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Near Field

Innovative mechanical designs for scanners
J. Demas,T. Speicher, November 1997

Nearfield Systems Incorporated (NSI) provides antenna measurement systems to domestic and foreign, commercial and government customers with sophisticated requirements that demand custom solutions for RF, mechanical, thermal or software applications. NSI is continuously adapting existing designs to seek cost effective solutions for each customer's demanding specification. This paper discusses numerous near-field scanner designs to meet a variety of applications. Presented are designs for several vertical planar scanners, horizontal scanners, tilted planar scanners, and special scanners designed to attach to structures to test antennas in-situ.

Implementation of a spherical near-field measurement system in mainland China
G. Hindman,Ye, W-B. Hanjian, November 1997

Far-field range testing has been the standard at the Southwest China Research Institute of Electronic Equipment (SWIEE) and at other facilities in mainland China. SWIEE has recently commissioned a new spherical near-field measurement system from Nearfield Systems Inc. (NSI) and Hewlett Packard (HP) to improve its antenna measurement capability. The near-field system provides significant advantages over the older far-field testing including elimination of weather problems with outdoor range testing, complete characterization of the antenna, and improved accuracy. This paper will discuss the antenna types at SWIEE tested with the NSl/HP near-field system, and the results being achieved.

Large combination horizontal and vertical near field measurement facility for satellite antenna characterization, A
J. Demas, November 1997

A large horizontal near field measurement facility has been validated and commissioned at Lockheed Martin's Sunnyvale, CA facility. The new measurement facility will be used for characterizing antennas for a variety of satellites over a frequency range of 1 26.5 GHz. A horizontal near field scanner with a 14m x 7.8m (46' x 26') effective scan area has been designed to allow for 9.8m (32') of vertical clearance permitting zenith oriented satellites to be easily positioned within the range and tested in an efficient manner. The facility will soon support the measurement of antennas that are in a vertical orientation. This is accomplished with a novel add-on that allows vertical planar near field scanning on the same range. The vertical scanner has an effective coverage area of 13.6m (45') horizontal x 9m (30') vertical. The system is being used to test commercial communications satellites.

Compensation of unknown position-induced phase errors in a driveby imaging radar
P.N.R. Stoyle, November 1997

One approach to getting near-field ISAR measurement costs down is to dispense with track or turntable, and instead mount the SAR antenna on a vehicle and simply 'drive by' the target of interest, which might be a vehicle or aircraft standing on tarmac. In this situation the antenna path will depart from a perfect straight line or circular arc, and there will also be vibrational wobble at the antenna phase center. These effects can defocus the images obtained. One way to overcome the focus problem is to mount strategically placed corner reflector(s) in front of the target, each in a different range cell. These then act as phase references, used to refocus the image. However it is not strictly necessary to employ reflectors - a good focus can normally be obtained by suitably processing just the target return itself. This paper will describe autofocus procedures which have been sucessfully used in conjunction with chirp radar data, in the 'driveby' situation.

Holographic near-field/far-field for TeraHertz antenna testing
G. Junkin,J.C. Bennett, T. Huang, November 1997

Gabor holography is an appropriate technique for near­ field measurements at THz frequencies when apertures of the order of thousands of wavelengths are involved. The method permits pattern prediction over a restricted angular range from intensity measurements, providing a direct method of recovering phase which overcomes cable, planarity and atmospheric effects; problematic to conventional near-field phase measurements. We demonstrate the feasibility and convenience of the method with an example planar near-field measurement at 94GHz for a 1.1m Cassegrain reflector and we determine the relationships governing dynamic range and the requirements for sampling. Finally, two-dimensional numerical simulations for a lm antenna at 0.5THz, with a 10m scan distance, will be presented to demonstrate the feasibility of the method for large terahertz antennas.

Alignment errors and standard gain horn calibrations
M. Dich,H.E. Gram, November 1997

The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex­ tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.

Alignment errors and standard gain horn calibrations
M. Dich,H.E. Gram, November 1997

The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex­ tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.

Effect of data coherence on a waterline bistatic near field to far field transform
M.A. Ricoy,E. LeBaron, November 1997

A waterline bistatic algorithm, based on the exact near field to far field transformation (NFFFT) and previously exercised on numerical data, is here applied to actual measured data taken at a traditional RCS range reconfigured for near field measurements. The resulting far field predictions for a lOA and 20A conesphere were initially worse than expected. Further examination of the data yielded two important observations. First, the data were found to have relative alignment errors from set to set, leading to a significant broadening of the predicted far field peaks. Second, a few data sets exhibited a constant phase offset inconsistent with the other measured data. This paper discusses the detection of the data misregistration issues highlighted above, along with their ad hoc correction. Predictions are give for the waterline bistatic NFFFT algorithm applied to the measured near field data, both before and after the corrections have been applied. The results are compared with analogous results for numerical input data.

Experiences with near field measurements of the active phased array radar PHARUS
M.H. Paquay, November 1997

Measurements of antennas with integrated electronics is an upcoming topic. In many cases the antennas can only work in pulsed mode which requires synchronisation between radar and measurement equipment. Up and down mixing by internal LO's causes additional problems, especially with Near Field measurements where amplitude and phase data is required. Based upon hands-on experience, this paper treats some of the problems and pitfalls related to the Near Field measurements of an active antenna and alignment of the elements by means of backtransformation of the data.

Technique for collecting and procesing flight-line RCS data, A
G. Fliss,J. Burns, November 1997

Recently, several deployable, ground-to-ground col­ lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.

Time domain near-field far-field transformation using optimal plane-polar sampling representation
O.M. Bucci (Universita di Napoli “Federico II”),G. D'Elia (Universita di Napoli “Federico II”), M.D. Migliore (Universita di Napoli “Federico II”), November 1996

A time domain near-field far-field transformation technique based on a non redundant plane-polar sampling representation of the field is presented. The method allows to obtain the far-field with a minimum number of samples and/or a reduction of the scanning area. Various computational schemes are presented.

Near field interferometric techniques for array antenna performance evaluation
M.D. Migliore (Universita di Napoli “Federico II”),G. Panariello (Universita di Napoli “Federico II”), November 1996

A new holographic technique for array diagnosis is discussed. Numerical and experimental results are shown in case of linear arrays and compared with the performance of the “classical” holographic approach based on FFT. Furthermore, the working progress on a sub-optimal algorithm specifically developped for diagnosis of large arrays is presented.

Phaseless measurements of antenna near fields employing holographic phase retrieval
C.F. Stubenrauch (National Institute of Standards and Technology),Katie MacReynolds (National Institute of Standards and Technology) Allen C. Newell (National Institute of Standards and Technology) Robert H. Cormack (Computational Optics) John E. Will (University of Colorado) John D. Norgard (University of Colorado), November 1996

We describe a technique which employs amplitude-only measurements of an unknown antenna combined with a synthetic reference wave to produce a hologram of a near-field antenna distribution. The hologram, which may be recorded by amplitude-only receiving equipment, is digitally processed using an enhanced theory which allows complete removal of the spurious images normally encountered with optical hologram reconstruction. The recovered near-field data are then processed using standard algorithms to calculate antenna far-fields. We present the theoretical formulation and results of measurements obtained on an 1.2 m reflector antenna.

Phaseless measurements of antenna near fields employing holographic phase retrieval
C.F. Stubenrauch (National Institute of Standards and Technology),Katie MacReynolds (National Institute of Standards and Technology) Allen C. Newell (National Institute of Standards and Technology) Robert H. Cormack (Computational Optics) John E. Will (University of Colorado) John D. Norgard (University of Colorado), November 1996

We describe a technique which employs amplitude-only measurements of an unknown antenna combined with a synthetic reference wave to produce a hologram of a near-field antenna distribution. The hologram, which may be recorded by amplitude-only receiving equipment, is digitally processed using an enhanced theory which allows complete removal of the spurious images normally encountered with optical hologram reconstruction. The recovered near-field data are then processed using standard algorithms to calculate antenna far-fields. We present the theoretical formulation and results of measurements obtained on an 1.2 m reflector antenna.

The Planar near-field measurement of an antenna tilted with respect to the scan plane
P.R. Rousseau (The Aerospace Corporation), November 1996

Planar near-field antenna measurements have developed into a mature science. Nonetheless, unique difficulties arise when measuring some modern antennas, such as high gain satellite antenna systems. In a typical planar near-field measurement, the antenna under test (AUT) has a collimated beam in the near-field which is perpendicular to the scan plane (i.e. the AUT boresight is parallel to the normal of the scan plane). On the other hand, the scan plane is positioned close to the AUT to maximize the valid angular range in the far-zone patterns. Unfortunately, it is not always possible to place the AUT very close to the scan plane and keep the near-field beam perpendicular to the scan plane. An investigation of the benefits and pitfalls of a planar near-field measurement where the AUT beam is not perpendicular to the scan plane is presented. The measurements of antennas tilted 45 degrees with respect to the scan plane normal are used as examples. With this atypical arrangement, some of the usual errors in a near-field measurement are emphasized. Procedures to identify and reduce these errors will be presented.

Planar near-field antenna measurements using non-ideal measurement locations
R.C. Wittmann (National Institute of Standards and Technology),B.K. Alpert (National Institute of Standards and Technology), M.H. Francis (National Institute of Standards and Technology), November 1996

The standard planar near-field to far-field transformation method requires data points on a plane-rectangular lattice. In this paper we introduce a transformation algorithm in which measurements are neither required to lie on a regular grid nor are strictly confined to a plane. Computational complexivity is O (N log N), where N is the number of data points. (Actual calculation times depend on the numerical precision specified and on the condition number of the problem.) This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable for other measurement approaches, such as plane-polar scanning, where data are collected on a non-rectangular grid.

Windows 96 for planar near-field measurements
E.B. Joy (Georgia Institute of Technology),C. Rose (Georgia Institute of Technology), November 1996

This paper reports on the results of computer simulations of planar near-field test-zone-fields. Techniques for the improvement of the quality of the test fields are presented and demonstrated. These techniques include the use of larger scan areas and the use of window functions applied to the measured near-field data. Test-zone-field quality is measured by the angular spectrum of the error of the test-zone-field as compared to an ideal plane wave test-zone-field. This investigation sought the minimum scan length, L, for a given critical angle, ?c and separation, S. It is shown that significant improvements in test-zone-field quality can be realized if the test zone is extended from the standard length, Ls=D+2S(tan(?c)) by an amount 20?/cos(?c). This scan length is approximately 30? larger, for a critical angle of 50 degree and 60? larger, for a critical angle of 70 degrees, than the standard length. A raised cosine amplitude/quadratic phase window applied to the measured near-field data can significantly reduce scan length requirement while maintaining the increased accuracy of the extended scan length. The recommended scan length with window is given by Lw=D+2S(tan(?c))+2W, where W is the length of the window applied to each end of the scan measurements. The window description and required length are presented.

Efficient near-field measurements of antennas, radomes, and scattering targets via the modulated scattering technique
B. Cown (SATIMO),J.P. Estrada (Georgia Tech) Ph. Garreau (SATIMO) D. Picard (SUPELEC) J. Ch. Bolomey (SUPELEC), November 1996

This paper summarizes the state of the art for using one-dimensional and two-dimensional arrays of modulated scattering elements to rapidly measure the near-field electromagnetic fields 1) radiated by antennas with or without radomes and 2) scattered by targets located in free-space or buried in lossy dielectric media. The application of rapid near-field scanning via measurement arrays based on the Modulated Scattering Technique (MST) in both France and the U.S. is discussed in this paper.

Planar, time domain, near-field measurements
A. Dominek (Analytic Designs, Incorporated),H. Shamansky (Analytic Designs, Incorporated), November 1996

In this paper, a near-field time domain radiation measurement is described, similar to the traditional frequency domain near-field radiation measurement. This time domain measurement approach borrows many of the principles developed in the frequency domain and is ideally suited for the measurement of broadband devices. The goal of determining the radiated far-fields of an antenna is accomplished by the transformation of near-field data collected over a planar sampling surface. The near-fields are generated with an antenna excited by a short duration transient pulse. In particular, the near-fields of an aperture antenna are collected using a digital sampling oscilloscope. The bandwidth of the excitation pulse is approximately 10 GHz.

Performance analysis of the image-based near field-to-far field transformation
I. LaHaie (ERIM),E. LeBaron (ERIM), November 1996

At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.







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