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Far Field
Improved Procedure for NFR Error at Off-Probe-Calibration Frequencies
R. Wilson (Space Systems/Loral),W. Scott (Space Systems/Loral), November 2002
Calibrated probe complex pattern data is used in planar NFR (near field range) data processing to remove the effects of the probe on the measurement. In a prior paper [1] we proposed a procedure to estimate the measurement error (uncertainty) introduced into a near field antenna radiation pattern measurement due to test frequencies that do not coincide with available calibration frequencies of the range probe. Our prior paper resulted in a “19th term” which was added to the well known NIST NFR 18 Term Error Table used to evaluate the unavoidable uncertainty of far-field radiation patterns derived from a near field scan of a given AUT (antenna under test). A limitation of this procedure, pointed out in our prior paper, is that it was most accurate for a test frequency falling midway between two nearest neighbor probe calibration frequencies. The estimated uncertainty became overly pessimistic as the test frequency of interest moved closer to one of the neighboring calibrated frequencies. The procedure is improved in the present paper by the inclusion of a new term that is a function of the test frequency and the two nearest neighbor probe calibration frequencies. Examples are shown of the use of the new procedure to obtain an improved estimate of this measurement uncertainty and to create the 19th term for use with the standard 18 Term Error Table.
Statistical Analysis of Near Field-to-Far Field RCS Transformation Performance
I.J. LaHaie (Veridian Ann Arbor Research and Development Center),D.J. Infante (Veridian Ann Arbor Research and Development Center), E.I. LeBaron (Veridian Ann Arbor Research and Development Center), P.K. Rennich (Veridian Ann Arbor Research and Development Center), November 2002
In previous AMTA presentations, we developed and evaluated an image-based near field-to-far field transformation (IB NFFFT) algorithm for monostatic RCS measurements. We showed that the algorithm’s far field RCS pattern prediction performance was quite good for a variety of frequencies, near field measurement distances, and target geometries. In this paper, we quantify the statistical RCS prediction performance of the IB NFFFT using simulated data from a generalized point scatterer model and method of moments (MoM) code, both of which allow modeling of targets with single and multiple interactions. It is shown that the predicted RCS statistics remain quite accurate under conditions where the predicted far field patterns have significantly degraded due to multiple interactions and other effects.
Chamber Design 101
G. Sanches (Advanced ElectroMagnetics, Inc.), November 2002
This paper will deal with basic rectangular chamber design and the choices that most affect the performance characteristics of a typical Rectangular Anechoic Chamber. The first and foremost criterion that needs to be addressed is “What is the chamber for”. The answer to this question is the primary driving factor regulating the overall chamber design. Is the chamber to be used to evaluate low gain, low frequency antennas? Is the chamber going to be used for RCS measurements of unique test bodies? Is the chamber going to be used to test high gain high frequency antennas? Is the chamber going to be used for far field measurements? Is the chamber going to be used for near field measurements? On and on. The answers to these very basic questions have a dramatic effect on the overall design of the anechoic chamber. Since there are so many preliminary criteria that have to be decided before we can even attempt a design I will make the following assumptions: 1) The chamber is to be a far field antenna measurement facility 2) The chamber is to operate from 2.0 Ghz to 18.0 Ghz 3) The chamber is to be of a rectangular design 4) The quiet zone is to be a 4’ diameter sphere 5) The range length is to be 20’ 6) The desired Quiet Zone performance is a. –30 dB @ 2.0 Ghz b. –40 dB @ 4.0 Ghz c. –50 dB @ 10.0 Ghz d. –50 dB @ 18.0 Ghz With these parameters we will first look at the effect that source antenna selection has on the chamber deign. The first design example will be with a low gain broadband antenna chosen as the source and the second case will be with a high gain antenna chosen as the source. This paper will detail the different design approaches that this choice has on the overall size and absorber placement in the chamber. These will have a dramatic effect on overall chamber size and cost.
The New Anechoic Test Range at NPL
P.R. Miller (National Physical Laboratory),A. Beardmore (National Physical Laboratory), D.G. Gentle (National Physical Laboratory), Edward Johnson (National Physical Laboratory), P.D. Lovelock (National Physical Laboratory), November 2002
NPL has recently commissioned a new indoor test range. This test range has been designed to offer Extrapolation Gain Measurements, Far-Field Probe Calibrations, and eventually, a Spherical Near-Field Test Capability. This paper describes this new range and the results of the initial validation measurements. It also compares the gains of a standard gain horn calibrated in NPL’s old Extrapolation Range with those from the new one.
Uncertainty Analysis for Spherical Near-Field Measurements
M.H. Francis,R.C. Wittmann, November 2003
A general approach is introduced for estimating uncertainties in far-field parameters obtained from spherical near-field measurements. Although the analysis is incomplete at present, we expect that as the measurement radius increases, our results will transform smoothly into the far-field case, where uncertainties depend on the on-axis gain and polarization of the probe and on the measurements in the far-field direction of interest.
Design of Dielectric Rod Antenna for Near-Field Probe
H. Lee (ElectroScience Laboratory),C-C Chen (ElectroScience Laboratory), K. Sickles (WPAFB/SRL), R. Lee (ElectroScience Laboratory), November 2003
A prototype design of the dielectric rod antenna is discussed. This novel design is suitable for nearfield probing application in that it provides broad bandwidth, dual-polarization and low RCS. The design details are provided in this document along with measurement data associated with important antenna characteristics such as VSWR and far-field radiation pattern
On the Estimation of Far-Field RCS From Monostatic Near-Field Data
A. Bhatia (Defence Laboratory),P. Vasistha (Defence Laboratory), R. Shejwar (Defence Laboratory), November 2003
Monostatic backscatter measurements made in the near-field have been used to generate high resolution images of complex targets; however, the appropriate use of this data for obtaining far-field RCS values needed further examination. In this paper we comment on some of the available methods, and discuss in some more detail the concept that Fourier Transform of monostatic backscatter data collected over a planar array indeed provides samples in Fourier Space directly.
Intra Range Measurement Comparisons at SNF/FF Antenna Measurement Facility of Fraunhofer IIS
R. Wansch (Fraunhofer-Institute for Integrated Circuits),P. Betjes (Nearfield Systems, Inc.), November 2003
The Institute for Integrated Circuits of the Fraunhofer Gesellschaft recently acquired a combined spherical nearfield / far-field (SNF/FF) antenna measurement range with a shielded anechoic chamber for verifying passive and active antenna design concepts. A single 9-pin digital control connector allows the range to remain sealed from external RF, while maintaining full motion and data acquisition control. This set-up uses two different illuminators, separated 180° as seen from the AUT. This combined SNF/FF configuration gives the opportunity to perform intra-range measurement comparisons (SNF vs. FF) with not only the distance between AUT and illuminator being varied, but also with the measurement zone being reversed. In this manner, a comparison between SNF and FF measurements also compares the quality of two sides of the measurement chamber.
Far-Field Range Design by Using Finite-Difference Time-Domain Method
H-T Chen (Chinese Military Academy),E. Chang (Wavepro, Inc.), November 2003
An indoor far-field range consists of the appropriate instrumentation and an anechoic chamber. In most of cases, the construction of the anechoic chamber is a laboring task and costs at a great expense. To save the money and labor, efforts for the range design are needed before the chamber been constructed. In this paper, the finite-difference time-domain (FDTD) method is employed to establish the design criteria for the far-field ranges. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the numerical calculations. To emulate the test procedure of the free-space VSWR technique, the electric fields of the points on the scanning axis are recorded during the simulation. And then, by plotting the amplitude ripples calculated from the recorded data, the range performance can be evaluated. The criteria of chamber layout, absorber arrangement, and source antenna selection and placement will be presented and discussed.
Estimating the Uncertainties Due to Position Errors in Spherical Near-Field Measurements
A.C. Newell (Nearfield Systems Inc.), November 2003
Probe position errors, specifically the uncertainty in the theta and phi position of the probe on the measurement sphere, are one of the sources of error in the calculated far-field and hologram patterns derived from spherical near-field measurements. Until recently, we have relied on analytical results for planar position errors to provide a guideline for specifying the required accuracy of a spherical measurement system. This guideline is that the angular error should not result in translation along the arc of the minimum sphere of more than ?/100. As a result of recent simulation and analysis, expressions have been derived that relate more specifically to spherical near-field measurements. Using the dimensions of the Antenna Under Test (AUT), its directivity, the radius of the sphere (the minimum sphere) enclosing all radiating surfaces and the frequency we can estimate the errors that will result from a given position error. These results can be used to specify and design a measurement system for a desired level of accuracy and to estimate the measurement uncertainty in a measurement system.
Wireless Test Cell Design Considerations
C.W. Sirles,A.R. Howland, M.H. Sewell, November 2003
This paper describes a family of new measurement systems, termed “test cells”, designed to satisfy the certification requirements of the Cellular Telephone & Internet Association’s (CTIA) “Method of Measurement for Radiated RF Power and Receiver Performance” test plan for wireless subscriber stations. These test cells employ simultaneous dual-axis mechanical scanning and operate in both far-field and near-field modes over the 750MHz to 6 GHz frequency range. Operation can be extended to higher frequencies through the use of suitable sampling antennas. Test cell facility configuration is detailed. Scanner layout and RF sampling antenna designs are discussed. Anechoic chamber characterization data is presented along with typical measured pattern and efficiency data for both broadbeam and directive AUT’s. Measurement test times for various test scenarios are discussed.
Preliminary Investigations of Cohering Distributed Aperture Measurement Data
J. Kemp (Georgia Tech Research Institute),J. Holder (Georgia Tech Research Institute), November 2003
Preliminary investigations for cohering multiple apertures into a single distributed aperture were performed at the Georgia Tech Research Institute. Data were collected on complex targets in near realtime with two individual HP8510 Network Analyzer systems controlled by a single data acquisition computer as an interferometeric measurement. The data were analyzed and presented for high-accuracy angular resolution by examining the amplitude and phase difference between the two network analyzers. In addition, further upcoming tests on the Georgia Tech Research Institute far-field range will be outlined, showing how both measured angular resolution improvement and power-aperture gain product will be collected over a wideband frequency range.
Near-Field and Far-Field Characterization of the Reflector IRA
M. Manteghi (University of California, Los Angeles),Y. Rahmat-Samii (University of California, Los Angeles), November 2003
Over the last decade there has been great interest in ultrawideband (UWB) communication systems. Ultrawideband antennas that are able to transmit or receive short pulses with no distortion are called Impulse Radiating Antennas (IRA). One of the most commonly used IRA.s consists of a parabolic reflector fed by conical transmission lines that propagate a spherical TEM wave. The reflector IRA was constructed, analyzed and measured at UCLA. A method of moments based software, Hybrid EFIE and MFIE Iterative (HEMI), is employed to simulate the antenna. The software has to be run many times for a wide frequency range. The simulation results for the current distribution on the conical coplanar feeds show that one of the arms can be used as an UWB balun and the unbalanced line can be connected to the antenna. The aperture field is studied by calculating the surface current on the reflector. These current distributions show that the aperture field is tapered from edge to center and the center part is less illuminated in comparison with the edges. This increases the side lobe level for reflector IRA. To measure the time domain characteristics of an IRA, we have to use either short pulses and a time-domain setup or many frequencies in a wide frequency band and use an inverse Fourier transformation to calculate the time-domain results. In this work, we used frequency domain measurement setup to measure the antenna characteristics. The recently constructed spherical near-field measurement chamber at UCLA is used to measure the radiation characteristics of the antenna. The far-field calculated from the near-field measured data is compared with the HEMI results. Calculated and measured results show good agreement.
Implanted Antennas Inside a Human Body: Characterization and Performance Evaluation
J. Kim (University of California, Los Angeles),Y. Rahmat-Samii (University of California, Los Angeles), November 2003
In this paper, the electromagnetic (EM) characteristics of various antennas implanted in both the human head and the human body are analyzed for biomedical applications such as hyperthermia and biotelemetry. The implanted antennas are studied in two ways: the near- and far-field patterns of the antenna are calculated and the potential effects on the human body are observed. To ensure the correctness of the results, we apply two simulation methodologies: dyadic Green’s function (DGF) expansions and finite difference time domain (FDTD). We characterize the performances of the low profile antennas designed for biomedical applications in terms of specific absorption rate (SAR), radiation patterns, maximum available power and safety issues. These results should also provide a good basis for validating the results of experimental data.
Antenna Pattern Correction for Near Field-to-Far Field RCS Transformation of 1-D Linear SAR Measurements
I.J. LaHaie (General Dynamics Advanced Informations Systems),S.A. Rice (General Dynamics Advanced Informations Systems), November 2003
In a previous AMTA paper [1], we presented a firstprinciples algorithm called wavenumber migration (WM) for estimating a target’s far-field RCS and/or far-field images from extreme near-field linear (1-D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. However, the algorithm assumes the radar antenna has a uniform, isotropic pattern on both transmit and receive. In this paper, we describe a modification to the (1-D) linear SAR WM algorithm that compensates for nonuniform antenna pattern effects. We also introduce two variants to the algorithm that eliminate certain computational steps and lead to more efficient implementations. The effectiveness of the pattern compensation is demonstrated for all three versions of the algorithm in both the RCS and the image domains using simulated data from arrays of simple point scatterers.
Antenna Pattern Correction for Near Field-to-Far Field RCS Transformation of 1-D Linear SAR Measurements
I.J. LaHaie (General Dynamics Advanced Informations Systems),S.A. Rice (General Dynamics Advanced Informations Systems), November 2003
In a previous AMTA paper [1], we presented a firstprinciples algorithm called wavenumber migration (WM) for estimating a target’s far-field RCS and/or far-field images from extreme near-field linear (1-D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. However, the algorithm assumes the radar antenna has a uniform, isotropic pattern on both transmit and receive. In this paper, we describe a modification to the (1-D) linear SAR WM algorithm that compensates for nonuniform antenna pattern effects. We also introduce two variants to the algorithm that eliminate certain computational steps and lead to more efficient implementations. The effectiveness of the pattern compensation is demonstrated for all three versions of the algorithm in both the RCS and the image domains using simulated data from arrays of simple point scatterers.
A Low-Cost Compact Measurement System for Diagnostic Imaging and RCS Estimation
R. Cioni (IDS Ingegneria Dei Sistemi SpA),A. Sarri (IDS Ingegneria Dei Sistemi SpA), G. De Mauro (IDS Ingegneria Dei Sistemi SpA), S. Sensani (IDS Ingegneria Dei Sistemi SpA), November 2003
The task of performing reliable RCS measurements in complex environments under near-field conditions is gaining more and more interest, mainly for a rapid assessment of RADAR performance of constructive details. This paper describes a low-cost compact measurement system fully developed by IDS, that allows fast and effective acquisition of diagnostic images under nearfield conditions and far-field RCS estimation in a nonanechoic environment. The hardware of the system is composed of a planar scanner, two horn antennas, a Vector Network Analyzer and a computer. The two axes scanner allows 2D scanning of antennas in a vertical plane. For each point of a predefined grid along the scanned area, the Analyzer performs a frequency scan. The acquisition software synchronizes scanner movements with data acquisition, transfer and storage on the computer’s HDD. The software has post-processing capabilities as well. A number of focusing algorithms permit to produce 2D and 3D diagnostic images of the target as well as 2D backprojection. It is moreover possible to reconstruct the RCS starting from near-field images. Along with system features, a summary of performances and some simple targets images are presented.
Characterization of a Fresnel Zone Antenna Using Bi-Polar Planar Near-Field Measurements
N.P. Sakungew (University of California, Los Angeles),Y. Rahmat-Samii (University of California, Los Angeles), November 2003
A fully-functional Fresnel Zone (FZ) antenna was designed and measured using PO simulation programs and the bi-polar near-field facility. The results from these measurements and simulation are presented in this paper. First, a detailed description of an FZ antenna and its operation is given. Then, a discussion of the design and construction procedure for both the FZ antenna and supporting structure is included. The resulting far-field pattern, near-field plots, and holographic images are shown in this paper. The antenna was measured with different feed positions to observe how it affects the overall antenna performance.
Measurement of Directive Antennas Using Rapid Probe Array Within a Spherical Near-Field Test Range
N. Robic (SATIMO SA),L. Duchesne (SATIMO SA), P. Bellocq (SATIMO USA), P. Garreau, (SATIMO SA) Per Olav IVERSEN (SATIMO USA), November 2003
This paper will discuss the capability of spherical near-field test ranges using probe arrays to measure electrically large directive antennas. More particularly, the operating range of the existing Stargate equipment in terms of antenna dimensions has been identified. The advantages of using such equipment to measure quasi isotropic antennas will be first reminded. Then a study that aims to give a limit of the dimensions of the antenna under test will be presented. The sources of error that contribute to the limits will be described. Finally it will shown how an extension of Stargate equipment can be implemented in order to increase its the capability to the measurement of directive antennas, i.e. largest in dimension. This paper will be illustrated with real measurements of directive antennas. A comparison with both probe array near-field scanner and a far-field test range will be commented.
How Far is Far Enough for System-Level Testing of DF Interferometer Arrays
N. Isman (ORBIT/FR Engineering ltd.), November 2003
The restriction of ? 2D2 R = is a commonly employed criterion for the minimum required separation between the range antenna and the Antenna Under Test (AUT) in a Far-Field (FF) antenna test range. However, this criterion, which is suitable for most common and simple cases, may not be adequate for more specialized test applications. Direction-finding (DF) interferometer antenna array testing is one such example. In a DF interferometer antenna array the phase difference between any two antennas serves as an Angle-Of-Arrival (AOA) discriminator for the radiation impinging on the array. At the system level, the array must be tested in order to calibrate its AOA discrimination function and to evaluate its accuracy, which, in many cases is done using a FF test range. In this paper, interferometer array FF testing is analyzed and an expression is developed for estimating the required separation between the range antenna and the array under test, in order to satisfy certain angle discrimination accuracy requirements. The results are compared with the common FF criterion and with restrictions imposed by other considerations.


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