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This paper presents the initial field probe characterization results for an RF scattering compact range using a high precision calibration sphere. This approach uses an Inverted Stewart Platform to position the ultra-sphere through the target quite zone. The Inverted Stewert Platform and optical target tracking system provide a fast and efficient for performing a volumetric incident illumination field characterization of the compact range quite zone using a backscatter RF measurement. The Inverted Stewert Platform system uses six small diameter strings attached to the ultra-sphere to provide the ultra-sphere positioning over the entire quiet zone of the compact range. The inverted Stewart platform also offers increased stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. This presentation will discuss an in-house development of the sphere field probe and discuss advantages and disadvantages of the ultra-sphere volumetric field probe.
In the Flight Model (FM) of the PLANCK telescope, the feed horns are connected to either HEMTs or bolometers operating at cryogenic temperatures to detect the Cosmic Microwave Background radiometric signal. For the purpose of an overall alignment verification at ambient temperature, RCS measurements have been performed using an auxiliary feed horn that is terminated with a switching diode. This verification test has been conducted at 320 GHz, to benefit from the narrow beam and a high sensitivity to misalignment. To perform the RCS measurements, an additional “circulator” with low propagation loss and high isolation from transmit to return channel had to be developed. Besides that, the circulator also co-locates the phase centres of both Tx and Rx range antennas on the focal point of the CATR, which allows mono-static RCS measurements. Quasi-optical techniques have been used to design a circulator that meets these requirements. To test the feasibility of determining the feed location from the RCS measurements with an uncertainty of ±1 mm, a test campaign was conducted with the so called RF Qualification Model (RFQM). In this campaign, 9 feed locations with 1 mm separation were tested. With the Flight Model, the test was on the critical path of the planning and only one test could be conducted to verify the overall alignment.
We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.
At the present time at the end of a measurement of RCS of a target, it is possible to obtain either the value of the RCS of the target as a whole for a given frequency, bearing and elevation or a RADAR image of this target. The aim of this RADAR image is generally to locate the bright points that constitute the target and not to estimate the energy of these bright points. That is why the calculation of these energies is not generally the subject of an elaborate rigorous processing. Yet it may be necessary to be able to give the RCS of any part of the target when this target has been measured as a whole. In answer to this need it is necessary to isolate and calculate sharply the energies of the bright points that constitute the target, because the RCS of each part of the target is the sum of the energies of the bright points which constitute it. This article exposes a method of processing which allows this calculation, while resolving the problems linked to the interpolation and to the discrete nature of the measurements and calculations.
Phased arrays antennas have desirable features in terms of simplicity, compact dimensions and low weight for low frequency applications requiring dual polarization and medium gain such as RCS measurements. However, a fundamental problem with phased arrays technology in wide band applications is grating lobe limitations due to the grid topology of the phased array elements. The spacing of the array elements cannot be to close in order to limit element coupling and not to large to avoid grating lobes. Consequently, conventional phase array antenna applications are generally limited to a useable frequency bandwidth of 1:2. A unique grid topology has recently been developed to overcome this problem [1, 2]. By interleaving three separate phased arrays, each dedicated to a different subband with close to 1:2 bandwidth, the useable bandwidth of the combined phased array antenna can be extended to as much as 1:7 while maintaining the nice performance features of the basic phase array technology. Based on this technology a large dual polarized phase array antenna has been designed for indoor RCS testing in the frequency range from 140MHz to 1000MHz. The operational bandwidth of the array is split into three subbands: 140-260 MHz, 260-520 MHz and 520-1000 MHz. The array is 6.34 x 6m and weighs less than 250Kg. Due to the element spacing and topology the phased array is sensitive to excitation errors so the beam forming network (BFN) feeding the elements must be wellbalanced. A uniform amplitude and phase distribution for the array excitation coefficients has been selected to simplify the BFN design and minimize possible excitation errors throughout the bandwidth. This paper describe the antenna electrical design and performance trade-off activity, the manufacturing details and discuss the comprehensive validation/testing activity prior to delivery to the final customer.
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
Dual polarized probes for modern high precision measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. As a consequence, the probe design is a trade-off between performance requirements and the usable bandwidth of the probe. For measurement applications in circular polarization the choice is between measuring the linear polarization components separately and derive the resulting circular polarized by computation or to measure directly with a circular polarized probe. Dual polarized probes in circular polarization with high polarization purity is difficult to achieve on a wide bandwidth. Dual linear polarized probe technology has recently been developed capable of achieving as much as 1:4 bandwidth while maintaining the high performance of traditional probe designs [1–7]. This paper describes the development, manufacturing and test of dual circular polarized probes with as much as 1:2 bandwidth as shown in Figure 1.
An effective near-field – far-field transformation technique with spherical spiral scanning tailored for antennas having two dimensions very different from the third one is here proposed. To this end, an antenna with one or two predominant dimensions (as, e.g., an elongated or quasi-planar antenna) is no longer considered as enclosed in a sphere, but in a prolate or oblate ellipsoid, respectively, thus allowing one to remarkably reduce the number of required data. Moreover these source modellings remain quite general and contain the spherical one as particular case. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
Carbon nanotube antennas and antenna arrays are discussed as possible nanoantennas in the GHz frequency range. Due to their exceedingly small radius, carbon nanotubes present unique measurement and simulation challenges, unlike those encountered in ordinary antenna applications. In this paper, we present preliminary results for measurement and simulation of carbon nanotube based antennas.
Recent advances in computational electromagnetic tools have made antenna design possible along with integration of antennas on various ground, sea and air platforms. Numerical computations can be performed to evaluate the effects of antenna placement, radiation hazard, EMC/EMI, etc. The typical numerical approaches include full wave techniques such as Method of Moments (MoM), Multilevel Fast Multipole Method (MLFMM) and asymptotic techniques such as Physical Optics (PO) and Uniform Theory of Diffraction (UTD). For many practical applications, sometimes it is necessary to study the electromagnetic behavior on a specific structure over a broad frequency band, and therefore it is important to have some benchmark data on computational resources needed for some commonly used numerical techniques. In this study, representative full-size air, ground and sea platforms are considered and the frequency limit is pushed at different bands using several numerical techniques. The accuracy and computational resources are compared.
This paper presents a technique for calculating the antenna pattern distortion caused by supporting structures such as buildings, towers, etc. The technique is based on ray tracing and the uniform theory of diffraction. The resulting distorted pattern can then be added to antenna databases and used as input to, for example, wireless network planning tools. The present method is fast and can considerably improve the accuracy of propagation calculations of radio frequency signals. A representative example from the application of this technique to an antenna mounted on the top of a building is presented.
Dipoles are a typical reference antenna in measurements. Because its performance is calculable even in the near field it is commonly used as a reference. But while the ideal dipole is a calculable device, the actual reference dipole used in the lab can be far from the ideal. In this paper end fed sleeve dipoles commonly used as references in wireless measurements and traditional quarter wavelength dipoles used in a wide variety of applications including RFID testing are study. Misalignment, manufacturing tolerances, variations on dielectric, and messy solder points will be analyzed numerically and in some cases compared with measured data to see the effects of these problems on the final performance of a reference dipole unit.
PLANCK is one of the scientific missions of the European Space Agency, devoted to observe the Cosmic Microwave Background radiation with unprecedented accuracy. One of the key factors for the performance is the radiation pattern of the telescope, especially the sidelobe performance in the direction of hot celestial bodies like Sun, Earth and Moon. The satellite will operate around the L2 Lagrangian point in deep space under cryogenic conditions. These conditions can not be realized in an antenna test range for a payload of this size. Therefore, the predictions for the performance under flight conditions depend highly on numerical simulations. The model to be used had never before been verified to this level of confidentiality. The challenge was to conduct a test campaign at frequencies up to 320 GHz (far beyond the normal range of the used CATR) with a very large object (the PLANCK RF Qualification Model with an aperture size of 1.5 m, i.e. more than 1500 wavelength at 320 GHz) to demonstrate Sidelobe Levels down to -90 dB. A selection of the measurement results and comparison with predictions will be presented.
We introduce a new way to visualize electromagnetic waves. The sum total of wave information, essentially everywhere save for the source current distribution, can be determined from just a single data bank acquired across a near-field surface. The waves can be recovered and displayed in terms of their intensity or phase differential over the entire region of interest. A field mapping algorithm is proposed which obtains the field everywhere, both interior and exterior to the measurement surface, based on a single near-field data acquisition. The field mapping algorithm is a direct, closedform solution which is numerically straightforward and efficient. Verification is demonstrated by analytic examples, numerical simulation results, and hardware measurement. Excellent agreement is evident in all cases.
This paper describes an application of well known microwave holography to the practical case of the space antenna for the European Navigation System GALILEO. The antenna consists in an array of 45 patch elements, divided into six sectors, fed by a two level beam forming network. In fact, the procedure described in this paper has been used in the frame of the development of the GALILEO Navigation antenna to identify element feeding errors. A planar hologram on the aperture plane of the array has been obtained by a set of spherical near field measurements. Sampling the resulting aperture field distribution (in amplitude and phase) allowed reconstructing the excitation law and identifying errors. The developed procedure was validated with a number of test cases assessing numerical errors introduced by the process. Applying the back-projection to the measured far-field led to discover that some sectors of the array were overfed and that errors were present in the central power divider responsible of the first power distribution in the antenna. A new power divider was then manufactured and integrated into the array leading to a well performing antenna.
This paper describes or deals with a quality analysis and comparison of three radar reflectivity information or data types. The information or data types include radar cross section (RCS) as defined by IEEE Standard 100, the bowtie sector average, and the gross estimate radar return (commonly known as the fuzzball). The paper discusses the uncertainty analysis of measured RCS, and the paper provides analysis on the uncertainty of bowtie sector averages and “fuzzballs” (gross estimate radar returns). The comparison of the information or data types, their quality, uncertainties, and usefulness represents a significant part and focus of the study.
In this paper a method to identify faulty elements in a planar array using Artificial Neural Networks (ANN) is presented. The input to the neural network is amplitude of deviation pattern and output of neural network is the location of faulty elements. A planar array of 5×5 number of isotropic elements with uniform excitation and spacing ?/2 is considered. Either one faulty element or two faulty elements can exist in the array. The network is trained with some of the possible faulty deviation patterns and tested with various measurement errors. ANN is implemented with Radial Basis Function neural network (RBF) and Probabilistic neural network and their performance is compared.
Practical ISAR measurements must often be made in the near-field. Scatterers are illuminated by a spherical wavefront, generating a continuum of incident angles due to parallax. Ignoring this, radar image processing produces geometrically distorted images whose utility diminishes the more deeply into the near-field the measurements are made. The underlying assumption that a target may be accurately modeled as a collection of isotropic point scatterers can enormously widen in angle. Yet, by considering parallax (with attention to phase), near-field measurements can produce quasi-far-field images, whose Fourier transform bears a greater likeness to a far-field RCS signature. A technique is presented and explored whereby each image pixel is focused at angles normal to the incident spherical wavefront by compensating for parallax. The focused coordinates are spatially variant, but for a pixel exactly containing a point scatterer, the resulting focused IQ pairs are identical with those in the far-field.
An Integral Equation-based method for Near-to-Far Field Transformation method and antenna diagnostics is presented. This technique, called the Sources Reconstruction Method (SRM) makes use of the Equivalence Principle jointly to the Integral Equations in order to find an equivalent problem so that the fields radiated by the original problem and by the equivalent one are the same. While most of the antenna diagnostics techniques limit their application to canonical geometries (planar, cylindrical, spherical), the SRM extends the diagnostics capabilities to arbitrary geometries. Thus, if the surface where the equivalent electromagnetic currents are reconstructed fits the Antenna-Under-Test (AUT) geometry it is possible to diagnose the fields and currents distribution over the AUT surface. This generalization for arbitrary geometries increases the SRM computational cost if compared to other diagnostics methods. The paper describes the latest SRM improvements, which are mostly related to the computational cost reduction by means of the Fast Multipole Method (FMM). Examples showing the SRM capabilities for antenna diagnostics are included.
The slot line, a transmission line suitable for application to microwave integrated circuits, may be used in place of or in association with microstrip. This paper presents an alternative method based on the adaptive neuro-fuzzy inference system (ANFIS) for computing the characteristic impedances of slot lines. The ANFIS is a class of adaptive networks which are functionally equivalent to fuzzy inference systems. The ANFIS has the advantages of the expert knowledge of the fuzzy inference system and the learning capability of neural networks. Different optimization algorithms, hybrid learning, genetic, simulated annealing, and least-squares, are used to determine optimally the design parameters of the ANFIS. The algorithm performances for the optimization of the ANFIS model parameters are compared with each other. The results of ANFIS are compared with the results of a commercial electromagnetic simulator IE3D and closed form expressions (CFE) obtained by curve fitting technique to the numerical results.