Interactions Between Probe Arrays and Antenna Under Test in Cylindrical and Spherical Near-Field Test Ranges: Numerical Assessment and Compensation Schemes
While probe arrays are now recognized to allow rapid and accurate near-field measurements, the interaction with the Antenna Under Test (AUT) is still sometimes considered as a potential limitation, especially for electrically large directive antennas . Based on numerical simulations, this paper reports the results of a thorough investigation of the interaction mechanism and analyses its impact on the far-field pattern accuracy. The most often, interaction effects can be maintained at an acceptable level, thanks to an appropriate design of the probe array element and structure. However, the efficiency of a posteriori compensation schemes has also been investigated. The Pattern Coherent Averaging Technique (PCAT) , which is well known for compensating plane wave deviations in the quiet zone of antenna far-field test ranges or interactions from single probe near-field facilities, also proved very efficient to reduce the interaction effects with a probe array.
Readily Made Comparison Among the Three Near-Field Measurement Geometries Using a Composite Near-Field Range
In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software.
Accurate alignment of the range is enabled by a tracking laser interferometer. This composite nearfield scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coodinate systems. Comparison data are presented.
Antenna Beam Shape - Effects on RCS Measurement
This work is part of the UK Ministry of Defence initiative to examine causes of uncertainties in RCS measurements and to establish a network of certified facilities.
Having developed a ‘best practice’ guide where causes of uncertainty were listed, the effect of polar diagram was selected as a priority topic.
Correction algorithms for RCS measurements require knowledge of the beam shape and resolution in crossrange of the significant scatterers. Accordingly, the accuracy of polar diagram measurement, the effect of amplitude ripple and the applicability of the correction algorithms to near-field data were addressed.
Measurements were made on two targets, a long cylinder and a small aircraft.
Two antennas and two ranges were used to achieve 1dB, 3dB and 6dB illumination tapers across the cylinder. The 6dB taper situation was modelled for three different numbers of points. The work demonstrated that polar diagram effects are significant for point scatterers or simple targets, like the cylinder; however, for the small aircraft with a large number of distributed scatterers, the overall effect is less significant.
Test and Assessment of a Direction Finding Antenna Measured on the Nose of an MH-47A Helicopter
One basic Direction Finding (DF) technique for Radar is Amplitude Based Comparison DF. Multiple directional antennas are placed around an aircraft to get a 360 deg view of the area. By placing these antennas on the aircraft, the antennas are subjected to reflections from the aircraft, which distorts the antenna characteristics. This antenna distortion causes errors in the measurement of the angle of arrival.
The work presented here describes the measurement of the antenna characteristics of a cavity backed spiral antenna both by itself and attached to the nose of an MH- 47A helicopter nose measured in an anechoic chamber.
The spiral antenna’s pattern was changed when it was measured on the helicopter. The effect this change in pattern has on the DF accuracy is discussed.
Data Comparison of Two Reflectivity Arches
Abstract Standardization, accuracy and uncertainty are important considerations to the electromagnetic material measurement community. Test requirements, available hardware and material sample limitations can all add variance to each of these factors. This paper presents comparative data from the Boeing-Tulsa and Boeing-Philadelphia RF reflectivity arches for the purpose of illustrating a process of system performance verification. This initial study is intended to foster discussion within the community and to better understand discrepancies among the various test systems.
A Feed Scanning Based APC-Technique for Improving the Measurement Accuracy in a Sub-MM CATR
It is vital for many future scientific remote sensing satellite missions to develop accurate measurement techniques for high-gain sub-mm wave antennas. At microwaves and longer millimeter wavelengths, the measurement techniques are well established and several error compensation methods have been introduced. This paper proposes a novel error compensation technique suitable for compact antenna test ranges (CATRs) at sub-mm wavelengths. The method is based on antenna pattern comparison (APC). In the APC-technique, several antenna patterns are recorded at different positions in the quiet-zone field and the corrected pattern is obtained by averaging the measured patterns. In the proposed technique, the relatively small feed antenna of the CATR is moved instead of moving the heavy combination of the antenna under test (AUT) and the rotation stage. This is much easier to accomplish. The applicability of the proposed method is studied and the method is demonstrated by a combination of quiet-zone measurements and simulations of the antenna measurements in a hologram based compact antenna test range at 310 GHz. For verification purposes the results with this method is compared to the results with the conventional APC-technique.
New Network Analyzer Methodologies in Antenna/RCS Measurements
This paper is designed to illustrate the technical advances in Network Analyzers and how they can be effectively utilized in an RCS test range. The Hewlett-Packard 8530A [1 - 4] has been utilized in antenna test ranges since the 1980’s and will be used as a reference comparison. Advances in network analyzer hardware and software provide increased functionality, speed and accuracy for RCS measurements. A typical RCS full polarization matrix imaging measurement will be used to illustrate these advances in technology. Range gating, digital and down-range resolution and alias-free range topics will be discussed illustrating the technical advances that can be utilized in an RCS test range. Flexibility of network analyzer hardware will also illustrate the effectiveness of reducing measurement hardware complexity resulting in an increase in measurement speed and accuracy.
RCS measurement Errors Associated with Calibration Spheres on Foam Columns
There is a trend within the RCS community to use squatty cylinders in place of spheres for calibration. A higher degree of accuracy can be achieved; however, cylinder calibrations require much more precision in the alignment procedures. This effort is doubled when the dual calibration target is also a cylinder. The dual calibration test article could be a sphere thus reducing calibration efforts as long as good correlation exists between theory and measurement sphere data. A series of measurements were collected at the NASA Langley Research Center Compact Range Pilot Facility to study measurement errors of spheres atop foam columns to determine their feasibility for dual calibration use.
A Sphere String Reel Calibration Technique for Improved RCS Measurements
In recent years the need for higher quality RCS calibrations has lead to several different calibration technique investigations, such as squat cylinders, bi-cones and hybrids of both. A desirable calibration technique requires: easy implementation, a known theoretical or calculable solution and minimal interaction. The sphere as a calibration target satisfies two of the three requirements. It has no alignment issues and can be easily calculated, but the sphere-holder interaction introduces several dB of error. To reduce this interaction error, a 3D string-reel support system has been developed and demonstrated that significantly improves sphere calibration accuracy. The string-reel sphere positioning system utilizes low dielectric and highly swept strings to achieve minimal calibration error. An additional benefit of this technique allows for field probing and quick quiet zone evaluations.
Active Antenna Measurement System with High speed Time Synchronization
Phased arrays antennas are designed to control their radiation characteristics by accurately setting the phase and amplitude distribution of the elements. Inaccurate control of the phase and amplitude can significantly alter the radiation pattern of an array. In fact, the operating principle of scanning arrays of elements for applications such as target tracking or mobile satellite communications, where the requirements for low side lobes and high gain are of very high importance, is primarily based on precise control of the phase and amplitude of the elements. For these reasons, the complexity of antenna measurement system design for phased array antennas measurements involves high accuracy and precise time synchronization between all the components of the system. This paper presents a comprehensive solution for accurate and reliable measurement of very large phased array antennas at high frequencies. The presented solution addresses the following issues: • Accurate positioning of the RF sensor / probe. • High-speed multi – frequency data collection. • High-speed multi - port data collection. • Programmable and real-time TTL position event triggers. • Pulse measurement. • Multi beam measurement. • Synchronization with the radar computer.
Practical Implementation of Probe-Position Correction in Near-field Planar Scanning Measurements
This paper discusses the use of a laser-tracking device to provide position information in x, y, and z that can be used in position correction algorithms to correct for any displacement error in the actual measurement. Planar near-field measurements require taking amplitude and phase information at accurate and equal point spacing on a plane in front of the antenna under test. The required position accuracy on this plane has been determined to be approximately ./50. As frequencies increase higher, the accuracy in point spacing position on the planar grid becomes more difficult to achieve.
Evaluation of Hard Gating in the ESA/ESTEC CPTR
Compact antenna test ranges such as the ESA/ESTEC CPTR are large facilities for the characterization of electrically and physically large antennas as well as end to end radiated payload testing. To achieve high accuracy measurements, time gating is used to filter out as many room effects as possible. The most common implementation of time gating is to perform a frequency sweep, Fourier transformation to the time domain followed by windowing, gating and back transformation to the frequency domain. All of this is at a time penalty. An alternative is to have a synchronised switching system to switch on and off the transmit power as well as switching on and off the receiver. Such a solution has been devised in a cooperative effort between EADS Astrium and the Munich University of Applied Sciences. The paper will present the capabilities of the Astrium HG2000 Hard Gate system (1) in the ESA/ESTEC CPTR, its implementation in the facility as well as presenting direct comparison of results obtained by the hard gate system with the conventional soft gate on both low gain and high gain antennas
On the Number of Modes in Spherical Expansions
Since the early days of spherical near-field far-field transformations a recommendation for the necessary number of polar modes has been given by , being the wavenumber and or the radius of the minimum sphere. The almost explosive development in computer speed and storage capacity witnessed during the last two decades has made trans-formations of fields from antennas exceeding thou-sands of wavelengths feasible, and a closer investiga-tion of the above expression seems to be appropriate. An improved expression for the number of modes, N, related to the antenna size and the required accuracy will be developed. The impact of truncation of the modal expansion at a given level will be illustrated. This is especially important for measurements where noise is present, or where there is undesirable scatter-ing from objects.
An Efficient and Highly Accurate Technique for Periodic Planar Scanner Calibration with the Antenna Unter Test in Situ
This paper describes the development, testing and evaluation of a new, automated system for calibration and AUT alignment of a planar near-field scanner that allows the calibration system to remain in place during AUT measurement and which can be used to support AUT alignment to the scan plane. During scanner calibration, probe aperture position measurements are made using a tracking laser interferometer, a fixture that positions the interferometer retro reflector at a precise location relative to the probe aperture and a probe roll axis that maintains the proper orientation between the retro reflector and the interferometer as the probe position is moved. Aperture scan path information is used to construct a best-fit scan plane and to define a Cartesian, scanner-based coordinate system. Scan path data is then used to build a probe position error map for each of the three Cartesian coordinates as a function of the nominal position in the scan plane. These error maps can be used to implement software-based corrections (K-corrections) or they may be used for active Z-axis correction during measurements. By using a set of tooling points on the antenna mount, an AUT coordinate system is measured with the interferometer. The system then directs an operator through a set of AUT adjustments that align the AUT with the planar near-field scanner to a desired accuracy. This paper describes the implementation and testing of the system on an actual planar scanner and AUT test environment, showing the improvement in effective scanner planarity.
A Low Cost and High Accuracy Optical Boresighting and Alignment System using Video Cameras
ABSTRACT This paper describes a novel optical boresighting and alignment system used to mechanically align antennas on a compact antenna range at the North Island Naval Air Depot in San Diego, CA. The antenna range has a 5-axis (roll/upper slide/azimuth/elevation/lower slide) positioner used to measure various airborne antennas for production testing. The video alignment system implemented on this range uses two video cameras outfitted with telephoto lenses, one on the roll stage and the other on an antenna-mounting fixture. The system has been demonstrated to yield an accuracy of ±0.005 degrees. Prior to the start of testing the positioner is commanded to a “0” position and the cameras focus on a fixed optical target to provide the operator with a quick visual confirmation that the positioner is accurately aligned prior to testing. The video alignment system described has numerous advantages over other mechanical alignment techniques, is low cost, easy to use, and can be adapted to a variety of testing configurations.
Compact Range Rolled Edge Reflector Design, Fabrication, Installation and Mechanical Qualification
This paper describes the methodologies and processes used for the development, installation, alignment and qualification of a Compact Range Rolled Edge Reflector purchased by the MIT Lincoln Laboratory and installed at their test facility located at Hanscom Air Force Base. The Ohio State University, under contract to MIT Lincoln Laboratory, performed the electromagnetic design and analysis to determine the desired surface shape and required mechanical accuracy of various zones of that surface. The requirement for operation over a very broad frequency range (400 MHz to 100 GHz) resulted in a surface specification that was both physically large (24 ft × 24 ft) and included extremely tight tolerance requirements in the center section.
The mechanical design process will be described, including the generation of a solid “Master Surface” created from the “cloud” of data points supplied by The Ohio State University, verification of the “Master Surface” with The Ohio State University, segmentation of the reflector body into multiple panels, design, fabrication and factory qualification of the structural stands, panel adjustment mechanisms, and panels. Results of thermal cycling of the reflector panels during the fabrication process will be presented.
The processes used for installation of the reflector and the alignment of each panel to the “Master Surface” will be presented and discussed. Final verification of the surface accuracy using a tracking laser interferometer will be described. Color contour plots of the reflector surface will be provided, illustrating the final surface shape and verifying compliance to the surface accuracy requirement
Rapid Spherical Near-Field Antenna Test System for Vehicle Mounted Antennas
More and more wireless services such as satellite radio (SDAR), navigation systems, OnStar, and mobile telephones are installed in GM vehicles. This has created a need to make quick and accurate vehicle antenna measurements. For the frequency range of 500 MHz to 6 GHz, one solution is to use a spherical near-field system. The Satimo rapid probe array technology was selected to develop a vehicle antenna test system (ATS) to reduce test time and maintain data accuracy. The ATS was designed to operate inside of an existing GM electromagnetic compatibility (EMC) anechoic chamber equipped with a nine-meter turntable. The ATS was completed and received XM certification in the first quarter of 2004. The ATS performs multi-frequency dual-polarized complex measurements for every one-degree in azimuth and elevation, over a full hemisphere, in approximately five minutes. The autonomous transport and deployment system, allows the ATS hardware to be removed and the chamber returned to its EMC configuration. This paper presents the ATS design and a summary of the verification test results. A detailed uncertainty budget, as defined by NIST, is also presented.
Implementation of a Geometric-Error Correction System for Extremely High Probe Position Accuracy in Spherical Near-Field Scanning
In this paper, we describe a new method for improving the true-position accuracy of a very large, spherical near- field measurement system. The mechanical positioning subsystem consists of 10-meter diameter, 180 circular- arc scanner and an MI Technologies MI-51230 azimuth rotator and position controller.
The principle components of the error correction method are the error measurement system, the position correction algorithm, and a pair of very high precision, mechanical error correction stages. Using a tracking laser interferometer, error maps are constructed for radial, planar and elevation errors. A position correction algorithm utilizes these discrete-point error maps to generate error correction terms over the continuous range of the elevation axis. The small position correction motions required in the radial and planar directions are performed using the mechanical correction stages. Corrections to the position of the elevation axis are made using the primary elevation axis drive.
Results are presented that show the geometry of the spherical scanning system before and after error correction. It is observed that the accuracy of the radial, planar and elevation axes can be significantly improved using the error correction system.
High Accuracy Horizontal Scanner Operating in X-band for the Measurement of a Spaceborne Synthetic Aperture Radar (SAR) Antenna
ELTA is now in the process of designing and building a new spaceborn SAR “TECSAR” – Israel Synthetic Aperture Radar (SAR) X-Band lightweight satellite.
TECSAR contains an ultra-light weight high accuracy Paraboloid deployable reflector antenna. TECSAR’s electronic beam steering capability is achieved by using a feed array in the focal plane.
For future testing at ELTA, Israel, an horizontal Planar near-field antenna test range (7m x 8m scan) has recently been completed by ORBIT/FR to allow testing of large fully integrated space antennas as stand alone as well as integrated with a satellite The paper will describe: o Short TECSAR SAR antenna description o The special requirements of the measurement system o System design and measured performance
Probe Array Concepts for Fast Testing of Large Radiating Structures
Satimo’s STARGATE probe array systems are now well established as an efficient tool for testing radiated performances of wireless devices and antennas. Since 1998, about forty STARGATE measurement systems have been successfully installed worldwide. Recently, a range of new applications have also demonstrated the suitability of probe arrays for large radiating structures and directive antennas. These new generation of measurement set-ups present innovative aspects regarding their rapidity, dynamic range, and accuracy. This paper will describe several novel antenna testing concepts based on probe arrays that cover automotive, aerospace, and military applications and a wide range of frequencies. The basic difference between traditional approaches using single probe and the STARGATE approach using an array of probes will be explained along with probe array calibration procedures. An error analysis budget using the conventional NIST error terms will be presented including the specific terms related to the use of probe arrays. Also a discussion will be made on some of the key technical challenges to making large probe arrays including such issues as dynamic range, mechanical tolerances, and data truncation effects.