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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.
This paper investigates the effect of alignment errors on near-field cylindrical ranges at frequencies over 18 GHz. This is of particular interest because the small probe sizes and wavelengths above 18 GHz can make the alignment of the near-field system a difficult task. Previous probe alignment investigations have been done at frequencies below 18 GHz. This paper will determine if the conclusions from the previous work are valid at higher frequencies and will expand on that previous work. Measured data will be presented to demonstrate the effect of the probe axis not intersecting the azimuth axis as well as the probe not being orthogonal to the azimuth axis of rotation.
This paper will explore the effects of the source antenna's axial ration on the apparent gain of an Antenna Under Test (AUT). A technique will be given to correct these errors. Finally, experimental test results will be given.
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.
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.
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.
Inverse synthetic aperture radar (ISAR) imaging on a turntable-tower test range permits convenient generation of high resolution two- and three dimensional of radar targets under controlled conditions, typically for characterization of the radar cross section of targets or to provide data for testing SAR image processing and automatic target recognition algorithms. However, turntable ISAR images suffer zero-Doppler clutter (ZDC) artifacts and near-field errors not found in the airborne SAR images they seek to emulate. In this paper, we begin by reviewing a technique to suppress ZDC while minimizing effects on the target signature. Next, turntable ISAR images of a vehicle formed at Georgia Tech's Electromagnetic Test Facility are used to demonstrate a computationally-efficient implementation of a backprojection (BP) image former. BP-formed ISAR images are free of all first order near-field errors. Finally, images generated using these techniques are compared to images obtained using electromagnetic prediction codes.
Recently there has been a large effort to improve RCS range performance. Reducing errors associated with an RCS measurement requires the identification of stray signal sources, highly accurate calibration, and an understanding of the target mount interactions. This paper will illustrate the potential errors resulting from target mount interaction. A complex RCS target of generic shapes was designed to illustrate target support interactions. Target features include a front wedge shape, a rear circular shape and a vertical fin. All the target features are separable in time using a 2-18 Ghz measurement system. The target features were designed to strongly interact with the ogival pylon. Measurements using the metal ogival support show strong interactions resulting from the shadowing effect produced by the metal ogival pylon. The measurements were repeated using a foam column mount. Since the foam column interacts much less strongly than the metal ogive, the foam column results are much more accurate.
Radar Cross Section measurements require the target to be in the far field of the illuminating and receiving antennas. Such requirements are met in a compact range in the SHF band, but problems arise when trying to measure at lower frequencies. Typically, below 500 MHz, compact ranges are no more efficient, and one should only rely upon direct illumination. In this case, the wavefront is spherical and the field in the quiet zone is not homogeneous. Furthermore, unwanted reflections from the walls are strong due to the poor efficiency of absorbing materials at these frequencies, so the measurement that can be made have no longer something to see with RCS, especially with large targets. We first propose a specific array antenna to minimize errors caused by wall reflections in the V-UHF band for small and medium size targets. Then an original method based upon the same array technology is proposed that allows to precisely measure the RCS of large targets. The basic idea is to generate an electromagnetic field such that the response of the target illuminated with this field is the actual RCS of the target. This is achieved by combining data collected when selecting successively each element of the array as a transmitter, and successively each other element of the array as a receiver. Simulations with a MoM code and measurements proving the validity of the method are presented.