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The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. The phaseless techniques have been discussed in the antenna measurements community and the theories behind these techniques are well explained in the literature. Unfortunately the issue of the noise and the presence of measurement errors are not investigated in details to provide strong impetus to the importance of phaseless measurements. In this paper the near field of a number of different types of antennas with high, medium and low side lobes is simulated to create as realistic case as possible. The effects of the probe positioning errors are investigated by injecting random errors in the position of the probe samples along x-, y- and z-axis. It is also illustrated how the positioning errors can distort the phase distributions. Through detailed characterizations of the constructed far field patterns, robustness of the Iterative Fourier technique even at the presence of very high probe positioning errors is demonstrated. It is shown how the utilization of the phaseless techniques will significantly reduce the probe positioning error effects when it is compared to the commonly used amplitude and phase near field measurement techniques.
The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. The phaseless techniques have been discussed in the antenna measurements community and the theories behind these techniques are well explained in the literature. Unfortunately the issue of the noise and the presence of measurement errors are not investigated in details to provide strong impetus to the importance of phaseless measurements. In this paper the near field of a number of different types of antennas with high, medium and low side lobes is simulated to create as realistic case as possible. The effects of the probe positioning errors are investigated by injecting random errors in the position of the probe samples along x-, y- and z-axis. It is also illustrated how the positioning errors can distort the phase distributions. Through detailed characterizations of the constructed far field patterns, robustness of the Iterative Fourier technique even at the presence of very high probe positioning errors is demonstrated. It is shown how the utilization of the phaseless techniques will significantly reduce the probe positioning error effects when it is compared to the commonly used amplitude and phase near field measurement techniques.
In this paper a technique is described that allows for the determination and correction of probe translation during polarization rotation in planar near-field measurements. The technique, which relies on the independent translation of coordinate systems for the two orthogonally polarized data sets, has significance for mm-wave testing, where bulky RF components makes probe alignment difficult. Measured data is presented to demonstrate the success of the technique.
This paper describes joint studies of Wroclaw University of Technology and Denmark Technical University on optimizing placement and performance of low-profile antennas on small satellite, such as ESMO Moon orbiter. After comprehensive electromagnetic studies with use of numerical analysis, a spacecraft mockup modeling its conductive surfaces was developed. Two to four antennas were mounted and several placement configurations were investigated. For verification purpose of numerical analysis and formulating design guidelines to an actual Moon probe, precise measurements of combined radiation pattern were performed at the Near-Field Antenna Test Facility, Denmark Technical University.
Full-wave modeling and far-field measurements are utilized to study the effects of a microstrip feeder on the band rejection and the far-field purity of planar log-periodic antennas. Three different configurations are investigated. Specifically, band rejection by relevant teeth removal (near/far-field), integration of the band-stop filter (near-field only), and the combination of the two are studied. Far-field contamination effects due to a microstrip feed line, and coupling to the antenna radiator, are evaluated for both radiating and band rejection regions. Important guidelines regarding the position and distancing of the feed to the radiator, as well as the trade-offs between substrate and superstrate configurations are derived. Antennas are developed to have a VSWR better than 2.5:1 in the 2-4 GHz and 7-11 GHz bands, and band rejection centered at 6 GHz. It is clearly shown that log-periodic antennas can be readily designed to have arbitrary, even reconfigurable, band rejection regions where overall realized gain is notched for more than 20 dB. A computer aided analysis was performed using commercial finite element and method of moments software tools. The measurements were conducted at Lockheed Martin in Denver, Colorado.
Highlights of work at the Naval Research Laboratory in evanescent near-field electromagnetic holography (ENEH) will be presented. This work grew out of extensive experimental work in near-field acoustical holography at our laboratory that has been recognized formally by the Laboratory as one of the 75 most innovative technologies over the past 75 years. This new electromagnetic approach differs from the usual nearfield imaging in that it provides much better than halfwavelength resolution due to the inclusion of evanescent waves. Furthermore ”imaging” to a source surface provides a reconstruction of the surface currents, Poynting vector as well as the E and H field vectors. These quantities are derived from two measured holograms (phase and amplitude) of two polarizations of the electric and/or magnetic fields over a 2-D surface (the hologram). Experimental work in both low (100 Hz) and high frequencies (10GHz) is of interest, although we present here results of the latter along with the theory. Two approaches will be discussed for backtracking the measured fields: one that uses wave function expansions in plane, cylindrical or spherical geometries, highlighting the cylindrical geometry in this paper, and a second more general formulation that uses the field expanded using an array of equivalent dipole sources especially useful in arbitrary geometries. Both approaches represent inverse methods and are ill-posed and require regularization to stabilize the reconstructions. We hope that these methods will provide high resolution new diagnostic tools for antenna analysis, as well as diagnostics for applications in EMC and EMI among others. Currently we are seeking partnership with other laboratories and universities to direct this technology towards problems that could benefit from its unique diagnostic capabilities. Work supported by the Office of Naval Research.
An error simulator based on virtual cylindrical near-field acquisitions has been implemented in order to evaluate how mechanical or electrical inaccuracies may affect the antenna parameters. In outdoor ranges, where the uncertainty could be rather important due to the weather conditions, an uncertainty analysis a priori based on simulations is an effective way to characterize measurement accuracy. The tool implemented includes the modelling of the Antenna Under Test (AUT) and the probe and the cylindrical near-to-far-field transformation. Thus, by comparing the results achieved considering an infinite far-field and the ones obtained while adding mechanical and electrical errors, the deviations produced can be estimated. As a result, through virtual simulations, it is possible to determine if the measurement accuracy requirements can be satisfied or not and the effect of the errors on the measurement outcomes can be checked. Several types of results were evaluated for different antenna sizes, which allowed determining the effect of the errors and uncertainties in the measurement for the antennas under study.
The near-field aperture distribution excited on the guiding surface of various planar leaky-wave antenna designs is examined. The investigated antennas (for millimeter wave applications) are realized by circular, straight and elliptical metallic strip gratings on a high permittivity dielectric substrate. With such straight and curvilinear grating configurations, analytical determination of the near-field, and hence the leaky-wave phase and attenuation constants along the guiding surface, can be mathematically intensive. To assist in such complex characterizations, the near-field/far-field extrapolation techniques can provide insight and thus illustrate such 2- D aperture field distributions. Specifically, by taking the inverse Fourier transform of measured 2D far-field beam patterns, the near-field distribution along the aperture can be estimated.
NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or far-field range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical near-field measurements, as well as in improving chamber performance.
Planar near-field probing is used in the optimisation of the quiet-zone of a hologram-based compact antenna test range (CATR). In this paper, the measurement instrumentation for 650 GHz operation is introduced and the potential measurement errors in the quiet-zone measurements are identified. Applicable error correction and compensation methods are discussed and the total measurement accuracy is calculated.
A probe-compensated near-field-to-far-field transform algorithm has been developed that can generate far-field patterns from near-field measurements made on an arbitrary surface. We present the concept, the algorithm, and computer simulated and measured test results for measurements on a conical surface. The prototype conical near-field measurements were made in a planar near-field range on a horn antenna under test (AUT) mounted on an azimuth-over-elevation positioner to produce a conical measurement surface. This system is especially applicable for producing full-hemisphere far-field patterns for antennas mounted on vehicles where other standard measurement systems may not adapt to the profile well, may not provide full-hemisphere coverage, or may require large, mechanically complex systems.
Calibration of planar near field probes is generally required to obtain accurate cross-polarization measurements of satellite antennas; however, probe calibration is costly and time consuming. One way to avoid probe calibration is to ignore the probe cross-polarization and use the probe co-polarized patterns alone for probe correction. Then the probe can be easily characterized by standard, in-house measurements or by analytical models. Of course, if the probe cross-polarization is ignored, additional errors are introduced in the co- and cross-polarized pattern measurements, but the errors can be manageable, depending on the probe and Antenna-Under-Test (AUT) polarization properties. Complete formulas and/or tables for near field measurement errors for three popular measurement configurations are presented, along with experimental verification of the error estimates for one case.
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Yet CELAR RCS measurement facilities are not compact bases and therefore the measured field is a near field. This article proposes a solution allowing the transformation of this near field to a far field and this in the three dimensions of space without limiting any dimension with Fraunhöfer criterion. Thanks to this method the RCS of a target is able to be known in any direction of space and moreover the calculation of a three-dimensional ISAR (Inverse Synthetic Aperture Radar) picture is thus possible. At first the theoretic part of our work is presented. Then a fast method in order to calculate the transformation of a near field to a far field by optimising the calculation time thanks to signal processing theory is given. Finally obtained results from simulated bright points are presented.
ABSTRACT A new probe compensated near-field – far-field transformation technique with planar spiral scanning is here proposed. It is tailored for quasi planar antennas, since an oblate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling is quite general (containing the spherical one as particular case) and allows one to consider measurement planes at a distance smaller than one half the maximum source size, thus reducing the error related to the truncation of the scanning surface. Moreover, it reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. 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.
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. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. 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. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.
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. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. 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. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.
A new method is here proposed to accurately evaluate the Specific Absorption Rate (SAR), e.g. of a mobile phone, through free-space measurements. The method takes advantage of the simple yet powerful plane-wave spectrum (PWS) representation of the electromagnetic (EM) field. The emitting device is tested in an anechoic chamber, where the two tangential components of the electric field are measured (amplitude and phase) and expanded into their PWS. These experimental data are subsequently fed to an equivalent transmission-line representation of the planar stratified structure composed by stacking the half-space made of free-space and the stratified flat phantom. Numerical simulations have shown that this method allows to accurately reconstruct the E field distribution inside a homogeneous phantom, with a worst-case error of 26 % in the estimation of the peak E field [1,2]. Furthermore, the proposed method is the first practical procedure for assessing the SAR in a stratified phantom, where the standard approach of moving a probe inside a liquid-filled phantom is no more feasible.
ACC has developed for the ESA-ESTEC CATR a compact but highly versatile 5-axis positioner. It is composed of a roll axis, upper azimuth, elevation, translation and lower azimuth axis. The clearance between the floor and the translation stage is designed to pass over a 12” walkway absorber while the roll axis height is only 155 cm (~5 feet). The standard configuration for medium or high gain antennas is the roll-over-azimuth or elevation-overazimuth configuration with a vertical interface for the AUT. For omni-directional antennas and RCS measurements, the positioner can be configured as a low profile azimuth positioner with a horizontal interface without a blocking structure behind the AUT. The positioner can also be configured for bistatic RCS measurements and Spherical Near Field. With the addition of a linear scanner, the Quiet Zone can be scanned in a polar way but also planar scanning is possible. Other key parameters are: angular accuracy: 0.01°, accuracy of the translation axis: 0.01 mm, load capacity 100 kg.
Modern day remote sensing spacecraft often feature multiple payloads sharing a common bus (spacecraft platform). RE02 emission testing (1, 2) characterizes the emission signature of a given payload in order to assess electromagnetic compatibility with respect to other payloads (i.e. “victims”) on the bus. Typically, a simple path loss model based on 1/r2 power variance (ref: Friis path loss equation) is used to account for the distance between the emitting and victim payloads using measured amplitudes taken during RE02 measurements. RE02 measurement technique (2) dictates that emissions testing take place at a fixed radial distance of one meter from the radiating instrument. At certain frequencies, however, this measurement takes place in the near field of the emitter. In general, power density amplitudes are greater in the near field than its far field counterpart. This paper investigates any potential error incurred by not accounting for this effect. A simple math model for a common mode radiator is developed to estimate this error and attempt to better understand the field relationships at lower frequencies where the near field predominates.
A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The technique is based on the transformation of the Spherical Wave Expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the Plane Wave Expansion (PWE), and it allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. While the fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously, we validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed and surface distortions are intentionally introduced. The effects of these errors will be detected in the antenna far-field pattern, and the accuracy and ability of the diagnostics technique to subsequently identify them will be investigated. Real measurement data will be employed for each test case.