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Far Field
Planar Near-Field Scan Plane Truncation Applied to the Measurement of Large Phased Array Antennas
C. Smith (Lockheed Martin), November 2001
An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.
Holographic Projection to an Arbitrary Plane from Spherical Near-Field Measurements
A.C. Newell (Nearfield Systems Inc.),B. Schluper (Nearfield Systems Inc.), R.J. David (The Mitre Corp.), November 2001
Holographic back-projections of planar near-field measurements to a plane have been available for some time. It is also straightforward to produce a hologram from cylindrical measurements to another cylindrical surface and from spherical measurements to another spherical surface1-7. In many cases the AUT is approximately a planar structure and it is desirable to calculate the hologram on a planar surface from cylindrical or spherical near-field or far-field measurements. This paper will describe a recently developed spherical hologram calculation where the farfield pattern can be projected on any plane by specifying the normal to the plane. The resulting hologram shows details of the radiating antenna as well as the energy scattered from the supporting structure. Since the hologram is derived from pattern data over a complete hemisphere, it generally shows more detail than holograms from planar measurements made at the same separation distance.
On the Use of Wavenumber Migration for Linear SAR Image Formation and Near-Field to Far-Field RCS Transformation
B. Fischer (AARDC),I.J. LaHaie (AARDC), J. Fliss (AARDC), November 2001
This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery [1], where it is often referred to as the “Range Migration Algorithm (RMA)”[2]. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.
Broadband Dielectric Probe for Near Field Measurements
C-C Chen (ElectroScience Laboratory),P.A. Diez (ElectroScience Laboratory), W.D. Burnside (ElectroScience Laboratory), November 2001
A novel broadband dielectric rod probe design that has the characteristics of broad bandwidth; symmetric probe pattern; low RCS; low antenna clutter and dual polarization operation is discussed. The RCS level reduces the interaction between the probe and antenna under test (AUT). The lower antenna clutter level improves the sensitivity in detecting responses from wide angles with greater time delays. During the transmission mode, the rod is excited with a broadband microwave launcher from one end. The radiation then occurs at the other terminal of the rod. Measurement results of the far-field patterns, RCS and reflection coefficient for a prototype rod probe (DRP) are presented.
Modeling of the Antenna-to-Range Coupling for a Compact Range
F. Jensen (TICRA),K. Pontoppidan (TICRA), November 2001
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Modeling of the Antenna-to-Range Coupling for a Compact Range
F. Jensen (TICRA),K. Pontoppidan (TICRA), November 2001
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Determination of the Phase Center Loci of H-plane Arrays of LPDAs
J. McLean (TDK RF Solutions), November 2001
Log Periodic Dipole Arrays (LPDAs) are widely used for certain metrology applications including site attenuation measurements. To accurately make such measurements, the location of the phase center of the antenna is required. However, the LPDA does not, in the strictest sense, exhibit a phase center. Approximate phase centers can be defined by computing the local curvature of a far-field constant-phase surface on the antenna’s principal lobe. However, because the E- and H-plane patterns are different, the phase centers computed from each pattern (or any two-dimensional cut) are not co-located at a given frequency and, moreover, track differently with frequency. An H-plane array of LPDAs with an appropriate taper can be made to exhibit very similar E and H plane patterns over a very broad frequency range. Such an antenna exhibits a much better defined phase center (the phase center still moves as a function of frequency) and is therefore much better suited for metrology applications. Here we present phase center calculations and measurements for two different H-plane arrays of LPDAs. One array is composed of two highly compressed LPDAs (ô=.88, ó=.05) fed with a corporate feed network, while the other is composed of two high gain LPDAs using the so-called “optimum” parameters (ô=.88, ó=.16) and fed with a hybrid feed network. Numerically predicted and experimentally measured results for the phase center loci are presented and compared with those of the component LPDAs.
Numerical Analysis of a Novel Tapered Chamber Feed Antenna Design
K-H Lee (ElectroScience Laboratory),C-C Chen (ElectroScience Laboratory), R. Lee (ElectroScience Laboratory), W.D. Burnside (ElectroScience Laboratory), November 2002
Tapered chambers have long been used for far-field antenna and RCS measurements. Conventional taper chambers used commercial antennas such as horns or log-period dipoles as wave launchers. One problem of this approach is the movement of the phase center associated with the antenna design. The positioning of the antenna inside the chamber is also critical. Undesired target-zone amplitude and phase distortion are caused by the scattering from the absorber walls. A novel feed antenna design for a tapered chamber is proposed here to provide broadband and dual polarization capabilities. This design integrates the absorber and the conducting walls behind the absorbers into to ensure a stationary phase center over a wider frequency range. In such a design, the dielectric constant of the absorber is utilized to maintain a clean phase front and a single incident wave at high frequencies. The conductivity of the absorber is also utilized to shape the field distribution at low frequencies. As a result, a wider frequency range can achievable for a given chamber size. One trade-off of this design is its reduced efficiency could be associated with the absorber absorption. Some simulation results from a 3-D FDTD model of a prototype design will be presented.
Measured Frequency Performance of a X-Band 4-Port Butler Matrix on a Planar Near-Field Range
J. Kemp (Georgia Tech Research Institute),G. Hampton (Georgia Tech Research Institute), November 2002
Single-beam and composite-beam performance of a 4-port X-band waveguide Butler matrix was measured on the Georgia Tech Research Institute planar near-field range for wideband frequency performance. The techniques necessary to perform accurate measurements on a broad-beamed antenna in a near-field range will be discussed, and measured far-field pattern data collected at the design frequency of 9.3 GHz are presented and compared with predicted results of the Butler matrix. In cases where the measured data and the expected results do not compare well, aperture amplitude and phase data, transformed from the near-field data, are shown as a diagnostic tool for corrections. After correction, new data at 9.3 GHz are presented for comparison with predicted results, and selected farfield pattern data collected at 8.6 GHz and 11.0 GHz are presented.
Methods to Estimate and Reduce Leakage Bias Errors in Planar Near-Field Antenna Measurements
A.C. Newell (Newell Near-Field Consultants),J. Guerrieri (National Institute of Standards and Technology), K. MacReynolds (National Institute of Standards and Technology), November 2002
This paper describes two methods that can be used to measure the leakage signals in quadrature detectors, predict the effect on the far-field pattern, and correct the measured data for leakage bias errors without additional near-field measurements. One method is an extension and addition to the work previously reported by Rousseau1. An alternative method will be discussed to determine the leakage signal by summing the near-field data at the edges of the scan rather than summing below a threshold level. Examples for both broad-beam horns and narrowbeam antennas will be used to illustrate the techniques.
FDTD Simulations for Far Field Range Evaluation
H-T Chen (Chinese Military Academy),H-D Chen (Cheng-Shiu Institute of Technology), R-C Liu (Wavepro, Inc.), T-Z Chang (Wavepro, Inc.), November 2002
The free-space VSWR technique, which involves scanning a field probe through the quiet zone area and plotting the amplitude and phase ripples over this region, is generally used for evaluating the performance of a farfield range. In this paper, this free-space VSWR technique is simulated by the finite-difference time-domain (FDTD) method to demonstrate the relationship between the ripple amplitude and the absorber reflectivity. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the simulations. The pyramidal absorbers on the walls of the far-field range are modeled by using effective layer model. That is, in the FIDELITYTM simulation setup, the absorbers are replaced with several homogeneous but uniaxially anisotropic layers. The amplitude ripples for both cases of 12-in-pyramid chamber and 18-in-pyramid chamber are presented and discussed.
FDTD Simulations for Far Field Range Evaluation
H-T Chen (Chinese Military Academy),H-D Chen (Cheng-Shiu Institute of Technology), R-C Liu (Wavepro, Inc.), T-Z Chang (Wavepro, Inc.), November 2002
The free-space VSWR technique, which involves scanning a field probe through the quiet zone area and plotting the amplitude and phase ripples over this region, is generally used for evaluating the performance of a farfield range. In this paper, this free-space VSWR technique is simulated by the finite-difference time-domain (FDTD) method to demonstrate the relationship between the ripple amplitude and the absorber reflectivity. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the simulations. The pyramidal absorbers on the walls of the far-field range are modeled by using effective layer model. That is, in the FIDELITYTM simulation setup, the absorbers are replaced with several homogeneous but uniaxially anisotropic layers. The amplitude ripples for both cases of 12-in-pyramid chamber and 18-in-pyramid chamber are presented and discussed.
Outdoor Low Frequency Bistatic Far Field Radar Cross Section Measurements
B. Schardt (NAVAIR Weapons Division),P. Liesman (NAVAIR Weapons Division), R. Young (NAVAIR Weapons Division), November 2002
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
UHF Antenna Phased Array Measurements
L.D. Poles (Air Force Research Laboratory),E. Martin (Air Force Research Laboratory), E. Wisniewski (Air Force Research Laboratory), J. Kenney (Air Force Research Laboratory), R. Wing (Air Force Research Laboratory), Ryan Thomas (Air Force Research Laboratory), James Kenney (Air Force Research Laboratory), November 2002
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
Pattern Measurement of X-Band Standard Gain Horn Antenna Using Photonic Sensor and Planar Near Field Scanning Technique
M. Hirose (National Metrology Institute of Japan),J. Ichijoh (NEC TOKIN Corporation), K. Komiyama (National Metrology Institute of Japan), S. Torihata (NEC TOKIN Corporation), November 2002
We have measured the amplitude and the phase of the electric field on a planar area very near (about 0.3 wavelengths) to the aperture of a X-band standard gain horn antenna using a photonic sensor and transformed the aperture field distribution to the far field pattern. The measured aperture field distributions and antenna patterns agreed well with those calculated by the method of moments. Comparing the far field patterns by the photonic sensor and the conventional open-ended rectangular waveguide probe reveals that the antenna measurement using the photonic sensor has advantages over the conventional probe.
How to Choose an Antenna Range Configuration
D. Gray (Nearfield Systems, Inc.), November 2002
Choosing the proper antenna range configuration is important in making accurate measurements and verifying antenna performance. This paper will describe the steps involved so the antenna engineer can select and specify the best antenna range configuration for a given antenna. It will describe the factors involved in choosing between near-field systems versus far-field systems, and the different scan types involved. It will explain the advantages of each type of antenna range and how the choices are affected by such factors as aperture size, frequency range, gain, beamwidth, polarization, field of view, sidelobe levels, and backlobe characterization desires. This paper will help the antenna engineer identify, understand, and evaluate the applicable characteristics and will help him in specifying the proper antenna range for testing the antenna.
The Theory and Architecture of a Plane Wave Generator
C. Courtney (Voss Scientific),D. Voss (Voss Scientific), L. LeDuc (Edwards Air Force Base), R. Haupt (Utah State University), November 2002
The radiation properties of an antenna are defined in the far field, since this is the environment that they will operate. Creating far field conditions when testing a large aperture antenna is quite challenging. This is particularly true if testing occurs within the confines of an anechoic chamber, or if other complicating field characteristics (like angle-of-arrival simulation) are desired. Rather than attempt to generate a true planewave in the usual manner, we propose an instrument that creates a field distribution in the near field of a transmit array that is planewave-like in nature only over specified regions of interest (a region occupied by an antenna under test, for example); we do not require that the incident field be a true planewave at other locations. In these other locations the field is free to assume any value demanded by the governing equations of electromagnetics. By relaxing the requirement on the electromagnetic field in the test volume, we considerably reduce the complexity of the problem and define a tractable problem with a potential engineering solution.
Near Field Calibration for Large Reflectors
W.L. Lippincott (Naval Research Laboratory),M. Smythers (Microstar Corporation), T. Gutwein (Microstar Corporation), Peter J. Souza (Blaise Engineering), November 2002
This paper presents a 'mid-range' calibration technique, now being developed for a 60 ft. diameter reflector site. With this technique, near-field amplitude and phase is collected at a calibration tower as the reflector scans across it. The mid-range 'near-field' data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to standard measurement techniques. A particular advantage is that the system, once set-up, can be used on a regular basis without impacting site operations.
Inaccuracy in Spherical Near Field Antenna Measurements Due to Anechoic Chamber Reflectivity
S.N. Pivnenko (Technical University of Denmark),J.M. Nielson (Technical University of Denmark), O. Breinbjerg (Technical University of Denmark), November 2002
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.
An Expanded Approach to Spherical Near-Field Uncertainty
D. Hess (MI-Technologies), November 2002
We at MI Technologies have employed the Hansen error analysis [1] developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design. I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges [2]. The result is an appropriate extension of the TUD uncertainty analysis. Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field [5]. In this paper, I offer such an extended tabulation for spherical measurements.


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