Effect of data coherence on a waterline bistatic near field to far field transform
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.
Technique for collecting and procesing flight-line RCS data, A
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 . The techniques are described and demonstrated on numerically simulated and experimentally measured data.
Advanced MST Probe Arrays for Rapid Diagnostic Imaging
Electromagnetic field measurement systems based on the Advanced Modulated Scattering Technique (A MST) permit fast and accurate diagnostic testing to be performed in the near-field (NF) or the far-field (FF) of antennas and scattering objects. A-MST probe arrays are particularly effective for rapid diagnostic testing applications where it is desired to obtain overall measurement duration reductions of 80% to 98% compared to conventional single-probe measurement times.
Full Scale Automotive Antenna Near Field and Far Field Range
This paper describes an antenna measurement system which combines three types of measurements into one integrated range operating from 45 MHz through 18 GHz. A vehicle can be measured at short ranges inside a protective enclosure at various elevation angles for both low frequency Far Field (FF) measurements and higher frequency Near Field (NF) measurements. The vehicle can also be measured on an extended FF 120m range by radiating through the transparent enclosure.
The vehicle enters a 12m radius radome and is mounted on a 6m diameter turntable which enables continuous rotation of the vehicle at a maximum speed of 3 rpm. An elevation positioner moves a lOm arm equipped with linear and roll axes at the top, which provide the NF probe movement. Azimuth rotation of the vehicle and elevation movement of the arm provide a complete hemispherical scan. During FF measurements from outside of the radome, the arm is stored below ground level and is covered.
Time Domain Near-Field Scattering Measurements
In this paper, a near-field time domain scattering measurement technique is described. Near-field measurements are typically performed for radiation applications but not scattering applications. This time domain measurement approach borrows from many of the principles developed in the frequency domain and is ideally suited for broadband scattering characterization.
The goal of determining the scattered far-fields of a structure is accomplished by the transformation of near-field data collected over a planar sampling surface. The scattered near-fields were generated with a probe excited by a fast rise time step. In particular, the near-fields were sampled with a second probe and digitized using a digital sampling oscilloscope. The bandwidth of the excitation pulse was approximately 15 GHz.
The overall accuracy of this approach is examined through a comparison of the transformed far-field pattern to a numerical calculation.
Far-Field to Near-Field Test Comparison Results for Evaluation of Test Article
Over the past six years the Navy has developed a portable measurement capability. As part of the validation of this tool a comparison test was developed to understand the issues involving testing complex targets in a near-field cluttered environment. The test was designed to evaluate not only the effects of near field curvature, but how clutter from ceiling and walls have an effect on the accuracy of the measurement.
The test measured all test objects in the far-field as a baseline, then repeated the same measurements at five different near-field configurations. The results of the test will be shown on a simple 15 ft. pole target, along with the metrics for evaluation of the results.
Physics Based Modeling of Target Signatures
The scattered field from an arbitrary target may include a variety of scattering mechanisms such as specular and diffraction terms, creeping waves and resonant phenomena. In addition, buried within such data are target-mount interactions and clutter terms associated with the test environment. This research presents a method for decomposing a broadband complex signal into its constituent mechanisms. The method makes use of basis functions (words) which best describe the physics of the scattered fields. The MUSIC algorithm is used to estimate the time delay of each word. A constrained optimization refines the estimate and determines the energy for each. The method is tested using two far-field radar cross section (RCS) measurements. The first example identifies targetmount interactions for a common calibration sphere. The second example applies the method to a low observable (LO) ogive target.
DATE - Depot Aperture Tuning Equipment for the ERIEYE Airborne Early Warning System
DATE is a portable, rapid assembled, planar near field measurement system for ERIEYE Airborne Early Warning System. DATE shall be used both as a production range at Ericsson Microwave Systems (EMW) and as a maintenance equipment delivered with the ERIEYE AEW System.
Up to now ERIEYE has been measured and phase aligned at EMW's large nearfield range. The active antenna is interfaced through a Beam Steering Computer (BSC) and hardware interface. The disadvantages with this approach is a slow communication speed and reduced Built In Test.
Since the large nearfield range is designed to meet the requirements from many different antenna types the transport, mounting, alignment and range error analysis are very time and personnel consuming.
The DATE-scope is to provide a portable planar near field test system that's custom-made for ERIEYE. The time from stored system to completed measurement shall be very short and performed by a "non antenna test engineer". This is done by: • Incorporate the BSC as a radar-mode.
• Use the radar receiver and transmitter for RF measurement.
• Reduce alignment time and complexity by a common alignment system for antenna and scanner. Scanner alignment for very high position accuracy.
• Automatic Advanced Data Processing: Transformation from near field to far field to excitation to new T/R-module setting-up-table in one step.
Probe Calibration Using Time Domain Gating and Off-Bench Optical Alignment
Probe correction is required to accurately determine the far-field pattern of an antenna from near-field measurements. At Raytheon Primary Standards Laboratory (PSL) in El Segundo, CA, data acquisition hardware, instrument control software, and a mechanical positioning system have been developed and used with an HP Network Analyzer/Receiver system to perform these measurements. Using a three antenna technique, the on-axis and polarization parameters of a linearly (or circularly) polarized probe are calibrated. The relative far-field pattern of the probe is then measured utilizing the two nominal, orthogonal polarizations of the source antenna. All measurements are stepped in frequency and use a time domain gating technique. The probe and the source antenna are optically aligned to the interface and unique, kinematic designed interface flanges allow repeatable mounting of the antennas to the test station.
Experimental Validatoin of a New Technique to Reduce the Truncation Error in Near-Field Measurement Techniques
The technique is based on a non-redundant and non-uniform representation of the near-field on the measurement plane and performs an estimation of the fields samples outside the measurement region. Thanks to the non-uniformity distribution of the samples, also the estimation of a limited number of them allows a significant improvement in the far field reconstruction. The numerical and experimental investigation presented in this paper confirms the effectiveness and flexibility of the technique, which requires a low computational effort.
Locating Defective Array Modules Using Planar Near-Field Measurements
The backtransformation in (planar) Near Field processing is often claimed to be a very powerful tool for antenna diagnostics. Less known is a kind of defocusing effect which is introduced by the processing. Selecting the visible space in the Far-Field domain has a similar effect as a bandfilter in the frequency domain of an electric signal. In that analogous case it is better known that after the transform to the time domain, one has to deal with sin(x)/x behavior, limiting the resolution. The mathematics and convolution effects of both the onedimensional time-frequency transform as the two dimensional Near-Field Far-Field transform will be explained. Some measurement procedures are proposed, including S/N requirements. It turns out that the back transformation technique has some nasty properties which limit the use for alignment purposes. Some alternatives are discussed.
Measurement of the Sirius 2 Telecommunication Satellite Antenna
The Sirius 2 telcommunication satellite was build in France by Aerospatiale. As a subcontractor Saab Ericsson Space (SES) developed the telecommunication antenna for direct television broadcast. The satellite was successfully launched November 13, 1997.
Three antennas were manufactured by SES: a quality model (QM), a flight model (FMl) and a flight spare (FM2). Each antennas consists of a 1.4 meter in diameter shaped main reflector fed by a shaped subreflector and a dual polarized feed horn. For the test of the antennas, spherical near-field antenna test ranges located at Ericsson Microwave System (EMW)/SES in Sweden and at the Technical University of Denmark (DTU) were used. Each of the three antennas was measured twice. Between the two measurements mechanical and thermal tests were performed.
The paper presents the measurements on the satellite antennas together with a discussion of the advantages of using the spherical near-field technique for this type of measurements.
Compared to a far-field range the advantages are evident: At both SES and DTU a measurement distance of ten and six meters respectively were used on the indoor ranges. On a far-field range a measurement distance in the excess of 250 meters must be applied. To decrease the measurement time the near fields were only measured in a certain region on the near field sphere. The influence of this truncation will be discussed. Coordinate systems for the antennas were defined using mirror cubes. The RF measurements as well as the optical measurements on the cubes were performed without dismounting the antenna from the antenna positioner.
The radiation patterns are therefore precisely decined with respect to the coordinate systems of the cubes.
Measurement and Analysis of a Microstrip Patch Array Antenna
This paper is concerned with the measurement and analysis of a circularly polarized, flat plate patch array receiving antenna at 12.5 GHz. Input impedance and far field pattern measurements of the antenna over the frequency band from 10 to 15 GHz were performed. The small Compact Range (CR) facility of the Ohio State University Electro Science Laboratory OSU/ESL was used to measure the gain pattern. Gain pattern measurement of the antenna was done by using the gain comparison method. A broadband (2-18 GHz), constant phase pyramidal horn antenna was used as a reference. The data were analyzed to determine the radiation efficiency of the antenna.
Demonstration of Plane Wave, Pattern Subtraction, Range Compensation
Ths paper demonstrates the plane wave, pattern subtraction method for performing range compensation of full-sphere antenna patterns measured on a fixed line-of-sight far-field range. The range field is measured on the surface of a sphere and a plane wave model of the range field illuminating the antenna under test (AUT) is determined. The range compensation algorithm uses information contained in both the plane wave model and the AUT pattern measurement to estimate the error pattern that is added to the measured AUT pattern by an extraneous source. This estimated error pattern is subtracted from the antenna pattern measurement to obtain a compensated pattern. The compensated pattern and estimated error pattern are improved iteratively. This paper demonstrates the technique using measured data. The AUT is measured in a far-field anechoic chamber that contains a secondary horn antenna located 20 degrees off-axis from the range antenna, which is used as an extraneous source. The AUT is a 474 element planar array operating at a frequency of 9.33 GHz.
Crosspolar Correction in Far-Field and Compact Range Antenna Measurements
Offset parabolic reflector Compact Ranges are limited for cross polarization measurements in comparison to compensated dual reflector systems. This means that, in some cases, the crosspolar measurements at low levels show a significant content of the compact range reflector cross polar. An investigation has been carried out at INTA to reduce the crosspolarization measurement errors levels to those of a compensated dual reflector system by the application of vector deconvolution techniques. Results are shown of the validation of the algorithm in a far-field range where a crosspolar field is introduced by depointing the transmitter antenna.
Dual Compact Range for Bistatic Signature Exploitation
We are developing a new indoor bistatic measurement technique for scale model targets. This procedure will collect far-field data at bistatic angles from 60° to nearly 180° and near-field data over a 10' high, 10' radius cylinder surrounding the target. A stationary parabolic reflector illuminates the target while a duplicate parabolic reflector, rotated to its bistatic position, acquires far-field data. The independent, concentrically mounted near-field scanner gathers comparison data. Most compact range reflectors employ shaped edges to avoid edge diffracted signals entering the measurement volume. We report results of using shaped absorber material over otherwise unmodified reflector edges to reduce diffraction. High-resolution 3D images of sample structures demonstrate the practicality of this approach.
Multifunctional Anechoic Chamber for Near/Far Field Antenna Measurement and EMC/EMI, A
Due to the high cost of constructing anechoic chamber, the multi-usage of a chamber in various applications is very effective in terms of cost as well as space. In this paper, we describe an anechoic chamber, currently used at SK Telecom in Korea. This is designed for the measurements of both far/near field antenna and EMC/EMI in the identical chamber.
This anechoic chamber and measurement system support antenna test in the frequency range of 150 MHz to 40 GHz and satisfy the requirement of ANSI C63.4 and CISPR16.1for EMC/EMI. The near field measurement system supports planar, cylindrical and spherical methods to test various types of antennas. For the far field and EMC/EMI measurement, the planner near field scanner is hidden by movable absorber wall. The AUT positioner is foldable and can be stored under the chamber floor. Brief description of the chamber and the measurement system with measured results are also provided.
Development of Standardized Procedures for Antenna Measurement Ranges
The DREO-DFL Antenna Research Lab (DDARLing), contains far-field and planar near-field antenna measurement ranges. Measurements can be made on both ranges from 1.0 to 62.5 GHz.
In the early implementation stages of our antenna measurement ranges, most of our energy was absorbed in mastering the mechanics of the positioners and the intracies of the operation of the software, and addressing component failures. To make useful measurements, it is necessary to minimize system errors. Early experience and frustration has led us to the development of an ordered series of standardized procedures that are aimed at careful set-up, calibration, and operation of the ranges. Within these procedures, attention is paid to the identification and minimization of errors due to alignment, equipment calibration, linearity, leakage, multipath, and drift. Following a brief description of the two ranges in the DDARLing facility, the paper provides details of one of these procedures.