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Reconfigurable antennas provide the ability to electronically change the antenna’s performance, which allows the antenna’s band of operation and gain pattern to be rapidly adapted to meet system requirements. A cylindrical, conformal reconfigurable antenna is presented which tunes over a wide band and provides full 360° azimuth coverage. The antenna maintains a realized gain (with mismatch and loss) better than a dipole from 800 MHz to 3 GHz, using the antenna’s gain to compensate for losses in the antenna. The antenna is designed and characterized with the cylinder’s bottom over a finite ground plane (no other antenna ground planes are used). The antenna is constructed using a modular approach out of a series of identical boards which act as antenna pixels. Each pixel contain four RF switches (one for each side of the board) along with contacts for control and ground wires. By fragmenting the reconfigurable antenna into individual pixel boards, one can construct elements of arbitrary size and shape with the primary physical constraint being how densely the electronics can be fabricated. By providing flexibility to scale in size, the antenna implementation can be optimized for more gain or for a smaller footprint. Two scaled versions of the same architecture have been constructed out of the same pixels to demonstrate the flexibility of the approach. In this paper we present data demonstrating more than 2 dBi gain from 1.2 GHz to 2.5 GHz band with beamwidths as narrow as 60°. Beam patterns are presented for GSM-900, GMS-1900, and WiFi frequencies. Finally, we will show the antenna element’s ability to maintain gain in a specific direction while forming a null over a series of offset angles.
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.
A rotated uniaxial anisotropic parallel plate Green’s function is developed to support non-destructive material characterization techniques. Uniaxial media possess equivalent transverse tensor element and a unique longitudinal element aligned along the optic axis. Previous non-destructive characterization techniques using rectangular waveguide- flange type probes demonstrate excellent transverse permittivity measurements due to the strong transverse electric field from the rectangular waveguide, while the longitudinal permittivity result are generally poor. Poor longitudinal permittivity results are due to the weak interrogating longitudinal electric field. Accommodating the strong rectangular waveguide transverse field component, measurement of unique transverse permittivity components is accomplished by aligning the rectangular waveguide-flange probe to each transverse axis. This affords the opportunity to measure unique transverse component media. A rotated uniaxial material possess unique transverse elements and a longitudinal element which is equivalent to one of the transverse components. A non-destructive measurement system measuring rotated uniaxial media requires a rotated uniaxial Green’s Function to support a Magnetic Field Integral Equation (MFIE) and subsequent moment method forward model. Electric and magnetic field components in a rotated uniaxial material interact with transverse parallel plate boundary conditions differently than uniaxial media due to the unique transverse permittivity and/or permeability components. In this paper, a field-based approach is used to derive both the principle and scattered solutions to the rotated uniaxial anisotropic parallel plate Green’s function. Field structure inside a uniformly filled rotated uniaxial parallel plate region is explored using various magnetic currents. A companion paper: “Rotated Uniaxial Anisotropic Material Characterization – Experiment” then employs the rotated uniaxial anisotropic Green’s function to evaluate and demonstrate measurement performance.
Non-destructive evaluation of anisotropic media requires sufficient measurement diversity to characterize a material. Measurement diversity may involve using unique field applicators, orienting the sample into unique positions, or using multiple sample thicknesses to support characterization. Measurement diversity challenges are increased if the sample has a fixed thickness and is backed by a metal substrate. A Single Port Waveguide Probe (SPWP) non-destructive material characterization technique is proposed to accommodate measuring a metal backed, known thickness, rotated uniaxial anisotropic material. A rotated uniaxial sample possesses unique transverse constitutive components and a longitudinal constitutive component which is the same as one of the transverse values. The SPWP consist of a rectangular waveguide aperture cut in the center of a square flange. The flange is place upon a metal-backed material surface, which forms a parallel plate region. Two orthogonal transverse plane measurements aligned with the sample’s transverse constitutive parameter components offers measurement diversity. The rectangular waveguide aperture fields are measured as calibrated S-parameter reflection coefficients using a Vector Network Analyzer. An assumed TE10 mode-only moment method forward model is presented utilizing a rotated uniaxial anisotropic parallel plate Green’s function developed in a companion paper entitled “Rotated Uniaxial Anisotropic Material Characterization – Theory”. The Green’s function is employed in a Magnetic Field Integral Equation (MFIE) and Love’s Equivalence principle relates the waveguide aperture fields to an equivalent current in the parallel plate region. Constitutive parameter results are generated by minimizing error between the forward model and the experimental S-parameter reflection coefficients, using a least squares minimization algorithm. In this paper, measurements and simulations of rotated uniaxial samples are made assessing SPWP measurement performance using a fundamental mode moment method technique.
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.
Among the near-field – far-field (NF-FF) transformation techniques, the one employing the bi-polar scanning is particularly interesting, since it retains all the advantages of that using the plane-polar one, while requiring a mechanically simple, compact, and cheaper measurement facility [1]. In fact, in this scan, the antenna under test (AUT) rotates axially, while the probe is mounted at the end of an arm that rotates around an axis parallel to the AUT one. An effective probe voltage representation on the scanning plane requiring a minimum number of bi-polar NF data has been developed in [2], by properly exploiting the nonredundant sampling representations of electromagnetic (EM) fields [3] and considering the AUT as enclosed in an oblate ellipsoid. A 2-D optimal sampling interpolation (OSI) formula is then employed to efficiently recover the NF data required by the traditional plane-rectangular NF-FF transformation [4] from the acquired nonredundant bi-polar samples. It is so possible to considerably reduce the number of the needed NF data and corresponding measurement time with respect to the previous approach [1], which did not exploit the nonredundant sampling representations. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. Therefore, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF-FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy [5] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in [5] to the uniform samples retrieval in the case of cylindrical and spherical surfaces. The latter, based on the singular value decomposition (SVD) method, does not exhibit this constraint and has been applied to the nonredundant bi-polar [6] scanning technique based on the oblate ellipsoidal modeling. However, it can be conveniently used only when the uniform samples recovery can be split in two independent one-dimensional problems. The goal of this work is not only to provide the experimental validation of the SVD based technique [6], but also to develop the approach using the iterative technique and experimentally assess its effectiveness. [1] L.I. Williams, Y. Rahmat-Samii, R.G. Yaccarino, “The bi-polar planar near-field measurement technique, Part I: implementation and measurement comparisons,” IEEE Trans. Antennas Prop., vol. 42, pp. 184-195, Feb. 1994. [2] F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF-FF transformation with bi-polar scanning,” Microw. Optic. Technol. Lett., vol. 36, pp. 32-36, 2003. [3] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop., vol. 46, pp. 351-359, March 1998. [4] E.B. Joy, W.M. Leach, Jr., G.P. Rodrigue, D.T. Paris, “Application of probe-compensated near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-26, pp. 379-389, May 1978. [5] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop., vol. 141, pp. 77-84, April 1994. [6]F. Ferrara, C. Gennarelli, M. Iacone, G. Riccio, C. Savarese, “NF–FF transformation with bi-polar scanning from nonuniformly spaced data,” Appl. Comp. Electr. Soc. Jour., vol. 20, pp. 35-42, March 2005.
Millimeter wave vehicular radar operating in the 77 GHz band for automatic emergency breaking (AEB) applications in detecting vehicles, pedestrians, and bicyclists, test data has shown that the radar cross section (RCS) of a target decreases significantly with distance at short range distances typically measured by automotive radar systems, where the reliable detection is most critical. Some attribute this reduction to a reducing illumination spot size from the antenna beam pattern. Another theory points to the spherical phase front due to measurement in the Fresnel region of the target, when the distance for the far-field zone is not met. The illumination of the target depends on the antenna patterns of the radar, whereas the Fresnel region effects depend on the target geometry and size. Due to fluctuations in measured data for RCS as a function of range in the near-field, upper and lower bounds for the target RCS versus range have been determined empirically as a method for describing the expected RCS of target. So far, the range-dependent RCS bounds used in AEB test protocols have been determined empirically. The study discussed in this paper aims to study the underlying physics that produces range-dependent RCS in near field and provide analytical model of such behavior. The resultant analytical model can then be used to objectively determine the RCS upper and lower bounds according to the radar system parameters such as antenna patterns and height. A comparison of the analytically predicted model and empirical near-field RCS as a function of range data will be presented for pedestrian, bicyclist, and vehicle targets.
Among the near-field - far-field (NF-FF) transformations, that with spherical scan [1] is the most appealing due to its feature to allow the whole radiation pattern reconstruction of the antenna under test (AUT). To get a considerable measurement time saving, spherical NF-FF transformations for AUTs with one or two predominant dimensions, requiring a minimum number of NF data, have been developed in [2], by using the nonredundant sampling representations of the electromagnetic (EM) fields [3] and adopting a prolate or oblate ellipsoid to shape the AUT. Another effective possibility to save the measurement time is to make faster the scan by collecting the NF data through continuous and synchronized movements of the probe and AUT. To this end, NF-FF transformations with spherical spiral scan have been recently proposed. They rely on the nonredundant representations and use optimal sampling interpolation (OSI) formulae [3] to effectively recover the NF data needed by the traditional spherical NF-FF transformation [1] from the acquired ones. The nonredundant sampling representation on the sphere from spiral samples and the related OSI expansion have been developed in [4-6] by adopting a spherical AUT model and choosing the spiral pitch equal to the sample spacing needed to interpolate along a meridian. Then, NF-FF transformations with spherical spiral scan for long or quasi-planar AUTs [7] have been obtained by applying the unified theory of spiral scans for non-volumetric AUTs [8]. Unfortunately, due to practical constraints, it is not always possible to mount the AUT in such a way that it is centered on the scanning sphere centre. In this case, the number of NF data required by the NF-FF transformation [1] and the related measurement time can remarkably increase, due to the corresponding grow of the minimum sphere radius. Aim of this work is the development of a fast and accurate nonredundant NF-FF transformation with spherical spiral scan suitable for quasi-planar antennas, which requires practically the same number of NF data both in the centered and offset mountings of the AUT. To this end, an offset mounted quasi-planar AUT is modeled as contained in a oblate ellipsoid, and an effective representation of the probe voltage over the scanning sphere, using a minimum number of samples collected on a proper spiral wrapping it, is developed by applying the unified theory of spiral scans for non-volumetric AUTs [8] in the spherical coordinate system having the origin coincident with the AUT centre at distance from the scanning sphere one. The related OSI expansion allows to accurately reconstruct the NF data required for the NF-FF transformation. [1] J. Hald, J.E. Hansen, F. Jensen, F.H. Larsen, Spherical near-field antenna measurements, J.E. Hansen, (ed.), London, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl., vol. 15, pp. 755-775, June 2001. [3] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop., vol. 46, pp. 351-359, March 1998. [4] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “NF–FF transformation with spherical spiral scanning,” IEEE Antennas Wireless Prop. Lett., vol. 2, pp. 263-266, 2003. [5] J F. D’Agostino, F. Ferrara, J.A. Fordham, C. Gennarelli, R. Guerriero, M. Migliozzi, “An experimental validation of the near-field - far-field transformation with spherical spiral scan,” IEEE Antennas Prop. Magaz., vol. 55, pp. 228-235, Aug. 2013. [6] F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Theoretical foundations of near-field–far-field transformations with spiral scannings,” Prog. in Electr. Res., vol. 61, pp. 193-214, 2006. [7] R. Cicchetti, F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Near-field to far-field transformation techniques with spiral scannings: a comprehensive review,” Int. Jour. Antennas Prop., vol. 2014, ID 143084, 11 pages, 2014. [8] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “The unified theory of near–field–far–field transformations with spiral scannings for nonspherical antennas,” Prog. in Electr. Res. B, vol. 14, pp. 449-477, 2009.
Modern cars are equipped with a large number of antennas which are strongly integrated with the car. A full characterization of the radiating properties of the entire vehicle is thus typically required. In order to characterize the radiating properties of the installed antennas, large measurement systems accommodating the full vehicle are required. As in standard antenna measurements, a full spherical near field (NF) scanning around the car is desirable in order to perform an accurate NF/FF transformation. However, due to size and weight of the Device Under Test (DUT) and/or economic factors a full spherical scan is often unfeasible. For this reason, truncated spherical scanners (such as hemispherical) are typically involved. A classic solution is to combine hemispherical scanning with a metallic ground plane which is assumed to be a Perfect Electric Conductor (PEC) in the NF/FF transformation. However, the PEC ground-plane is less representative of realistic automotive environments such as asphalt that is strongly dielectric. A further drawback is the strong scattering from the large metallic ground-plane which highly compromises the NF measurements at low frequencies. In many situations, it is thus desirable to perform the NF measurements in a condition similar to free-space by using absorber materials on the floor. It is well-known that standard NF/FF transformations applied to partial spherical acquisitions generates the so called truncation errors. Such errors are stronger at lower frequencies due to the lower number of spherical modes for fixed DUT size. Moreover, typical antennas for automotive applications are generally low directive thus, the impact of the truncation on the measured pattern is often non-negligible. In such cases advanced post-processing techniques must be involved to mitigate the effect of the truncation errors. In this paper two truncation error mitigation techniques will be compared when applied to automotive measurements performed in free-space conditions. The first technique is an iterative process which at each iteration applies a modal filtering based on the size of the DUT. The second technique is based on the computation of the equivalent currents of the DUT over an equivalent surface which acts as spatial filter. Both techniques give excellent mitigation performance with different computational effort. The good agreement between two different techniques effectively defining the lower bound for what can be successfully mitigated by post processing techniques.
The Translated Spherical Wave Expansion (Translated-SWE) has been recently proposed as a powerful Near Field to Far Field (NF/FF) transformation tool which allows to reduce the number of samples in offset spherical NF measurements. The algorithm is based on the definition of a new reference system located on the Device Under Test (DUT) rather than on the center of the measurement sphere. The translation of the measurement system on the DUT allows to represent it with a minimum number of spherical modes (smaller minimum sphere) thus the reduction of the NF sampling points (down-sampling). The validation of the Translated-SWE have been presented in previous publications in case of DUT offset displaced along the Z-axis. This may occur in case of mechanical constraints of the measurement system such as mast or stand-offs of fixed length, used to handle the DUT. Similarly, in other measurement situations, the DUT is intentionally displaced offset wrt the center of rotation to enhance the echo reduction capabilities of the modal filtering performed on the SWE spectrum. It has been also shown that in such measurement scenarios the Translated-SWE can be effectively used allowing a significant reduction of the sampling points and thus of the testing time. Antennas installed on complex structure, like cars, is another example of offset radiating devices. In many practical case, the currents induced by the fed antenna on the structure have only a localized effect (e.g. higher directive antennas and/or antennas working at higher frequencies). In such situations a down-sampled acquisition can be performed taking advantage of the Translated-SWE which is run moving the reference system on the fed antenna so that only the portion of structure surrounding that antenna is taken into account. The size of the measured portion of the structure will of course depend on the density of the applied sampling while the remaining part will be neglected. In this paper the Translated-SWE algorithm will be applied to antenna installed on cars in generic offset position. To this purpose the algorithm has been updated in order to be able to deal with generic XYZ-offsets.
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.
Antenna measurements above a material half-space are becoming an interesting aspect of near-field measurements especially for automotive antenna tests. Upcoming measurement facilities will be equipped with a dielectric or metallic ground. The near-field is sampled on a measurement surface in the vicinity of the device under test (DUT) above the ground, e.g. on a hemisphere. Thus, the effect of the ground has to be considered in the subsequent near-field to far-field transformation in order to obtain the far-field of the DUT above the ground plane. Assuming the metallic ground of the facility to be perfectly conducting, the ground effects are considered by introducing image sources below the ground plane in addition to the primary sources of the DUT above the ground plane. If coupling effects between the DUT and the ground plane are negligible, the primary sources correspond to the sources of the DUT in free-space. As a consequence, by separating the primary sources from the image sources, the free-space far-field of the DUT can be obtained from near-field measurements above ground. This means that measurement ranges with a ground plane can also be used to obtain free-space far-fields. In electromagnetic simulations, the primary sources can be placed in arbitrary environments, e.g. for communication channel evaluations. The quality of the primary sources extraction process mainly depends on the distance of the DUT sources from the ground plane as well as on the localization property of the employed equivalent sources which e.g. can be electric and/or magnetic surface currents or spherical modes. In this contribution, the numerical properties of the forward operator describing the relation between the DUT sources and the signal of the probe antenna above ground are analyzed in detail. The requirements for the unique determination of the primary sources from the near-field observations by inverting the operator are identified. Based on numerical investigations and real measurements obtained in a hemispherical near-field measurement facility, it will be shown that dependent on the ratio of the geometrical extensions of the DUT and its height above the ground as wells as on the strength of the coupling between the DUT and the ground, the free-space DUT far-field can be extracted with high quality.
This paper presents the results of a recent study concerning the computational electromagnetic simulation of a spherical near-field (SNF) antenna test system. The new plane-wave scattering matrix approach [1, 2] allows many of the commonly encountered components within the range uncertainty budget, including range reflections, to be included within the model [3]. This paper presents the results of simulations that verify the utility of the spherical mathematical absorber reflection suppression (S-MARS) technique [3, 4] for the identification and subsequent extraction of artifacts resulting from range reflections. Although past verifications have been obtained using experimental techniques this paper, for the first time, corroborates these findings using purely computational methods. The use of MARS is particularly relevant in applications that inherently include scatterers within the test environment. Such cases include instances where a SNF test system is installed within an existing compact antenna test range (CATR) as is the configuration at the recently upgraded Queen Mary University of London (QMUL) Antenna Laboratory [5, 6]. Thus, this study focuses on this installation with results of CEM simulations and actual range measurements being presented. The method enables a quantitative measure of the levels of suppression offered by the MARS system. References A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 34th Annual Meeting & Symposium, Bellevue, Washington October, 2012. A.C. Newell, S.F. Gregson, “Computational Electromagnetic Modelling Of Spherical Near-Field Antenna Test Systems Using Plane Wave Spectrum Scatting Matrix Approach”, Antenna Measurement Techniques Association (AMTA) 36th Annual Meeting & Symposium, Tucson, Arizona, October, 2014. C.G. Parini, S.F. Gregson, J. McCormick, D. Janse van Rensburg “Theory and Practice of Modern Antenna Range Measurements”, IET Press, 2014, ISBN 978-1-84919-560-7. G.E. Hindman, A.C. Newell, “Reflection Suppression in a large spherical near-field range”, Antenna Measurement Techniques Association (AMTA) 27th Annual Meeting & Symposium, Newport, RI, October. 2005. A.D. Olver, C.G. Parini, “Millimetre-wave Compact Antenna Test Range”, JINA Nice, November 1992. C.G. Parini, R. Dubrovka, S.F. Gregson, "CATR Quiet Zone Modelling and the Prediction of 'Measured' Radiation Pattern Errors: Comparison using a Variety of Electromagnetic Simulation Methods" Antenna Measurement Techniques Association (AMTA) 37th Annual Meeting & Symposium, Long Beach California, October 2015.
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.
In this paper, an analysis of the intermodulation effects observed in radio communication signals measurement systems, placed in vicinity of highly congested FM stations sites is presented. This work is required due to intermodulation issues have been a widely-studied topic in frequency analysis for communication stations, nevertheless the perspective of those analysis is mainly focused on avoiding the generation of intermodulation products that represents real spectrum occupancy but there is no an analysis for the intermodulation caused by the susceptibility of monitoring devices that, due to the non-linear behavior of the electronic components, present intermodulation products like real signals. The causes of intermodulation phenomena are discussed, which are mainly due to the non-linear behavior of one or several components of the monitoring system, and due to the proximity between transmission sites or between the measurement system and the site of the transmission. As a complement, a review of technical specifications for different monitoring instrumentation like receptors, active and passive antenna system, amplifiers, and filters and its effects on unwanted Intermodulation generation is done. With the goal of suppressing or minimize the Intermodulation Distortion of the electronic devices used for monitoring, a procedure for identification, according ITU recommendations, based on attenuators for common spectrum analyzers and using passive antennas have been designed and tested in different sites in different countries. The measurements obtained with different active devices and passive devices in the measurement system are presented and compared, identifying with the procedure the spectral characteristics of the intermodulation products and the reduction or filtering of these effects analytically and graphically.
Flared notch (“Vivaldi”) arrays have been a subject of great interest since the mid 1990s for use in broadband phased-array systems. These arrays are popular in large part due to their ultra-wide bandwidths, which can span multiple octaves, exceeding the bandwidths of the individual flared notch elements themselves. This effect is achieved via strong inter-element coupling, a departure from the conventional wisdom of minimizing mutual coupling between elements in a phased array. The benefits of this design choice have been widely reported on in the literature - however, this dependence on element coupling also places serious constraints on array performance, especially with regards to scan angle, active impedance, and array efficiency, which often go unreported. In addition, reliance on inter-element coupling necessitates an array that can be safely approximated as “infinitely” planar. If an array does not strictly meet this condition, significant VSWR issues can result, especially for elements near the edges of the array. This paper discusses the common pitfalls inherent in practical flared-notch array design that are often overlooked in the literature. To aid in this analysis, a network-centric approach to array modeling is demonstrated that allows for an examination of both element- and array-level performance metrics in a way that minimizes computation time and resources. Special attention is paid to parameters such as active impedance as a function of scan angle, which, though vital to array performance, are often mischaracterized by “infinite array” approximations commonly used by engineers in the design phase. The effects of mutual coupling on different array performance metrics, both beneficial and detrimental, are examined in detail so that an informed decision can be made on the suitability of the flared-notch topology for a given application.
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
Unmanned Aerial Vehicles (UAVs) are realizing their potential in both military and commercial sectors. Effective UAV operation must maintain wireless communication channels to execute flexible mission objectives. Antenna arrays establish this connectivity with increased directivity and beam-steering capabilities. Efficiently operating the antenna arrays within the installed environment is critically important. The antenna array design must therefore be optimized in-situ by altering the physical dimensions and electronically steering the beam over several directions. The resulting design space can potentially become too overwhelming even for modern computers within practical timeframes unless a strategic process is undertaken. A workflow has been demonstrated to achieve optimal installed antenna performance with minimal computational resources. An asymptotic computational electromagnetic (CEM) numerical technique, Ray Launching – Geometrical Optics (RL-GO), is initially leveraged to search over the surface of an aircraft to almost instantly determine a few suitable install locations for the antenna array. Method of Moments hybridized with Physical Optics (MoM/PO), a more accurate solver, is then used to simulate a few strategic perturbations to the physical model and ultimately characterize the installed antenna performance with a mathematical model. This process reduces the design space and facilitates computing thousands of configurations almost instantly to identify both the optimal install location and antenna array specifications. Finally, the most rigorous electromagnetic solver(s), MoM and/or MLFMM, are used to verify the final installed antenna performance.