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Accuracy

Error Analysis of Circular-Polarization Components Synthesized From Linearly Polarized Measurements
P.N. Betjes (Nearfield Systems, Inc., Europe), November 2001

A usual way of performing pattern-measurements on circularly polarized antennas is by measuring the linear components of the field and mathematically converting those to the left-hand and right-hand circular components. These synthesized circular components are sensitive for a number of factors: The exact orthogonality of the measured linear components, the measurement-accuracy of both phase and amplitude of the measured linear components, the polarization-pureness (or the accuracy of the description of the polarization-characteristics) of the probe, etc. This paper analyzes these factors, using a computer-model. An indication on the requirements to be imposed on the measurement-equipment is provided.

Broadband Radar Cross Section Predictions and Measurements for a Canonical 3-Meter Ogive Body of Revolution
B.M. Kent (Air Force Research Laboratory),K.C. Hill (Air Force Research Laboratory), November 2001

In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets [1]. In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system [2]. In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.

Limitations of Near-Field Back Projection for Phased Array Tuning Applications
D.J. Van Rensburg (Nearfield Systems Inc.), November 2001

Simulated data is presented for a planar array to demonstrate the limitations of planar near-field back projections. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations are further illustrated through the data presented here. The impact of probe to AUT separation distance is shown as well as the correspondence between array excitation perturbations and that detected through the back projection technique. Results are shown for a simple iterative array excitation adjustment process. The purpose of this paper is to provide guidelines for the application of the planar near-field back projection technique.

Phased Array Calibration Method with Evaluating Phase Shifter Error
N. Takemure (Mitsubishi Electric Corporation),I. Chiba (Mitsubishi Electric Corporation), M. Ohtsuka (Mitsubishi Electric Corporation), T. Takahashi (Mitsubishi Electric Corporation), Y. Konishi (Mitsubishi Electric Corporation), November 2001

In this paper, the authors propose an improved Rotatingelement Electric-field Vector (REV) method taking into account amplitude and phase error of phase shifters in order to achieve more precise calibration. The conventional REV method has been used in order to determine and/or adjust amplitude and phase of electrical field radiated from each antenna element -element fieldin phased array antennas. However, amplitude and phase deviations due to phase shifter errors, and so on, reduce the measurement accuracy because the conventional REV method assumes no deviation. On the other hand, the proposed REV method can evaluate element fields without error and error electrical fields -error fields- due to phase shifter errors in each bit, by measuring both amplitude and phase value of array composite electrical field. In a simulation for a 31- element array with 5-bit phase shifter, the evaluated element fields and error fields agree well with the expected values. This result shows that the proposed method allows the phased arrays to be calibrated more accurately as considering phase shifter errors.

A Simple Analysis of Near-Field Boresight Error Requirements
D.W. Hess (MI Technologies), November 2001

The need to measure the boresight pointing direction of radar antennas to a high degree of accuracy yields a requirement for excellent positioning accuracy on near-field antenna ranges. Evaluation of this requirement can be accomplished by a full and complete sensitivity analysis. Alternatively, to gain an understanding of the effects of errors more simply, one can approach the question of accuracy required in the setup, by use of a physical model and straightforward physical reasoning. The approach starts with the assumptions of a collimated wave with planar phase fronts and the premise that the boresight direction of such a sum beam is along the normal to the phase fronts. A sensitivity analysis of the simple trigonometric boresight relationship between mechanical boresight and phase front normal, shows how accurate the receiver and the positioner must be to achieve a given boresight determination. Such an approach has been known for many years as it regards planar scanning; and, the results are known to be applicable. In this paper this consideration is extended to spherical scanners to arrive at estimates of the mechanical positioner accuracies and electrical receiver accuracies needed to make boresight measurements of radar antennas with spherical near-field ranges.

Experimental Studies With Comparisons to Computational Model for Automobile Antennas
Y. Kim (ElectroScience Laboratory),E.K. Walton (ElectroScience Laboratory), November 2001

A series of experimental and theoretical tests designed to develop techniques for reliable computational modeling of automobile antenna performance is presented. The results from the experimental measurements are compared with the results of computational techniques to verify their accuracy and reliability. The Electromagnetic Surface Patch (ESP5) code, a theoretical Method of Moment (MoM) general-purpose code developed at the Ohio State University, is used for computational modeling. We progress from the simple geometry of a single square plate and a monopole, to the more complex structure of a small copper-coated plastic model of an automobile. The computational simulation and measurements are configured with both a monopole antenna mounted at the center of the automobile roof and a backlite heater grid FM antenna. The input impedance, pattern, and polarization are all measured. Comparisons between the results of the computational simulations are presented, as well as the procedures used to measure the antenna characteristics and compare the experimental data with the measured data.

Wide Band Compact Antenna Test Range
P. Bengtsson (Ericsson Microwave Systems AB),H. Eriksson (Ericsson Microwave Systems AB), M. Boumans (ORBIT/FR-Europe), November 2001

Ericsson Microwave Systems (EMW) in Sweden has several outdoor and indoor test ranges in operation [1], [2], [3]. In line with future needs and requirements EMW has started building a new Compact Antenna Test Range to be used for a large range of projects and applications. The Compact Antenna Test Range will cover the frequency range of 800 MHz to 75 GHz. The test range will have the possibility for both active and passive antenna measurements at both system and subsystem / unit levels. The test zone will be 3 meters diameter. The maximum load the positioner can carry will be 700 Kg with very high position accuracy for special applications. Due to the relatively low design frequency and the desired size of the test zone, special considerations have been taken in the conceptual design of the reflector system as well as the choice of absorbers. Another important parameter in the design of the facility will be the access to the quiet zone and the time needed to change frequency bands and test objects. To accomplish this, preparations have to be made for easy alignment, very precise interfaces and a fast access to the test area.

Calibration and Verification Measurements in Compensated Compact Ranges Up to 500 GHz
J. Hartmann (Astrium GmbH, EADS),H.J. Steiner (Astrium GmbH, EADS), J. Habersack (Astrium GmbH, EADS), J. Lemanczyk (ESA/ESTEC), P. De Maagt (ESA/ESTEC), November 2001

Compensated Compact Ranges (CCR) represent a high standard of state-of-the-art test facilities with a fast and real time measurement capability up to the submm wave range. Future scientific and earth observation instruments of ESA/ESTEC such as MASTER, PLANCK and HERSCHEL are working within this frequency ranges and require a high measurement accuracy for large antenna apertures. Within the ADMIRALS study for ESA/ESTEC, transmit and receive modules up to 500 GHz and an appropriate large offset reflector antenna with precise surface accuracy in form of a Representative Test Object (RTO) were applied. Related tests in the CCR 75/60 of Astrium were performed in order to qualify the test facility and verify the antenna measurements with theoretical pattern calculations. The present paper shows measurement results with the highly accurate Plane Wave Scanner (PWS) of Astrium GmbH and the RTO. Through the measurements performed, the accuracy of the plane wave field as well as pattern accuracy in the quiet zone of the CCR 75/60 have been qualified up to 500 GHz.

Mitigation of Multipath and Ground Interactions in RCS Measurements Using a Single Target Translation
I.J. LaHaie (AARDC),M.A. Blischke (AARDC), November 2001

Translating pylon terminations are often used in narrowband RCS background measurements as means of separating the returns of the termination from those of the pylon itself. Typically, this is done by measuring the pylon while the fixture continuously translates in the range direction through a distance of at least half a wavelength. This paper describes a translated target processing (TTP) algorithmw hich is an extension of this technique to RCS measurements of rotating targets. The technique is applicable to both narrowband and wideband measurements. The algorithm is applied to the problemof mitigating multipath and ground interaction contamination in indoor and outdoor RCS measurements, respectively. Its performance was evaluated as a function of signal-to-noise ratio, target-tocontamination ratio, and translation distance and accuracy using point target simulations. We conclude with a demonstration of the TTP algorithm using actual measurements from the Boeing 9-77 compact range.

Correcting for Range Measurement Errors Using Quiet Zone Synthesis
A. Shroyer (Ball Aerospace and Technologies Corp.),L. Diaz (Ball Aerospace and Technologies Corp.), N. Zawistowski (Ball Aerospace and Technologies Corp.), November 2001

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 [1].

A Large Aperture 650 GHz Near-Field Measurement System for the Earth Observing System Microwave Limb Sounder
D. Slater (Nearfield Systems Inc.),J. Hardy (California Institute of Technology), P. Stek (California Institute of Technology), R. Cofield (California Institute of Technology), R. Dengler (California Institute of Technology), R. Jarnot (California Institute of Technology), R. Swindlehurst (California Institute of Technology), November 2001

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
W. Forster (Mission Research Corporation), November 2001

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
J.T. Shaffer,R.B. Dybdal, November 2000

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
C.A. Rose, November 2000

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
D.J. Van Rensburg,A. Newell, M. Hagenbeek, November 2000

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
A.C. Newell,D. Lee, November 2000

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)
R.L. Eigle,A. Buterbbaugh, W.J. Kent, November 2000

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
D. Bird, November 2000

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
W.G. Scott,R.E. Wilson, November 2000

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
J. Habersack,H-J Steiner, J. Hartmann, J. Lemanczyk, P. De Maagt, November 2000

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.







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