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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.
When using an array antenna for measurements, small phase imbalances between the sum and difference channels may occur due to a variety of factors. This imbalance may arise due to unequal line lengths, twists or bends in the cabling, leakage, improper connections, or a variety of other factors. In the ideal phase array antenna one lobe of the difference pattern should be inphase with the sum pattern, and the other lobe should be 180° out-of-phase. When a phase imbalance occurs between the sum and difference channels, errors occur in determining the antenna beam deflection due to the presence of a radome. By taking the ratio of the difference to sum channel data a phase correction factor may be determined. Application of this phase factor to the difference channel data will phase align the sum and difference channels so the correct deflection may be determined. This correction factor will be frequency dependent.
We at MI Technologies have employed the Hansen error analysis [1] developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design. I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges [2]. The result is an appropriate extension of the TUD uncertainty analysis. Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field [5]. In this paper, I offer such an extended tabulation for spherical measurements.
Test results of the compact range facility in the National University of Singapore are presented in this paper. The tests were performed for antenna and RCS measurements from L-band to Ka-band. Errors of experimental measurements are compared to errors in measurements calculated by results of field measuring in the quiet zone.
This paper reviews the Helendale Measurement Facility (HMF) ground plane range uncertainty analysis and associated data collection. Range uncertainty analysis is a requirement for ISO-25/ANSI-Z-540 range certification and is a priority one section in the Helendale Range Book. Targets used for the analysis were two sets of right circular “squat” calibration cylinders. These cylinders are the dual calibration cylinders for HMF. Calibration measurement uncertainties are established statistically from a large number of repeated measurements at S, C, X, and Ku bands. Each measurement was taken at two target support locations down range. The field data collected included monostatic scattering from two calibration cylinders, backgrounds with no target and support, and drift data for quality control. I and Q imbalance, frequency stability, range accuracy, linearity, and field uniformity at target locations were considered in the analysis. The uncertainty analysis is based on RSS addition of errors and assumes all errors are additive and that targets are not LO. The statistical approach used to perform the uncertainty analysis reported in this paper was developed cooperatively at AFRL and Mission Research Corporation.
While there are standard test methods to characterize the performance of passive antennas, active antennas (with integrated amplifiers) and more complex systems with adaptive functionality create new testing challenges, both in definition and approach. Active antenna gain is a combination of the antenna gain and the embedded amplifier gain. Since these amplifiers may be distributed throughout the array with gain variations between amplifiers, there is a challenge in performing measurements that separate the two gain components. For adaptive antennas, the pattern changes with the incident angle of the test signal, so the adaptive function is often disabled to provide a snapshot of the system, like antenna patterns, for a particular set of conditions. In other cases of adaptive antennas, the composite system performance is measured for angular changes while the system adapts. This paper presents an overview of the testing of both active antennas and adaptive antenna combining systems. Examples of the types of test metrics and errors will be given.
Flexing of cables in planar near-field test systems may introduce significant phase errors to the measured vector values of the field. Submm-wave receivers require several flexible cables to be connected to them. The phase errors originated in the bending cables get multiplied and added to the phase of the final detected submm-wave signal. A complete submm-wave antenna measurement system with on-the-fly measurement of the phase errors in a flexing microwave cable is presented. The phase error measurement is based on the use of a pilot signal. Correction of the detected vector values is done as a postprocessing step. Quiet-zone fields and the corresponding phase error planes have been measured at 310 GHz for two different-sized CATRs based on a hologram. The measured maximum phase errors were 7o and 11o for 30 cm and 60 cm holograms, respectively.
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.
In practice, accurate VHF Antenna radiation patterns are usually difficult to achieve due to high level multipath present in the measurement test range. Special range geometry’s and source arrangements have been devised over the years [1] to mitigate the measurement errors produced by test range multipath. In this paper we will describe a new illumination source method designed to accurately control the influence of ground path illumination and in turn reduce quiet-zone amplitude ripple. An array of VHF elements with adaptive complex weights will be used to produce a controlled illumination line source for a given range geometry. Simulated quietzone performance will be shown.
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.
Compact RCS measurement ranges all suffer from some level of non-ideal field illumination. Stray fields from interactions with the chamber wall and diffraction effects are major contributors to the non-uniformity of the incident field at the target. This non-uniformity gives rise to unavoidable errors in RCS measurements. We present a detailed analysis of how non-uniform illumination manifests itself into RCS measurement errors. The analysis approach is based on the plane wave spectral decomposition of the illumination. We compute the energy scattered by the planar components of the illumination and determine how much of this energy is coupled backi nto the radar antenna. We model the target as a diffuse scatterer by using a collection of point scatterers distributed within a specified volume. We present uncertainty results based on a simulation as well as field probe data collected from AFRL’s Advanced Compact Range (ACR).
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 transmitting multi-beam frequency-reuse antenna on an orbiting satellite has N co-polarized spot-beams with each beam driven by a separate transmitter (all transmitters sharing a common band) and each pointed in a different azimuth and elevation direction. The interference effect of N-1 beam side-lobes falling simultaneously on any receiving ground user in a satellite main beam can be estimated by combining the N-1 radiation pattern side-lobe levels which coincide on each user. To predict this effect, the radiation pattern of each beam can be measured in a near field pattern range (NFR) on the ground. When this is done, the measurement error (uncertainty) of each side-lobe falling in the direction of a given main beam ground terminal can also be obtained by a series of special error measurements. The measured error terms for a given side-lobe can be combined in an NFR error table to obtain the measurement error for that side-lobe in the direction of the given terminal location. This process can be repeated for each of the N-1 side-lobes. In this paper we present a method for combining the measured errors of the N-1 side-lobes to yield a combined uncertainty for the combined interference level of the N-1 side-lobes. This process can be repeated for each main beam terminal location. Several tables are presented showing how the combined side-lobe error varies as a function of the levels of the individual side-lobes and the measurement uncertainty of each side-lobe.