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Far Field

Near-field to far-field transformation using an equivalent magnetic current approach
P. Petre (Syracuse University),T.K. Sarkar (Syracuse University), November 1992

An alternate method is presented for computing far-field antenna patterns from planar near-field measurements. The method utilizes near-field data to determine equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna, and these currents are used to ascertain the far-fields. An electric field integral equation (EFIE) is developed to relate the near-fields to the equivalent magnetic currents. Method of moments (MOM) procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method (CGM), and in the case of a rectangular matrix, a least squares solution for the currents is found without explicitly computing the normal form of the operation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions by exploiting the block Toeplitz structure of the matrix and using CGM and Fast Fourier Transform (CGFFT) thereby drastically reducing comparison and storage requirements. Numerical results are presented by extrapolating the far-fields using experimental near-field data.

Theoretical comparison of modal expansion and integral equation methods for near-field to far-field transformation
P. Petre (Syracuse University),T.K. Sarkar (Syracuse University), November 1992

A theoretical comparison for the application and derivation of modal expansion and integral equation methods is presented. It is shown that one formulation can be transformed into the other one using Fourier transform. From this point of view it can be stated that both method solves the same integral equation but for the modal expansion approach the integral equation is solved in the spectral domain while for the integral equation method the same equation is solved in the space domain. It is shown that for most of the practical antenna types the integral equation method gives more accurate far-field estimation than the modal expansion method, particularly in the planar scanning case.

A Practical technique for near field antenna testing
H. Tobin (USAF Rome Laboratory),J. Simmers (USAF Rome Laboratory), P.R. Franchi (USAF Rome Laboratory), November 1992

In recent years different techniques have been developed for measuring large aperture antennas on smaller ranges. Problems still exist with these techniques, though, such as impracticality and size restrictions. This paper presents a new method for measuring a phased-array antenna at approximately one tenth the far-field distance. This method involves focusing the test array to a probe a certain distance away, then moving the probe along an elliptical path. Since different elliptical paths can be easily generated with the same test hardware, this new method promises to yield a measurement technique that can be readily adapted to different sized antennas. This paper also presents the results of computer simulations showing the validity and limitations of this technique.

Accurate planar near-field probe correcion using dual-port circularly-polarized probes
J. Guerrieri (National Institute of Standards and Technology),A. Repjar (National Institute of Standards and Technology), D. Tamura (National Institute of Standards and Technology), November 1992

When the planar near-field method is used for antenna characterization, two probes are required to measure an antenna under test (AUT). The receiving patterns (both amplitude and phase) of these probes, obtained from planar near-field measurements, must be utilized to accurately determine the far field of the AUT. This process is commonly called planar near field probe correction. When the AUT is nominally circularly polarized (CP), the measurements are more accurate and efficient if nominally circularly-polarized probes are used. Further efficiency is obtained when only one probe which is dual-polarized is used to allow for simultaneous measurements of both components. However, when using dual-port CP probes to measure the antenna, we must apply the probe correction even for on-axis measurements.

Validation testing of the planar near-field range facility at SPAR Aerospace Limited
W.K. Dishman (Scientific-Atlanta, Inc.),S.J. Manning (Scientific-Atlanta, Inc.), November 1992

A series of measurements to validate the performance of a Planar Near-Field (PNF) Antenna Test Range located at the Satellite and Aerospace Systems Division at Spar Aerospace Limited were made by Scientific-Atlanta during the month of February 1992. These measurements were made as a part of a contract to provide Spar with a Model 2095 Microwave Measurement System with planar near-field software options and related instrumentation and hardware. The range validation consisted of a series of self-tests and far-field pattern comparison tests using a planar array antenna provided by Spar that had been independently calibrated at another range facility. This paper describes the range validation tests and presents some of the results. Comparisons of far-field patterns measured on the validation antenna at both the Spar PNF facility and another antenna range are presented.

The UCLA bi-polar near-field range: processing techniques and measurement comparisons
L.I. Williams (University of California, Los Angeles),R.G. Yaccarino (University of California, Los Angeles), Y. Rahmat-Samii (University of California, Los Angeles), November 1992

A novel planar near-field antenna measurement and diagnostic system is described. This bi-polar near-field system offers a large scan plane size with reduced "real estate" requirements and a simple mechanical implementation resulting in a highly cost effective antenna measurement system. A brief description of the bi-polar near-field range and its associated data processing methods are given. Measured results are compared with those obtained on a far-field range and a plane rectangular planar near-field range. It is shown that the UCLA facility produces highly accurate results which rival those of modern production antenna measurement facilities. Holographic images produced from measured data are provided to demonstrate the diagnostic capabilities of the antenna range and to provide electromagnetic field visualization for educational purposes.

Quasi real time antenna testing by means of a 2D modulated scattering array in the focal plane of a compact range
P. Garreau (SATIMO France),Kees Van't Klooster (ESA-ESTEC The Netherlands) J.Ch. Bolomey (SUPELEC France), November 1992

This paper presents the feasability (sic) to explore the Focal Plane (FP) of a Compact Antenna Test Range (CATR). We first introduce the interest of getting very fast the Far Field Pattern of an antenna with a 2D modulated scattering array located at the focus of a CATR. Then, we discuss the geometric, electrical and optical constraints involved when using this technique. A comparison with a classical measurement performed at ESA-ESTEC is shown and we conclude by emphasizing the potentialities of this technique.

Error simulation, estimation and correction in probe corrected planar near field antenna measurements
A. Lopez (Polytechnic University of Madrid ),J. Molina (Polytechnic University of Madrid ), J.L. Besada (Polytechnic University of Madrid ), November 1992

A Planar Near Field to Far Field (PNF/FF) Transformation Program has been developed. This PNF/FF package includes probe correction, spectral filtering, position errors correction and sampled data expansion. In order to evaluate how measurement system errors affect PNF/FF transformation results, a whole set of simulation routines have also been implemented. In this paper, main modules of the PNF/FF package are discussed and error simulation models together with correction routines are described.

Antenna testing by phaseless near zone data: experimental results in the cylindrical case
R. Pierri (2nd University of Naples),G. D'Elia (University of Naples) T. Isernia (University of Naples) G. Leone (University of Salerno) P. Langsford (GEC Marconi Research Center), November 1992

A new near-field far-field transformation procedure, based on only amplitude measurement, is tested from both simulated and measured data. The measurements have been collected at Marconi Research Center and refers to a parabolic reflector working at 9 Ghz. This first experimental validation of the procedure fully support (sic) the feasibility of phaseless near field measurement in the antenna testing.

Refurbishment of the TUD-ESA spherical near field antenna test facility
J. Lemanczyk (Technical University of Denmark),J.E. Hansen (Technical University of Denmark), November 1992

The anechoic chamber housing the TUD-ESA Spherical Near Field Far Field Antenna Test Facility at the Technical University of Denmark dates back to 1967 while the present RF and data collection and control systems were designed and installed in several stages between 1978 and 1985. This paper undertakes to describe the definition and realization of a refurbished and upgraded radio anechoic facility for antenna measurements given as a starting point the already existing facility. In a parallel effort, both the RF and data collection and control subsystems are being renewed and upgraded.

A High Speed Fiber Optic Remote Receiver Link for Improved Antenna Measurements
Gerard J. Matyas (ORBIT Advanced Technologies, Inc.), November 1992

The remote capability of the ORBIT AL-8000-5 Microwave Receiver is described. The use of a high speed fiber optic link between the remote receiver and the control room unit allows range distances of up to 19,000 feet. With repeaters, the range distance limitation is removed. This eliminates many of the distance cable and EMI problems associated with receivers which use a remote LO. The small size and weight of the remote unit, allow the system to be mounted on the probe carriage of near-field scanner systems. This eliminates the high frequency phase errors as well as the phase error due to cable bending and temperature variation during the measurement. The result is a lower cost and more accurate measurement system. The advantages of this type of remote system are discussed for both near-field and far-field applications. Measurement data which show the performance of the fiber-optic system are presented. A description and pictures of recent installations are to be provided.

Antenna range performance comparisons
E.H. England (Defense Research Agency),H. Hezewijk (TNO Labs) J. Bennett (University of Sheffield) N. Williams (ERA Technology Ltd.), November 1991

The radiation patterns of a low (40dB) sidelobe antenna have been measured on a variety of antenna test ranges including Near Field, Far Field and Compact versions. Originally intended to validate new Near Field Ranges, some of the early results will be presented and the variations examined. The need for some form of range validation is shown. There is also some explanation of the fundamental effects that various ranges have on results.

A Methodology for diagnostics and performance improvement for large reflector antennas using microwave holography
D.J. Rochblatt (California Institute of Technology), November 1991

Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.

Measurement techniques for cryogenic KA-band microstrip antennas
M.A. Richard (Case Western Reserve University),K.B. Bhasin (NASA Lewis Research Center) C. Gilbert (Ball Communications Systems Division) S. Metzler (Ball Communications Systems Division) P.C. Claspy (Case Western Reserve University), November 1991

The measurement of cryogenic antennas poses unique logistical problems since the antenna under test must be embedding in the cooling chamber. In this paper, a method of measuring the performance o cryogenic microstrip antennas using the closed cycle gas-cooled refrigerator in a far field range is described. Antenna patterns showing the performance of gold and superconducting Ka-band microstrip antennas at various temperatures are presented.

Ship mounted antenna measurements using GPS
Millington. T.A. (Southwest Research Institute),J.H. Nixon (Southwest Research Institute), R.W. Robinson (Southwest Research Institute), November 1991

Antenna amplitude and phase pattern measurements on combat ships and other large ships have typically relied on traditional methods which include circling a fixed buoy in the far field, tracking a shore-based transmitter with an optical device, or circling the subject ship with a smaller boat outfitted with a transmitter. These techniques required the measurement of many independent variables using less than precise methods to compute antenna patterns relative to the ship’s structure. Using the global positioning system to precisely locate the ship relative to the transmitter site location and combining this with the ship’s heading, antenna measurements can be accurately and quickly obtained. This paper will describe the traditional fixed buoy and optical follower techniques and contrast these against the more accurate and faster GPS antenna measurement technique.

Antenna measurements for advanced T/R module arrays
J.S. DeRosa (Rome Laboratory), November 1991

Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.

Compact range bistatic scattering measurements
E. Walton (The Ohio State University ElectroScience Laboratory),S. Tuhela-Reuning (The Ohio State University ElectroScience Laboratory), November 1991

This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.

On the errors involved in a free space RAM reflectivity measurement
F.C. Smith (University of Sheffield),B. Chambers (University of Sheffield), J.C. Bennett (University of Sheffield), November 1991

Edge and corner diffraction and non-planewave illumination both cause measured free space relativity data to deviate from the infinite sample/planewave result which is predicted when using the Transmission Line Methos (TLM) for planar surfaces. The amount by which each of the two factors perturbs the measured data depends on the measurement system used; compact ranges, near field focused antennas and far field antennas on an NRL arch are all susceptible to the effects of non-planewave illumination and perimeter diffraction. Perimeter diffraction is virtually eliminated in the case of a near field focused system or where the sample is semi-infinite; however, the truncated illumination inevitable yields additional angular planewave components. In a far field system, the quadratic phase variation at the sample surface is shown to cause significant errors in the depth of resonant nulls. A uniform illumination is required to accurately map the depth of resonant nulls, but the consequent perimeter diffraction causes errors in null position. Perimeter diffraction does not cause errors in the null depth providing the illumination in uniform.

Error budget performance analysis for compact radar range
M. Arm (Riverside Research Institute),L. Wolk (Riverside Research Institute), R. Reichmeider (Riverside Research Institute), November 1991

The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.

Range field compensation
D.N. Black (Georgia Institute of Technology),E.B. Joy (Georgia Institute of Technology), M.G. Guler (Georgia Institute of Technology), R.E. Wilson (Georgia Institute of Technology), November 1991

The accuracy of antenna measurements can be improved by compensating for the effects of extraneous fields present in an antenna range using analytical compensation techniques. Range field compensation is a new technique to provide increased measurement accuracy by compensating for extraneous fields created by refection and scattering of the range antenna field from fixed objects in the range and by leakage of the range antenna RF system from a fixed location in the range. The range antenna field must be the dominant field in the range, and the range field cannot change for different AUTs. Existing compensation techniques are limited in the amount of compensation they can provide. The range field is measured over a spherical surface encompassing the test zone using a low gain probe. The measured range field is used in subsequent antenna measurements to compensate for the effects of extraneous fields. This technique is demonstrated using measurements simulated for an anechoic chamber far-field range.







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