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Near Field

Revival of the Northrop Grumman CTS 10K Far-Field Range
Jeff Way,John Luzwick, Mark Hozlevcar, Dan Lang, November 2010

Outdoor far- field antenna test ranges have declined in popularity due to the advent of alternative test methods, e.g., Near-Field Antenna ranges and Compact Antenna Test Ranges. They are also costly to maintain. A natural consequence of that trend is that far-field ranges are either shut down or rendered dormant for long periods of time. The latter was the situation for the NGAS (Northrop Grumman Aerospace Systems) CTS 10K Far-Field range. The Far-Field was an outdoor range with a 10,000’ range length, open transmit site and radome enclosed receive site. It had been dormant for 7 years and was needed for a unique test before the test site was vacated completely. This paper provides a brief description of the range, the upgrades made to address equipment obsolescence and the checkout process to ensure that the range would meet performance requirements. The range needed to operate from 100 MHz to 18 GHz. Therefore, range diagnostics were performed at various frequency points and swept measurements also executed. A Range Readiness report was created and presented internally. Elements of that report are shared in this paper.

GlobalStar II RX L-Band Antenna Spherical Near Field Measurement
Trenta Damiano, November 2009

The 18 terms technique, for the evaluation of the measurement error, was used to justify the differences between the measurement data obtained from the three facilities. In Addition a general description of the test setup and the principle error sources found during the finalization of the test setup are given.

Imaging by Near-Field Measurements
George Cheng,Jan Grzesik, Yong Zhu, November 2009

Allwave Corporation, 3860 Del Amo Blvd., #404, Torrance, CA 90503, USA

MEASUREMENTS OF POWER HANDLING OF RF ABSORBER MATERIALS: CREATION OF A MEDIUM POWER ABSORBER BY MECHANICAL MEANS
Vince Rodriguez,Garth d'Abreu, Kefeng Liu, November 2009

The rise in the number of active antennas used in radar applications calls for changes to the common absorber treatment used in chambers. The electronic circuits that are imbedded into these scanning arrays are non-linear in nature so they must be tested at the correct power outputs to get the correct pattern behavior. The combination of higher power and narrow beams means that areas of the anechoic treatment in a chamber can be subjected to high power densities. High power absorber has been used in the industry for many years. The substrate used in these absorbers makes the material very expensive. While in the past it was common to use this material only in regions where the main beam was going to be illuminating the absorber treatment the new electronically scanned arrays will have main beams that can illuminate several areas of the chamber. In cases where Near Field systems are used the absorber material will be in a radiation region where the main beam has not been formed, but the Absorber is closer to an array that is radiating high power so a large area of higher power absorber is needed to treat the chamber. In the present paper the authors present a medium power absorber 3kW per square meter (versus 775w per square meter) using the same material used in common RF absorber. Mechanical changes to the absorber are performed to increase the thermal dissipation of the EM energy absorbed. A series of measurement of the absorber is performed with a without additional air flow for cooling. The result is an absorber that can handle higher power densities without the need for exotic substrates.

Accurate Radiation Pattern Measurements in a Time-Reversal Electromagnetic Chamber
Andrea Cozza, PhD,Abd el-Bassir Abou el-Aileh, November 2009

In a recent paper [1], we have introduced the concept of the time-reversal electromagnetic chamber (TREC), a new facility for creating coherent wave-fronts within a reverberation chamber. This facility, based on the use of time-reversal techniques in a reverberating environment, is here shown to be also a useful tool for the characterization of the field radiated by an antenna under test (AUT). The TREC is proven to be capable of providing real-time measurements, with an accuracy comparable to that of spherical near-field facilities, while using a very limited number of static probe antennas. This performance is made possible by taking advantage of the reflections over the chamber’s walls, in order to gain access to the field radiated along all the directions, with no need to mechanically displace the probes, or to have a full range of electronically switched ones. A 2D numerical validation supports this approach, proving that the proposed procedure allows the retrieval of the free-space radiation pattern of the AUT, with an accuracy below 1 dB over its main lobes.

A Novel Phaseless Spherical Near-Field Antenna Measurement Including the Issue of Robustness
Carsten Schmidt,Thomas Eibert, Yahya Rahmat-Samii, November 2009

The radiation characteristics of antennas can be deter-mined by measuring amplitude and phase data in the ra-diating near-field followed by a transformation to the far-field. Accurate phase measurements especially at high frequencies are very demanding in terms of the required measurement equipment and tolerances. Phaseless mea-surement techniques have been proposed, which often deal with a second set of amplitude only measurement data in order to compensate the lack of phase information. In this paper the concept of phaseless spherical near-field measurements will be addressed by presenting a phaseless near-field transformation algorithm for spherical antenna measurements, working with amplitude only data on two spheres. In particular the measurement of a patch antenna is considered to demonstrate the utility of the technique for low gain antennas. To address the issue of robustness, inaccurate measurement distances as well as spherical rotation angles are considered in order to evaluate the accuracy of the method against probe positioning errors. Furthermore noise contributions are introduced to emu-late measurement inaccuracies in general.

Near-field Electromagnetic Holography in Conductive Media
Earl Williams, November 2009

This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.

Generalization of the Circular and Linear Near Field-to-Far Field RCS Transformations to Off-Waterline Collection Geometries
Ivan LaHaie, November 2009

In previous papers [1]-[4] we have presented formulations for the circular and linear near field-to-far field RCS transformations (CNFFFT and LNFFFT, respectively). These formulations assumed that the target did not have significant extent above or below a central (waterline) plane, and that the circular or linear near field scans lied in this waterline plane. In this paper, the CNFFFT and LNFFFT formulations are generalized to scans that lie in a plane parallel to and above or below the waterline plane. These scans correspond to conical or great circle RCS cuts, respectively, in the far field at elevation angles other than 90°. We will show that the generalization can be accomplished by modifying just the frequency domain processing steps that are common to both algorithms, while leaving the spatial processing portions (apart from a minor variable redefinition) unchanged. The paper focuses on the mathematical derivation and numerical implementation of the algorithms; examples of numerical and experiment results are deferred to future papers.

Reflection Suppression In Cylindrical Near-Field Antenna Measurement Systems – Cylindrical MARS
Stuart Gregson,Allen Newell, Greg Hindman, November 2009

Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements [2, 3, 4, 5]. Whilst the technique presented herein is also a general purpose measurement and post-processing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the post-processing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.

Nearly-Orthogonal Hierarchical Vector Basis Functions Employed for the Discretization of Inverse Equivalent Surface Currents
Ismatullah Ismatullah,Thomas Eibert, November 2009

Inverse equivalent current method has recently gained popularity in the applications of near-field far-field (NF­FF) transformations especially when near-field (NF) measurements are carried out on irregular measurement grids around the arbitrarily shaped object under test. Usually low order (LO) Rao-Wilton-Glisson (RWG) basis functions or even point based low order basis functions are used for the discretization of the unknown surface current densities on the triangular discretization elements. Better accuracies are achievable when equal number of higher order (HO) basis functions is employed to represent unknown surface current densities. Nearly-orthogonal hierarchical vector basis functions complete to full first order with respect to the curl space are therefore utilized for the discretization of inverse equivalent surface currents defined on flat triangular domains. Various numerical examples are presented and comparison is made with the results of LO discretization.

Single Antenna Method for Determining the Gain of Near-Field Waveguide Probes
Russell Soerens, November 2009

Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.

A NONREDUNDANT NF–FF TRANSFORMATION WITH SPHERICAL SPIRAL SCANNING USING A FLEXIBLE AUT MODEL
Francesco D'Agostino, November 2009

ABSTRACT In this work, a probe compensated near-field – far-field transformation technique with spherical spiral scanning suitable to deal with elongated antennas is developed by properly applying the unified theory of spiral scans for nonspherical antennas. A very flexible source modelling, formed by a cylinder ended in two half-spheres, is considered as surface enclosing the an­tenna under test. It is so possible to obtain a remark­able reduction of the number of data to be acquired, thus significantly reducing the required measurement time. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.

Numerical Calibration of Standard Gain Horns
Don Bodnar, November 2009

The gain-transfer technique is the most commonly used antenna gain measurement method and involves the comparison of the AUT gain to that of another antenna with known gain. At microwave frequencies and above, special pyramidal horn antennas known as standard-gain horns are universally accepted as the gain standard of choice. A design method and gain curves for these horns were developed by the US Naval Research Laboratory in 1954. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach. Similar computational electromagnetic modeling is applied to predict the gain and pattern of open-ended waveguide probes which are used in near-field antenna measurements. This approach provides data for probes that are not available in the literature.

Identifying Pointing Errors for the NIST 18 Term Error Technique
Zachary Newbold,Allen Newell, Bruce Williams, November 2009

The NIST 18 Term Error Analysis Technique uses a combination of mathematical analysis, computer simulation and near-field measurements to estimate the uncertainty for near-field range results on a given antenna and frequency range. A subset of these error terms is considered for alignment accuracy of an antenna’s RF main beam. Of the 18 terms, several have no applicable influence on determining the beam pointing or the terms have a minor effect and when an RSS estimate is performed they are rendered inconsequential. The remainder become the dominant terms for identifying the alignment accuracy. There are six terms that can be evaluated to determine the main beam pointing uncertainty of an antenna with respect to dual band performance. Analysis of the near-field measurements is performed to identify the alignment uncertainty of the main beam with respect to a specified mechanical position as well as to the main beam of the second band.

The Effect of the Absorber Collar on Open Ended Waveguide Probes
Allen Newell, November 2009

This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for near-field measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the far-field patterns of the probe and this in turn could affect the probe correction process when the probe was used in near-field measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.

The Effect of the Absorber Collar on Open Ended Waveguide Probes
Allen Newell, November 2009

This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for near-field measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the far-field patterns of the probe and this in turn could affect the probe correction process when the probe was used in near-field measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.

Radiation Pattern Measurements of NASA Mars Science Laboratory UHF Entry Antenna using a Spherical Near-field Range
John Demas,David P. Farr , Hilda Hernandez, Jesus Aguilar, Paula R. Brown, November 2009

Measurements of a conical micro-strip Wraparound™ antenna array mounted on a portion of the entry vehicle for NASA’s Mars Science Laboratory mission were completed at Nearfield Systems, Inc.’s new spherical near-field range facility. The Wraparound™ antenna, designed and manufactured by Haigh-Farr, Inc., provides nearly full spherical coverage and operates in the UHF frequency band for telecommunications to orbiting assets at Mars. A summary of the measurements techniques and results are presented, along with a comparison of the measured and calculated patterns.

Implementation of Extrapolation and Planar Near-Field Antenna Measurement System for W-band Antennas
Noweon Kang,Jeong-Hwan KIM, Jin-Seob KANG, Tae-Weon KANG, November 2009

Korea Research Institute of Standards and Science (KRISS) has upgraded antenna measurement system to W-band (75 GHz ~ 110 GHz). This paper describes the antenna measurement facility that consists of a planar near-field scanner, a microwave subsystem and an extrapolation range, and measurement results. The microwave subsystem is adjusted to meet system power budget. Comparison result of on-axis horn antenna gains from extrapolation method based on three-antenna technique with those from planar near-field measurement shows good agreement within 0.15 dB.

Improvement of the Signal to Noise in spherical near field antenna measurement through mode truncation and spatial filtering
Sara Burgos,Francisco Cano, José Luis Besada-Sanmartín, Manuel Sierra Castaner, Manuel Sierra-Castañer, November 2009

This paper analyzes the reduction of the noise effect in spherical near-field antenna measurements. Two techniques have been evaluated: the first one is based on the mode truncation and the second one consists of a spatial filtering after a diagnosis process. The antenna under test (AUT) used for this evaluation is the 12 GHz Validation Standard antenna (VAST12). The VAST12 measurements have been performed in the Spherical Near-Field Antenna Test Facility of the Technical University of Madrid (UPM). These measurements have been corrupted adding a White Gaussian Noise (WGN) with different levels. First, the effect of the number of spherical modes considered in the near-to-far-field transformation has been evaluated, analyzing also the error due to the mode truncation versus the reduction of the noise uncertainty associated to each spherical mode. Second, a diagnosis process based on a holographic technique has been carried out. A spatial filtering including the AUT aperture has been applied and then, the far-field is reconstructed and compared with the uncorrupted far-field. Several results illustrate the signal In this paper, the effect of White Gaussian Noise (WGN) in spherical near-field measurements and the improvement of the signal to noise ratio (SNR) through mode truncation and spatial filtering are evaluated employing simulations and measurements. Section 2 and 3 respectively explain the mode truncation and spatial filtering to minimize the noise. In section 4 and 5 the results achieved when applying both techniques are illustrated. Finally, section 6 summarizes the conclusions drawn.

Improvement of the Signal to Noise in spherical near field antenna measurement through mode truncation and spatial filtering
Sara Burgos,Francisco Cano, José Luis Besada-Sanmartín, Manuel Sierra Castaner, Manuel Sierra-Castañer, November 2009

This paper analyzes the reduction of the noise effect in spherical near-field antenna measurements. Two techniques have been evaluated: the first one is based on the mode truncation and the second one consists of a spatial filtering after a diagnosis process. The antenna under test (AUT) used for this evaluation is the 12 GHz Validation Standard antenna (VAST12). The VAST12 measurements have been performed in the Spherical Near-Field Antenna Test Facility of the Technical University of Madrid (UPM). These measurements have been corrupted adding a White Gaussian Noise (WGN) with different levels. First, the effect of the number of spherical modes considered in the near-to-far-field transformation has been evaluated, analyzing also the error due to the mode truncation versus the reduction of the noise uncertainty associated to each spherical mode. Second, a diagnosis process based on a holographic technique has been carried out. A spatial filtering including the AUT aperture has been applied and then, the far-field is reconstructed and compared with the uncorrupted far-field. Several results illustrate the signal In this paper, the effect of White Gaussian Noise (WGN) in spherical near-field measurements and the improvement of the signal to noise ratio (SNR) through mode truncation and spatial filtering are evaluated employing simulations and measurements. Section 2 and 3 respectively explain the mode truncation and spatial filtering to minimize the noise. In section 4 and 5 the results achieved when applying both techniques are illustrated. Finally, section 6 summarizes the conclusions drawn.







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