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An innovative method for the diagnosis of large reflector antennas from far field data in radio astronomical application is presented, which is based on the optimization of the number and the location of the far field sampling points required to retrieve the antenna status in terms of feed misalignments. In these applications a continuous monitoring of the Antenna Under Test (AUT), and its subsequent reassessment, is necessary to guarantee the optimal performances of the radiotelescope. The goal of the method is to reduce the measurement time length to minimize the effects of the time variations of both the measurement setup and of the environmental conditions, as well as the issues raised by the complex tracking of the source determined by a prolonged acquisition process. Furthermore, a short measurement process helps to shorten the idle time forced by the maintenance activity. The field radiated by the AUT is described by the aperture field method. The effects of the feed misalignments are modeled in terms of an aberration function, and the relationship between this function and the Far Field Pattern is recast in the linear map by expanding on a proper set of basis functions the perturbation function of the Aperture Field. These basis functions are determined using the Principal Component Analysis. Accordingly, from the Far Field Pattern, assumed measured in amplitude and phase, the unknown parameters defining the antenna status can be retrieved. The number and the position of the samples is then found by a Singular Values Optimization (SVO).
A number of European Space Agency's (ESA) initiatives planned for the current decade require metrology level accuracy antenna measurements at frequencies extending from L-band to as low as 400 MHz. The BIOMASS radar, the Galileo navigation and search and rescue services could be mentioned among others. To address the needs, the Technical University of Denmark (DTU), who operates ESA’s external reference laboratory “DTU-ESA Spherical Near-Field (SNF) Antenna Test Facility”, in years 2009-2011 developed a 0.4-1.2 GHz wide-band higher-order probe. Even though the probe was manufactured of light-weight materials -- aluminium and carbon-fibre-reinforced polymer (CFRP) -- it still weighs 22.5 kg and cannot be handled by a single person without proper lifting tools. Besides that, a higher-order probe correction technique necessary to process the measurement data obtained with such a probe is by far more demanding in terms of the computational complexity as well as in terms of calibration and post- processing time than the first-order probe correction. On the other hand, conventional first-order probes for SNF antenna measurements utilizing open-ended cylindrical waveguides or conical horns fed by cylindrical waveguides operating in the fundamental TE11-mode regime also become excessively bulky and heavy as frequency decreases, and already at 1 GHz an open-ended cylindrical waveguide probe is challengingly large. For example, the largest first-order probe at the DTU-ESA SNF Antenna Test Facility operates in the frequency band 1.4–1.65 GHz and weighs 12 kg. At 400 MHz, a classical first-order probe can easily exceed 1 cubic meter in size and reach 25-30 kg in weight. In this contribution, a compact P-band dual-polarized first-order probe is presented. The probe is based on a concept of a superdirective linear array of electrically small resonant magnetic dipole radiators. The height of the probe is just 365 mm over a 720-mm circular ground plane and it weighs less than 5 kg. The probe covers the bandwidth 421-444 MHz with more than 9 dBi directivity and |µ| ? 1 modes suppressed below -35 dB. The probe design, fabrication, and test results will be discussed.
The possibility to use Near Field (NF) representation of antenna measurements in terms of equivalent currents, implemented in the commercial tool INSIGHT, is recently available in most CEM solvers. This method allows to use measured data to enhance numerical simulations in complex and/or large scenarios where antennas are installed. In the past this approach has been investigated and validated by determining the antenna radiation pattern in different antenna placement conditions. The aim of this paper is to present how this method can be extended for simulation of antenna coupling. Indeed using this innovative approach, after antennas are measured, their measured models can be imported in CEM tools and coupling with other radiators in arbitrary configurations can be simulated. No information about mechanical and/or electrical design of the measured antenna model are needed by the CEM tool, since the measured NF model in terms of equivalent currents already fully represents the antenna. Investigations have been performed on a H/V polarized array of three identical elements. Only the radiation pattern of the central element of the array has been measured, then starting from the measured data, the coupling between the other elements has been simulated by numerical tools. Accuracy of the procedure has been checked comparing the simulated results with the measured data of the entire array antenna. The testing procedure combining measurements and simulations consists of the following stages: · Measurement of the single element of the array and creation of the measured NF source representation. · Importing NF source in the CEM tool and placement in the array configuration. · Numerical simulation of the antenna coupling between the measured model and the other two elements of the array. Each element has two feeding ports implementing the dual H/V polarization. Preliminary analysis of the coupling is simulated and comparison with the measured data of the entire array agreement is acceptable. This study is currently under development for improving the accuracy of the results and including new test cases of different complexity.
The efficiency of use of the parabolic reflector of a single offset reflector compact antenna test range (CATR) is affected largely by the illumination provided by the range feed and the reflector edge treatment. Thus, when these factors are taken together it is commonly found that the realized quiet zone (QZ) diameter is typically as little as 30% of the diameter of the reflector for the commonly encountered case of a single offset CATR. Furthermore, single offset CATR performance is known to degrade as the wavelength of the illuminating fields becomes more comparable with the physical dimensions of the reflector because the physical optics (PO) assumption needed for collimation of the reflected field becomes less effective. Different reflector edge treatments such as rolled or serrated edges are commonly employed to taper the intensity of the reflected fields at the reflector aperture boundary, seeking to minimize the level of diffracted fields in the quiet-zone (QZ). Such strategies mean that at higher frequencies the transverse dimensions of the QZ are unnecessarily reduced thereby decreasing the spatial efficiency of the CATR and limiting the effective bandwidth of the antenna test system. In this paper we report preliminary results that begin to investigate the alternative strategy for controlling the signal illuminating the CATR reflector by utilizing a shaped beam feed antenna. Building on our previously reported work of efficient CATR computational electromagnetic simulation, we report the use of an array feed whose excitation is optimized to achieve maximum QZ size. We illustrate the concept by employing the technique with a sector-shaped, reflector single offset CATR having no edge treatment and then using the same reflector with an edge treatment and by examining the impact that this has on the amplitude taper and the amplitude and phase peak-to-peak ripple.
In wireless communication technology, the growth of 5G and MIMO (multiple- input and multiple-output) systems has revealed a gap in the methods to characterize and calibrate hardware for high frequency and coherent MIMO applications. For coherent array configurations and ad hoc systems we need to measure transmission loss and phase/delay over every element. We demonstrate the use of standard RF hardware to generate and receive multiple signals in a system that is a tabletop analogy for an ad hoc system. The initial test system consists of using a single WR-15 VNA extender to detect two separate modulated signals. As our sources, we individually modulate WR-15 VNA extenders to generate continuous waveform, modulated signals around 60 GHz. On the receive side, our IF signal is first measured with a high-dynamic- range spectrum analyzer and then later collected in a digital oscilloscope. All the signal generators for the receiver LO and transmitter(s) RF IN are tied together with a common 10 MHz reference. Characterizing this initial 2x1 system is then extensible to multiple-receiver applications. We will use these coherent sources to get full complex waveform characterization element-by-element in a receiving array. We report on measurement and calibration methods to characterize the response of these systems for continuous waveforms, modulated signals, and multi-frequency applications needed for next generation coherent MIMO systems.
A high-gain 6-40 GHz circularly polarized antenna system has been designed for a CubeSat Radiometer Radio Frequency Interference (RFI) Technology Validation mission, which is to demonstrate wideband (6-40GHz) RFI mitigating backend technologies vital for future space-borne microwave radiometers. In stowed configuration, the antenna system needs to fit within a small volume of 10cm (L) by 8cm (W) by 5cm (H). The deployed length of the antenna is 25cm. The total antenna payload including deployment mechanism needs to be less than 0.2kg. The desired gain is 14 dBic gain at 6 GHz and linearly increased to 22 dBic at 40 GHz in order to minimize the coverage footprint on earth. The proposed antenna system include three continuous-taper helical antennas due to its simple feeding, circular polarization (CP), and wide bandwidth. They also have desirable light weight and flexible structures. The three helical elements operate at 6-11 GHz, 11-22 GHz and 21-40 GHz, respectively. The diameter of each helical antenna is specially profiled as a function of height to achieve the desired linear gain vs. frequency property. Since the three antenna elements are co-located within a small cavity, their positions were carefully investigated to minimize mutual coupling and coupling to cavity. This paper presents the antenna design specifications, simulated performances, and preliminary measurement data.
A detailed study of several analytic methods for the simulation of compact antenna test range (CATR) quiet zone field is presented. The focus of the present paper is to develop a fast simulation model suited to an optimization process of the CATR geometry in order to minimize the desired quiet zone parameters, such as amplitude taper and ripple, phase variation and crosspolar level, for a wide range of frequencies. The algorithms introduced are based on physical optics (PO) which provides accurate results at high frequencies, when the reflector size is very large in wavelengths. Unlike conventional PO algorithms where the surface current integration is performed with a finite integral, here the problem is addressed by modeling the reflector surface as a set of equivalent radiating planar triangles. Comparisons between the developed method and the direct integration are given to justify the computational savings. A Fast Physical Optics algorithm is implemented for evaluating the quiet zone field to further reduce computational times.
Free Space VSWR measurement has been the de facto standard method for anechoic chamber performance evaluation for more than 50 years. In this method, a probe antenna is kept at a fixed angle while traveling along a linear path to record the standing wave pattern. The probe antenna is then rotated to a different angle to repeat the measurement. Reflectivity, which is used as the chamber performance metric, is calculated for each probe rotation angle. In this paper, we show that the reflectivity is affected by the antenna patterns of the probe antenna. When the probe antenna is aimed at the specular reflection point of a chamber surface, measurement dynamic range is improved, and the method provides a measure of the reflectivity primarily from that surface. When the probe is not directed at a specular point, other reflections in the chamber can contribute to the VSWR, and the chamber reflectivity becomes more dependent on the probe antenna pattern.
This contribution presents a universal antenna construction toolkit with slotted waveguide elements that can flexibly combined to form a reconfigurable antenna array capable of providing arbitrary symmetric radiation patterns. The design and the arrangement of radiating elements allow adjusting arbitrary real amplitudes of single radiating elements in a solely mechanical way without any electrical feeding network. Additional modular connecting elements even allow two dimensional and conformal antenna designs with circular and multiple polarizations. With a single toolkit in the Ku-band several design and measurement examples are presented, such as a linear array forming a desired main lobe down to -20dB, and a universal two dimensional antenna array that can switch between vertical, horizontal, LHC and RHC polarization. Given a desired antenna pattern the design procedure allows an automated generation of the physical antenna layout that can mechanically be combined without the need of additional full wave simulations. The waveguide toolkit is easily scalable to any other frequency band just being limited in the upper frequency by manufacturing issues. Another major benefit is that the modular concept of connecting and radiating elements eases the manufacturing where otherwise integral waveguide antennas require much more demanding processes. Different physical realizations of the modular waveguide concept are presented and discussed in the paper and related to the antenna performance. Beside several applications for the universal antenna toolkit, such as investigating illumination issues in scattering theory, educational aspects of teaching group antenna theory are also discussed in this contribution.
Anisotropic material characterization requires versatile sample fixtures in order to provide sufficient measurement diversity for material parameter extraction. However, extensive sample preparation is often required prior to making a measurement, especially for anisotropic materials. An alternative nondestructive material measurement approach using a Single Port Waveguide Probe (SPWP) is proposed to simplify measurement of uniaxial anisotropic media. Instead of cutting a material sample to fit into a given fixture, nondestructively interrogating a sheet of material via the SPWP greatly simplifies sample preparation and measurement. The SPWP system measures a metal-backed sample of a known thickness. A flange with a waveguide aperture cut in the center is placed on the metal backed sample (thus forming a parallel plate region) and a length of rectangular waveguide is connected to the flange aperture. A Vector Network Analyzer port is connected to the end of the rectangular waveguide to collect calibration and sample data. Measurements of two different thicknesses of a sample are performed to provide sufficient data for extracting the sample permittivity tensor. The sample permittivity tensor is computed via comparison of the measured and theoretical S-parameters using a least squares minimization algorithm. The theoretical S-parameters are derived using a magnetic field integral equation which utilizes a uniaxial parallel plate Green’s function to constitute the fields in the parallel plate region. Love’s Equivalence Principle is used to relate the fields in the parallel plate flange region to the fields in the waveguide (assumed to be the dominant TE10 mode only). In this paper, the SPWP theoretical development, measurement and material parameter extraction are discussed. Measurements and simulations of isotropic and uniaxial samples are made to assess the SPWP performance.
Raytheon, El Segundo, CA chamber #2 is a dual reflector, indoor compact range that is the largest facility of its kind within the company. A series of tests were performed to characterize the measured transfer function of the chamber because of a recent capital upgrade of the range measurement system. The purpose of this paper is to document and discuss the results of the characterization testing, review how the measured transfer function of the range was determined, and compare the current results with both past data and analytical predictions, and demonstrate how this transfer function is used for antenna and radar cross section (RCS) measurement characterization. The measured transfer function of the range is used for both antenna and RCS measurement characterization. For antenna measurements, the transfer function is used in the Friis transmission equation to determine, for example, the expected power at the receiver given the transmit power and gain of both the transmit antenna and the antenna under test. Appropriate amplification and/or attenuation can determined as part of the test planning process saving time during test setup and test execution. For RCS measurements, the transfer function was recently utilized to study the benefits and challenges of relocating our instrumentation radar from a smaller compact range to this large compact range. The motivation for the study was enhanced measurement capability for larger targets and lower frequencies. This study utilized noise equivalent RCS (NERCS) as the metric and transmit power, pulse width, and pulse integration as the study parameters to find a practical solution for optimizing NERCS.
In Near Field (NF) measurements different echo reduction techniques can be applied to mitigate echoes or stray signals deriving from the surrounding environment. A very promising echo reduction technique is based on the so called spatial or modal filtering. The spatial filtering is very efficient in measurement scenarios with stationary AUT but necessitates an offset of the AUT in scenarios where the antenna is rotating. Unfortunately, the measurement of the AUT in an offset configuration requires the acquisition of a higher number of samples. An innovative spherical NF/FF transformation algorithm for offset measurements based on a Translated Spherical Wave Expansion (TSWE) has been recently proposed. In this paper we investigate by experiment the echo reduction properties of offset AUT measurements using TSWE.
Probe correction in standard spherical near field measurements are typically limited to probes with |µ|=1 spherical wave spectrum when performing spherical wave expansion. The design of such probes is often a trade-off between achievable performance, modal purity and bandwidth. Compensation techniques for probes with higher or full order modal spectrum have recently been proposed. The advantages of such techniques are more freedom in the selection of the probe for a given measurement scenario and increased bandwidth. The technique reported in this paper is valid for probes with a known modal spectrum of arbitrary order. Probe compensation is performed directly on each spherical wave function before expanding the measured field. This leads to a computationally very effective algorithm. In this paper, the accuracy of the new algorithm is validated experimentally for different higher order probes in the measurement of a standard gain horn. For each scenario, the accuracy and computational requirement of the new algorithm is compared to standard transformations.
Compact Antenna Test Range (CATR) provide convenient testing, directly in far-field conditions of antenna systems placed in the Quiet Zone (QZ). Polarization performance is often the reason that a more expensive, complex, compensated dual reflector CATR is chosen rather than a single reflector CATR. For this reason, minimizing the QZ cross-polarization of a single reflector CATR has been a challenge for the industry for many years. A new, dual polarised feed, based on conjugate matching of the undesired cross polar field in the QZ on a full wave-guide band, has recently been developed, manufactured and tested. The CXR feed (cross polar reduction feed) has shown to significantly improve the QZ cross-polar discrimination of standard single reflector CATR systems. In previous papers, the CXR feed concept has been discussed and proved using a limited scope demonstrator and numerical analysis. In this paper, for the first time, the exhaustive testing of the dual polarised feed operating in the extended WR-75 waveguide band (10-16 GHz) is presented. Accuracy improvements, achieved in antenna cross-polar testing, using this feed is also illustrated by measured examples.
An inverse source reconstruction method with great potential in radome diagnostics is presented. Radomes are designed to enclose antennas to protect them, from e.g. weather conditions. Frequency selective surface (FSS) radomes are designed to conceal the antennas and provide stealth properties, by transmitting specific frequencies and be reflective for other frequencies. Ideally, the radome is expected to be electrically transparent. However, tradeoffs are necessary to fulfill properties such as aerodynamics, robustness, lightweight, weather persistency, stealth properties, etc. One tradeoff is the existence of inevitable defects. Specifically, for examples, seams in large radomes, lightning strike protection, Pitot tubes, rain caps, or lattice dislocations in frequency selective radomes. In all these examples of defects, it is essential to diagnose their influences, since they degrade the electromagnetic performance of the radomes if not carefully attended and analyzed. In this contribution, we investigate if source reconstruction can be employed to localize and image the disturbances from the defects on the surface of the radome. Employing far-field measurements remove the need for probe compensation. An artificial puck plate (APP) radome with dislocations in the lattice is investigated. An APP radome is a frequency selective surface (FSS) and it consists of a thick perforated conducting frame, where the apertures in the periodic lattice are filled with dielectric pucks. Due to the double curvature of an FSS surface, gaps and disturbances in the lattice may cause deterioration of the radome performance. Source reconstruction methods determine the equivalent surface currents close to the object of interest. The reconstructions are established by employing an integral representation in combination with an integral equation. The geometry of the object on which the fields are reconstructed is arbitrary. However, the problem is ill-posed and needs regularization. The equivalent surface currents are reconstructed on a body of revolution with the method of moment (MoM), and the problem is regularized with a singular value decomposition (SVD). The aim is to back-propagate a measured far field to determine the field components on the radome surface. The purpose is to investigate if defects on a frequency selective surface (FSS) lattice can be localized.
A high quality antenna feed is an essential element of a compact antenna test range (CATR) in order to ensure the range can achieve the necessary stability in beam width, phase center and the necessary purity of polarization throughout the range’s quiet zone. In order to maintain the requisite quality, such feeds are typically 1) single-port and 2) cover a relatively limited band of frequencies. It is desirable to have a single dual ported, broadband feed that covers multiple waveguide bands to eliminate the need for a polarization positioner and avoid the difficulty associated with changing feeds for a single antenna measurement. Though some such feeds exist in the market, with such feeds, we often see a reduction in polarization purity across the band of interest relative to the more band limited feeds. Previous attempts to utilize dual-port probes and/or extend the bandwidth of the feed have resulted in degraded performance in terms of beam pattern and polarization purity. In an attempt to overcome some of the deficiencies above, the authors have applied polarization processing to dual-pol antennas to correct for the impurity in polarization of the antenna as a function of frequency. We present here a broadband CATR feed solution using a low-cost, dual-port sinuous feed structure combined with polarization processing to achieve low cross-pol coupling throughout the quiet zone. In the following paper, the feed structure, polarization theory, and processing algorithm are described. We also present co- and cross-pol coupling results before and after correcting for the polarization distortion using data collected in two CATRs in Atlanta, GA and Asia.
In order to achieve high accuracy in measuring sidelobes and/or nulls in antenna patterns, it is necessary to use a test system with very high dynamic range. This is particularly important when the antenna has extremely high gain such as those used for certain satellite communications or radio astronomy applications or when transmit power is limited relative to range loss as is often the case in millimeter wave applications. For several years, commercially available antenna measurement receivers have offered a dynamic range as high as 135dB for such applications. This dynamic range has been made possible, in part, by a simple correlator in the receiver’s DSP chain. In this paper, we model the various sources of error in a test signal due to imperfections and uncertainties of the test equipment and the physical environment and analyze these models as they propagate through the receive chain. The results of that analysis demonstrate the correlator’s ability to reduce carrier frequency offset (CFO) and local oscillator (LO) phase noise to offer the fidelity of test signal necessary to achieve extremely high dynamic ranges of up to 135dB.
Dual polarized probes are convenient for accurate and time efficient Planar Near Field (PNF) antenna testing. Traditional probe designs are often bandwidth limited and electrically large leading to high scattering in PNF measurements with short probe/AUT distances. In this paper, an octave band probe design with minimum scattering characteristics is presented. The scattering minimization is largely obtained by a very small axially symmetric aperture of 0.4? diameter at the lowest frequency. The aperture also provide a near constant directivity in the full bandwidth and very low cross polar. The probe is fed by a balanced ortho-mode junction (OMJ) based on inverted quad-ridge technology and external feeding circuitry to obtain high polarization purity.
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment. This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. This paper details a RCS measurement technique and the associated-post processing of raw data dedicated to the localization of the scatterers of a target under test. A specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. Compared to classical radar images, the main issue is linked with the variation of polarization induced by the near-field 3D RCS facility. This method is based on a fast and efficient regularized inversion that reconstructs simultaneously HH, VV and HV 3-D scatterer maps. The approach stands on a simple but original extension of the standard multiple scatterer point model, closely related to HR polarimetric characterization. This algorithm is tested on simulated and measured data from a metallic target. Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Needs of antenna measurements at low VHF range require the development of specific facilities. Costs saving could be found by reusing existing chambers and extending the frequency band down to few tens of MHz, especially if the implementation of such a system is performed in undersized chambers with already existing absorbers. CNES began such an adaptation in the 2000’s by adding a VHF measurement probe (80-400 MHz) in their CATR chamber which allows performing spherical single probe Near Field measurement thanks to the existing positioner. In the past four years, intensives studies have been led to reduce uncertainties onto measurements results and to wide again the lower frequency down to 50 MHz. Major error terms were identified and both a new measurement probe and post processing tools have been designed and implemented. This paper focuses on the hardware and software upgrades. Details will be first provided on the mechanical upgrades of the probe positioner, aiming to improve the accuracy and the repeatability of the positioning, as well as the ergonomic usage for saving installation time. A dedicated reference antenna in gain and polarization has been developed and validated. Such reliable reference antennas at this frequency range are a key point to reduce uncertainties onto measurement results. Finally, optical tool for aligning the measurement probe and the AUT as well as the post processing tool will be presented.