AMTA Paper Archive
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Correcting for Range Measurement Errors Using Quiet Zone Synthesis
A method is presented for correcting for range measurement errors resulting from non-uniform quiet zone illumination in indoor tapered antenna chambers. The interaction of the source antenna with the throat of the chamber causes undesirable amplitude and phase variations over the quiet zone, the region where the antenna under test (AUT) is located. These variations can impact the accuracy of the antenna pattern measurements, especially when the AUT has a significant aperture. These quiet-zone anomalies can be measured and removed from the antenna patterns by quiet-zone probing. The quiet zone can be probed planar, cylindrical, or spherical quiet zone probe configurations. A planar quiet-zone probe is used here. This process of calibrating the antenna pattern measurements for quiet-zone range errors is called quietzone synthesis (QZS) and is implemented here using MATLAB .
A Large Aperture 650 GHz Near-Field Measurement System for the Earth Observing System Microwave Limb Sounder
This paper describes a large aperture, 650 GHz, planar near-field measurement system developed for field of view characterization of the Earth Observing System Microwave Limb Sounder (EOS MLS). Scheduled for launch in 2003 on the NASA EOS Aura spacecraft, EOS MLS is being developed by the Jet Propulsion Laboratory to study stratospheric chemistry using radiometers from 118 to 2500 GHz. The combination of a very high operating frequency and a 1.6-meter aperture, coupled with significant cost and weight restrictions, required a new look at near-field scanner design approaches. Nearfield Systems Inc. (NSI) developed a planar scanner that provides a planar accuracy of 4 microns RMS over the entire 2.4 x 2.4 meter scan area. This paper presents an overview of this system including the sub-millimeter wave RF subsystem and the ultrahigh precision scanner. Representative measurement results will be shown.
Positioning System Upgrade of an Existing Measurement System
An accurate and reliable target positioning system is mandatory for a good antenna and/or radar cross section (RCS) measurement facility. Most measurements involve characterizing the radiation or scattering of the unit under test as a function of angle and frequency. Accuracy and repeatability become increasingly important in RCS measurements where background subtraction is utilized. Any error in target position will reduce the subtraction effectiveness. Wear and tear of existing equipment coupled with improvements in motion control technology may compel some measurement facilities to upgrade their positioning system. Doing so, while keeping the rest of the measurement system intact, poses integration challenges that cannot be over emphasized. Problems will inevitably be encountered. Their source could be the new positioning system, the old measurement system, or the communication between the two. Subtleties of how the motion control system works can be overlooked during the requirements definition phase of the project. Further idiosyncrasies can be missed during acceptance testing of the system. The Air Force Research Lab compact range has recently upgraded their target positioning system and will share the lessons learned as a result.
Using Standard Gain Horns
Standard gain horn antennas are commonly used as reference antennas in establishing absolute gain levels of antennas under test. However, their high sidelobes and backlobes can interact with the structure surrounding the horn in the measurement setup. These interactions degrade the accuracy of the gain values. Thus, while the gain of the horn may be carefully calibrated in free space, its gain value in the measurement environment can differ from its free space value. Examples will illustrate this problem and ways are described to reduce the sensitivity to the environment.
Accuracy Estimation of Microwave Holography From Planar Near-Field Measurements
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as "back transformation", is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space. The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna. This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.
Impact of Alignment Errors on Cylindrical Near-Field Antenna Measurements, The
This paper addresses the sensitivity of the cylindrical near-field technique to some of the critical alignment parameters. Measured data is presented to demonstrate the effect of errors in the radial distance parameter and probe alignment errors. Far-field measurements taken on a planar near-field range are used as reference. The results presented here form the first qualitative data demonstrating the impact of alignment errors on a cylindrical near-field measurement. A preliminary conclusion is that the radial distance accuracy requirement may not be as crucial as was stated in the past. This paper also shows how the NSI data acquisition system allows one to conduct such parametric studies in an automated way.
Application of the NIST 18 Term Error Model to Cylindrical Near-Field Antenna Measurements
This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the pattern and probe polarization terms. The analysis is also extended to estimate the effect of position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.
Bistatic Radar Cross Section Study of Complex Objects Utilizing the Bistatic Coherent Measurement Systems (BICOMS)
The NRTF and MRC have recently completed the first bistatic RCS test utilizing the Bistatic Coherent Measurement System (BICOMS). BICOMS is the first true far-field, phase coherent, bistatic RCS measurement system in the world and is installed at the NRTF Mainsite facility. The test objects include a 10 foot long ogive and a 1/3 scale C-29 aircraft model. Full pol rimetric, 2-18 GHz monostatic and bistatic RCS measurements were performed on both targets at 17 degree and 90 degree bistatic angles. BICOMS data demonstrates excellent agreement to method-of moments RCS predictions (ogive) and indoor RCS chamber measurements (monostatic, ogive). This paper describes the BICOMS system and the test process, highlights some process improvements discovered during testing, assesses the quality of the collected data set, and analyzes the accuracy of the bistatic equivalence theorem.
Characterization of an Outdoor RCS Measurement Range
The Radar Signature Management Group of Racal Defence Electronics Limited specializes in the measurement, prediction and analysis of radar signatures. Types of measurement ranges used by the Group fall into three categories: • Indoor instrumented ranges • Outdoor measurement ranges • Full-scale trials, in which dynamic measurements are made of the target in its normal operational environment This paper describes a methodology used for characterizing the uncertainties within data from one of the outdoor RCS measurement ranges, at frequencies from 8 to 12 GHz. The results are summarized and uncertainties arising from the following sources are quantified: • Linearity • Absolute Accuracy • Stability and Repeatability • Polar Diagram The effects of background and target-to-pylon support interface are also discussed. The individual uncertainties are combined in a simple manner in order to obtain an overall uncertainty bound for the range, and recom mendations are made for reducing uncertainties against the difficulty and cost of implementation.
NFR Cross Polarized Pattern Errors Using a Linear Probe to Measure a Circularly Polarized Antenna
For greatest efficiency and accuracy in measuring patterns of a circularly polarized antenna on a planar near field range (NFR), a recommended procedure is to use a fast switched, dual circularly polarized probe. With such equipment one obtains complete pattern and polarization data from a single scan of the antenna aperture. For our task of measuring high gain shaped beam apertures, measurement efficiency is further improved by using a moderately high gain (about 12 dBi) probe that has been accurately calibrated for patterns, polarization, and gain over the test frequency band. Such a probe allows scan data point spacing to be typically at least one wavelength, thus keeping scan time minimized with acceptably small aliasing (data spacing) error. The measured near field amplitude and phase data is transformed via computer to produce the angular spectrum that is further processed to remove the effect of the probe patterns, i.e. probe correction. The final output is a set of (principal and cross) circular polarized far field patterns. However on one occasion, due to fast breaking changes in requirements, we were unable to obtain a calibrated circular polarized probe in the available time. For this test we used an available calibrated 12 dBi fast-switched dual linear-polarized probe with software capable of processing principal and cross circular-polarized far field patterns. As anticipated, we found from preliminary tests that the predicted low cross-polarized shaped beam pattern was not achieved when using the calibrated fast Ku band probe switch. Further tests showed the problem to be due to small errors in calibration of the probe switch. This paper will discuss test and analysis details of this problem and methods of solution.
Facility Trade-Off for Measurements up to 500 GHz
Future European Space Agency (ESA) earth observation and space science missions such as MASTER and PLANCK will have instruments and associated antennas working well up into the Terahertz frequencies. The large sizes of the antenna apertures and the need to accurately verify their performance, place high demands on test facilities and test techniques. In recent decades, different types of facilities have been developed. ESA has identified that for measurements up to at least 500 GHz, existing facilities and techniques could be applied with a relatively modest investment. A trade-off between the cylindrical near-field and compact antenna test ranges at Astrium has been carried out to identify which of the two existing ranges would provide better accuracy.
Evaluation of the Accuracy of the PTP Phase Retrieval Algorithm by Means of a Numerical/Statistical Approach
Obtaining far-field radiation patterns of high frequency antennas (>80Ghz) from near-field measurements has been an important issue in the last twenty years. However with frequencies increasing into the millimetre and sub-millimetre bands, questions have been raised about possible limitations on the assessment of such antennas and in particular the measurement of phase. The PTP phase retrieval algorithm addresses the problem by extracting the phase from the knowledge of two amplitude data sets in the near-field. The accuracy of the algorithm is studied by simulation and measurement by means of a numerical/statistical approach. Pseudo-random phase apertures can be generated using Zernike polynomials, which in turn can be used as initial estimates for the algorithm. This paper shows some simulated and measured results for various separations. It can be seen that different pseudo-random phase functions can affect the accuracy of phase retrieved results in particular when the distance between planes is considerably small in relation to the AUT size.
Precision Positioner Alignment Techniques for Spherical Near Field Antenna Measurements Using Laser Alignment Tools
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970's around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna's electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate. MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment. This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.
Measured Error Terms for the Three-Antenna Gain-Measurement Technique
This paper will detail the implementation and results of a gain calculation performed on standard gain horns (SGHs) in the LS and XN microwave bands. The three-antenna method was used to ensure the highest accuracy possible, and extensive efforts were made to minimize the error budget. The measurement was performed in a large anechoic chamber, with the receive and transmit antennas placed 4.6 meters high in opposing corners. The resulting fifteen meters of aperture separation (approximately 10D2/l. for LS band and 15D2/l for XN band) eliminated all measurable aperture interactions and greatly reduced multipath interference from chamber reflections. Rigorous analysis of the error terms proved this method to be both accurate and reliable. Typical values of measured error terms will be presented.
Cramer-RAO Bound System-Level Analysis for Multi-Mode Spiral Antennas; Single-Element and Arrayed
This paper considers the use of Cramer-Rao bound (CRB) to aid in providing accurate and quantitative system-level trades for antenna direction finding (DF). Past work has focussed on the use of spectral estimation techniques (e.g., MLM and MUSIC) to obtain needed DF accuracy. Here, the CRB is used to quickly assess tradeoffs in determining optimal antenna array positioning on a platform system. We develop the necessary CRB mathematical relations and demonstrate the potential advantage of using multimode spiral antennas over a standard linear phase interferometer (LPI). The standard LPI configuration is used as a baseline for comparison.
Accuracy and Calculation Sensitivity for AFRL Squat Cylinder RCS Calibration Standards
(U) Precise radar cross-section (RCS) calibration are needed for all RCS measurement facilities. In 1996, AFRL began to advocate the use of a series of precision, short cylinder RCS calibration standards, demonstrating consistently greater accuracy than traditional sphere targets. Previous AMTA publications [1,2,3,4] demonstrated the overall measurement fidelity of these targets. However, questions regarding the accuracy and stability of the numerical RCS solutions to these cylinders continue to be raised. This paper will strictly and thoroughly examine the accuracy of several numerical techniques used to predict the AFRL calibration cylinder RCS, and will examine such "real world" issues as gridding sensitivity, conductivity vanat1ons, frequency bandwidth, and practical manufacturing tolerances.
New Compact Antenna Test Range at Allgon Systems AB
Allgon Systems AB has put a new compact antenna test range into operation in July 2000. The investment was triggered by Allgon's planned move to a new building. An indoor facility was preferred for fast and efficient operation. The present primary application is the measurements of base station antennas. The compact range is constructed using a single reflector with serrated edges. A sophisticated feed carrousel enables automatic changing of 3 feed systems. The size of the quiet zone is 3 meters. The initial frequency range is from 800 to 6000 MHz. However, the reflector accuracy allows future extensions to 40 GHz and higher. The cha mber size is 21 x 12 x 10.5 m (L x W x H). Absorber layout comprises 24, 36 and 48 inch absorbers. An overhead crane spans the entire facility. The positioner system is configured as roll over azimuth with a lower elevation over azimuth for pick-u p and small elevation angle measurements. Different sizes of masts and roll positioners are available, depending on the AUT. Instrumentation is based on a HP 8753. Software is based on the FR-959 Plus. Antenna measurement results show the performance of the facility.
Error Statistics in RF Measurements
Error budget projections of measurement accuracy require statistical descriptions of the individual error sources. Thermal noise errors are well known and commonly used. Such statistics, however, have a zero mean Gaussian distribution and sadly are misapplied to the distributions of other error sources. The class of coherent RF errors, for example, has non-zero mean values and variances that differ from Gaussian values. Such statistics are described.
RCS Measurements of LO Features on a Test Body
The paper presents an example of the design process undertaken to determine the RCS response of LO features mounted on a test body. Although not unique, the example considers the various aspects which determine the accuracy of the final data in the design of the experiment and signal processing. The high quality of experimental results illustrate the potential of using an integrated approach in which the designs of the test body, the measurement process, the signal processing techniques, and validation of results are optimally applied to meet the objective not achievable by conventional means.
Characterization of Antenna Patterns by Means of Statistical Image Classification
The accuracy of near field measurements have in the past largely been judged by inspection however the authors have developed an objective measure of the accuracy and repeatability of such measurements. This paper illustrates the measurement process and the techniques associated with statistical image classification used to confirm its accuracy and repeatability. The technique will be illustrated via the correlation of data sets acquired over a variety of different frequencies and scan plane areas. The examination of these measurements will demonstrate the applicability and sensitivity of the technique when the accurate assessment of highly correlated patterns is required.
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