative to investigate the electrical dimension, D, as a function of frequency for the broadband horn antennas to accurately yield the far-field distances needed to characterize the different antenna parameters like half-power beam width, first null level, side lobe level, etc. This paper explores the utilization of the spherical modes and underlying Minimum Radial Extend (MRE) from Nearfield to Far-field transformation theory to extract the electrical dimension, D, so as to accurately characterize the HPBW across the frequency range. Firstly, the near fields are transformed to far-fields by incorporating spherical modes. The transformed farfields are compared to the ideal far-field pattern for standard gain horns, with respect to the equivalent noise level parameter over the HPBW solid angle, to compute the acceptance criteria. Based on the acceptance criteria of the equivalent noise level for standard gain horns, the same exercise is repeated for a broadband quad-ridge horn over the HPBW solid angle across the frequency range. The MRE is computed from the number of spherical modes across the frequency range and the electrical dimension, D, is calculated to be twice of the MRE value. The far-field distance is calculated based on the computed electrical dimension and compared to the far-field distance calculated per the physical dimension of the antenna structure.

Sub-6 5G Dual-Port 16 Element Patch Antenna Array Designed and Built with The Brick-Based Antenna Design Methodology
UmutBulus, October 2021

The brick-based antenna design is a new concept to the literature. Metals and dielectrics are in brick-form to let the antenna designers connect and disconnect the cells easily. Designing and prototyping an antenna takes only a few minutes with this concept. Antenna engineers directly build their design in front of a network analyzer and iterate to reach their requirements. This hardware-based antenna design solution also creates a design cycle of measure-iterate instead of simulate­iterate. This study starts with introducing this new method and continues with a dual-port 3.5 GHz patch antenna design and measurement. After the single antenna reaches the target frequencies, the 16 element 4x4 planar patch antenna array is built and measured.

A Near-Field to Far-Field Transformation with Noncanonical Plane-Rectangular Scan Using an Effective AUT Modeling
F. Bevilacqua, F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, October 2021

This communication provides the experimental validation of an effective probe-compensated near-field to far-field (NFFF) transformation with a nonconventional plane-rectangular scan suitable for flat antennas under test (AUTs). It is based on the nonredundant sampling representations of the electromagnetic fields, on the use of optimal sampling interpolation expansions, and assumes a flat AUT as enclosed in a dish having diameter equal to its maximum dimension. This source modeling results to be very effective from the NF data reduction viewpoint, since, by fitting very well the geometry of such a kind of AUT, it is able to reduce as much as possible the residual volumetric redundancy related to the use of the other modelings suitable for quasi-planar AUTs (an oblate spheroid or a double bowl). Experimental results, assessing the practical feasibility of the proposed NF-FF transformation technique, are shown.

Nonredundant NF-FF Transformation with Planar Spiral Scan Optimized for Flat Antennas Under Test
F. Bevilacqua, F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, October 2021

A probe-compensated near-field-far-field (NF-FF) transformation with planar spiral scan, particularly suitable for flat antennas under test (AUTs), is proposed in this communication. It relies on the nonredundant sampling representations of electromagnetic fields and has been achieved by properly applying the unified theory of spiral scannings for nonvolumetric antennas, when such a kind of AUT is considered as enclosed in a dish with diameter equal to its maximum dimension, thus better shaping its geometry. An efficient two-dimensional optimal sampling interpolation (OSI) algorithm is then developed to recover the NF data required by the standard NF-FF transformation with plane-rectangular scan from those collected along the spiral. Since the number of NF data and spiral turns is related to the area of the modeling surface, the here proposed NF-FF transformation technique allows one to further reduce the measurement time with respect to those based on the modelings for quasi-planar AUTs, which instead involve, in such a case, a residual volumetric redundancy. Some numerical simulations, assessing the accuracy of the OSI algorithm and of the so developed NF-FF transformation, are shown.

Method to Experimentally Verify Far-Field Conditions in a Direct Far-Field Measurement Setup
A.J. van den Biggelaar, A.B. Smolders, U. Johannsen, October 2021

In this paper, a method is presented that allows for the experimental verification of far-field conditions in a direct far-field measurement setup. The method is based on a relativedistance sweep (i.e., increasing the distance by linearly translating one antenna) and on the Friis equation. The presented method is only valid for one specified direction and is therefore well suited to assess whether or not far-field conditions are achieved when performing an absolute measurement, such as a maximum gain or effective-isotropic-radiated-power (EIRP) measurement. It is shown that antenna measurement uncertainties due to the finite antenna separation, scattering, positional inaccuracies, drift and noise on the order of hundredths of dBs around 30 GHz for a separation on the order of 1 m can be obtained. Using this method, it is also experimentally shown that whether or not farfield conditions are met depends not on one but on both antennas in a two-antenna measurement setup. This implies that, strictly speaking, the far-field distance cannot be determined by solely considering the largest antenna in a two-antenna measurement setup.

Measurements of a Deployable Tightly Coupled Dipole Array for Small Satellites
Maxence Carvalho and John L. Volakis, October 2021

An origami-based Tightly Coupled Dipole Array (TCDA) is proposed for small satellite applications. The array is formed by a two-layered structure using rigid and flexible substrates to enable accordion-like folding. The proposed TCDA operates across 0.4-2.4 GHz with VSWR < 3 at broadside and across 0.6-2.4 GHz with VSWR < 3 when scanning down to 45 in the E-, D-, and H-plane. An 8 prototype was fabricated using Kapton Polyimide and FR4 and tested to verify the bandwidth and gain of the origami array. The fabricated prototype was demonstrated to be packable, low-profile, and lightweight (only 1.1kg). Notably, when packed, the array has a one-dimensional size reduction of 75%. As will be discussed, the packing compression is made possible by eliminating vertical PCB boards and incorporating the balun feeds within the dipole layer. To our knowledge, this is one of the first foldable, low profile, and low-scanning ultra-wideband arrays in the literature.

Integrated Microstrip Antenna for Satellite Simulators
Shahin Salarian, Dariush Mirshekar-Syahkal, October 2021

A Novel Microstrip patch antenna have been designed for satellite communication, to be used in satellite simulator system for transmitter and receiver antenna, at X and Ku frequency band, integrated as transceiver antenna. The transmitter antenna is designed for the uplink at 14.25 GHz and receiver antenna is designed for the downlink at 11.45 GHz. The transmitter and receiver antennas are integrated into a microstrip patch with microstrip transmission feedline on two sides for each frequency band. Quarter-wavelength structure is used for matching. Simulation results reveal a broadband structure for reflection, with a gain of 6.5 dB and high efficiency.

Phase Retrieval of 2-D Radiated Fields from Phaseless Data: A new Crossword Approach
R. Palmeri, G. M. Battaglia, A. F. Morabito, T. Isernia, October 2021

We tackle the problem of recovering 2-D complex fields starting from the spectral amplitude data, the support of the source, and a few additional information. In particular, we further elaborate on our ‘crosswords-solution like’ approach where the solution is found by solving 1D problems and congruence arguments. Its argued that intersecating lines and circles (rather than just lines) is more effective, and we show how the resulting approach, initially developed for the case of continuos (aperture) sources, is also effective in determining the excitations of discrete (array) sources.

Open Source Antenna Pattern Measurement System
Christian Hearn,Dustin Birch,Daniel Newton,Shelby Chatlin, November 2020

An open-source antenna pattern measurement system comprised of software-defined radios (SDRs), standard PVC tubing, and 3-D printer hardware will measure the radiation patterns of student-built prototype antennas. The antenna pattern measurement system developed at Weber State University (WSU) was inspired by the published work of Picco and Martin [1]. Their low-cost and practical system utilized commercially-available 2.4 GHz wireless routers. Open-source firmware was loaded on the routers to access the received signal strength indicator (RSSI) data. The RSSI was recorded versus antenna-under-test orientation using National Instruments LabVIEW. The WSU antenna pattern measurement prototype utilizes wideband software-defined-radios to generate, transmit, and receive the test signal. Synchronous belts, gears, and 3-D printer parts were chosen and designed to address mechanical problems described by Picco and Martin. Position control is achieved using an Arduino microcontroller with open-source software developed for 3-D printer systems. Measured principal plane gain patterns for three antenna prototypes are compared to simulated results. Models were constructed using commercial Method-of-Moments (FEKO) for comparison. Measured Radiation pattern data was scaled to the simulated Gain values for a quarter-wave monopole over a finite ground plane, a Yagi-Uda directional antenna, and an air-backed circular microstrip patch antenna. The low-cost, open-source nature of the measurement system is ideal for undergraduate-level investigation of antenna theory and measurement. It is anticipated the SDRs will permit future research of modulation methods and encoding to improve measurements in non-anechoic environments.

Aircraft Antenna Placement Investigation Utilizing Measuered Sources in Simulation Model
Bj”rn M”hring,Bernd Gabler,Markus Limbach, November 2020

Antenna placement or antenna in-situ performance analysis on large and complex platforms such as ships, airplanes, satellites, space shuttles, or cars has become even more and more important over the years. We present a systematic investigation of different antenna types for space applications in G- and S-band on an experimental aircraft. In this process, the individual antennas are measured with the help of a dual reflector compact antenna test range (CATR) under far-field conditions in various configurations. These results are validated and compared utilizing a finite element method (FEM) based solver simulation model. At first, the antennas are simulated and measured alone without any supporting or mounting structure. Subsequently, the effect of mounting structures on the overall radiation performance is added by analyzing the antennas over a large conducting ground plane, on top and the side of winglets, and on top of a cylinder body with dimensions on the order of the actual aircraft. For the detailed in-situ investigations, a second method of moments (MoM) based simulation tool is employed which works on measured sources. These measured sources are obtained from the CATR measurements of the isolated antennas. By means of a spherical wave expansion (SWE), they are transformed into a near-field source for the simulation model. These measured data based results are again compared and validated with the full FEM simulation for the complete aircraft setup and the simplified cylinder body. By this means, the expensive design and measurement of a full-scale electromagnetically equivalent mock-up of the aircraft could be saved. Furthermore, the pure simulation of the installed antenna performance often suffers from the limited availability of exact antenna design parameters. In some cases, the antenna design data remains undisclosed deliberately due to IP reasons. The presented results reveal the influence of physical structure on the radiation characteristics and demonstrate the benefits of working with measured data in simulation tools.

Element Failure Detection of Antenna Array using Far-field Measurement with Shallow Neural Network
Michitaka Ameya, November 2020

In the 5G communication, antenna array has been widely used for high-speed wireless communication. For reliable antenna array system, the failure diagnosis of antenna array is one of the most important problems that has been studied for a long time. The back-projection method using near-field measurements is a one of the failure diagnosis technique based on the plane-wave expansion. However, when antenna elements are densely placed, it is difficult to estimate the excitation coefficients of the antenna elements with the back-projection method, because the obtained images from the conventional back-projection method has only a resolution of one wavelength. In addition, since there is usually a trade-off between measurement accuracy and measurement time. Therefore, it is difficult to satisfy the both requirements of accuracy and short measurement time. We have reported the element failure detection algorithm using a 2-layer shallow neural network with planar near-field measurement last year. In this report, the element failure detection of antenna array is performed with a minimum number of measurement points while maintaining enough accuracy by learning the relationship between excitation coefficients of antenna array and the electric far-field distribution by a shallow neural network. In the case of 64-elements short dipole antenna arrays, the estimation error of excitation coefficients of antenna array less than 1% are achieved by our trained neural network with a minimum number of far-field measurements with 50 dB SNR. The detailed algorithm and simulation results will be reported in the full-paper and the presentation.

Correction of Non-ideal Probe Orientations for Spherical Near-Field Antenna Measurements
Rasmus Cornelius, Dirk Heberling, October 2017

Positioning in near-field antenna measurements is crucial and often an absolute position accuracy of ?\50 is required. This can be difficult to achieve in practice, e.g. for robotic arm measurement systems and/or high frequencies. Therefore, optical measurement devices are used to precisely measure the position and orientation. The information can be used to correct the position and orientation during the measurement or in the near-field to far-field transformation. The latter has the benefit that the measurement acquisition is typically faster because no additional correction movements are needed. Different methods for correction of non-ideal measurement positions in r, ? and f have been presented in the past. However, often not only the relative position but also the orientation between the antenna under test (AUT) and the probe coordinate system is not perfect. So far, correction and investigation of the related non-ideal probe orientations has been neglected due to the assumption that the probe receiving pattern is broad. In this paper, non-ideal probe orientations will be investigated and a spherical wave expansion procedure which corrects non-ideal probe orientations and positions will be presented. This is achieved by including an arbitrary probe pointing in the probe response calculation by additional Euler rotations of the probe receiving coefficients. The introduced pointwise higher-order probe correction scheme allows an exact spherical wave expansion of the radiated AUT field. The transformation is based on solving a system of linear equations and, thus, has a higher complexity compared to Fourier-based methods. However, it will be shown that most of the calculations can be precomputed during the acquisition and that solving the linear equation system can be accelerated by using iterative techniques such as the conjugate gradient method. The applicability of the proposed method is demonstrated by measurements where an intentional misalignment is introduced. Furthermore, the method can be used to include full probe correction in the translated spherical wave expansion algorithm. In conclusion, the proposed procedure is a beneficial extension of spherical wave expansion methods and can be applied in different measurement scenarios.

Determination of the Far-Field Radiation Pattern of a Vehicle Mounted VHF Antenna From a Set of Sparse Near-Field Measurements
Scott Kordella, Kenneth Grimm, October 2017

The paper summarizes the performance of a new near-field to far-field (NF/FF) transform approach for a VHF vehicle mounted AUT test case, and compares the approach with the spherical measurement approach. The NF/FF transformation is based on the solution of an inverse problem in which the measured NF and predicted FF values are attributed to a set of equivalent electric and magnetic surface currents which lie on a convex arbitrary surface that is conformal to the antenna under test (AUT). The NF points are conformal to the AUT, reducing the number of samples and relaxing positioning requirements used in conventional spherical, NF/FF geometries. A pseudo inversion of the matrix representing the mapping of the equivalent sources into the near-field samples is obtained by using the singular value decomposition (SVD), which is used to form an approximation of the inverse of the matrix. This inverse, when multiplied by the NF measurement vector, solves for the efficiently radiating components of the current, which are used to compute the FF in a straightforward manner. Keywords—Antenna Near-Field to Far-Field Transformation, Electromagnetic Inverse Problems.

Radiation Center Estimation from Near-Field Data Using a Direct and an Iterative Approach
Cosme Culotta-López, Kui Wu, Dirk Heberling, October 2017

Spherical Near-Field (SNF) measurements are an established technique for the characterization of an Antenna Under Test (AUT). The normal sampling criterion follows the Nyquist theorem, taking equiangular samples. The sampling step size depend on the smallest sphere that, centered in the measurement’s coordinate system, encloses the AUT, i.e. the global minimum sphere. In addition, a local minimum sphere can be defined as the sphere with minimum radius which, centered in the AUT, encloses it alone. The local minimum sphere is always equal or smaller than the global minimum sphere, being equal when the AUT is centered in the measurement’s coordinate system. It is assumed that the local minimum sphere’s center coincides with the radiation center. Furthermore, it is possible to compute a Translated Spherical Wave Expansion (TSWE) centered in the local minimum sphere, thus needing less measurement points, as long as the relative position of its center is known. Due to practical reasons, it is not always possible to easily locate the radiation center. In this paper, the relative position of the radiation center of an AUT with respect to the measurement's coordinate system’s center is estimated from SNF data using two approaches. The first approach takes the phase center as an estimation of the radiation center and is based on the method of moving reference point, strictly valid for the far-field case, analyzing its error at different near-field distances. The second approach is based on a spherical modes' spectrum analysis: the closer the AUT’s radiation center is to the coordinate system's center, the larger the power fraction in the lower modes will be. The proposed algorithm iteratively displaces the SWE and checks the power in a predefined number of modes until the convergence criterion is fulfilled. It is important to note that no near-field to far-field transformation is used, for the less measurement points taken do not allow it. A thorough analysis of the estimation error is done by simulation for different cases and antennas. The estimation error of both methods is compared and discussed, highlighting the convenience of each method depending on the requirements.
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