Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The results of integrated antenna measurements are often severely distorted by reflections from the measurement environment. In order to feed passiveintegrated antennas wafer probes have to be used. Wafer probes are not only electrically large, but are also located in the immediate environment of the antenna undertest (AUT) and reflect part of the radiated signals. This causes significant distortions and erroneous results in radiation pattern, directivity, and gain measurements.Custom wafer probes have been used to reduce reflections for meaningful measurement results, but these special probes are difficult to fabricate and expensive. If the antenna is measured within an active system that generates the transmit signal, wafer probes are not required to feed the AUT, but bond wires, circuit elementsclose to the antenna, and parasitic radiation of surface waves also add distortions, which still limit the achievable accuracy of the measurements. In this paper modal filtering is used to mitigate the influence of these unwanted distortions in post-processing for both standard wafer probe and active antennameasurements. In the first part of the paper the performance of the post-processing technique is assessed for standard probe measurements at 160 GHz by comparing the post-processed results to a measurement of the same antenna using a custom made wafer probe that was designed for minimum reflections. In the second part modal filtering is used to reduce unwanted reflections for an active antenna measurement at 160 GHz. When the active circuitry that generates thetransmit frequency is integrated on the same chip as the AUT, the phase of the transmit signal is unknown. As the phase information is required for the post processing,a static external probe antenna is used as a reference to eliminate the phase drift of the measured signal. It is shown that modal filtering can be applied to integrated antenna measurements above 100 GHz and that reflections from wafer probes, bond wires, and the PCB canbe reduced significantly for passive and active antenna measurements, respectively.
A technique of visualizing a surface current or a near-field equivalent source distribution is required for an antenna evaluation and its failure diagnostic. An inverse problem reconstructs the equivalent wave source distribution inside the measurement region by solving the propagation coefficient of the electromagnetic wave inversely from the near-field measured electromagnetic field the antenna under test (AUT) Since this method can set an arbitrary shape surface enclosing the AUT as a estimation surface, it is effective to visualize the internal equivalent source distribution. However under interference wave environments, the inverse source technique reconstructs the equivalent currents, including interference wave component as a part of internal source distribution. The estimation accuracy is particularly deteriorated under interference wave conditions. We propose a method of 2-step source reconstruction on arbitrary 3-D surface using dual measured electromagnetic surface, for reconstruct internal equivalent source accurately. First step, we set a shape of estimation surface similar to the shapes of measurement surface, to estimate the internal distribution accurately under well-posed conditions. Not only the shape of the estimation surface but also the sampling density is made same to the density of the measurement surfaces. In second step, to reconstruct an arbitrary shape surface from reconstructed internal field distribution in previous step. Since the inverse problem in this step is generally under ill-posed conditions, the regularization is applied to improve the accuracy of the solution. By this 2-step reconstruction, the interference wave is eliminated and the internal equivalent source on arbitrary surface is reconstructed. For example, we apply the proposed method to a 4-element linear patch array antenna, and the effectiveness of the proposed method is clarified. We also showed that the internal source distribution accurately reconstructed on an arbitrary surface even in the interference wave environments and identify the defective operation part of the AUT.
The measured source or Huygens box antenna representation has become an increasing popular solution to create accurate computational models of measured source antennas for the numerical analysis of antenna placement on complex platforms such as satellites. The equivalent representation of the measured antenna is obtained through the equivalent current (EQC) or inverse source technique, which is a measurement post-processing method that represents the measured antenna in equivalent electric and magnetic currents on a surface conformal to the antenna. The highly accurate representation of the measured antenna can be used for both suspended and flush mounted antenna and the format is compatible with most commonly used commercial CEM solvers. This technique enables computation of complex antenna scenarios in which the source antenna is physically available but the computational details are unknown. This is often the case for space antenna testing in which antennas from different suppliers are integrated on a platform representing the complex scenario. In this paper, the validation of this technique in space antenna testing application is presented. The test object is a GNSS antenna mounted on a Sentinel satellite mock-up working at 1227 and 1575 GHz. The GNSS antenna and Sentinel satellite structure have been designed, manufactured and measured by RUAG SPACE. Simulations of the sentinel satellite using the measured source technique are compared to measurement of the satellite mock-up model at the working frequencies of 1227 MHz and 1575 MHz. Preliminary results of this validation activity have been previously presented. This paper reports on the full validation activity including the possibility to use different CEM solvers. The activity has been partly supported by ESA ESTEC contract 4000116755 “Time Efficient satellite antenna testing technique based on NF measurement and simulation with controlled accuracy”.
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.
The accurate alignment of antennas and field probes is a critical aspect of modern antenna metrology systems, particularly in the millimeter-wave region of the spectrum.Commercial off-the-shelf robotic arms provide a sufficient level of positional accuracy for many industrial applications.The Antenna Metrology Project in the Communications Technology Laboratory at the National Institute of Standards and Technology has shown that path-corrected commercial robotic arms, both in hardware and software analysis, can be used to achieve sufficient positioning and alignment accuracies (positioning error ~ /50) for antenna characterization measurements such as gain extrapolation and near-field pattern out to 183 GHz [1]. Position correction is achieved using a laser tracker with a 6 degree of freedom sensor attached to the robot end effector.The end effector’s actual position, measured using the laser tracker, is compared to its commanded position and a path correction is iteratively applied to the robot until the desired level of accuracy is achieved in the frequency range of interest.At lower frequency ranges (< 40 GHz), sufficient positional accuracy can be achieved, without path correction, using a using a calibrated kinematic model of the robot alone [2].This kinematic model is based on knowledge of the link frame transformations between adjacent links and captures deviations due to gravitational loading on the joints and small mechanical offsets between the joints.Additionally, the calibration procedure locates the robot’s base frame in the coordinate system of the robot’s end effector.Each link frame is described by four physical quantities, known as Denavit-Hartenberg (DH) parameters [3]. We performed calibration measurements of our CROMMA system’s DH parameters over a working volume of ~1 m3.We then use the laser tracker to compare the robot’s positional accuracy over this working volume with and without the calibrated kinematic model applied.The path errors for the calibrated case set an upper frequency limit for uncorrected antenna characterization measurements. [1]D. R. Novotny, J.A. Gordon, J.R. Guerrieri, “Antenna Alignment and Positional Validation of a mm Wave Antenna System Using 6D Coordinate Metrology, ” Proceedings of the Antenna Measurements Techniques Association, pp 247-252, 2014 [2]R.Swanson, G. Balandran, S. Sandwith, “50-micron Hole Position Drilling Using Laser Tracker Controlled Robots, ” Journal of the CMSC, Vol 9, No 1, Spring 2014 [3].J.J. Craig, “Introduction to Robotics: Mechanics and Control, 3rd ed.,” New Jersey, Prentice Hall, 2004, pp. 62-69
Nowadays many antennas are planar and processed on single or multilayer PCBs. These antennas are often simulated before with a full-wave solver like CST Microwave Studio. In these simulations the reflection coefficient on the feeding transmission line is determined (in parallel to the farfield characteristics but which is of no interest in this paper). The antenna is optimized for minimal reflection. In the simulation the parameters of the antenna feeding line is chosen in the same manner than in the practical design where the antenna is directly connected to the output of an active circuitry via a planar transmission line (e.g. microstrip line, coplanar line, etc.). It is recommended to measure the input reflection coefficient of the antenna with a network analyzer before using it in the final design. For this reason the antenna is processed as prototype on a PCB and linked to the measurement equipment via an RF connector, eg. a Rosenberger 32K243 connector. It is common knowledge that every connector has an influence on signal transmission and provokes reflections. Thus the combination of antenna with connector can cause non-negligible reflections although the sole antenna has not. In order to determine the correct reflection coefficient of the antenna the transfer function of the connector must be known. In two former papers the authors presented methods to determine the scattering parameters of connectors by narrowband and broadband measurement of symmetrical structures. By knowing the scattering parameters the antenne input reflection coefficient can be determined by deembedding. Nevertheless the antenna is optimized for the unembedded design and not for the embedded version with connector. If the connector has non-neligible reflections in the high frequency domain the performance of the complete antenna prototype is bad and could not be used for other purposes (e.g. modular electronic setup where different components are linked together via cables). Furthermore if less power is transmitted to the PCB the calculation accuracy for the deembedding is reduced. A matching network can help to reduce the input reflection loss towards the antenna prototyp which is in general inserted in front of a device. A ”self-made” passive matching network between connector and coaxial cable at 24 GHz again requires a structure on a PCB which again introduces two cable to PCB transitions and additional losses. Thus the idea is to develop a matching on the PCB-side ”behind” the connector which reduces the input reflection of the connector independently of the termination structure on the PCB.
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.
A complete characterization of the radiation and scattering phenomena is essential to the ray tracing simulators. In the ray tracing modeling, the electromagnetic field quantities are traced along the ray paths and determined by the antenna radiation pattern and the scattering patterns of the obstacles. The polarimetric patterns may be prepared in advance from the measurements or numerical simulations, and reused by the ray tracing simulators for various situations. However, the prefabricated pattern data set usually contain only a limited quantity of samples at discrete angular directions and frequencies. The lack of full representations of the desired patterns hinders the accurate calculation of ray field quantities. Although interpolation can be done using multidimensional splines or polynomials, the accuracy is not assured by the problems’ physics. Especially, it is difficult to tackle the phase wrapping problem in the multidimensional case, which might lead to wrong phase interpolation. In this paper, model based parameter estimation (MBPE) is used to circumvent the requirement of obtaining all samples of the desired radiation and scattering patterns in both spatial (angular) and frequency domains. Since any function defined on the surface of a sphere can be represented by a sum of spherical harmonic functions, we utilize the spherical harmonic expansion in the spatial domain firstly. Specifically, in order to avoid the singularities of conventional vector spherical harmonics at the north and south poles, scalar spherical harmonics is used instead. The expansion coefficients are vector-valued frequency domain responses, independent to the angular variables. Then each coefficient is expanded by using the singularity expansion method, which leads to a rational function characterized by its poles and residues in the frequency domain. Since the poles are the characteristic of the considered object (antenna or scatter), it is reasonable to assume that all spherical harmonic components have the same poles. Therefore, the parameters to be estimated are the frequency domain poles and the corresponding residues for each spherical harmonic component. By following this method, a physically-based, closed-form, reduced-order parametric model can be established from the sampled pattern data. The proposed method will be validated by simulations and measurements.
The maintenance of aircraft radomes is of particular importance for the commercial aviation industry due to the necessity to ensure the correct functioning of the radar antenna, housed within such protective enclosures. Given that the radar component provides weather assessment, as well as guidance and navigation functions (turbulence avoidance, efficiency of route planning in case of storms, etc.), it is imperative that every repaired radome be tested with accuracy and reliability to ensure that the enclosed weather radar continues to operate in accordance with the after-repair test requirements of the RTCA/DO-213. Recently, this quality standard was updated and published under the name RTCA/DO-213A, establishing more stringent measurement requirements and incorporating the possibility of measuring radomes using Near-Field systems. Consequently, a compliant multi-probe Near- Field system concept – AeroLab – has been specifically designed to measure commercial aircraft nose-radomes, in order to meet the new standard requirements. AeroLab performs Near-Field measurements. Near-Field to Far-Field transformations are then applied to the results. Such a Near-Field system allows the test range to be more compact than traditional Far-field test ranges, and thus be independent from the updated Far-Field distance which has progressed from D²/2l to 2D²/l in the new standard RTCA/DO-213A. AeroLab enables the evaluation of the transmission efficiency and beamwidth. It also allows for accurate evaluations of the side-lobe levels by providing improved visualization of principal cut views selected from 3D patterns. Moreover, depending upon the weather radar system inside the radome under test, 2 distinct scan sequences must now be taken into account: “elevation over azimuth” and “azimuth over elevation”. AeroLab emulates both of these motion sequences through a monolithic gimbal. Furthermore, thanks to its multi-probe array, such measurements are performed in a fraction of the time spent in current mono-probe test facilities (less than 4 hours, i.e. 1/3 less time than single probe scanners). Keywords: RTCA/DO-213A, radome measurement system, after-repair tests, multi-probe measurement system, Near-Field system.
Comparison activities in which a number of measurement facilities compare their measurements of the same antenna in a standard configuration have become important for documentation and validation of laboratory expertise and competence. It is also mandatory to have regular participation in such activities to obtain and maintain accreditations like ISO 17025. The main goal of the facility comparison activities is to provide a formal opportunity for the participants to validate and document their achieved measurement accuracy and procedures by comparison with other facilities. Since 2004, comparison campaigns with different scopes have been conducted on antenna measurements within various European activities: EurAAP (European Association on Antennas and Propagation) supported by the European Cooperation in Science and Technology (COST) in the programs ASSIST IC0603 and VISTA IC1102 and the 6th EU framework network “Antenna Centre of Excellence” (ACE). Results of these activities have led to improvement in antenna measurement procedures and protocols in facilities and contributions to standards. Due to the direct benefits to the participants, the activities have been very successful and partial results have been published in IEEE referenced papers during the years. The large amount of measured data available have fostered fruitful discussion and research on the improvement of standard procedures, protocols and tools for performance verification like the facility comparison campaigns. As a further benefit, the campaigns have initiated a dialogue among different laboratories throughout Europe and USA and is spreading into Asia. In this paper we report on a recent EurAAP facility comparison campaign involving a medium gain ridge horn, MVI-SH800. The campaign covers measurement in the L and C band frequencies in different facilities in Europe and USA. The horn is equipped with an absorber plate to enhance the correlation in different facilities by reducing the sensibility to the measurement set-up. The results of 8 facilities will be shown in terms of gain and directivity patterns, equivalent noise level and the declared uncertainty will be checked against the whole set of measurements.
When antenna near-field (NF) measurement within small electrical distance is needed, such as miniaturization of the measurement device or measurement of a low-frequency DUT, the coaxial cables connected to the probes will significantly but inevitably disturb the fields. The measurement accuracy is therefore compromised. In this paper, a novel probe design is proposed by replacing coaxial cable with optical fiber to minimize the disturbance. In this design, the RF-over-Fiber (RoF) technology is applied in signal transmission with Vertical-Cavity Surface-Emitting Laser (VCSEL) and photodiode (PD) as the transmitter and receiver respectively. The VCSEL is powered via optical fiber with Power-over-Fiber (PoF) technology. A power laser emits optical power which is guided by optical fiber to illuminate a miniaturized photovoltaic (PV) element. The PV element serves as a voltage source for the VCSEL. A spherical, multi-probe, NF measurement design with 60cm-diameter is built for portable DUT operated between 0.6 to 2.6GHz. There are 64 probes installed along the two arches for both theta and phi polarizations, so mechanical rotation is needed only on phi axis. Thanks to the high RF transparency of the probes, there is no need to wrap absorbers around the probes to shield the cables. Another spherical NF measurement prototype is also under development. It is half-spherical (10m-diameter) for large DUT, such as vehicles, with low frequency antenna, namely, 70MHz to 600MHz. At this frequency range, to the best of our knowledge, there is no effective and accurate way to measure the radiation performance because the disturbance on the EM fields by the coaxial cables is obviously not negligible.
Low-profile vertically polarized end-fire radiating antennas with wide bandwidth have been used in the wireless systems of a variety of vehicles and aircraft. Many antennas conforming to these specifications have been designed in recent years for operation in microwave frequencies; however, there is interest in the ability to scale these designs to support millimeter wave applications. This work will utilize modeling and simulation, fabrication, and measurements to characterize, scale, and optimize a coupled microstrip resonator antenna. All modeling and simulation will be performed using CST Microwave Studio. The antenna will consist of a trapezoidal launcher along with several rows of coupled microstrip resonators that will be milled from Rogers RT/Duroid 5880 printed circuit board. The coaxial fed trapezoidal launcher will be utilized as a reflector and will allow realization of quasi-plane waves at the interface between the launcher and the microstrip resonators. The rows of coupled microstrip resonators decrease in the length along the endfire direction to provide increased endfire directivity. The resonant frequencies of the antenna will be determined mainly by the dimensions of the trapezoidal launcher, microstrip resonators, and the gaps between them. The design for this antenna is based off the antenna documented in Compact Wideband and Low-Profile Antenna Mountable on Large Metallic Surfaces (Zhang, Pedersen,IEEE TAP, VOL. 65, NO. 1, JANUARY 2017). Initial modeling and simulation of the documented antenna have shown good agreement with the authors findings. Simulating a reduction to the antenna dimensions has shown promising performance into millimeter wave frequencies including a 73.7% fractional bandwidth (36-78GHz) for VSWR < 2. Since these reduced dimensions can’t be easily fabricated a slightly modified version of the antenna will be modeled and simulated as described above. The antenna will then be fabricated and measured to validate the accuracy of the model. Once validated, the antenna model will be scaled up to millimeter wave frequencies and simulated to predict performance.
In the last years different advances in Near-Field (NF) measurements have been proposed. Among the others, the ones of interest here are: the determination of the number and spatial distribution of sampling points, the introduction of scanning strategies aimed to reduce the measurement time, the adoption of a proper representation, for the unknowns of interest, able to improve the reliability of the characterization [1]. In particular, the use of Prolate Spheroidal Wave Functions (PSWFs) for the expansion of the aperture field has proven effective to take into account for the quasi-band-limitedness of both the aperture field and the Plane Wave Spectrum. Furthermore, using a proper expansion is an important step of the Singular Value Optimization (SVO) approach, wherein the number of the spatial distribution of the NF samples are determined as the ones reducing the ill-conditioning of the problem [1]. Up to now, rectangular PSWFs has been successfully exploited to perform optimized NF characterizations of rectangular aperture antennas. Recently, we tackled the extension to the case of circular apertures. The difficulties related to the stability and accuracy of the numerical evaluation of the Circular PSWFs have been assessed in [2], showing the benefits due to the use of a proper expansion, with respect to standard backpropagation. Furthermore, the circular PSWFs expansion correctly takes into account for the spectral radiating support, with respect suboptimal representation of the rectangular case. The aim of the paper is to show how the circular PSWFs expansion can be fruitfully exploited in the NF characterization of circular aperture antennas. Experimental results will be presented to support the performance of the method. [1] A. Capozzoli, C. Curcio, G. D’Elia, A. Liseno, “Singular value optimization in plane-polar near-field antenna characterization”, IEEE Antennas Prop. Mag., vol. 52, n. 2, 103-112, Apr. 2010. [2] A. Capozzoli, C. Curcio, G. D’Elia, A. Liseno, “Prolate Function Expansion of Circularly Supported Aperture Fields in Near-Field Antenna Characterization”, European Conference on Antennas and Propagation 2017, Paris 19-24 March 2017.
Gain is a principal property of antennas; it is essential in establishing the link budget for communication and sensing systems through its presence in Friis’ transmission formula and the radar range equation. The experimental determination of antenna gain is most often based on a gain-transfer technique involving a reference antenna for which the gain has been calibrated to high accuracy; this is typically a pyramidal horn antenna [1]. The required accuracy of antenna gain obviously depend on the application; in some cases it can very high, ±0.1 dB or less, and this implies an even higher accuracy, of the order of ±0.01dB, for the gain reference antenna. This work investigates the accuracy to which a gain reference antenna can be calibrated; the investigation is based on experimental spherical near-field antenna measurements [2] and computational integral equation / method of moments simulations [3]. While calibration of gain reference antennas has been studied in many previous works, even works from early 1950s [4]-[6], this work is novel in systematically supporting measurements with full-wave simulations. Such simulations facilitate the study of e.g. the effect of multiple reflections between antennas at short distances. We study two absolute calibration techniques for the gain of pyramidal horn antennas. The first technique determines gain as the product of directivity and radiation efficiency; this technique has been referred to as the pattern integration technique [7] (which is not an entirely adequate designation since gain cannot be determined from the radiation pattern). The second technique determines the gain from Friis’ transmission formula [8] for two identical antennas; this technique is generally referred to as the two-antenna technique [1]. These two calibration techniques involve very different steps and contain very different sources of error; for both techniques our investigation involves measurements as well as simulations. For the pattern integration technique we compare experimental and computational results for the directivity and demonstrate agreement within one-hundredth of a dB. The radiation efficiency is calculated by different techniques based on the surface impedance boundary condition for the metallic walls of the pyramidal horn. This technique is not influenced by proximity effects or by impedance mismatch between the measurement system and the gain reference antenna. For the two-antenna techniques we compare experimental and computational results for the gain and we compare the calculated distance-dependence with that of the extrapolation technique [9]. It is demonstrated how the use of the phase center distance in Friis’ transmission formula notably decreases the necessary separation between the antennas for a required accuracy, but that multiple reflections may then become a limiting factor. This technique is highly influenced by the impedance mismatch that must be accurately accounted for. We compare the gain values resulting from the pattern integration technique and the two-antenna technique, including their very different uncertainty estimates, for a C-band standard gain horn. The work is related to an on-going ESA project at the DTU-ESA Spherical Near-Field Antenna Test Facility for the on-ground calibration of the scatterometer antennas of the EUMETSAT MetOp Second Generation B-series satellites. IEEE Standard – Test Procedures for Antennas, Std 149-1979, IEEE & John Wiley & Sons, 1979. J.E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Perigrinus Ltd., London 1987. www.wipl-d.com W.C. Jakes, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 160-162, February 1951. E.H. Braun, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 109-115, January 1953. W.T. Slayton, “Design and Calibration of Microwave Antenna Gain Standards”, Naval Research Laboratory, Washington D.C., November 1954. A. Ludwig, J. Hardy, and R. Norman, “Gain Calibration of a Horn Antenna Using Pattern Integration”, Technical Report 32-1572, Jet Propulsion Laboratory, California Institute of Technology, October 1972. H.T. Friis, “A Note on a Simple Transmission Formula”, Proceedings of the I.R.E. and Waves and Electrons, pp. 254-256, May 1946. A.C. Newell, R.C. Baird, P.F. Wacker, “Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique”, IEEE Transactions on Antenna and Propagation, vol. 21, no. 4, pp. 418-431, July 1973.
The expanding market for millimeter wave antennas is drivinga need for high performance near-field antenna measurement systems at these frequencies. Traditionally at millimeter waves, acquisition of two orthogonal polarizations have been achieved through mechanical rotation of a single polarized probe and an associated frequency conversion module. This generally results in the collection of two complete spherical data sets, one for each polarization,with both acquisitions significantly separated in time. To enable improvements in both measurement speed and accuracy, MVG have developed a new high performance dual polarized feed in V-band (50GHz-75GHz). This probe has been integrated in a millimeter wave Spherical Near-Field (SNF) system via two parallel receiver channels that are simultaneously sampled. This architecture more than doubles the acquisition speed and additionally ensures that the two polarization components are sampled at precisely the same point in space and time. This is particularly important when performing accurate polarization analysis (e.g. conversion of dual linear polarization to spherical/elliptical polarizations). The two measurement channels are calibrated via radiated boresight measurements over a range of polarization angles, generating a four term “ortho-mode” correction matrix vs. frequency. The SNF probe is based on an axially corrugated aperture providing a medium gain pattern (14dBi). The probe provides symmetric cuts and low cross-polarization levels in the diagonal planes. The directivity/beam-width of the aperture has been tailored to the measurement system, ensuring proper AUT illumination and sufficient gain to compensate for free space path loss. Dual polarization capability is achieved with an integrated turnstile OMT feeding directly into the probe circular waveguide and a conical matching stub at the bottom. Thanks to the balanced feed used for each polarization, the port-to-port coupling is sufficiently low to allow for simultaneous acquisition of the two linear field components. Input ports are based on standard WR-15 waveguide to simplify the integration with the front-end (dual channel receiver). The paper will present the detailed description and measured performances of the new dual polarized SNF probe. Additionally, measurement time and achieved accuracy will be compared between the single polarization probe architecture and the dual polarized probe installed in the same spherical near-field antenna measurement system.
This work introduces a new technique in electromagnetic antenna near-field to far-field transformation (NF/FF). The NF/FF transformation is based on the solution of an inverse problem in which the measured NF and predicted FF values are attributed to a set of equivalent electric and magnetic surface currents which lie on a convex arbitrary surface that is conformal to the antenna under test (AUT). The NF points are conformal to the AUT, reducing the number of samples and relaxing positioning requirements used in conventional spherical, cylindrical and planar NF/FF geometries. A pseudo inversion of the matrix representing the mapping of the equivalent sources into the near-field samples is obtained by using the singular value decomposition (SVD). The SVD is used to form an approximation of the inverse of the matrix. This inverse, when multiplied by the NF measurement vector, solves for the efficiently radiating components of the current, and not the essentially non-radiating components of current which are not visible in the measurements. The inversion technique used is robust in the presence of measurement noise and provides a stable solution for the unknown currents. The FF is computed from the currents in a straightforward manner. The work develops the theoretical foundation for the approach and investigates the FF reconstruction accuracy of the technique for a test case. Approved for Public Release; Distribution Unlimited. Case Number 16-0884 The author's affiliation with The MITRE Corporation is provided for identification purposes only, and is not intended to convey or imply MITRE's concurrence with, or support for, the positions, opinions or viewpoints expressed by the author.
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 [1]. 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.
The uncertainty of the far field, obtained from antenna planar near field measurements, against the dynamic range is investigated by means of statistical analysis. The dynamic range is usually limited by the noise floor for low level signals and by the receiver saturation for high level signals. The noise level could be important for high measurement rate, which requires the usage of a high signal level to ensure a sufficient signal to noise ratio. As a result the nonlinearities are increasing, thus a compromise must be accomplished. To evaluate the effects of the limited near field dynamic range on the far field, numerical simulations are performed for dipoles array. Initially, the synthetic near field data corresponding to a given antenna under test were generated and directly processed to yield the corresponding far field patterns. Many far field parameters such as gain, beam width, maximum sidelobe level, etc. are determined and recorded as the error-free values of these parameters. Afterwards, the synthetic near field data are deliberately corrupted by noise and receiver nonlinearities while varying the amplitude through small, medium and large values. The error-corrupted near field data are processed to yield the far field patterns, and the error-corrupted values of the far field parameters are calculated. Finally, a statistical analysis was conducted by means of comparison between the error-corrupted parameters and the error-free parameters to provide a quantitative evaluation of the effects of near field errors on the different far field parameters.
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.
Finding the monostatic radar cross section (RCS) of a structure using computational electromagnetics (CEM) is a challenging task, particularly when the structure is large in terms of wavelengths. Such structures are challenging due to the large computational requirements, often combined with high accuracy demands and/or complicated geometry. Previously, these challenges have resulted in algorithms that either relax the accuracy requirements by using asymptotic methods or, if full-wave methods are used, require extreme runtimes even on very large computing clusters. For full-wave methods based on an integral equation formulation, such as Method of Moments (MoM), the reason for the large computational requirements can be found in the O(f^6) computational time scaling of monostatic RCS, where f is the frequency. Acceleration algorithms such as the Multi-Level Fast Multipole Method (MLFMM) reduce this to O(C(f,v) f^2 log f), where C(f,v) is the number of iterations required for convergence of an iterative solver, and v is the number of incident angles. Unfortunately, in most state-of-the-art implementations of monostatic RCS, C(f,v) is very large, meaning that in practice MoM is preferred to avoid an iterative solver. In this paper, we describe a range of efforts towards developing an efficient algorithm for large-scale monostatic RCS, in particular for structures that are too large to handle for MoM. These efforts include an efficient discretization based on higher-order basis functions and quadrilateral meshing of the structure, an MLFMM implementation focused on keeping memory requirements low, and a highly efficient block Krylov solver. The efficient higher-order discretization has already proven its worth for scattering problems, and the paper will demonstrate how its advantages over traditional RWG discretizations make it perfectly suited for RCS computation. In particular, combining the low amount of unknowns with a strong preconditioner allows rapid convergence of the iterative solver. The use of a low-memory MLFMM implementation, tailored for higher-order basis functions, means that problems of unprecedented size can be handled even on ordinary workstations, i.e., without resorting to expensive computing clusters. Finally, recent work on block Krylov solvers, along with interpolation algorithms for linear systems with a large amount of right-hand sides and efficient stopping criteria, allows a short computing time by significantly reducing the number of iterations.