AMTA Paper Archive
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Electromagnetic Interference Measurements at the MIT Lincoln Laboratory RF Systems Test Facility
Robust and repeatable electromagnetic interference and compliance (EMI/C) measurements require specialized test equipment and adherence to a rigorous set of procedures corresponding to the necessary standard. In this work, we describe the EMI/C testing capabilities at the RF Systems Test Facility at MIT Lincoln Laboratory and share the findings from work done in accordance to MIL-STD-461G. Both conducted and radiated emissions were measured on an example RF test artifact in the large near-field anechoic chamber at the facility. CE102, CE106, and RE102 test setups and results are discussed.
Pointwise Probe Correction Applied to a Robot-Based mm-Wave Antenna Test Range
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>
Examination of EMC Chamber Qualification Methodology for Applications above 1 GHz Using Frequency Domain Mode Filtering
Anechoic chambers used for Electromagnetic Compatibility (EMC) measurements above 1 GHz are qualified based on the Site Voltage Standing Wave Ratio (SVSWR) method as per the international standard CISPR 16-1-4. The SVSWR measurements consist of a series of scalar measurements using a dipole-like antenna placed along several linear transmission paths that are located at the edge of the quiet zone (QZ). The measurement process is conceptually similar to measuring VSWR using a slotted line and a moving probe. A full set of tests is time consuming because of the number of positions, antenna heights, polarizations and frequencies that are generally required. To reduce the test burden, the SVSWR method intentionally under-samples the measurement by requiring only 6 measurement points along each 40 cm long linear path to characterize the standing wave. As a result, the test results are generally overly optimistic. At microwave frequencies (note the upper frequency limit is 18 GHz), this under-sampling becomes far more pronounced. In this paper, we explore the effectiveness of using Cylindrical Mode Coefficients (CMC) based frequency domain mode filtering techniques to obtain the VSWR. Here, we place the test antenna on the outer edge of the turntable to obtain a full rotational pattern cut of amplitude and phase data. The antenna is then mathematically translated to the rotation center, whereupon a band-pass filter that tightly encloses the test antenna mode spectrum is applied. The difference between the mode filtered antenna pattern and the original perturbed pattern is attributed to chamber reflections. The measurement is comparatively easy to implement with no special positioning equipment needed. In this paper we present measured results taken from two horizontal polarization measurements (where the antennas were oriented 90 degrees from each other), and one vertical polarization measurement. For an EMC chamber test at a fixed height, an entire measurement campaign reduces to taking three vector pattern cuts. In contrast to the conventional technique, the proposed novel method does not suffer from positional under-sampling, so it is well-placed to be applied at microwave frequencies and above.
Application of Kernel Density Estimation to Achieve Automated Near Real-Time Antenna Pattern Data Processing and Analysis in an Anechoic Chamber
The Benefield Anechoic Facility (BAF) at Edwards Air Force Base is the world's largest known anechoic chamber. Due to its unmatched size and complement of test equipment, the BAF hosts far-field pattern measurements at all azimuth angles and multiple simultaneous elevations of installed antennas on large aircraft across a frequency range of 0.1 - 18 GHz. Antenna tests at the BAF rapidly produce large quantities of data, which often require immediate analysis to allow system owners to make relevant improvements. Historically, the BAF had accomplished quality assurance manually. Analysis was accomplished post-test by customers and the BAF team. Today, the BAF team has developed scripts that use kernel density estimation and basic machine learning to automatically check incoming data for errors and highlight unusual results for review. During a 2019 test of over sixty installed antennas on a B-1B bomber, the BAF team used these scripts to produce calibrated, quality-assured antenna patterns in near real-time. Rapid processing brings deficiencies to the customer's attention fast enough to allow corrections to be applied and re-tested during the same test event ? highly significant and valuable as aircraft and BAF schedule times are limited and may be a one-time opportunity to gather required data. This paper will explore the algorithm used to evaluate antenna patterns, as well as the expected characteristics of patterns that enable the selection of relevant data. Development and application of this algorithm found that using kernel density estimation to calculate the number of maxima in a pattern's distribution of gain values, then performing this recursively over only the main lobe, can identify problems such as incorrect switching, mismatched transmission lines, and multipath. Algorithm optimization was achieved using generated data, then verified by applying the algorithm to previous test data. For the B-1B, the script searched for data that deviated from an expected pattern with clean main and side lobes, minimal frequency dependency, and a low-power noise distribution at all azimuth angles outside the lobes. Finally, this paper will discuss the results of using this algorithm during a live test, and future improvements and applications for this data processing technique.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.
Common Microwave Absorbers Evaluations in W-band (75-100 GHz)
Understanding absorber performance in the W band (75-100 GHz) has become increasingly important, especially with the popular use of W band radars for automotive range detections. Commercial absorber performance data is typically available only to 40 GHz. Measurements performed in the W band in anechoic chambers are often under the assumptions that high frequency absorber data can be extrapolated from the data below 40 GHz. In this paper, we provide a survey of common microwave absorbers in the W band. It shows that the extrapolated data from the lower frequencies are not accurate. Absorber analysis models for low frequencies such using homogenization concept are no longer valid. This is because, for the millimeter wave, microstructures of the foam substrate become important, and the dimensions of the pyramids are much greater than the wavelengths. We examine performance variations due to parameters, such as carbon loading, shape, and thickness of the absorbers. We will also show how paint on the absorber surface might affect the absorber reflectivity, and if the common practice of black-tipping (leaving the tip of the absorbers unpainted) is an effective technique to alleviate paint effects.
An RCS-based Wall-reflectivity Technique - The First On-site Test Results
Among different measurement techniques for the wall reflectivity, an RCS_based technique has been implemented and test results are reported. For most of the anechoic chambers, the factory acceptance test and a quality-control check is sufficient for the customers to be sure that the absorbers used to line their chamber are good enough. In some cases, a quiet-zone reflectivity measurement will certify that the chamber yields the quietness as needed for the specific application of the customer. This last technique is mostly used in the far-filed ranges. However, in some anechoic chambers, e. g. some compact ranges, the customer wants to know the effect of the installation and the shipment on the final absorber installed in the room. That is why, they ask for a wall reflectivity measurement to see the reflectivity of the absorbers after being installed. The main problem to be solved when talking about wall reflectivity is the un-wanted clutter in the room which needs to be compensated for. Last year at AMTA 2016, we have introduced a clutter-removal technique to reduce the unwanted shattering levels. That was supported by some lab implementations and accordingly some limitations in the implementation. This paper, explains the result of the first practical on-site test done in an anechoic chamber. Many different points in the chamber have been tested and a detailed discussion of the results are brought to view.
Ka-Band Measurement Results of the Irregular Near-Field Scanning System PAMS
The portable antenna measurement system PAMS was developed for arbitrary and irregular near-field scanning. The system utilizes a crane for positioning of the near-field probe. Inherent positioning inaccuracies of the crane mechanics are handled with precise knowledge of the probe location and a new transformation algorithm. The probe position and orientation is tracked by a laser while the near-field is being sampled. Far-field patterns are obtained by applying modern multi-level fast multipole techniques. The measurement process includes full probe pattern correction of both polarizations and takes into account channel imbalances. Because the system is designed for measuring large antennas the RF setup utilizes fiber optic links for all signals from the ground instrumentation up to the gondola, at which the probe is mounted. This paper presents results of the Ka-band test campaign in the scope of an ESA/ESTEC project. First, the new versatile approach of characterizing antennas in the near-field without precise positioning mechanics is briefly summarized. The setup inside the anechoic chamber at Airbus Ottobrunn, Germany is shown. Test object was a linearly polarized parabolic antenna with 33dBi gain at 33GHz. The near-fields were scanned on a plane with irregular variations of over a wavelength in wave propagation. Allowing these phase variations in combination with a non-equidistant grid gives more degree of freedom in scanning with less demanding mechanics at the cost of more complex data processing. The setup and the way of on-the-fly scanning are explained with respect to the crane speed and the receiver measurement time. Far-fields contours are compared to compact range measurements for both polarizations to verify the test results. The methodology of gain determination is also described under the uncommon near-field constraint of coarse positioning accuracy. Finally, the error level assessment is outlined on the basis of the classic 18-term near-field budgets. The assessment differs in the way the impact of the field transformation on the far-field pattern is evaluated. Evaluation is done by testing the sensitivity of the transformation with a combination of measured and synthetic data.
Additive manufacturing metallic sphere as a RCS measurement standard
RCS measurements are usually performed in 3 steps in an anechoic chamber. First, the reflectivity of the target is measured. Then a reference measurement (generally without the target) is performed. Finally, a calibration standard of known RCS is used as a reference target. The main goal of the calibration phase is to transform raw measurements of reflectivity (S11 parameter in dB) into RCS (in dBsm) through the determination of the inverse transfer function of the entire RCS measurement layout. This calibration process indirectly converts the received electric field into a complex scattering coefficient. Moreover, it establishes a phase reference relatively to the rotation center of the target positioning system. The most frequently used standards are metallic spheres which have advantageous characteristics: monostatic RCS is well known by Mie-Series and independent of azimuth and elevation. However, manufacturing a perfect metallic lightweight sphere using conventional techniques include many issues that can generate defects in the spherical shape. The purpose of this paper is to evaluate the geometric and RCS performances of metallic spheres obtained from metal additive manufacturing systems using Selective Laser Melting (SLM) solutions. SLM is a fast prototyping technique designed to melt and fuse metallic powders together. On the one hand, these metallic spheres were checked by a 3D scanner in order to quantify the potential shape defects and on the other hand, RCS measurements were performed in an anechoic chamber. All these results will be presented in the final paper and compared with theoretical RCS data.
Comparing Predicted Performance of Anechoic Chambers to Free Space VSWR Measurements
Abstract— Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials; however, the bi-static or off angle performance must also be known. In reference , a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance of several anechoic chambers. These predictions are compared with full wave analysis performed in CST Suite®. A 12 m wide by 22 m long with a height of 12 m chamber was analyzed at 700 MHz. The QZ performance was compared to the polynomial predictions showing a difference of less than 2.2 dB. In addition, comparisons are made with measurements of the QZ performance of anechoic chambers. Measurements performed per the free space VSWR method of three different chambers are compared with the prediction that uses the polynomials presented in . The chambers are: a 18 m long by 11.5 m wide and 11.5 m in height operating from 100M MHz to 12 GHz; a 13.41 m by6.1 m by 6.1 m operating from 800 MHz to 6 GHz; and a 14 m long by 4.12 m by 4.27 m operating in the X band. The results show that the polynomial approximations can be used to give a reasonably accurate and safe prediction of the QZ performance of anechoic chambers.  V. Rodriguez and E. Barry, “A polynomial approximation for the Prediciotn of Reflected Energy from Pyramidal RF Absorbers,” Proceedeings of the 38th annual Symposium of the Antenna Measurement Techniques Association (AMTA 2016), pp. 155–160, October 2016.
On the Disadvantages of Tilting the Receive End-Wall of a Compact Range for RCS Measurements
Abstract— Tilting the receive end wall of a compact range anechoic chamber to improve Radar Cross-Section (RCS) measurements has been a tool of the trade used since the earliest days of anechoic chambers. A preliminary analysis using geometrical optics (GO) validates this technique. The GO approach however ignores the backscattering modes from the reflected waves from a field of absorber. In this paper, a series of numerical experiments are performed comparing a straight wall and a tilted wall to show the effects on both the quiet zone and the energy reflected back towards the source antenna. Two Absorber covered walls are simulated. Both walls are illuminated with a standard gain horn (SGH). The effects of a wall tilted back 20° are computed. The simulations are done for 72-inch long absorber for the frequency range covering from 500 MHz to 1 GHz. The ripple on a 10 ft (3.05 m) quiet zone (QZ) is measured for the vertical wall and the tilted wall. In addition to the QZ analysis a time-domain analysis is performed. The reflected pulse at the excitation antenna is compared for the two back wall configurations Results show that tilting the wall improves measurements at some frequencies but causes a higher return at other frequencies; indicating this method does not provide a broadband advantage. Keywords: Anechoic Chamber Design, Radar Cross Section Measurements, Geometrical Optics
A Broadband Patch Antenna with an Anisotropic Superstrate - Design and Measurement Challenges
Microstrip patch antennas are well known in the field of communications and other areas where antennas are used. They consist of a metallic conducting surface deposited onto a grounded dielectric substrate and are widely used in situations where a conformal antenna is desired. They are also popular antennas for array applications. But most patch antennas are typically resonant structures owing to the standing wave of current that forms on them. This resonant behavior limits the impedance bandwidth of the antenna to a few percent. In this paper we shall present an approach for improving the bandwidth of a resonant patch antenna which employs an engineered anisotropic superstrate. By proper design of this superstrate and its tensor, and proper alignment of it with the axis of the patch, an antenna with improved impedance bandwidth results. Some of the challenges associated with the measurement of the anisotropic superstrate will be discussed, ranging from 3D simulations to physical models tested in the laboratory. A final working model of the antenna will be discussed; this model consists of a stacked patch arrangement and was designed to operate at the GPS L1 and L2 frequencies. Data collected from 3D simulations using CST Microwave Studio along with laboratory and anechoic chamber measurements will be presented, showing how the bandwidth at both of these frequencies can be increased while maintaining circular polarization in both passbands. Tolerance to errors in alignment and fabrication will also be presented. Additionally, some lessons learned on anechoic chamber measurements of the antenna’s gain and axial ratio will be discussed.
Measurements of Low Gain VHF Antennas in Spherical Multi-Probe NF Systems
Measurement of the radiation properties of low gain antenna operating at VHF frequencies is well known to be a challenging task. Such antennas are sometimes tested in outdoor Far Field (FF) ranges which are unfortunately subject to errors caused by the electromagnetic pollution and scattering from the environment. Near Field (NF) measurements performed in shielded anechoic chambers are thus preferable to outdoor ranges. However, also in such cases, the accuracy of the results may be compromised by the poor reflectivity of the absorbing material which might be not large enough wrt the VHF wavelength. Other source of errors may be caused by the truncation of the scanning area which generates ripple on the FF pattern after NF/FF transformation. Spherical multi-probe systems developed by MVG are optimal measurement solution for low directive Device Under Test (DUT). Such systems allow to perform a quasi-full spherical acquisition combining a rotation of the DUT along azimuth, with a fast electronically scanned multi-probe vertical arch. The DUT can be accommodated on masts made of polyester material which allows to minimize the interaction with the DUT. Measurements of low directive device above 400 MHz performed with such type of systems have been demonstrated to be accurate and extremely fast in previous publications. In this paper, measurements of a low directivity antenna, performed at VHF frequencies in a MVG spherical multi-probe system, will be presented. The antenna in this study is an array element, part of a larger array, which has been developed for space-born AIS applications. Gain and pattern accuracy of the measurement will be demonstrated by comparison with full wave simulation of the tested antenna.
The effect of the receiving-antenna pattern on the results of the free-space VSWR technique
The free-space VSWR technique as the standard method to extract the quiet-zone reflectivity in anechoic chambers has been explained in short. Among different uncertainty factors, the effect of pattern of the probe/receiving antenna has been investigated and some points how to reduce this effect has been suggested.
Phase Error Characterization of a Space-Fed Array
GTRI has been developing a method for insertion phase calibration, as discussed in the paper “Insertion Phase Calibration of Space-Fed Arrays,” which was presented at AMTA in 2015 . This method has been implemented to characterize the phase response of phase shifters in a system currently under fabrication at GTRI. One of the primary requirements for the phased-array antenna of this system is a maximum RMS phase error. The RMS phase error for this array is influenced by a variety of error sources, including phase shifter quantization, beam steering computer (BSC) algorithmic error, phase shifter unpredictability error, test fixture induced error, phase shifter thermal drift, and phase shifter frequency dependency. Each of these error sources has been categorized as either a non-deterministic error, whose behavior can be statistically characterized but not calibrated out, or as a deterministic error, whose behavior can be characterized and potentially calibrated out. The non-deterministic errors include element unpredictability, which is induced by the inability of an individual phase shifter to precisely repeat a given phase command, and errors induced by the calibration test fixture itself. The deterministic errors include phase shifter quantization error, which is a function of the phase state bit precision, BSC algorithmic error, which is driven by the numerical preciseness of calculation of the commanded phase states for each element, thermal driven phase drift, and phase shifter frequency dependency across the band of operation. To calibrate the insertion phase and phase-state response curves for all phase shifters used in the system, a custom-built calibration fixture was constructed into a septum wall that separates two semi-anechoic chambers. The realized phase-error budget of the system under fabrication was affected directly by the accuracy of both the calibration method and this fixture. We will present our analysis of all phase-error sources as they contribute to the overall phase-error design goal of the system. We have shown how the design and implementation of both the calibration fixture and methodology meet that goal.
Improved clutter removal for measuring wall reflectivity using the RCS technique
Absorber lining is an important part of an indoor antenna measurement chamber design. During the design phase different absorber types are selected for minimizing the expected reflection from given locations in the chamber. By the time of installation, these absorbers have already been measured as part of the production quality control. The question however arises if after installation, these absorbers still meet the requirements of the design. The free-space-VSWR  measurement technique is a method to assess the overall reflectivity of the chamber at a certain location, i.e. quiet-zone reflectivity, but cannot be easily limited to measure the reflectivity of a single wall. In this work the RCS technique  is revised. The reflection of the wall is measured using a quasi-monostatic RCS setup which is mounted on a linear sliding system. The linear sliding system is positioned perpendicular to the wall. After measuring at several positions the measurement results are shifted in distance such that the reference target or wall add coherently and clutter or other walls destructively. Using this technique it will be shown that the reflectivity of an absorber-lined wall can be determined during installation where not all walls or floor have been covered yet.  J. Appel-Hansen, “Reflectivity level of radio anechoic chambers,” IEEE Trans. Antennas Propag., vol. 21, no. 4, pp. 490–498, Jul. 1973.  G. Cottard and Y. Arien, “Anechoic Chamber Measurement Improvement,” Microw. J., no. March, 2006.
A Rotating Source Polarization Measurement Technique Using Two Circularly Polarized Antennas
This paper combines the standard two-antenna gain measurement technique with the rotating source method for measuring the gain as well as the polarization ratio and tilt angle of the polarization ellipse of a circularly polarized antenna. The technique is illustrated with two identical helical antennas, one for the source and one for the antenna-under-test (AUT), facing each other. Measurements of the voltage transfer ratio are made over one full 360 degree on-axis rotation of the source while the AUT remains stationary. The rotation causes the phase of the electric field of the principal polarization to rotate in one direction and the phase of the cross polarization to rotate in the opposite direction. A Fast Fourier Transform (FFT) of the data from a single rotation is insufficient to resolve the two polarization components. Leakage from the principal polarization will most likely cover up the low-level opposite polarization signal. However, the FFT resolution can be artificially increased by appending to the measured data, precisely M-1 copies of the data. Now the polarization components will be separated by 2M rotations. Application of a heavy weighting function to the augmented data and a phase compensation to the FFT allows an unambiguous decomposition of the measured voltage transfer ratio into a principal and a cross polarization component. These are then used to calculate antenna polarization characteristics. The technique was verified in an anechoic chamber with two 6-turn 5.8 GHz helical antennas separated by 4 feet. There was very good agreement between electromagnetic simulations and measurements of the polarization ellipse tilt angle and a -20 dB polarization ratio.
Insights Into Spherical Near Field Probe Correction Gained From Examining the Probe Response Constants
Several recent articles [1-9] have focused on assessing spherical near field (SNF) errors induced by using a non-ideal probe, i.e. a probe that has modal content. This paper explores this issue from the perspective of the probe response constants, defined by , which are the mathematical representation of the effect of the antenna under test (AUT) subtending a finite angular portion of the probe pattern at measurement distance . The probe response constants are a function of the probe modal coefficients, the size of the AUT (i.e. the AUT minimum sphere radius ), and the measurement distance , and thus can be used to evaluate the relative contribution of probe content as both measurement distance and AUT size varies. After a brief introduction, the first section of this paper reviews the theory describing the probe response constants; the second section provides some examples of the probe response constants for a simulated broadband quad-ridge horn, and the final section examines measured AUT pattern errors induced by using the corresponding probe response constants in a conventional SNF-to-FF transform. References:  A. C. Newell and S. F. Gregson, “Effect of Higher Order Modes in Standard Spherical Near-Field Probe Correction,” in AMTA 2015 Proceedings, Long Beach, CA, 2015.  Y. Weitsch, T. F. Eibert, and L. G. T. van de Coevering, “Investigation of Higher Order Probe Corrected Near-Field Far-Field Transformation Algorithms for Preceise Measurement Results in Small Anechoic Chambers, in AMTA 2015 Proceedings, Long Beach, CA, 2015.  A. C. Newell and S. F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction,” in EuCAP 2014 Proceedings, The Hague, 2014.  A. C Newell and S. F. Gregson, “Higher Order Mode Probes in Spherical Near-Field Measurements, in EuCAP 2013 Proceedings, Gothenburg, 2013.  A. C. Newell and S. F. Gregson, “Estimating the Effect of Higher-Order Modes in Spherical Near-Field Probe Correction,” in AMTA 2012 Proceedings, Seattle, WA, 2012.  T. A. Laitinen and S. Pivnenko, “On the Truncation of the Azimuthal Mode Spectrum of High-Order Probes in Probe-Corrected Spherical Near-Field Antenna Measurements,” in AMTA 2011 Proceedings, Denver, CO, 2011.  T. A. Laitinen, S. Pivnenko, and O. Breinbjerg, “Theory and Practice of the FFT/Matrix Inversion Technique for Probe-Corrected Spherical Near-field Antenna Measurements with High-Order Probes,” IEEE Trans. Antennas and Prop., Vol. 58, No. 8, August 2010.  T. A. Laitinen, J. M. Nielsen, S. Pivnenko, and O. Breinbjerg, On the Application Range of General High-Order Probe Correction Technique in Spherical Near-Field Antenna Measurements,” in EuCAP 2007 Proceedings, Edinburgh, 2007.  T. A Laitinen, S. Pivnenko, and O. Breinbjerg, “Odd-Order Probe Correction Technique for Spherical Near-Field Antenna Measurements,” Radio Sci., Vol. 40, No. 5, 2005.  J. E. Hansen ed., Spherical Near-Field Antenna Measurements, London: Peregrinus, 1988.
BIANCHA: A spherical indoor facility for bistatic electromagnetic tests
BIANCHA (BIstatic ANechoic CHAmber) is a singular facility located at the premises of the National Institute for Aerospace Technology (INTA), Spain, and was devised to perform a wide variety of electromagnetic tests and to research into innovative measurement techniques that may need high positioning accuracy. With this facility, both monostatic and bistatic tests can be performed, providing capability for a variety of electromagnetic measurements, such as the electromagnetic characterization of a material, the extraction of the bistatic radar cross section (RCS) of a target, near-field antenna measurements or material absorption measurements by replicating the NRL arch system. BIANCHA consists of two elevated scanning arms holding two antenna probes. While one scanning arm sweeps from one horizon to the other, the second scanning arm is mounted on the azimuth turntable. As a result, BIANCHA provides capability to perform measurements at any combination of angles, establishing a bistatic, spherical field scanner. In this regard, it is worth noting that in the last years, a renewed interest has arisen in bistatic radar. Some of the main reasons behind this renaissance are the recent advances in passive radar systems added to the advantages that bistatic radar can offer to detect stealth platforms. On the other hand, with the aim of developing new aeronautic materials with desired specifications, research on the electromagnetic properties of materials have also attracted much attention, demanding engineers and scientists to assess how these materials may affect the radar response of a target. Consequently, this paper introduces BIANCHA and demonstrates its applicability for these purposes by presenting results of different tests for different applications: a bistatic scattering analysis of scaled aircraft targets and the extraction of the electromagnetic properties of composite materials utilized in an actual aeronautical platform.
Limitations of the Free Space VSWR Measurements for Chamber Validations
Free Space VSWR measurement has been the de facto standard method for anechoic chamber performance evaluation for more than 50 years. In this method, a probe antenna is kept at a fixed angle while traveling along a linear path to record the standing wave pattern. The probe antenna is then rotated to a different angle to repeat the measurement. Reflectivity, which is used as the chamber performance metric, is calculated for each probe rotation angle. In this paper, we show that the reflectivity is affected by the antenna patterns of the probe antenna. When the probe antenna is aimed at the specular reflection point of a chamber surface, measurement dynamic range is improved, and the method provides a measure of the reflectivity primarily from that surface. When the probe is not directed at a specular point, other reflections in the chamber can contribute to the VSWR, and the chamber reflectivity becomes more dependent on the probe antenna pattern.
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