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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.
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.
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.
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.
CubeSats represent a major paradigm shift for the satellite community and access to space in general. Traditionally, the design trend for satellites focused on durability, long lifetimes, and high performance, which translates to a high cost and lengthy time to deployment. CubeSats, on the other hand, compromise performance and lifetime with the goal to dramatically reduce costs and development time. Their small size allows launching as a secondary payload which is a key factor in cost reduction. The chassis dimensions (typically 3U to 6U) makes it difficult to integrate high performance wireless systems into the CubeSat chassis. A major limitation is the volume required for a high-gain antenna. Deployable, high-gain antennas are attractive for CubeSats, especially ones that can be stowed in a compact footprint. Deployment strategy is a major consideration for designers, and current attention has focused on developing large apertures that deploy outside the CubeSat chassis (e.g. umbrella or truss-net reflectors). However, little work has been done to develop ultra-compact, deployable antennas with moderate aperture sizes that seamlessly integrate with the chassis. We propose a novel CubeSat antenna concept that utilizes a near-field Gregorian reflectarray. The feed is a planar array conformal to the CubeSat surface that radiates an effective plane wave in its near-field zone. A parabolic subreflector focuses the plane wave towards the focus of the main reflectarray aperture, which has been designed to emulate an offset parabolic main reflector. This reflectarray is also conformal to the CubeSat chassis. The novelty of this design lies in the deployment strategy, where only the small subreflector is reoriented for deployment. This overall design avoids cable movement and maintains a compact volume when stowed, while achieving desirable efficiencies. We will present our compact antenna arrangement for Ka-band operation with an aperture of 209mm which integrates with a 6U CubeSat chassis. The challenges in the design are threefold: developing a compact, efficient planar array for plane wave generation; developing an offset, efficient reflectarray; and developing tools for diffraction analysis of the full system. We also present measured performance for various segments of this system using UCLA bipolar planar near-field system to avoid gravitational loading and issues with linear probe motion.
CubeSats represent a major paradigm shift for the satellite community and access to space in general. Traditionally, the design trend for satellites focused on durability, long lifetimes, and high performance, which translates to a high cost and lengthy time to deployment. CubeSats, on the other hand, compromise performance and lifetime with the goal to dramatically reduce costs and development time. Their small size allows launching as a secondary payload which is a key factor in cost reduction. The chassis dimensions (typically 3U to 6U) makes it difficult to integrate high performance wireless systems into the CubeSat chassis. A major limitation is the volume required for a high-gain antenna. Deployable, high-gain antennas are attractive for CubeSats, especially ones that can be stowed in a compact footprint. Deployment strategy is a major consideration for designers, and current attention has focused on developing large apertures that deploy outside the CubeSat chassis (e.g. umbrella or truss-net reflectors). However, little work has been done to develop ultra-compact, deployable antennas with moderate aperture sizes that seamlessly integrate with the chassis. We propose a novel CubeSat antenna concept that utilizes a near-field Gregorian reflectarray. The feed is a planar array conformal to the CubeSat surface that radiates an effective plane wave in its near-field zone. A parabolic subreflector focuses the plane wave towards the focus of the main reflectarray aperture, which has been designed to emulate an offset parabolic main reflector. This reflectarray is also conformal to the CubeSat chassis. The novelty of this design lies in the deployment strategy, where only the small subreflector is reoriented for deployment. This overall design avoids cable movement and maintains a compact volume when stowed, while achieving desirable efficiencies. We will present our compact antenna arrangement for Ka-band operation with an aperture of 209mm which integrates with a 6U CubeSat chassis. The challenges in the design are threefold: developing a compact, efficient planar array for plane wave generation; developing an offset, efficient reflectarray; and developing tools for diffraction analysis of the full system. We also present measured performance for various segments of this system using UCLA bipolar planar near-field system to avoid gravitational loading and issues with linear probe motion.
NIST has been using coordinated robotics in the Configurable Robotic Milli-Meter wave Antenna (CROMMA) system to assess antenna performance and radiative emissions since 2010. The focus to date has been using coordinated motion to arbitrarily position and correct antenna alignment for high frequency (>60 GHz) applications. Coordinated robotic motion was originally chosen to overcome the systematic alignment and range configurability limitations inherent in legacy stacked-stage ranges. A limitation with CROMMA is the relatively small spatial reach of the robotic arm (2.5 m), which limits antenna size and the number of wavelengths in separation achievable for lower frequencies. To overcome these limitations and address other dynamic testing requirements, NIST proposed the Large Antenna Positioning System (LAPS). The LAPS consists of two kinematically linked robotic systems, one of which is integrated with an 8 m linear rail system. The stationary robot is a 6 axes, 2.5m horizontal/4.4m vertical reach robotic arm, while the robot integrated with the linear rail system is a 6 axes, 3m horizontal/5.5m vertical reach robot arm. This configuration allows antennas to be positioned within a 5m x 6m x 10m volume. The motion system can operate in either a coordinated or independent motion control state, allowing independent or dynamic dual-robot motion. The coordinated capabilities of the system are designed to support not only traditional antenna measurement geometries (i.e. spherical, cylindrical, planar, gain-extrapolation), but also intended to be used to dynamically interact with changing RF conditions. The robots can independently scan or interrogate multiple bearings of a device under test, or trace out complex 6D paths during system testing. Initial data on performance of the system, including comparison of robot kinematics, RF acquired data, and physical locations verified by a laser tracker, will be presented.
Cellular Base Stations require efficient performance validation methods. One performance criterion is the Station radiation pattern. In directive pattern measurements, it is well documented that the Compact Antenna Test Range (CATR) and the Spherical Near Field (SNF) methods produce equivalent patterns. However, Base Station radiation patterns are not necessarily directive to the extent necessary for equivalent patterns among CATR and SNF methods. In deploying a number of Spherical Near Field and point source Compact Antenna Test Range (CATR) test facilities, we have observed the radiation pattern of base station antennas are more sensitive to the mount in a CATR than in a Spherical Near Field Antenna Test Range. This fact conflicts with intuition and theory. A barbeque spit positioner has been deployed in both spherical near field and point source compact ranges. Recently the point source compact range has been observed to yield patterns noticeable different depending on the antenna mount to the spit. On the other hand, the Spherical Near field implementation, in at least two deployments to Germany, has NOT manifested such a dependence on the mount, or, perhaps, such a dependency exists and yet has not been recognized. Measured Data will be presented showing radiation dependencies upon the mount in a CATR and SNF implementation. Explanations as to the Root Cause will be stated.
As part of the Lockheed Martin (LM) Additive Manufacturing (AM) Initiative, the Rotary Mission System antenna group has been developing a new and improved Additive Spiral Antenna (ASA) for both transmit and receive applications. This is a collaboration effort between LM engineering and LM manufacturing for a low cost and high performance antenna for manyultra-wide band(UWB) applications in both military and commercial market sectors. Unlike other conventional spiral designs, thisrecently emerging Additive Manufacturing capabilities allow extra spiral antenna miniaturizations without additional gain bandwidth performance penalties. This is achieved by leveraging unique low cost AM abilities to form complex and thus much more efficient 3D shapes to increase spiral antenna radiation efficiency, approaching the Chu’s gain bandwidth limitation. An initial prototype ASA was designed and tested in 2016 and showed very encouraging results. The measured ASA performance indicated nearly the same antenna performance as our current conventional production spiral antenna having multi-decade frequency band performance. More importantly, the ASA aperture size was significantly reduced by more than 50% with excellent transmit and receive gain efficiency and power handling capabilities. This paper will describe this ASA prototype design approaches and antenna near field and far field compact range measurement results along with material characterizations to demonstrate Additive Manufacturing technology can enhance antenna performance that otherwise not realizable with conventional fabrications. In addition, an integrated optimum balun length electromagnetic band gap (EBG) cavity design further reduces the antenna depth by over 70% will be presented. This is realized by use of high power and high temperature honeycomb absorbers in conjunction to electromagnetic band gap (EBG) cavity design for achieving high efficiency and low cavity profile, with total antenna volume reduction by nearly 3x. Some discussions will be provided for solving high thermal issues associated with ASA’s transmit capabilities.
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.
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.
Rotating the coordinate system of an antenna pattern can be problematic due to the need to interpolate complex data in spherical coordinates. Common approaches to 2D interpolation often introduce errors because of polarization discontinuities at the spherical coordinate system poles. To overcome these difficulties, it is possible to transform an antenna pattern from field-space into spherical mode-space, perform the desired coordinate system rotation in mode-space, and then transform the modes in the rotated coordinate system back into field-space. This method, while more computationally intensive, is exact and alleviates all of the interpolation-related issues associated with rotations in field-space. Although rotations in mode-space have been implemented in commercially available software (e.g., the ROSCOE algorithm provided by TICRA), these algorithms may not be well understood by the general antenna measurement community. Therefore, the first goal of this paper is to present an easy-to-understand algorithm for performing rotations in mode-space. Next, the paper will address the challenge of computing the rotation coefficients, which are required by the mode-space coordinate system rotation algorithm. Although J. E. Hansen presented a method for recursively computing the rotation coefficients, this method is numerically unstable for large values of N (where N is the upper limit of the polar index). Therefore, this paper will present a numerically stable method for the recursive computation of the rotation coefficients. Finally, this paper will show the relationships between Euler angles and both Az-over-El angles and El-over-Az angles. These relationships are quite useful because it is often desired to rotate an antenna pattern based on Elevation and Azimuth angles, whereas the inputs for the mode-space rotation algorithm are Euler angles. Knowing these relationships, the Euler angles may be computed from the Azimuth and Elevation angles, which can then be used as the inputs to the mode-space rotation algorithm.
The spherical multipole based near-field far-field transformation is extended to near-field antenna measurements above a perfectly electrically conducting (PEC) ground plane. As the effect of the ground plane is considered in the transformation by applying the image principle to the spherical modes radiated by the device under test (DUT), the near-field measurement points above the ground plane are sufficient to fully characterize the radiation behavior of the DUT above PEC ground. The nonequispaced fast Fourier transform (NFFT) is employed in the forward operator of the inverse problem in order to apply the transformation to e.g. spiral scans which are favorable to large and heavy scanner systems. If the elevation axis is located above or below the ground plane, an additional translation operator is integrated into the transformation to consider such an offset in the mechanical system. The proposed method is applied to synthetic and simulated automotive antenna near-field data in order to show its effectiveness.
Time gating is one of the most widespread techniques to suppress the effect of unwanted echoes on antenna measurements. It just requires the measurement of the antenna under test (AUT) for a carefully chosen bandwidth and frequency step size and the isolation of the direct AUT signal contribution from the echo contribution in time domain is quite intuitive. Although a frequency sweep is usually fast compared to the axes movement, it might become the speed limiting factor for large measurement bandwidths. Thus, time gating techniques that need a minimum bandwidth are beneficial. Therefore, a gating method is presented that reconstructs a time domain signal with high resolution from a minimum measurement bandwidth based on the assumption that the time domain signal is sparse, i.e. it mainly consists of samples with low amplitude and only few samples with high amplitude which are related to the peaks of the direct and the echo signal. The effectiveness of the proposed method is compared with the well known fast Fourier transform (FFT) and matrix pencil method (MPM) based techniques using echoic near-field antenna measurement data.
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.
The Submillimeter Wave Instrument is a 600GHz spectrometer with a 30cm reflector antenna, part of the payload of the ESA JUICE mission. Due to the difficulty of producing reliable phase measurements at such high frequency a phaseless planar nearfield measurement based on the Iterative Fourier Technique (IFT) is explored. The IFT is a well-known technique which has shown good results with aperture-type antennas; furthermore, probe correction has been demonstrated to be possible in one experimental case. In this paper a series of numerical results are presented pointing to the feasibility of a phaseless planar measurement for the SWI. In particular, the effect of the initial guess is evaluated with an accurate guess leading to exceptional results and a very simple constant-phase guess resulting in a less accurate result, but still remarkably accurate for the main beam. Additional simulations concern the use of coarser spatial sampling rates, showing that the sampling spacing can be increased to 32 lambda without significant aliasing error in the main beam, owing to the the high directivity of the SWI. Results from preliminary experimental investigations will also be reported, if available, at the time of the presentation.
The bi-polar scanning proposed by Rahmat-Samii et al. in [1, 2] is particularly attractive for its mechanical characteristics. The antenna under test (AUT) rotates axially, whereas the probe is attached to the end of an arm which rotates around an axis parallel to the AUT one. This allows the collection of the NF data on a grid of concentric rings and radial arcs. Such a scanning maintains all the advantages of the plane-polar one while providing a compact, simple and cost-effective mem. In fact, only rotational motions are required and this is convenient since rotating tables are more accurate than linear positioners. Moreover, being the arm fixed at one point and the probe attached at its end, the bending is constant and this allows one to hold the planarity. An efficient probe compensated NF–FF transformation with bi-polar scanning requiring a minimum number of NF data has been developed in [3] by applying the nonredundant sampling representations of electromagnetic (EM) fields [4, 5] to the voltage measured by the scanning probe and assuming the AUT as enclosed in an oblate ellipsoid. Thus, the plane-rectangular data needed by the classical NF–FF transformation [6] can be efficiently recovered from the nonredundant bi-polar samples by means of an optimal sampling interpolation algorithm. It is so possible to significantly reduce the number of required NF data and related measurement time without losing the efficiency of the previous approaches [1, 2]. Goal of this work is just the experimental validation of the nonredundant NF–FF transformation with bi-polar scanning [3], which will be carried out at the Antenna Characterization Lab of the University of Salerno. [1] L.I. Williams, Y. Rahmat-Samii, and 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] R.G. Yaccarino, Y. Rahmat-Samii, and L.I. Williams, “The bi-polar near-field measurement technique, Part II: NF to FF transformation and holographic methods,” IEEE Trans. Antennas Prop., vol. 42, pp. 196-204, Feb. 1994. [3] F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Data reduction in the NF-FF transformation with bi-polar scanning,” Microw. Optic. Technol. Lett., vol. 36, pp. 32-36, Jan. 2003. [4] O.M. Bucci, C. Gennarelli, and 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. [5] O.M. Bucci and C. Gennarelli, “Application of nonredundant sampling representations of electromagnetic fields to NF-FF transformation techniques,” Int. Jour. Antennas Prop., vol. 2012, ID 319856, 14 pages. [6] E.B. Joy, W.M. Leach, G.P. Rodrigue, and D.T. Paris, “Applications of probe-compensated near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-26, pp. 379-389, May 1978.
The Meteosat Third Generation series will comprise four imaging and two sounding satellites. The MTG-I imaging satellites will carry the Flexible Combined Imager (FCI) and the Lightning Imager. The MTG-S sounding satellites – a first for Meteosat – will carry an Infrared Sounder (IRS) and an Ultraviolet Visible Near-Infrared spectrometer, which will be provided by ESA as the GMES Sentinel-4 mission. On the MTG-I satellites, FCI will scan the full Earth disc every 10 minutes using 16 spectral channels at very high spatial resolutions, from 2 km to 0.5 km. In fast imagery mode it will be capable of a repeat cycle of 2.5 minutes over a quarter of the disc. The MTG-I satellites include a Data Collection System (DCS) & Geostationary Search and Rescue (GEOSAR) payload. The DCS supports meteorology and weather prediction. The GEOSAR transponder will be operated within the COSPAS-SARSAT system. Distress alert signals are received by MTG-I in UHF band and transmitted to ground in L-band for distribution to rescue mission control centers. Developed by Thales Alenia Space Italy, the DCS and GEOSAR UHF and L-band patch array antennas have been designed to operate aboard MTG-I satellites. The Engineering Model of the MTG antenna assembly with mockup has been tested inside ESA’s Hybrid European RF and Antenna Test Zone (HERTZ) chamber. The spherical near field tests performed on the antenna stand-alone and on the antenna mounted on the mockup were aimed at identifying impact of the large satellite structure on radiation pattern of the two medium gain antennas at UHF- and L-band. Taking into account the frequency of operation and the type of antenna under test, the major contributors to the measurement error are the room scattering and the probe-AUT mutual coupling. For this reason, dedicated measurements and analysis have been performed, in order to estimate the uncertainty in the most realistic way. The other parameters have been estimated based on past experience and knowledge on the measurement system. Several additional measurements were performed in order to produce dedicated uncertainty budgets for the stand-alone and with mockup tests and for the two frequency bands UHF and L-Band.
Among the near-field–far-field (NF–FF) transformations, that adopting the spherical scanning is particularly interesting, since it allows the complete antenna pattern reconstruction and avoids the error due to the scanning zone truncation. The classical spherical NF–FF transformation [1] has been modified in [2] by exploiting the spatial quasi-bandlimitation properties of the electromagnetic (EM) fields [3]. In particular, the choice of the highest spherical wave has been rigorously determined by these properties instead to be fixed by a rule-of-thumb related to the minimum sphere enclosing the antenna under test (AUT). The nonredundant sampling representations of the EM fields [4] have been properly applied to develop effective NF–FF transformations, requiring a number of NF data remarkably lower than that needed by the classical transformation [1] when considering nonvolumetric antennas. In particular, a quasi-planar AUT has been modelled by an oblate ellipsoid [2] or by a double bowl [5], whereas a long AUT has been shaped by a prolate ellipsoid [2] or by a cylinder with two hemispherical caps (rounded cylinder) [5]. Unfortunately, for practical constraints, it is not always possible to mount the AUT in such a way that it is centred on the scanning sphere centre. In such a case, the number of NF data needed by the classical NF–FF transformation [1] and the related measurement time can considerably grow, due to the corresponding increase of the minimum sphere radius. To overcome this drawback, a new spherical NF–FF transformation has been recently proposed in [6], by developing a properly modified version of the spherical wave expansion, wherein the spherical wave functions are defined with respect to the AUT centre instead of the scanning sphere one. Although the number of needed NF data is drastically reduced with respect to that fixed by the rule of the minimum sphere radius, it results to be slightly greater than the one corresponding to a centred mounting. Aim of this work is to properly exploit the nonredundant representations of EM fields to develop a nonredundant spherical NF–FF transformation for long antennas, based on rounded cylinder modelling, which requires the same number of NF data in both cases of centred and offset mounting of the AUT. It will be so possible to remarkably reduce the number of NF data and the related measurement time with respect to that required by the approach [6]. [1] J. Hald, J.E. Hansen, F. Jensen, and F.H. Larsen, Spherical near-field antenna measurements, J.E. Hansen, (ed.), London, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, G. Riccio, and C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electromagn. Waves Appl., vol. 15, pp. 755-775, June 2001. [3] O.M. Bucci and G. Franceschetti, “On the spatial bandwidth of scattered fields,” IEEE Trans. Antennas Prop., vol. AP-35, pp. 1445-1455, Dec. 1987. [4] O.M. Bucci, C. Gennarelli, and 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, 1998. [5] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, and M. Migliozzi, “Effective antenna modellings for NF–FF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop., vol. 2011, ID 936781, 11 pages. [6] L.J. Foged, P.O. Iversen, F. Mioc, and F. Saccardi, “Spherical near field offset measurements using downsampled acquisition and advanced NF/FF transformation algorithm,” Proc. of EUCAP 2016, paper 1570229473, Davos, Apr. 2016.
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.