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Calibration

A Benchmark Biconical Antenna for Standardized Antenna Measurement and Simulation: Contribution to IEEE P2816
Satyajit Chakrabarti, Vikass Monebhurrun, Ashim Chakraborty, October 2022

The IEEE Antennas and Propagation Standards Committee (APS/SC), sponsored by the IEEE Antennas and Propagation Society (AP-S), is currently developing a recommended practice tailored for the modeling and simulation of antennas (IEEE P2816). Different numerical modeling techniques such as the finite element method (FEM), integral methods e.g. method of moments (MoM), the finite difference time domain (FDTD) method and the transmission line matrix (TLM) method are described. In addition, benchmark problems are also considered to ease the use of the recommended practice. For example, the biconical antenna is proposed as a benchmark problem for the numerical simulations using the above methods. Several international laboratories have already performed the numerical simulations of this biconical antenna. To confront the theoretical and numerical results with measurements, the same biconical antenna was proposed for fabrication and inter-laboratory measurement campaign. Herein, the fabrication and initial measurements of the prototype are discussed. An important difference between the theoretical, simulated and fabricated antenna is the feeding point. In theory, it is considered infinitely small whereas in the proposed numerical model, the gap in-between the two cones is assumed to be 0.3 mm. In practice, such a small gap cannot be enforced during the fabrication process. Since the off-the-shelf available SMA connector has a minimum diameter of 0.7 mm, the minimum diameters of the central and outer conductors of the 50W feed-line (Teflon filled) section were kept at 1 mm and 3.34 mm, respectively. In the actual fabrication, only a gap of 4 mm could be achieved in-between the two cones. An external quarter-wave skirt is further used as a balun for reasonable impedance matching. A Rohacell section is used to hold the arms of the antenna which are heavier. The realized prototype, therefore, differs significantly at the feeding point. The relevant simulation and experimental results are presented.

Testing of a 60 MHz Cubesat in an Electrically Small Environment with the Synthetic Probe Array Technique
Francesco Saccardi, Rubén Tena-Sánchez, Enrico Tartaglino, Andrea Giacomini, Lars Foged, Paul Moseley, Luis Rolo, October 2022

The ESA HERA-JUVENTAS mission relies on 50-70MHz dipole antennas mounted on a cubesat [1]. The mission requires an accurate verification of the radiation properties of the whole antenna system including the matching and amplification boards. The performance verification of low gain antenna systems below 400MHz is a challenging task because of the reflectivity of the measurement environments. Spherical Near-Field (SNF) measurements are the most suitable approach for such Devices Under Test (DUT) [2] but require a sufficiently large anechoic chamber equipped with absorbers able to provide low reflectivity. Meeting these requirements at low frequencies is often too expensive, as for the case of the HERA-JUVENTAS antenna system verification. An outdoor testing solution could be an alternative but at the expenses of measurement accuracy and repeatability. The SNF system installed in the HERTZ testing facility at ESA-ESTEC has been selected as cost-effective solution for the verification of the HERA-JUVENTAS cubesat. The HERTZ anechoic chamber was originally designed for measurements down to 400 MHz, hence, due to limited electrical size (~5λx2λx2.5λ) and poor absorption provided by the treatment of the chamber walls (~2dB), a high reflective environment is expected at 60MHz. The so-called Synthetic Probe Array (SPA) technique is a very effective solution to significantly improve the measurement accuracy in case of reflective environments. With the SPA technique each sample point on the NF sphere is measured with several probe positions generating a “virtual” array able to properly shape the equivalent probe radiation pattern, minimizing the illumination of the chamber walls. Validation of the SPA technique, combined with the λ/4-averaging technique to also minimize the effect of the backwall of the DUT, have been recently performed by means of simulations and scaled measurements as presented in [3]-[4]. In this paper the actual measurement results of the HERA-JUVENTAS cubesat performed in HERTZ with the SPA and λ/4-averaging techniques will be presented for the first time. Comparisons in terms of radiation pattern and gain, with the conventional single probe SNF approach will be shown to highlight the effect of the measurement environment at 60MHz and the improvements obtained with the considered techniques.

Design and Verification of Innovative Wideband Spherical Near Field Probes with High Modal Purity
Andrea Giacomini, Vincenzo Schirosi, Francesco Saccardi, Lars Foged, Jean-Marc Baracco, Anders Jernberg, Kazi Alam, Joseph Byström, Dan Karlsson, October 2022

Measurements of modern multi-service antenna systems require ever increasing bandwidths of the measurement equipment. The main bandwidth limiting factor of traditional Spherical Near Field (SNF) systems is mainly the probe as it should radiate only first-order azimuthal spherical modes to apply the first-order Probe Compensation (PC). Even though full PC techniques are becoming standard, enabling the use wideband antennas with more than 10:1 bandwidth as probes, high-purity first-order probes are still required in many applications, because of the simplification of data processing and calibration. Conventional dual-polarized first-order probes are based on Ortho-Mode Junctions (OMJ) with externally balanced feeding. The OMJ is fully symmetrical using two pairs of excitation pins fed by high precision 3dB, 0º/180º hybrid couplers to achieve good matching and maximize the cx-polar performance. Unfortunately, realistic couplers provide some excitation errors which are one of the main contributors of the generation of unwanted higher order spherical modes. Even a small unbalancing in amplitude or phase of the coupler will excite higher order modes at frequencies where these modes are allowed to propagate. Beside the possibility to compensate the effect of the higher-order modes in post-processing (e.g. full PC), the propagation of the spurious spherical modes can be controlled directly on the probe with improved designs of the feeding mechanism or considering ad-hoc designed hybrid couplers. In this paper, two innovative and high performance SNF probes will be presented. Both probes are based on an inverted quad-ridge waveguide technology. An advanced feeding mechanism allows the first probe to provide a high modal purity in the 617-960 MHz band, rejecting errors introduced by the external coupler. In the second one, a highly accurate coupler has been designed to minimize the higher order modes on a large bandwidth, 1427-4200 MHz. The two probes have been designed as part of the upgrade of a gantry arm system used to test modern base station antennas. The same measurement system has been used to calibrate the two probes and to verify the expected performance both in terms of radiation pattern and spherical modal content. The achieved measurement results will be shown in this paper.

Crosstalk is Good: Antenna Design to Enable Polarimetric Compressive Sensing
Jeffrey Massman, Julie Jackson, John Becker, October 2022

The dropped-channel polarimetric synthetic aperture radar (PolSAR) compressed sensing (CS) model [1,2] is able to recover an unmeasured polarimetric channel by utilizing antenna crosstalk and compressed sensing techniques. For successful recovery of a dropped channel, a sufficient amount of crosstalk is required to mix the information from the dropped channel into the measured channels. Recently, Monte Carlo simulations were conducted on the dropped-channel PolSAR CS model, and a range of crosstalk values of -9 dB to -3 dB was found to produce low recovery error for a variety of SAR image point spread functions and scene sparsity levels [3]. However, dual-polarized antennas are typically designed to have very high channel isolation, with crosstalk much less than the – dB minimum desirable value. To lend credibility to the dropped-channel PolSAR CS model, a new antenna is needed that can provide such high amounts of crosstalk without sacrificing gain, bandwidth, and radiation pattern. In this paper, we design a new, high crosstalk, dual-polarized patch antenna, using Ansys/HFSS to optimize pin placement and patch size for the desired gain, center frequency, and crosstalk values. The designed antenna is constructed, and S-parameters, gain, and radiation patterns are measured. The measured crosstalk values are then tested in the dropped-channel PolSAR CS model over a few deterministic scenes, demonstrating sufficient expected performance of the physical antenna for sparse scene recovery. 1. J. A. Jackson and F. A. Lee-Elkin, “System, Method, and Apparatus for Recovering Polarization Radar Data," United States of America Patent US11 194 104B1, Dec., 202 2. J. A. Jackson and F. A. Lee-Elkin, “Exploiting Channel Crosstalk for Polarimetric SAR Compressive Sensing," IEEE Transactions on Aerospace and Electronic Systems, vol. 56, no. 1, pp. 475-485, Feb. 2020. 3. J. Becker, Theory and Design of a Highly Compressed Dropped-Channel Polarimetric Synthetic Aperture Radar, PhD Dissertation, Air Force Institute of Technology, June 2022.

Genetic Optimization of Edge Treatments of Single Offset Reflector Compact Antenna Test Ranges
Marc Dirix, Stuart Gregson, October 2022

Direct far-field (DFF) testing has become the baseline test methodology for sub-6 GHz over the air (OTA) testing of the physical layer of radio access networks (RAN). However, the proliferation of mm-wave massive multiple input multiple output (Massive MIMO) antennas for 5G New Radio (NR) rollout and the use of complex waveforms for communication system testing and primary Figure of Merit (FoM) determination has necessitated the adoption of the Compact Antenna Test Range (CATR) as the preferred test solution. The CATR was initially conceived as comprising an efficient way of testing electrically large antennas at very much reduced, fixed, range lengths [1]. However, early workers quickly recognized that the reflector edge treatment and chamber wall illumination are significant factors determining the quality and purity of the collimated pseudo plane-wave with this becoming especially important at mm-wave frequencies [3]. Using modern powerful digital computational simulation techniques [2] in combination with genetic optimization, the edge treatment can be evolved for a specific CATR application as part of the design process for a range of reflector edge treatments [3, 4]. This paper extends the authors previous work to present a novel approach for the reflector edge treatments than have hitherto been considered within the design and genetic optimization procedure, while also taking into account both wall illumination and direct quiet-zone illumination. Resulting quiet-zone performances are compared and contrasted. [1] C.G. Parini, S.F. Gregson, J. McCormick, D. Janse van Rensburg “Theory and Practice of Modern Antenna Range Measurements”, IET Press, 2014, ISBN 978-1-84919-560-7. [2] S.F. Gregson, C.G. Parini, “Examination of the Effect of Common CATR Quiet Zone Specifications on Antenna Pattern Measurement Uncertainties”, Loughborough Conference on Antennas and Propagation, Loughborough, November 2017. [3] M. Dirix, S.F. Gregson, “Optimisation of the Serration Outline Shape of a Single Offset-Fed Compact Antenna Test Range Reflector Using A Genetic Evolution of the Superformula”, EuCAP virtual conference, March 22-26 2021. [4] M. Dirix, S. Gregson and R. Dubrovka, “Genetic Evolution of the Reflector Edge Treatment of a Single Offset-Fed Compact Antenna Test Range for 5G New Radio Applications,” in AMTA Annual Meeting and Symposium, Daytona Beach, Florida, 2021

Maximum Determinant Sampling Using Spline-Based Trajectories in a Robot-Based mm-Wave Antenna Test Range
Roland Moch, Dirk Heberling, October 2022

Robot-based measurement systems offer a high number of degrees of freedom for the configuration of the targeted antenna measurement. Especially in comparison to the conventional planar, cylindrical or spherical measurement chambers, complex measurement sequences can be exploited. In addition, the actual measurement path is not decisive for an antenna measurement since the data is acquired only at discrete sampling positions. Spline-based measurement trajectories provide a way to obtain the measurement data at the intended positions without specifying a fixed path. Instead, it is up to the robot controller itself to calculate and optimize the path between the various spline support points, for example regarding the greatest path speed. However, further optimization potential can be accessed by reducing the number of path-defining spline support points and instead recording the measurement data on the resulting path. This sampling data is thus not on a regular or uniform sampling grid, but on the path optimized by the robot controller, both in terms of position and orientation. Accordingly, a pointwise probe correction is essential so that the actual position and orientation of the probe can be considered in the calculation of the spherical mode coefficients. In order to evaluate the radiation pattern transformed into the far field of the antenna under test as well as the required measurement time, the spline-based measurements are compared to a conventional spherical measurement as usually performed with roll-over-azimuth positioners. The results show that spline-based motion sequences enable faster antenna measurements, since the actual trajectory is no longer predetermined, but can be optimized by the robot controller itself for the underlying measurement setup.

Estimating Shale Maturity from Ultra-Fast Microwave Heating
Jose Alvarez, David Jacobi, Poorna Srinivasan, October 2022

Currently to determine the basic parameters of shales, namely Maturity (which indicates production potential), geochemical analysis need to be performed. These analyses may take days to weeks, depending on laboratory availability. Moreover, by the time the samples get to the laboratory, they could be damaged or poorly preserved, thus creating a significant source of uncertainty in the measurements. Pyrolysis is a method that introduces a sample of rock, of known mass, into a sealed oven that is programmed to heat the sample according to a prescribed rate of increasing temperatures that terminates at 650°C. During the initial heating, upon reaching a threshold in temperature somewhere around 300-350°C, a significant amount of hydrogen is recorded which is called the “S1” peak. With further increases in temperature beyond 350°C, another threshhold is reached at 550-600°C, where yet more hydrogen is evolved from the rock and the peak recorded at that stage is called “S2”. A microwave heating and testing for geological applications was tested with different shale samples. The system consists of a dual-mode microwave cavity, where heating and measuring is performed simultaneously with two different microwave sources. The cavity has a diameter of 104.92 mm and a height of 85 mm. A small shale sample of 9.8 mm diameter by 15 mm height is introduced in a quartz vial with an inner diameter of 9.8 mm and 120 mm. Depending on the electrical losses, the sample could be heated up to 1200°C. Initial complex permittivity measurements show that the imaginary part exhibits relaxation processes at specific temperatures. These temperatures coincide with the expected temperatures of the S1 and S2 peaks of the pyrolysis method, from which we can compute the vitrinite reflectance of the sample, which is an indicator of Maturity. Thus, allowing for quick estimates of maturity, which allows for real time decisions on the development of unconventional resources.

Radiation and Scattering Pattern Characteristics of Chamfered-Tip Open-Ended Rectangular Waveguide Probes for Planar Near-Field Antenna Measurement Applications
Elbert H. Ko, Domenic J. Belgiovane, October 2021

The radiation and scattering pattern characteristics of open-ended rectangular waveguide with a chamfered tip are examined. Despite common and widespread use as a probe antenna for planar near-field antenna measurements, a methodical investigation of the chamfered-tip design and resultant performance has not been published. A computational electromagnetics (CEM) model for an open-ended rectangular waveguide probe with a parameterized chamfered tip has been constructed and results for both radiation and scattering patterns are presented. A comparison of results includes a probe without a chamfer and a probe typical of that available from commercial suppliers. It is shown that, for a series of standard waveguide size probes sharing a common thickness for the waveguide wall and chamfered tip, the radiation pattern is relatively insensitive to the chamfer tip designs studied until frequency increases into W-band (WR-10). The scattering pattern characteristics for the same series of standard waveguide size probes show a reduction in on-axis (boresight) monostatic radar cross section (RCS) for chamfered tip waveguides compared to blunt-ended waveguides and that this reduction increases for increasing frequency.

Shielding Effectiveness of Metal Mesh and Radio Frequency Shielding Film for Optical Applications
Nika Amralah, October 2021

In this paper, an investigation was conducted to find materials that are optically transparent and radio frequency (RF) shielding. Materials were first optically tested, followed by a shielding effectiveness test. The optical test evaluated metal mesh sizes 20, 22, 40 and 100, single layer RF film, double layer RF film, and combinations of film and mesh. Size 22 copper mesh and RF film demonstrated desirable optical properties and were then RF tested from 26 MHz to 40 GHz. The test was conducted using a shielded enclosure featuring an aperture in a wall panel to mount the material under test. Reference field strength measurements of the aperture were compared to measurements taken when material samples were placed within the aperture in order to characterize the shielding effectiveness of each shielding material. Test results for size 22 copper mesh, RF shielding film, and a combination of one layer of size 22 copper mesh with one layer of film demonstrate average shielding effectiveness results of 43 dB for the mesh, 46 dB for the film, and 69 dB for the mesh and film together. This information can be used when there is a requirement for a material to provide optically transparent RF shielding.

One-port Calibration of Free-space Material Measurement System Using Planar Offset Short
Jin-Seob Kang, October 2021

Electrical properties of materials are requisite to analyze and design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurand is the free-space scattering parameters of an MUT (material under test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing the MUT without prior machining and physical contact in high frequency ranges. This paper proposes a free-space two-tier one-port calibration method using three planar offset shorts with the respective offset of ???????? for the measurement of the full scattering parameters of a reciprocal planar MUT from two successive oneport calibrations. Measurement results of a glass plate of 4.775 mm thickness are shown in W-band (75-110 GHz).

Autonomous Spherical Passive/Active Radar Calibration System
Spencer K. Wallentine, R. Jerry Jost, Robert C. Reynolds, October 2021

A Spherical Passive/Active Radar Calibration System (SPARCS) has been designed as an advanced, airborne, radar calibration device (CD). SPARCS is currently under development as an autonomous, battery-powered, high-endurance, flying platform. This self-contained, multi-function radar calibration and diagnostic system functions as 1) a Passive Spherical Reflector, 2) an Active RF Repeater, 3) a Synthetic Target Generator, and 4) an UWB RF Sensor and Data Recorder of the radar under test or the localized RF environment. This innovative CD exploits major advances in commercial technology during the past decade associated with autonomous airborne drones and miniaturized digital RF systems on chips (RFSoCs), and other miniature electronics. Emphasis has been placed on a recoverable, reusable CD that enables precision calibrations over extensive open-air test volumes used for dynamic aircraft RCS measurement, test and verification, or time space position information (TSPI) test range tracking radars. This paper highlights early efforts to parameterize and develop SPARCS, including advances in autonomous navigation and flight time, electric ducted fan performance, radar screens for thruster inlet and outlet ports, active calibration functionality, and improving calibration uncertainty. SPARCS will provide an unprecedented capability for radar instrumentation calibration, target emulation, environmental assessment and in situ, real-time calibration.

Analysis of Probe Compensation Techniques for Fast Multi-Probe Planar Near Field Measurements
F. Saccardi, R. Tena-Sánchez, L. J. Foged, N. Gross, P.O. Iversen, K. Hassett, J. Hartzell, T. Schellenberg, G. Pinchuk, R. Braun, L. Shmidov, S. Solomon, M. He, X. Bland, October 2021

In Multi-Probe (MP) based measurement systems, the standard procedure is to calibrate the probe array with a well-known reference antenna [1]. This procedure equalizes amplitude, phase, and polarization characteristics of each probe array element. In Planar Near Field (PNF) systems, the probe pattern impact is usually more pronounced than in other near field scan geometries, such as spherical. Thus, the probe pattern must be compensated during post-processing for more accurate measurements at wider angles. While the probe array calibration ensures the on-axis equalization, the probe array elements still have individual pattern difference due to finite manufacturing accuracy and absorber interaction. Probe compensation using an equivalent probe pattern of the array has been shown to be very effective and accurate for MP PNF systems [2]. In this paper we compare two methods to determine the equivalent probe pattern for a given MP PNF system. We also discuss the acceptable limits of pattern variation within the array versus measurement accuracy as a design parameter for MP PNF systems.

Pointwise Probe Correction Applied to a Robot-Based mm-Wave Antenna Test Range
R. Moch, D. Heberling, October 2021

Robot-based measurement systems typically have a larger tolerance with respect to their positioning accuracy than conventional systems, e.g. roll-over-azimuth positioners. However, for spherical near-field measurements, the positioning accuracy of the probe is an important uncertainty in the required near-field-to-far-field transformation. One way to account for those non-idealities is to use the higher-order pointwise probe correction (PPC). It allows to consider the actual position and orientation of the probe by additional rotations and translations of the probe receive coefficients. To evaluate the PPC, the occurring position tolerances and the differences in the transformed farfield patterns of a standard gain horn are investigated at 60GHz. Using an onset measurement as reference, it is shown that the PPC provides improvements of 􀀀41dB and 􀀀65dB for the co- and cross-polarized measurements, respectively. In addition, an offset measurement is shown where the measurement sphere is shifted relatively to the AUT. The pointwise implementation of the correction method allows to reproduce the far-field pattern without additional measurement points, while the transformation without PPC fails. Thus, the implementation of the PPC not only enables the processing of irregular sampling grids, but also increases the measurement accuracy by including the actual position and orientation of the probp>

Unifying G/T and Noise Figure Metrics for Receiver Systems
Roy Monzello, November 2020

The conventional method for comparing the performance of antenna-receiver systems is the classical G/T metric. The G/T metric is the ratio of antenna-circuitgainrelative to the thermal noise temperature evaluated at the input of the low noise amplifier; the thermal noise at the input to the LNA consists of the received sky noise, the LNA's effective input noise temperature, and post LNA noise referenced to the LNA's input. While this has been a standard for many years, it will be shown that G/T does an incomplete job of describing the performance under all conditions. The noise figure metric was developed as a characteristic describing signal-to-noise degradation to be applied to circuit based input/output topologies, and cannot easily be applied to hybrid systems such as an antenna-receiver system in which the input power is described by spatial field density levels, and the output power is stated in terms of a circuit-based voltage-current environment. This paper presents a noise figure metric which has been expanded to include systems that are a hybrid of wave and circuit characteristics such as the marriage of an antenna and receiver. It will also be shown that whereas a system's noise figure is dependent upon a chosen noise reference temperature, the intrinsic Effective Input Noise Temperature of the system is an invariant that does not change when a different reference temperature is selected. It will also be shown that, in contrast to G/T, the effective input noise temperature of an antenna/receiver system will accurately predict the system's output SNR for all values of system input SNR. It will be shown in detail, how to measure the antenna/receiver system's Effective Input Noise Temperature (TE), resulting in the following equation: TE = (TD1 - Y£ TD2 )/(Y - 1) Where: TD1 , and TD2 are measured noise power densities at the face of the antenna, TE is the Effective Input Noise Temperature of the system, and "Y" is the classical "Y factor" metric.

Loadbox Design for EMC Testing in Automotive GNSS/SDARS Application
Ali Attaran,Adam Weiss,Ronald Missier,John Locke,Thomas Hermann, November 2020

In this paper, a loadbox was developed to perform theconductive and radiative Electromagnetic Compatibility (EMC) emission and immunity testing of the Global Navigation Satellite System (GNSS) and Satellite Digital Audio Radio Service (SDARS). To perform these tests, the supplier must purchase and build bias-tee, lowpass filter, choke, Diplexer and coupling circuitry to develop a loadbox. This means that same product made by different suppliers have different test set-up in place and therefore variability in the testing which create uncertainties in the test results and product approval or rejection. This is not reliable and can cause large amount of money wasted or bad product pass through. In this work, we propose an integrated loadbox design that can be built in-house with low cost and provide unique solution across the board. A loadbox consisting distributed Printed Circuit Board (PCB) made diplexer was developed that is easy to fabricate with low cost high durability and reliability during EMC testing and keep all the EMC testing consistent across the board which is enabling factor for proper decision making. A dual band diplexer was realized to separate the combined signal coming from the LNA's output port to two separate GNSS and SDARS ports. At the GNSS and SDARS frequencies due to the short wavelength of the RF signals, inductors and capacitors can be implemented using different transmission lines widths and lengths in short, open, parallel and straight-line formation. Distributed diplexer was designed using ADS RF Momentum simulator tool from Keysight and fabricated on a 2-layers PCB, FR2 of thickness 0.787 mm and a copper thickness of 35 um with overall size of 5.8x3.1 cm. Simulated and measured s-parameter for all of diplexer ports are in good agreement with measured insertion loss of better than 1.9 dB, return loss of 11.4 dB and GNSS-SDARS isolation of 16.8 dB at the GNSS frequency band and measured insertion loss of better than 2.1 dB, return loss of 14.1 dB and SDARS-GNSS isolation of 10.9 dB at the SDARS frequency band.

Correction of the Measured Phase of the Radiation Pattern of Millimeter-Wave Antennas
Antonius van den Biggelaar,Ben Jamroz,Dylan Williams,Bart Smolders,Ulf Johannsen, November 2020

To characterize the radiation characteristics of an antenna, determining the power pattern of the antenna is often sufficient. In some cases, however, both the amplitude and phase response are important. For instance, for accurate channel modeling, the antenna has to be de-embedded, requiring knowledge of the complex radiation pattern of the antenna. A vector network analyzer typically measures complex S-parameters, hence, determining the complex radiation pattern seems like a straightforward task. When measuring at higher frequencies, as the wavelength becomes shorter, antenna phase measurements are very sensitive to positioning and alignment errors. Using sophisticated measurement tools, the position and orientation of the antennas can be determined, and this information can be used to correct the measurement data. The stringent requirements on positioning and alignment at millimeter-wave frequencies, however, makes correcting the data based on physical insight, in some cases, a more practical solution. The results of a radiation pattern measurement of a WR-28 rectangular open-ended waveguide will be shown in the full paper. The magnitude of the radiation pattern is symmetric in its two principal planes, which is to be expected, but the phase of the radiation pattern is not symmetric. To explain this lack of symmetry, a two-parameter misalignment model will be presented. It will be shown that the measured phase is much more sensitive to the misalignment than the measured magnitude, explaining why the symmetry is only lacking in the measured phase. Based on the 1,708 available planar cuts, the two parameters in the misalignment model are determined with great confidence. Subsequently, the parameters are used to correct the phase of the measured radiation pattern, restoring the expected symmetry in the phase measurement.

Nearfield Antenna Measurements over Seawater - Challenges and Prospects
David Tonn, November 2020

The question of how to perform a nearfield antenna measurement in the presence of the air-sea interface is one that has been raised previously by the author[1]. When discussing spherical near field measurements various approaches have been proposed for addressing this problem, that are also applicable to measurements taken over a conducting ground plane. In this paper we shall discuss some of the practical challenges involved in data collection and measurement methods when performing this type of measurement. Examples shall be taken from both spherical nearfield measurements of simple sources along with single-point at-horizon measurements to examine the challenges associated with these approaches. A notional approach for measuring realized power gain at the horizon will also be discussed.

Balun Design for CISPR 16-1-5 Calibration and Reference Test Site Verification
James McLean, November 2020

The CISPR 16-1-5 standard requires site attenuation (SA) measurements for the validation of Calibration Test Sites (CALTS) and Reference Test Sites (REFTS). CALTS validation requires horizontally-polarized SA measurements, while REFTS validation requires both horizontally- and vertically-polarized measurements. These measurements are made with tuned linear dipole antennas driven from coaxial transmission lines via balancing networks (baluns). According to the CISPR standard, the effects of the baluns are removed with a substitution measurement. Specifically, the baluns are connected back-to-back (balanced to balanced) with the elements removed and the port-to-port insertion loss then measured. This insertion loss is then subtracted from the port-to-port insertion loss with the antennas assembled and in place on the OATS. Thus, the measurement is a true RF substitution measurement. The baluns must be perfectly symmetric for this measurement to be sound. It is then accurate only if the baluns are very well matched simultaneously to both to the coaxial transmission lines and the dipole antennas. Essentially, the dipole-to-dipole transmission, the 2-port network which is substituted, would have to behave as a matched attenuator. In the CISPR standard SA measurements are made a a minimum of 24 specific frequencies between 30 and 1000 MHz. The height of the transmitting antenna above the ground plane in all cases is 2 m, but the height of the receive antenna varies in order to avoid a transmission null. For each one of these measurements it is possible to obtain a perfect match for each dipole antenna. However, the matching network would be different for each frequency and also for the different heights involved. Thus, there is impetus to use broadband baluns and resistive matching pads. If this approach is selected, neither dipole can be perfectly matched. Moreover, if the balun is required to operate over a broad bandwidth, it is difficult for itsperformance to be made so good that it could be considered ideal. By employing a full 4-port model for antenna-to-antenna transmission on an OATS between linear dipoles with imperfect baluns and thus unbalanced antennas, we assess measurement error for topologies of balun/attenuator combinations for the CISPR 16-1-5 SA measurements.

Increasing 4-D Imaging Radar Calibration Accuracy Using Compact Antenna Test Range
Benoit Derat,Daniel Markert,Josef Schm?ller,Rong Chen Leng,Yaohui Liu,Ralf Reuter, November 2020

Advanced driver assistance systems (ADAS), such as blind spot warning and braking assistants, have been in use for years to improve road security. ADAS are currently further promoted through the autonomous driving trend. Due to their cost / performance trade-off, the automotive industry perceives 4-D high-resolution radar sensors, as one of the backbones of autonomous driving. With human safety being at stake, the topic of calibration of these sensors is obviously of the utmost importance. Performing an accurate calibration requires a test condition where the target is in the far-field of the radar under test (RUT). Due to the requirements for angular resolutions, 77 / 79 GHz radars with 15 cm radiation aperture or more are quite common. Applying Fraunhofer formula then results into a necessary measurement range length of 11.5m. Because of the high cost of ownership of an adequate anechoic range, radar manufacturers usually limit their measurements to the strict minimum and try to simplify the calibration process. A typical approach is to go for a diagonal calibration where the target is always at boresight for each beam-formed pattern of the RUT. This technique however delivers a sub-optimal compensation of the RUT biases. In particular, it creates high peak-to-side-lobe ratios (PSLR), where energetic echoes are observable in directions of side lobes of each beam. This paper introduces a new system for radar measurements, made of a short-size focal length offset-fed compact antenna test range (CATR), interfaced with an analog echo generator. With a chamber size of only 0.9 m x 2 m x 1.6 m, the setup has been designed to test apertures up to 30 cm size. The quality of the quiet zone achieved is discussed in the paper, as well as various uncertainty contributions relating to radar measurements. Tests are presented which involve a latest generation 4-D imaging radar on chip (RoC). Results obtained in the CATR are compared to a reference 7 m far-field range. Diagonal and full angular calibrations of the RoC are carried out and analyzed, demonstrating an improvement of 10 dB PSLR when the target is swept over the complete azimuth region.

Nearfield Measurements on Integrated Antennas with a Frequency Convertor and Embedded Local Oscillator
Michael Leffel,Thilo Bednorz, November 2020

Near-field measurements on antennas require magnitude and phase information dependent on the antenna position to support the near-field to far-field transformations. Modern active antennas are often integrated into frequency converters with embedded local oscillators (LO). For example, devices ranging from small 5G transceivers to large satellite payloads often need to be tested with the antenna integrated into the overall solution. There is no access to the embedded LO signal in these systems. The unknown phase of the embedded LO masks or corrupts the near-field phase measurement of the integrated antenna under test. A novel solution to this challenge is presented based on a new Vector Network Analyzer (VNA) platform. The system utilizes two stimulus signals (a measurement signal and a pilot signal) to characterize the antenna under test which is integrated into the frequency convertor. The pilot signal captures the phase information of the embedded LO, allowing the measurement signal to capture the antenna's magnitude and phase pattern as the antenna under test is moved within the near-field region.







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