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In the literature, one can find a low scattering photodiode modulated-probe for microwave near field imaging. The frequency response of the probe is computed at 2.45 GHz. In this paper, however a new formulation for computing the scattered field for low frequencies (from 150 MHz) by a broadband near field probe based on the impedance matrix is developed. In addition, a method to increase the scattered power by controlling matching will be shown.
Abstract—The measurement geometry and data processing techniques employed in spherical near-field (SNF) antenna measurements naturally quantify the directivity of an antenna under test. Computing antenna gain from these measurements requires additional information and processing. Equations that can be used to calculate the magnitude of antenna gain from spherical near-field measurements are provided in seminal SNF references, but equations that describe how to calculate the complex gain voltage of an antenna with spherical near-field methods have been largely absent in the literature. This paper presents equations that may be used to calculate the complex gain voltage of an antenna using the gain substitution method in a spherical near-field test range. The equations are presented in a more generalized format than previously seen, and will show how to use a combination of data collected in the near-field with data transformed to the far-field to calculate the gain voltage. Practical examples are provided for determining gain voltage using a single measurement set-up or multiple measurement set-ups, including a method to calculate gain voltage of each port of a multi-port antenna requiring only a single full sphere measurement of the standard gain antenna.
Abstract: In order to characterize the Boeing 9-77 compact range, the empty chamber background was measured as a function of frequency, polarization, and the azimuth angle of the upper turn-table (UTT). The results exhibited a near-field diffraction pattern with enlarged hot-spots on a 4-fold symmetry [1]. A 2-D FFT on the diffraction pattern yielded a mapping on the relative arrangement of the absorbers on the UTT [2]. In this paper, we take a closer look at the scattering geometry of the UTT as illuminated by the residual field above and beyond the quiet zone (QZ). The different responses in VV and HH are discussed. The enhanced diffraction due to a “blazed grating” condition is identified and analyzed.
Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Abstract— In order to achieve precise antenna pattern measurement in mm-wave frequency region, we propose a cylindrical near-field antenna measurement system using photomixing technique with UTC-PD. Due to this system, we can use an optical fiber as the transmission line of mm-wave signal and downsize the mm-wave signal source. Accordingly, we can achieve flexible cable movement and suppress the disturbance from the waveguide components. In this paper, we will show the measured near-field distribution on cylindrical coordinate by the proposed system and calculated far-field antenna pattern of standard gain horn antenna in W-band.
Abstract— This paper presents an original way for the design simulation, implementation, and measurement of a multiband flexible textile antenna. The aim is to realize an antenna with a dipolar radiation at several resonance frequencies. The radiating element is a monopole antenna. This antenna naturally exhibits a dipole and a quadripole radiation pattern for the first and second resonance frequency respectively. This behavior is due to the current distribution on the antenna. To constrain the second mode to change into a dipolar radiation pattern, two decorrelated and non-radiating parasitic elements are added to the antenna. At this second resonance frequency, the current distribution is different from the one of the quadripolar mode by the parasitic elements. The dimensions of these parasitic elements are defined by electromagnetic simulations and measurements. To validate this method, the monopole antenna is studied. The radiating element of the antenna is sewn on the textile flexible substrate. This substrate was previously characterized in terms of relative permittivity and losses. The near-field magnetic field and the far-field radiation pattern are studied in simulations and measurements.
Abstract—It is well-known that a complete bistatic set of near-.eld scattering data is required to compute far-.eld radar cross section (RCS) quantities. In many practical applications, however, only monostatic scattering data is available. Almost all algorithms for the transformation of monostatic near-.eld data are based on the synthetic aperture radar (SAR) image representation.Since these algorithms are often acceleratedbythe fastFouriertransform(FFT),they usuallypose manylimitations on the measurement procedure such as regularly spaced grids and separate treatment of the different polarizations due to scalar processing. In this paper, a novel and .exible algorithm is presented which is not based on the FFT but on multi-level fast multipole method (MLFMM) principles. Therefore, it is similar to the fast irregular antenna .eld transformation algorithm (FIAFTA) which has been designed for the transformation of antenna .elds and measurements. Numerical results of different scenarios show that these principles can also be successfully applied to monostatic scattering data. In summary, this approach is superior to existing algorithms, because it provides more .exibility while it is still very ef.cient.
The use of pre-conditioning interpolation schemes, as a possible means of enhancing the performance of previously introduced adaptive acquisition algorithms for spherical near-field (SNF) test time reduction, is evaluated. Investigations have been carried out to establish whether the adaptive SNF approach is suited to test engineering practice are reported. The pre-conditioning method involving the acquisition of two orthogonal polar cuts on the near-field sphere and the separate linear interpolation of two complex spherical components of the NF data is shown to be the preferred scheme. This method is evaluated for three different antennas using specific acquisition rules, and decision functions related to directivity, amplitude error, and side lobe level.
Abstract—The 3D reconstruction algorithm of DIATOOL is applied to the prototype feed array of the BIOMASS synthetic aperture radar, recently measured at the DTU-ESA Spherical Near-Field Antenna Test Facility in Denmark. Careful analysis of the measured feed array data had shown that the test support frame of the array had a significant influence on the measured feed pattern. The 3D reconstruction and further post-processing is therefore applied both to the feed array measured data, and a set of simulated data generated by the GRASP software which replicate the series of measurements. The results of the diagnostics and the corresponding improvement of the feed array field obtained by removal of the undesired effect of the frame are presented and discussed.
Abstract — Nearfield Systems Inc. (NSI) has been contracted by the Department of Electrical and Computer Engineering of the University of Waterloo to install a unique antenna test system with multiple configurations allowing it to characterize a wide variety of antenna types over a very wide bandwidth. The system employs a total of 10 positional axes to allow near-field and far-field testing in various modes of operation with great flexibility. A 4 m x 4 m planar near-field (PNF) scanner is used for testing directive antennas operating at frequencies up to 110 GHz with laser interferometer position feedback providing dynamic probe position correction. The PNF’s Y-axis can also be used for cylindrical near-field (CNF) testing applications when paired with a floor mounted azimuth rotation stage. A single phi-over-theta positioner permits both spherical near-field (SNF) testing from L-band to W-band and far-field testing down to 0.2 GHz. This positioner is installed on a translation stage allowing 1.8 m of Z-axis travel to adjust the probe-to-AUT separation. In addition, a theta-over-phi swing arm SNF system is available for testing large, gravitationally sensitive antennas that may be easily installed on a floor mounted rotation stage. In order to ensure system and personnel safety, a complex interlock system was designed to reduce the risk of mechanical interference and ease the transition from one configuration to another. The system installation and validation was completed in March 2013. We believe that this facility is unique in that it encompasses all commonly used near-field configurations within one chamber. It therefore provides a perfect environment for the training of young engineers and could potentially form the baseline of future academic test facilities. This paper will outline the technical specifications of the scanner and discuss the recommended applications for each configuration. It will also describe the details of the safety interlock system.
Abstract— Several gain measurement techniques exist for near-field antenna ranges. These include Comparison-gain, Direct-gain and Three-antenna gain methods. Each technique has its own unique advantages and disadvantages in terms of accuracy, cost and measurement time. Range operators must understand the differences between these techniques in order to properly configure their test system to best suit their requirements. This paper surveys each of the gain techniques and identifies the relative advantages of each. As part of the survey, all three techniques were performed on three types of near-field antenna measurement systems: Planar, Cylindrical and Spherical. The results of this paper provide the reader with a practical understanding of each technique, the formulas required, and real-world examples for the trade-offs needed to outfit a range for fast and accurate gain measurements while balancing cost and schedule.
Abstract -We propose a method of utilizing near-field spherical measurements so as to obtain the back lobes of high gain antennas without sacrificing the accuracy of the far-field, high-gain main lobe prediction. While a spherical scan is perfectly adequate to gauge the relatively broad back lobes, it is in general inadequate to capture the required details of a sharp forward peak. We overcome this difficulty through recourse to our Field Mapping Algorithm (FMA), which latter allows us to assemble planar near-field data based upon the spherical measurements actually acquired. In particular, planar data of this sort on the forward, main-lobe side offers the standard route to predicting the desired, high-gain, far-field pattern. Our spherical-to-planar FMA near-field data manufacture showed excellent agreement with direct planar near-field measurements for a slot array antenna, each one of them, naturally, underlying a common, far-field, high-gain pattern.
This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure [8] and can be used on small, and large, plane-polar near-field antenna test systems.
Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction
The ground based, Automatic Identification System (AIS) is a coastal tracking and messaging system used by vessels for maritime traffic monitoring purposes. The European SAT-AIS initiative aims at providing a space-based complementary system to extend the range of the existing AIS system to high seas via a satellite constellation in VHF band. In the course of the AISMAN development activity a miniaturized five element antenna array at 156/162 MHz, has been designed, manufactured and tested on a representative satellite mock-up [1]. The verification testing at satellite mock-up level has been performed in a hemispherical spherical near field antenna test range of Renault in Aubevoye, France [2]-[3]. Due to the scan truncation and the room scattering at VHF frequencies, advanced post processing based on the Equivalent Current expansion technique has been applied in the testing. This paper discuss the post processing issues and the findings of the verification testing.
In this communication, the experimental verification of a probe compensated near-field - far-field (NF-FF) transformation with spherical spiral scanning particularly suitable for elongated antennas is provided. It is based on a nonredundant sampling representation of the voltage measured by the probe, obtained by using the unified theory of spiral scans for nonspherical antennas and adopting a cylinder ended in two half-spheres for modelling long antennas. Its main characteristic is to allow a remarkable reduction of the measurement time due to the use of continuous and synchronized movements of the positioning systems and to the reduced number of required NF measurements. In fact, the NF data needed by the classical NF-FF transformation with spherical scanning are efficiently and accurately reconstructed from those acquired along the spiral, by employing an optimal sampling interpolation formula. Some experimental results, obtained at the Antenna Characterization Lab of the University of Salerno and assessing the effectiveness of such a NF-FF transformation technique, are presented.
This communication deals with the experimental validation of an efficient near-field - far-field (NF-FF) transformation using the planar wide-mesh scanning. Such a scanning technique is so named, since the sample grid is characterized by meshes wider and wider when going away from the center, and makes possible to lower the number of needed measurements, as well as the time required for the data acquisition when dealing with quasi-planar antennas. It relies on the use of the nonredundant sampling representation of electromagnetic fields based on the use of a very flexible modelling of the antenna under test, formed by two circular "bowls" with the same aperture diameter but eventually different bending radii. A two-dimensional optimal sampling interpolation formula allows the reconstruction of the NF data at any point on the measurement plane and, in particular, at those required by the classical NF-FF transformation with the conventional plane-rectangular scanning. The measurements, performed at the planar NF facility of the antenna characterization laboratories of Selex ES, have confirmed the effectiveness of this nonconventional scanning, also from the experimental viewpoint.