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Anechoic Chamber

Huffman Radar Site: Far Field Calibration and Testing Range
Tomas Kendo, Ryan Thompson Thompson, Thomas Corigliano, Chad Shaffer, Thomas Steffen, October 2022

This paper will describe the Huffman Radar Site (HRS), a unique in-situ remote radio frequency calibration and characterization capability located at the Air Force Research Laboratory Sensors Directorate, Wright Patterson Air Force Base (WPAFB), OH. HRS is a part of the OneRY Range complex which consists of Indoor and Outdoor Ranges used to conduct test, evaluation, integration, and demonstration of novel sensing systems and technologies. The Outdoor Range has diverse capabilities at several sites distributed across the local area. Within the Sensors Directorate complex there are three 100 foot antenna towers: the South Tower holds a dish-based S-Band Radar, the East Tower holds a large digital phased array radar, and the West Tower is reconfigurable as needed based on customer requirements. The Huffman Radar site is used to validate the proper functionality of systems on these towers, conduct experiment witness testing, and provide calibration signals for phased-array antennas. The site is primarily used as a Direct Illumination Far Field Range source standing approximately 2 miles away with direct line of sight to the South, East, and West towers. The capability includes full polarimetric transmit from 2.9 to 3.5 GHz and receive from 800 MHz to 6 GHz with future plans to expand the frequency range. This paper will include the design, link budget, hardware implementation, test, and validation of the site. Preliminary far-field antenna pattern data and calibration results for the S-Band Radar system and digital phased-array radar system will be presented. The discussion will include challenges and successes in standing up a multi-function outdoor remote testing capability.

Design and Measurements of a Small End-fed Normal Mode Helical Antenna with Integrated Microstrip Structure
Sri Lekha Srimat Kilambi, Herbert Aumann, Mauricio Pereira da Cunha, October 2022

Compact omnidirectional antennas are highly sought for a multitude of present-day wireless applications such as smart car keys, radio frequency identification (RFID) tags, tire pressure monitoring system, hand-held communication devices, and high-temperature harsh-environment wireless sensors. This paper discusses the performance and the unique challenges in measuring the radiation performance of a compact (~1/25th to 1/10th of a wavelength) helical and microstrip combined structure operating as a normal mode helical antenna (NMHA) around 300MHz. The helical wire structure (27-turn, 37mm high and 6.2 mm wide) is connected to the end of a 50 Ωmicrostrip line fabricated on 1.5 mm thick FR4 substrate. The microstrip line provides a ground plane to the helical structure, serving as an integral part of the radiating element. A tiny 1:1 balun transformer was used to partially decouple the integrated NMHA from the external sheath of the coaxial cable connected to a vector network analyzer, thus allowing proper NMHA impedance measurement. The NMHA S-parameters were simulated on two different platforms, ANSYS-HFSS and WIPL-D Pro, and compared to the frequency of the measured structures, with all simulations and measurements agreeing within 3.5%. Varying the length of the ground plane associated with the microstrip line from 13 mm to 76 mm resulted in the decrease of the measured NMHA operational frequency by 3.2%. The measured impedance of the fabricated NMHA (including the balun) was close to 50 Ω for the 51 mm long line without the need of additional matching circuit. The measured transmission loss for two identical antennas (each 26 cm3) placed about 1 m apart was 22 dB. This performance is comparable or better than the coupling between much larger antennas currently used in harsh environment power plant applications, such as suspended plate antennas (42,500 cm3) or planar inverted F-antennas (11,800 cm3) operating around the same frequency. In addition, the proposed NMHA structure can be implemented using substrates and wires capable of operation at temperature above 300 °C, which constitutes an appealing solution for high-temperature harsh-environment applications such as those found in industrial machinery, metallurgic industry, power plant boilers, and turbine engines.

A New Handheld Sensor for Measuring Intrinsic Dielectric Properties at 100 to 1000 MHz
John Schultz, Ren Geryak, October 2022

Electromagnetic materials characterization at UHF and VHF frequencies is typically done with laboratory fixtures such as the coaxial airline or rectangular waveguide. These conventional methods require material specimens to be cut or machined to precision tolerances for insertion within the transmission line fixture. Measurement accuracy dictates there should be little or no air gaps between the specimen and the transmission line walls. Transmission line methods also require significant handling and multi-step calibration procedures to characterize a material specimen. This paper describes a new handheld measurement device that overcomes these limitations with a simple calibration and non-destructive measurement procedures. This new method applies an open-ended stripline sensor tuned to maximize measurement sensitivity in the 100 to 1000 MHz range. The sensor footprint is approximately 100 mm square and utilizes an integrated one-port vector network analyzer. It operates by measuring the amplitude and phase of the reflection coefficient when placed adjacent to a material specimen. While traditional transmission line methods employ analytical expressions to relate scattering parameters to intrinsic properties, The open-ended stripline sensor geometry and its interaction with the material cannot be easily modeled with an analytical approximation. Instead, it is modeled with a full-wave Finite Difference Time Domain (FDTD) code to develop the relationship between measured reflection and complex permittivity. This inversion method precomputes a translation table by iteratively modeling the measurement fixture across a range of complex permittivities and specimen thicknesses. From this inversion database, interpolation is then used to calculate the frequency dependent complex permittivity or sheet impedance of a given specimen. This paper provides details about the calibration and use of this new device as well as the material property inversion algorithm. Measurement examples of low-loss and lossy materials as well as resistive sheets are also presented and compared to more conventional transmission line results, and are discussed in relationship to measurement uncertainties.

Co-Site Interference Analysis on Aerospace and Naval Platforms using Advanced Simulation Tools
V B Murthy D, CJ Reddy, October 2022

Modern Aerospace, Naval and Defense platforms are overwhelmed with radiofrequency (RF) signals competing for spectrum. RF co-site interference has become a major problem due to the RF interference from jamming, radio stations nearby, or even from civilian communications such as mobile phones. It can be a problem on military platforms like surface warships, land vehicles, and aircraft where many different RF transmit and receive antennas must share a relatively small space. This can be a communications nightmare in which separate RF systems inadvertently step on each other's signals, causing an RF communications fratricide problem that can also be compounded by intentional or accidental RF jamming. It has become more important than ever to address these issues that arise due to RF co-site interference. In this paper, we present advanced simulation tools for antenna placement, antenna coupling and cosite interference on electrically large naval and air platforms using Altair Feko and Wrap softwares. S-parameter coupling matrix of various antennas (both in-band and out-of-band) are computed using either full-wave solutions such as MoM, MLFMM or using asymptotic methods such as PO, RL-GO and UTD. Alternatively Coupling Loss Matrix, defined as the power ratio between powers at the terminals of the transmitting and receiving antennas can be computed using equivalent sources. S-Parameter matrix or Coupling Loss values are then used to study the parameters of co-site/collocation interference such as inter-modulation products, adjacent channel interference and harmonic interference. Furthermore, we will also discuss the options to mitigate collocation interference by adding appropriate filters.

Radiation pattern measurements using an active radar module
Anna Granich, Roland Moch, Amar Al-Bassam, Dirk Heberling, October 2022

In the automotive sector, driver assistance systems are playing an increasingly important role in automated driving, with radar sensors being a critical component for environmental perception. The implementation of safety-relevant functions places ever higher demands on sensor technologies in order to provide high-quality and reliable data. For radar sensors the radiation pattern of the antennas is a crucial factor for the performance of the overall system. As the technology moves towards highly integrated systems, the antennas are integrated directly on the circuit board or even the radar chip package. This complicates or even eliminates the possibility for classical antenna measurements, as there is no access to the antenna feed line. Here the integrated receiver and transmitter module of the radar system are used to measure the two-way radiation pattern with a reflector. However, this leads to a lot of unknown factors and influences that differ from classical antenna measurements. Within this study the formal built up radar measurement setup for the robot-based antenna measurement system of the Institute of High Frequency Technology of RWTH Aachen University is used for accurate two-way pattern measurements based on the sampled raw data of the radar system. A modular frequency modulated continuous wave radar setup with high configurability is used to build up measurements on well-known antennas. The flexibility of the modular radar allows for measurements on different antenna types as scalar feed horn and travelling wave antennas. Different parameters like the choice of the reflector, the measurement distance and repeatability of the measurements are examined on their influence on the measured two-way-patterns of these antennas. The radiation patterns are resolved over the frequency bandwidth of the chirps using the intermediate frequency signal of the radar sensor to investigate the influences on their frequency dependence. The possibility on measuring the co and cross polarization components of the patterns is studied.

A 77 GHz Microstrip Comb Line Antenna Array for Automotive RADAR application
Neha Pazare, Vivek Kamble, October 2022

In this paper, a 77 GHz microstrip comb-line antenna array for an automotive RADAR application with a low sidelobe level is proposed. The microstrip technology is used for the antenna due to its low fabrication cost, small size, and easy integration with other microwave circuitry. At very high frequencies such as millimeter waves, the gain of a single element patch antenna is not enough to withstand the RADAR application requirements, hence an array of antennae is beneficial. A The Phased array antenna configuration is needed to have a high gain and low sidelobe level of -20 dB and a beam steering mechanism. The design procedure used here is the implementation of a single comb antenna, that is further realized into a 1 x 10 uniform linear array of a comb line array. It has a gain of 14.81 dB and a sidelobe level of -15 dB. The radiation in the comb antenna is primarily due to the open sides with the lengths of the comb serving as transmission lines. The adjacent combs are placed at the distance of λ in order to co-phase the antenna elements at the desired frequency. Additionally, with an aim of reducing the sidelobe level, Taylor amplitude distribution is used, and the tapered array is designed. This methodology helped to achieve a sidelobe level of -20 dB. The gain of an overall array is increased to 20 dB by realizing the array of 4 x 10. Another requirement of the Automotive Radar is beam steering to accurately detect the target. Butler matrix is a beamforming network chosen to feed the phased array antenna. The proposed antenna array is simulated in Ansys HFSS with Rogers RO 3003 substrate of the thickness of 1.27 mm and has an overall dimension of 9 x 14.96 mm2. The goal of the design of this antenna is to acquire an appropriate radiation pattern with a low side lobe level better than -20dB and achieve beam steering using the Butler matrix to have a phased array configuration. Index Terms— RADAR, Antenna array, Comb line array, Butler Matrix, Phased array.

RCS Compact Range Focal Plane Array Antenna Feed Design Concept
William Carter, Jerry Jost, Gabriel M. Rebeiz, October 2022

Diagnostic and verification testing of Low Observable (LO) platforms and components requires an Ultra-Wideband (UWB) Inverse Synthetic Aperture Radar (ISAR) imaging capability. A Compact Range (CR) is a test instrument that, when fitted with an instrumentation radar and target positioner, can efficiently produce ISAR images and other Radar Cross Section (RCS) data products required for LO research, design and production programs. Key limiting factors for the instantaneous radar imaging bandwidth of a CR is the feed antenna, where the criteria of a good feed is frequency bandwidth and illumination pattern shape. Maintaining a relatively constant reflector illumination characteristic typically requires several feeds with constant patterns functioning over smaller operating bandwidths, to be mechanically sequenced in the measurements. These feed limitations increase operational costs and complexity for LO measurements, driving a need for improved illumination sources providing constant reflector illumination for UWB collections. Focal Plane Arrays (FPAs) can be utilized to resolve these issues while increasing instantaneous bandwidth and measurement quality while reducing operational costs. This paper presents a procedure for defining complex weights of an FPA aperture to optimize radiation pattern matching to the reflector. A simulated plane wave arriving from the CR quiet-zone impinges on a model of the reflector. The FPA is placed in a region near the focal point and contained within the beam waist envelope, and the FPA weights are computed using a Computed Electromagnetic (CEM) techniques. The computational complexity of CEM simulations of electrically large CRs are usually prohibitive, however this method exploits the large focal lengths of CRs to sparsely model reflectors, and produces a tractable solution even at millimeter wavelengths. Practical aspects of FPA designs are presented and discussed as applied to the large outdoor CR at the US Army, Electronic Proving Ground (EPG), Fort Huachuca, Arizona.

Electromagnetic Interference Measurements at the MIT Lincoln Laboratory RF Systems Test Facility
Cara Yang Kataria, Alan J. Fenn, Adam J. Chapman, and Peter T. Hurst, October 2021

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
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>

Examination of EMC Chamber Qualification Methodology for Applications above 1 GHz Using Frequency Domain Mode Filtering
Zhong Chen,Stuart Gregson, November 2020

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
Emily Mendoza,Shea Watson, November 2020

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
Andreas Olk, Kais-Ben Khadhra, Thiemo Spielmann, October 2017

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)
Zhong Chen, October 2017

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
Amin Enayati, Joachim Wesemael, October 2017

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
Alexander Geise, Torsten Fritzel, Maurice Paquay, October 2017

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
Pierre Massaloux, October 2017

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
Vince Rodriguez, October 2017

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 [1], 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 [1]. 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. [1] 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
Vince Rodriguez, October 2017

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
David Tonn, Susan Safford, October 2017

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
Andrea Giacomini, Francesco Saccardi, Vincenzo Schirosi, Francesca Rossi, Stephane Dooghe, Arnaud Gandois, Lars Foged, October 2017

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.
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