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This paper presents the design of an ultrawideband (UWB) open-boundary dual-polarized quad-ridged horn antenna (QRHA) having an impressive 3-decades bandwidth from 1 to 32 GHz, making it an ideal choice for a wide range of applications in radars and communication systems. Its unprecedented bandwidth performance in a single and compact antenna geometry covers various frequency bands including L, S, C, X, Ku, and K. The ridge tapering profile of the proposed QRHA ensures excellent impedance matching, while the aperture matching prevents any phase distortion. The back-cavity design as well as cavity flaring is carefully optimized to achieve wideband performance along with no presence of pattern degradation specifically at the higher end of the frequency band. The antenna design exhibits a favorable impedance matching below -10 dB, well-matched copolar antenna patterns in the principal planes, and good crosspolarization (exceeding -20 dB) over a 3-decades bandwidth.
This paper discusses the novel development of a lightweight RF front-end system aimed at enhancing airborne antenna measurements in the far-field. The proposed system leverages advancements in software-defined radio (SDR) technology and high-performance RF front-end systems. While there are instances of SDR applications in UAS measurements, these systems are predominantly designed for anti-UAS and communication purposes, lacking focus on antenna and radar characterization. In contrast, the proposed system is purposefully tailored for RF applications, completely self-contained and airborne, possessing both RX and TX capabilities, and minimizing reliance on ground-based components. The system facilitates transmission and reception in both H- and V-polarization, employing two independent channels. Consequently, it allows for the simultaneous measurement of antenna or radar properties in both copolar (Cp) and cross-polar (Xp) orientations. The comparison between antenna measurements conducted within an anechoic chamber and those carried out utilizing the proposed UAS system demonstrates a substantial level of agreement.
Compact Ranges are widely used for antenna measurements across wide frequency ranges spanning frequencies as low as 350MHz to as high as 60GHz and above. Advances in electromagnetic (EM) simulations have significantly improved the design process for compact ranges, resulting in reduced costs. Characterizing compact range including the anechoic chamber is computationally very challenging in terms of computer memory and time. In this paper, we will present the full wave method, MLFMM for characterizing compact range without the chamber and application of asymptotic method, RL-GO to characterize the compact range inside the anechoic chamber.
This paper describes two reflection methods to measure highly conductive coatings at VHF frequencies: 1) a resonant method based on eddy-current sensing at HF and VHF frequencies, and 2) a wideband method at VHF and UHF frequencies, based on a shorted transmission line combined with and computational electromagnetic (CEM) simulations to invert surface impedance. In both cases, the methods are able to determine surface impedances with sensitivities of a small fraction of an ohm. Both methods have strengths and weaknesses with respect to ease of calibration, sensitivity, frequency range, and use on non-flat surfaces. This paper describes both approaches and presents measurements on a variety of conducting materials and coatings. The resulting properties are also compared with DC conductivity measurements collected with a four-point probe system. The predicted accuracy for both methods is presented based on simulated data and empirical measurements.
Wireless devices supporting global navigation satellite systems (GNSS) services have become an essential tool in different areas of technology such as agriculture, construction, automotive, etc. Therefore the performance and reliability of such devices are important aspects that need to be addressed in the testing stage during the development of the units. The integration of the Over-the-Air (OTA) testing method with the 3D Wave Field Synthesis (3DWFS) technique offer not only the benefit of having tests under controllable and repeatable conditions but also the ability to recreate complex and realistic scenarios in a controlled environment with full polarimetric support for the testing of wireless devices. This contribution applies this technology to emulate a GNSS scenario within an anechoic chamber. For the results validation, a realistic GNSS outdoor scenario was recorded and compared with the emulated scenario where 3DWFS was applied for each individual satellite. This represents a significant step for the GNSS community and also for the future development and testing of wireless devices.
The existing IEEE-STD 1128 on “Recommended Practice for RF Absorber Evaluation in the Range of 30 MHz to 5 GHz” was published in 1998. The standard has been referenced frequently and used as a guide for RF absorber evaluations. The document has several aspects which need updating, including the frequency range of coverage, requirements for newer test equipment, advances in test methodologies and material property evaluation, measurement uncertainty considerations, and absorber high power handling and fire testing requirements. The working group is divided into task groups and is in the final stage of collecting inputs from these subgroups. The next step is to consolidate the inputs and produce a draft standard for a wider distribution before being submitted for balloting. The subgroup contributions can be found on the IEEE imeetcentral website (https://ieeesa. imeetcentral.com/p1128). The sections which have received substantive updates include bulk material measurements, instrumentation, absorber reflectivity measurements, and power handling test. In this paper, we will provide some detailed discussions on the planned updates from these contributions. For areas which did not receive sufficient input, the working group plan to table those topics for future considerations.
This paper addresses the circularly-polarized (CP) gain uncertainty when using linearly-polarized feeds to obtain circular polarization in Compact Antenna Test Ranges. In particular, our emphasis is placed on quantifying the inaccuracy caused by deviations in amplitude and in phase of the two orthogonal linear measurements. This is of paramount importance especially for highly directive CP antennas operating at high frequencies in that the CP gain will be adversely impacted even by a small deviation from an ideal 90- degree rotation, as well as by a situation when the rotation may cause a slight boresight misalignment. To characterize the gain uncertainty, we look at ratio differences between the peak amplitude of the linear measurements, as well as cases when the phase shift of the two orthogonal linear measurements is no longer 90 degrees. The former is done through mechanical and electrical boresighting technique in the initial setting. The latter, which is the focus of this paper, is carried out through several case studies in practice mimicking some non-ideal 90- degree rotation settings.
Spherical Near Field (SNF) measurement systems are
primarily limited in usable bandwidth by the probe frequency
coverage. This limitation mainly arises from the presence of
higher-order azimuthal modes in the probe pattern [1]. In case of
electrically large or offset AUTs, such a limitation may be
overcome by a full probe correction algorithm for the NF/FF
transformation [2]. However, probes approximating first order
performance over the full bandwidth are generally preferred.
Traditionally, first-order probes based on geometrically
symmetric Ortho-Mode Junctions (OMJ) with externally
balanced feeding have been widely accepted. These probe designs
rely on couplers that provide equal amplitude and opposite phase
distribution at their output ports [3]. In this paper, the design
and validation of a novel 3dB/180° coupler is presented. The
concept is based on the natural anti-symmetric properties of the
electric field within the component, providing a quasi-perfect
amplitude and opposite phase distribution. To achieve these
properties, an architecture based on slot coupling is selected. The
design has been implemented in several frequency bands, from
UHF to Ku-band, as stand-alone cased components.
Experimental data at L/S-band is presented in this paper,
showing excellent performance in terms of matching, balance,
and isolation between output ports, well in-line with full-wave
electromagnetic predictions. In addition, the impact of the
coupler accuracy is also assessed on a relevant SNF test case.
This paper introduces an innovative testing system designed for the characterization and calibration of W-band active phased array antennas utilizing antenna-in-package (AiP) technology. The proposed system is a multi-axis scanner with nine degrees of freedom, used to perform near-field and farfield measurements with same setup. The multi-axis system allows precise positioning of the antenna and the probe, allowing accurate measurements of the antenna radiation patterns in both near-field (NF) and far-field (FF) regions. Experimental results show that the proposed hybrid multi-axis scanner significantly improves the calibration accuracy of the antennas on-chip at 77 GHz compared to traditional far-field systems. Using the hybrid scanned (near-field and far-field) provides a versatile and effective test procedure to even characterize additional electromagnetic artifacts that may be present during the test. The proposed system enables accurate calibration and measurements by providing precise control over the positioning of the antenna and probe, while minimizing the effects of interference from the surrounding environment. Excellent agreement between the antenna array pattern measured in the near-field and far-field is achieved post-calibration. Moreover, the suggested system supports both automated and manual calibration, rendering it versatile and adaptable across various applications.
This paper presents research results conducted at the Naval Air Warfare Center Weapons Division (NAWCWD) Radar Reflectivity Laboratory (RRL) to characterize RCS measurement quality of a compact range anechoic chamber using a small resonant sphere as a test probe measured over a 3.17-octave bandwidth, which covers the first half of the resonance region. Specifically, tests were performed on 1-inch and 12-inch diameter spheres over 2-18 GHz, which is a very prevalent test spectrum for RRL customers. The spheres were tested at the quiet zone center and the 1-inch was rotationally scanned over a 1- meter radial arc within the test zone. Spectral and spatial analysis was performed using techniques developed by Dr. Dean L. Mensa [1].
Unmanned aerial systems (UASs) enable the in situ diagnostic of antennas operated in outdoor environments. Additionally, their flexibility introduces the possibility of performing several diagnostic methods. In this overview work, the challenges of performing outdoor measurements with UASs are discussed and some of the possibilities they introduce are outlined. The main diagnostics tool when performing outdoor far-field measurements with UASs is the so-called raster scan. This is the two-dimensional scanning of a limited portion of the measurement sphere about the main lobe. From the information raster scans provide, it is possible to retrieve antenna parameters critical for the deployment of large antennas, such as the Side Lobe Level (SLL) in all directions, as well as the First Null Level. Additionally, assuming a fine scan, i.e., sufficient resolution, the interpolation of any 1D cut for diagnostics is possible. Once a problematic cut is interpolated and assessed, it can be measured using the UASs and increasing the measured angular range for further assessment. Assuming the measured large antennas are reflector antennas, the finding of a higher-than-expected SLL may point to a problem with the positioning of the feed. Measuring with UASs allows for an iterative measurement-and-adjustment process directly in situ, which guarantees that the antenna’s performance is within the boundaries required by either the application or regulations. Additionally, the flexibility of UASs provide further advantages, such as the assessment of the impact of environmental reflections in the radiation characteristics by flying along the radial component of the measurement sphere and assessing the measured ripple, using a method similar to the Voltage Standing Wave Ratio (VSWR) method used for the characterization of anechoic chambers. With this technique, the impact of the environment of candidates for an antenna deployment site can be assessed before the antennas are installed, thus supporting the choice process, and reducing the risk of malfunction. The discussion of the introduced techniques is supported by measurements, and future possibilities and advantages are studied.
The plane-polar approach for near-field antenna measurements has attracted a great deal of interest in the open literature during the past four decades [1, 2, 3, 4, 5, 6, 7]. The measurement system is formed from the intersection of a linear translation stage and a rotation stage with the combination of the axes enabling the scanning probe to trace out a radial vector in two-dimensions facilitating the acquisition of samples across the surface of a planar disk, typically being tabulated on a set of concentric rings. In its classical form, the probe moves in a fixed radial direction and the AUT rotates axially. However, with the ever more prevalent utilization of industrial multi-axis robots and uninhabited air vehicles (UAV), i.e. drones, being harnessed for the task of mechanical probe positioning, such systems offer the possibility of acquisitions being taken across non-planar surfaces. In this paper an accelerated, rigorous, near-field to far-field transform for data that was sampled using a polar acquisition scheme that is based on a Fourier-Bessel expansion [4] is developed and presented that can be employed in the above circumstances. This highly efficient, robust, transform enables near-field data acquired on planar, and non-planar, surfaces to be transformed to the far-field providing the acquisition surface is rotationally symmetric about some fixed point in the x,y-plane with z being purely a function of the radial displacement. The utility of the non-planar acquisition interval stemming from the ability to minimize truncation effects without needing to increase the measurement size. The transform efficiency stems from the utilization of the fast Fourier transform (FFT) algorithm with the rigor and robustness deriving from the avoidance of recourse to approximation, e.g. piecewise polynomial interpolation cf. [7]. Numerical results are presented and used to verify the accuracy and efficiency of the novel transformation, as well as to confirm convergence of the requisite Bessel series expansion and sampling theorem.
A new far-field valid angle equation for rectilinear planar near-field measurements is presented. The new valid angle equation was derived by viewing the planar near-field to far-field transformation process as generating a set of pseudo plane waves by a synthetic phased array and subjecting the antenna-under-test to the radiation from this synthetic array. The synthetic phased array does not physically exist; rather, the array is formed during the post-processing of the planar near-field measurements. As part of this discussion, we present results from a numerical model, illustrating the total electric field present in the test zone due to the finite extent of the synthetic phased array. The new far-field valid angle equation accounts for the diffraction effects of the finite-sized synthetic array, and uses the industry accepted test-zone magnitude ripple of +/- 0.5 dB to limit the valid far-field angle for a fixed scan plane size. The resultant valid far-field angle computed with the new equation is compared against previously established and popularly accepted valid angle equations, such as the equations previously presented by Yaghjian, Maisto, and Joy [1, 2, 3]. Brief discussionsare offered on the measurement of low directivity antennas with a planar near-field measurement system, and on amplitude tapering of the near-field measurements to improve the quality of the pseudo plane wave. REFERENCES: [1] A. D. Yaghjian, "Upper-bound errors in far-field antenna parameters determined from planar near-field measurements, part 1: analysis," National Bureau of Standards (NBS), Boulder, Colorado, USA, vol. Technical Note 667, no. October 1975. [2] M. Maisto, R. Solimene and R. Pierri, "Valid angle criterion and radiation pattern estimation via singular value decomposition for planar scanning," IET Microwaves, Antennas & Propagation, vol. 13, no. 13, pp. 2342-2348, 2019. [3] E. B. Joy, C. A. Rose, A. H. Tonning, and EE6254 Students, “Test-zone Field Quality in Planar Near-field Measurements,” in Proceedings of the 17th Annual Meeting and Symposium of the Antenna Measurement Techniques Association, Williamsburg, 1995.
For some time now the CTIA W-IoT OTA Reverberation Chamber Ad-Hoc Group has been looking for an OTA test artifact that exhibited repeatable narrow band cellular desensitization so that it could be used to perform round-robin testing between labs and investigate the impact of different test methodologies (e.g. reverberation chamber vs. spherical antenna pattern measurement in an anechoic chamber) on intermediate channel desense testing. Since device manufacturers aren't eager to provide devices with known problems, some alternative was required. Attempts were made to modify a device by removing shielding cans, only to completely degrade device performance across the entire spectrum. Using an external signal generator feeding a coupler at the DUT was also considered, but cable effects and equipment variations would have impacted repeatability in the variety of anticipated test environments. Creating a custom device with an embedded interference source met with cost and other practical limitations that stalled progress along that avenue. A member of the working group related anecdotal evidence suggesting that SD card communication within the phone was known to be a problematic noise source that could cause cellular desensitization. Initial investigation centered on the use of an off-the-shelf SD card testing app to try to generate uniform traffic, but none of the evaluated tools had an option for continuous testing. Focus then turned to developing a custom app for the purpose, but later changes to the Android operating system have deprecated the use of external SD cards, removing standardized support and making the development of a custom application impractical. Another possible solution for long duration interference would be to just play a video from the SD card, but the variability of a typical compressed MPEG video, both in content and compression level, would likely cause surges of data transfer with pauses in between. What was needed was a solution to cause continuous data transfer with a constant signature (i.e. sending the same data constantly) to ensure stability and repeatability. Thus investigation turned to creating a customized video file in an uncompressed format to address these limitations. This paper will show the results of this effort.
Our work proposes a novel method for obtaining full-sphere radiation patterns from truncated measurements. This is achieved by stitching partially overlapping truncated field patterns, which together cover the whole measurement sphere. Measuring an antenna in different orientations results in a misalignment between the measurements which is not perfectly known and needs to be accounted for in order to stitch the patterns together. Our method first makes use of an iterative procedure to compute spherical wave coefficients capable of accurately describing the truncated patterns. Recent investigation of properties of radiation patterns from iteratively obtained spherical wave coefficients under rotation and translation has shown that, while coordinate system manipulation introduces additional errors, these errors are contained predominantly in the region near the angle of truncation. They are thus negligible if a sufficient overlap between the truncated patterns exists. To align truncated patterns, a bounded minimization of the normalized mean squared error in the overlapping range between patterns is done, varying through a range of different translation and rotation vectors for one truncated pattern while keeping the other pattern fixed. Finally, the fixed and the optimally aligned patterns can be stitched together. The proposed method was validated on spherical wave coefficients (SWCs)-based models and EM simulation models for randomly chosen misalignment offsets. For the SWCs-based models, the normalized mean squared error (NMSE) after pattern stitching was found to be below -53 dB for all tested misalignment offsets. Similar results were observed in the case of EM simulation models as well, where the error was found to be below -52 dB for all tested misalignment offsets. In the final validation step, the method was tested on actual measurement results of two low-gain antennas. For each of the validation steps, potential sources of error are identified. The method demonstrates promising results in achieving full-sphere characterization of low-gain antennas in typical non-full-sphere measurement chambers.
As radiocommunications and internet-based services have become ubiquitous, customer expectations for infotainment capabilities and reliability in vehicles have largely increased. As such, the optimization of the distribution and orientation of antennas within the car is required to deliver the adequate connectivity performance. Yet, making direct measurements of electromagnetic field distributions radiated by structure-integrated radiofrequency transceivers is extremely tedious, if not practically and economically impossible. Recent papers introduced the approach of simulation-augmented measurements, appearing as a relevant solution to that problem. This method relies on a three-step approach: (i) measure the phasor electric field radiated by the standalone or part-integrated antenna module around the test sample; (ii) use an algorithm to calculate equivalent electric and magnetic currents over a surface closely encompassing the device under test (DUT); (iii) inject these currents as a Huygens source into a full-wave solver, where the complete scattering and absorbing environment is then taken into account. This paper presents the concrete application of this approach to the evaluation of the electric field inside a vehicle, based on separate measurements of WiFi and Bluetooth antennas. These measurements are performed using a spherical near-field system, with either the standalone antennas as DUT or the antennas embedded into the physical middle console of a car. The equivalent sources generated from experimental data are then imported into the virtual car model, and interior electromagnetic fields are computed using the Finite-Difference Time-Domain technique. The assessment is realized for various conditions without and with driver and passengers. The results are analyzed and limitations, as well as uncertainties of the technique are discussed.
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.
This paper introduces a single-cut near-field measurement technique using only-amplitude data. The technique is based on measuring the near-field amplitude of an antenna over a ring, i.e. phi=0 cut, and performing a far-field transformation to obtain the radiation pattern over the same ring. This avoids the need of a full near-field spherical measurement if one is interested in only a few cuts of the far-field pattern. The lack of phase information complicates the field transformation. A common approach to solve this issue is to perform two near-field scans a different antenna-probe distance. This has the drawback of doubling the measurement time with respect to a complex measurement and a translation stage is required, which may be infeasible in some antenna measurement facilities. The technique proposed on this paper can retrieve the phase without measuring the near-field in two rings. Instead, the field is measured in one ring using a specialized probe. This probe provides partial coherence information between measurement samples, which can be exploited in a non-convex minimization solver to retrieve the phase of the near field with high convergence guarantee. The specialized probe can be implemented by using two separate probes connected two a dual channel Software Defined Radio (SDR) unit, so that the relative phase between measurement samples is known. Theoretical background of the proposed technique will be disclosed on the paper, along with simulated and measured transformation examples to demonstrate the potential capabilities: -Very fast near-field measurements (only one ring is measured instead of the full sphere). -Only amplitude information is required (no need of maintain stable phase reference, suitable for OTA testing). -No double-scan is required to retrieve the phase (measurement time reduced by half, no need for translation stage). -High reliability: Partial coherence provides a significant amount of independent information to the phase retrieval algorithm.
Inverse Synthetic Aperture Radar (ISAR) image gating for RCS extraction using backprojection is compared with image gating using smoothed reweighted L1-optimization in this study. The RCS of an object is measured by placing the object placed on a turntable which is rotated in an angular range while sweeping the frequency in the desired frequency range. A common model with isotropic point scatterers fixed in the object coordinate system is used in the ISAR imaging process. This model is used to define a forward operator. The ISAR image can be formed by operating with the backpropagation operator (i.e. backprojection), the adjoint of the forward operator, on the measured RCS. This robust method to solve the inverse problem gives an image with a resolution limited by the frequency bandwidth and the angular range. The RCS for a scattering feature is commonly determined by using the forward operator on the point scatterers in the image that are determined to belong to the scattering feature in ISAR image gating. L1-optimization is a method that can be used to get images with higher resolution and hence better separation of the different scattering features than backprojection. L1-optimization is well suited for naturally sparse ISAR images. One method to mitigate that the scatterers are restricted to a fixed grid is to use smoothed reweighting [1]. L1-optimizations are performed consecutively in a few steps where a smoothed version of the previous solution is used to determine a weighting matrix for the next step. Smoothed reweighted L1-optimization gives images with better separation of the scattering features in the ISAR image. Simulated and measured RCS data are used to compare image gating using backprojection with gating using smoothed reweighted L1-optimization. The main conclusion of this study is that the RCS can be extracted for scattering features, not resolved in backprojection images, using the smoothed reweighted L1-optimization. [1] D. Pinchera and M. D. Migliore, “Accurate reconstruction of the radiation of sparse sources from a small set of near-field measurements by means of a smooth-weighted norm for cluster-sparsity problems,” Electronics, vol. 10, no. 22, p. 2854, 2021.
The RCS of a target can be estimated using electromagnetic modeling if accurate geometries and material descriptions are available. An exact numerical calculation often requires prohibitive processing times. Moreover, numerical predictions with approximate techniques are difficult as it is challenging to consider all the physical phenomena. Therefore, a suitable RCS measurement facility adapted to the target size and specifications is required to estimate the RCS of a given target and to validate the numerical predictions. In general, the measurement of RCS takes place in anechoic chambers that simulate free-space and far-field conditions and where the unwanted reflections (walls, target mount, objects in the range, and the target interactions) are reduced. This paper presents a broadband measurement and validation of the RCS of a metallic trihedral corner reflector of 30 cm sides when fully anechoic conditions are not available, and consequently, some undesirable echoes are present in the measurements. Firstly, the measurement facility calibration and the target calibration are outlined. A single target reference approach is performed using a sphere as a reference, and its scattering response is shortly described. Then, the measurement of the target is performed. After these steps, a processing procedure is applied to isolate the target response from the background and the close responses due to unwanted reflections. The post-processing technique and the acquisition system are presented and discussed. The measurements are performed at X band as a function of the viewing angle for vertical transmit and receive polarization. To validate the technique, the RCS of the trihedral corner reflector is numerically simulated using the Integral Solver (I-Solver) of CST, with a Gaussian excitation, for vertical transmit and receive polarization. Measurements are compared with results obtained from CST software and show a good agreement with the numerical simulations. This setup will be used for RCS measurement of different complex targets and compared with measurements from other facilities to analyze and evaluate the RCS measurement uncertainty.