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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.
Probe correction aspects for the spherical near-field antenna measurements are investigated. First, the spherical mode analyses of the radiated fields of several antennas are performed. It is shown that many common antennas are essentially so-called odd-order antennas. Second, the errors caused by the use of the first-order probe correction [1] for a rectangular waveguide probe, that is an odd-order antenna, are demonstrated. Third, a recently developed probe correction technique for odd-order probes is applied for the rectangular waveguide probe and shown to provide accurate results.
Probe position errors, specifically the uncertainty in the theta and phi position of the probe on the measurement sphere, are one of the sources of error in the calculated far-field and hologram patterns derived from spherical near-field measurements. Until recently, we have relied on analytical results for planar position errors to provide a guideline for specifying the required accuracy of a spherical measurement system. This guideline is that the angular error should not result in translation along the arc of the minimum sphere of more than ?/100. As a result of recent simulation and analysis, expressions have been derived that relate more specifically to spherical near-field measurements. Using the dimensions of the Antenna Under Test (AUT), its directivity, the radius of the sphere (the minimum sphere) enclosing all radiating surfaces and the frequency we can estimate the errors that will result from a given position error. These results can be used to specify and design a measurement system for a desired level of accuracy and to estimate the measurement uncertainty in a measurement system.
For a planar near-field range, it is sometimes convenient to use a linearly polarized probe to measure a circularly polarized antenna. The quality of the circular polarization of the test-antenna is determined by the measured axial ratio. This requires the amplitude and phase from two near-field scans, one scan with the probe polarization oriented horizontally and another vertically. A lateral probe position error between the horizontal and vertical orientations can occur if the probe is not aligned properly with the probe polarization rotator. This particular probe position error affects the accuracy of the axial ratio in the main beam if the beam of the test antenna is not perpendicular to the scan plane. This paper presents analysis and measurement examples that demonstrate the relationship between the errors in the axial ratio and the lateral probe position. It is shown that the axial ratio, within the main beam, is not sensitive to the lateral probe position error when the beam is normal to the scan plane. However, the error in the axial ratio in the main beam can be quite significant with a small lateral probe position error if the antenna beam is tilted at an angle with respect to the scan plane. A simple phase correction algorithm is presented that is useful for measured data from an electrically large aperture.
At a Department of Defense antenna measurement laboratory, an important measurement is the accurate measurement of gain for circularly polarized antennas. An additional requirement is that a wide population of engineers and technicians that do not spend a significant amount of time using the facility make the measurements as they test the antennas for their projects. The objective was to create a highly automated, accurate test structure that was easily used by visiting engineers to make high quality measurements. Consistency of results across the user population was a paramount requirement. This paper describes the instrumentation and software used to meet this objective. The paper describes basic measurement techniques, the exploitation of instrumentation capabilities to make the measurements, the software processing of the data and the graphical user interface that was developed to make the test process essentially a “one button” operation. Significant components in the test scenario were the ability to accurately collect data on a linearly polarized Standard Gain Horn in orthogonal polarizations without inducing errors caused by various axes of motion and to provide channel imbalance correction for the orthogonal channels of the instrumentation and range.
This paper describes and discusses relevant performance issues concerning the quiet zone illumination of a baseline interferometer antenna using a compact range system. Typical baseline interferometer antennas are utilized for precision direction finding applications, and are designed on the principle of detecting the incoming phase wave front as a means to determine the direction of arrival of the detected signal. Quiet zone illumination of the antenna using a compact range deviates from the ideal illumination by introducing some levels of amplitude and phase taper and ripple. Unwanted relative differences in the illumination of the individual elements of the interferometer antenna will introduce errors in the subsequent analysis of the direction finding accuracy and precision of the array. Sources of these errors are examined in this paper, and relevant compact range performance trade-offs are discussed to optimize the range. Considerations are given to both utility of the range, as many interferometer antennas are broadband EW type arrays, and thus require single feed, single test broadband measurements, as well as to the accuracy in characterizing the performance of the interferometer over its full operating bandwidth. In addition, this paper discusses the analysis of high precision compact range field probe data, and the subsequent application of relevant statistical parameters to characterize the data. The analysis techniques utilized highlight the important performance features required of the compact range to effectively test baseline interferometers. The implementation of an automated utility is described that applies the relevant corrections, and applies the statistical algorithms, to the data to effectively reduce the data and summarize it in a fashion that provides immediate utility to the field probe test operator.
Microwave measurement of intrinsic material properties can be performed with transmission-line fixtures such as waveguides or free-space focused beams. However, analyses of measured data usually assume idealized sample geometries. In this paper, Finite Difference Time Domain (FDTD) calculations are used to study the systematic error from non-ideal geometries, in free-space and waveguide measurements of impedance sheets. Analytical models of these errors are developed. FDTD analysis can be used to numerically invert intrinsic material properties from measured freespace transmission coefficients. The focused beam is simulated in FDTD with a sum of weighted plane waves with a Gaussian spectral distribution. The transmission coefficient is predicted by propagating the focused beam through a material slab or sheet; and the dielectric or impedance properties are derived from the transmission coefficient. The focused beam diameter is preferably several wavelengths, which requires large sample size (>1 square meter) at low frequencies. A modified focused beam technique is described that incorporates a finite aperture in a metal groundplane to measure samples with reduced dimensions, even at low frequencies. Calculations are compared to laboratory measurements. FDTD calculations are also applied to study the effect of gaps in waveguide fixtures, since gap and edge effects in both waveguide or free-space aperture fixtures contribute to measurement error.
Modern radome measurements often involve scanning the radome in front of its antenna while the antenna is actively tracking an RF signal. Beam deflections caused by the radome are automatically tracked by the antenna and its associated positioning system, which is typically a two-axis (pitch & yaw) gimbal. The motion required to accurately track the beam can be very demanding of the gimbal. High structural stiffness, zero drivetrain backlash, and extremely accurate angle measurement are all necessary qualities for radome beam deflection measurement. This paper describes a new, advanced, two-axis gimbal that embodies those qualities. The new gimbal incorporates direct-drive motors to achieve zero backlash. The motors are mounted directly to the rotating gimbal elements, thereby eliminating the usual causes of drivetrain compliance. Rated torque of the motors is not high, and the antenna is therefore fully counterweighted. Each of two optical encoders is mounted on the same rotating gimbal element as its associated motor. The encoders are directly mounted; no flexible coupling is used. The antenna is mounted to those same rotating elements. Antenna positioning error due to windup of the structure and drivetrain is virtually eliminated. Eccentricity of the encoder disk, which is the primary source of direct-drive encoder errors, is adjusted by virtue of a remarkable in situ process.
We present the results of developing a MATLABbased Near-field Antenna measurements toolbox. The purpose of this package is two-fold. First, it functions as a training tool, to help the user understand the near-field measurement process. Second, it can also function as an analysis aid, providing insight into the effect of errors on the measurement process. Results obtained from using the beta version of the toolbox are presented and the toolbox will be available as a download from the website listed in the paper, to solicit feedback from the measurement community.
Coherent radar cross section measurements on a target moving along the line-of-sight in free space will trace a circle centered on the origin of the complex (I,Q) plane. The presence of additional complex signals (such as background, clutter, target-mount interactions, etc.), which do not depend on target position, will translate the origin of the circle to some complex point (I0,Q0). This type of phase-dependent I-Q data has been successfully analyzed. However, the presence of outliers can introduce significant errors in the determination of the radius and center of the IQ circle. Hence, we implement a combination of a robust and efficient Least-Median Square (LMS) and an Orthogonal Distance Regression (ODR) algorithm is used (1) to eliminate or to reduce the influence of outliers, and then (2) to separate the target and background signals. This technique is especially useful at sub-wavelength translations at VHF, where spectral techniques are not applicable since only a limited arc of data is available. We analyze data obtained as an Arrow III target moves relative to its supporting pylon. To demonstrate the effectiveness of the technique, we introduce rf interference signals into S band data and show that the uncontaminated parameters can be recovered with acceptable uncertainties.
Over the past few years, range certification activities have become more commonplace, as industry, government and academia have embraced the process and acted to implement documented procedures at their facilities. There is now a significant amount of documentation laying out the process, as well as templates to assist ranges in developing their range books. To date, however, there have been fewer examples of useful tools to assist the ranges in better understanding how the process will affect their specific range. The authors have developed a first generation MATLAB toolbox designed to provide ranges a “what-if” capability to see the impact of specific range errors on the range’s operations. Included within the toolbox are several types of additive and multiplicative errors, as well as means of modeling various aspects of radar operation.
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.
We discuss the mitigation of truncation errors in spherical scanning measurements. The main emphasis is the spherical harmonic representation of probe transmitting and receiving functions; however, our method is applicable to nearfield measurement of electrically small antennas for which fullsphere data are either unreliable or unavailable.
This paper describes two methods that can be used to measure the leakage signals in quadrature detectors, predict the effect on the far-field pattern, and correct the measured data for leakage bias errors without additional near-field measurements. One method is an extension and addition to the work previously reported by Rousseau1. An alternative method will be discussed to determine the leakage signal by summing the near-field data at the edges of the scan rather than summing below a threshold level. Examples for both broad-beam horns and narrowbeam antennas will be used to illustrate the techniques.
An error detection technique was developed for culling large masses of measured antenna pattern data by first removing information that is likely to be associated with the antenna. Since the maximum spatial frequency of radiation from the antenna can be determined by its electrical size, any energy outside that spatial band is not considered to be valid and may be used to flag suspicious data. This analysis can be accomplished rapidly and can be used to cull patterns containing such anomalies as spikes, notches, non-closures and multipath effects. This paper describes the method with examples from simulated and measured patterns.
The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
Free space, coherent radar cross section measurements on a moving target trace a circle centered on the origin of the complex (I,Q) plane. Noise introduces only small random variations in the radius of the circle. In real measurement configurations, additional signals are present due to background, clutter, targetmount interaction, instrumentation and the average of the time-dependent system drift. Such signals are important contributors to the uncertainty in radar cross section measurements. These time-independent complex signals will translate the origin of the circle to a complex point (I0,Q0). Such data are then defined by the three parameters (I0,Q0), the center of the circle, and st, the radar cross section of the target. Data obtained when a target is moved relative to its support pylon can be separated into phasedependent and phase-independent components using the techniques of (1) three-parameter numerical optimization, (2) least-median-squares fit, (3) adaptive forward-backward finite-impulse response procedure, and (4) orthogonal distance regression applied to a circle fit. We determine three parameters with known and acceptable uncertainties. However, the contribution of systematic errors due to unwanted in-phase electric signals must still be carefully evaluated.
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations ä z (x, y) can be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence. This paper describes a method for estimating the Zposition error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
The NUWC/NPT Overwater Arch Antenna Range consists of a 70 ft radius measurement arch located over an elevated 90 ft x 65 ft salt water pool. This facility, located outdoors, presents mechanical and electrical challenges. Measurement accuracy and precision are a function of environmental parameters (including unwanted signals), physical plant and instrumentation characteristics. Measured data variation will be presented along with techniques which could be employed to improve range performance.