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This paper discusses the configuration and performance of millimeter-wave measurement systems comprised of standard Harris Shaped Compact Ranges, Hewlett Packard (HP) 8510B Network Analyzer, and Millitech frequency extenders designed for use with the network analyzer. Millimeter-wave capabilities have been integrated into the Harris automated measurement system to allow computer controlled millimeter-wave compact range characterizations. This system offers a new measurement alternative for antenna and Radar Cross Section (RCS) measurements. Measured 35 GHz data from the Harris Model 1606 compact range, and 95 GHz data from the Model 1603 compact range are included.
This paper discusses the results of a recent study on the UHF performance of a Harris Shaped Compact Range. The design process for the dual polarized, 70% bandwidth UHF feedhorn is summarized. Measured data is presented for primary feedhorn patterns and for one-way CW field probe measurements with open-ended waveguide. The measured data is overlaid with computer predictions to validate the modelling tools and the measurement procedures. The automated quiet zone characterization procedure for amplitude and phase is also discussed.
In an earlier paper the virtual vertex compact range reflector was introduced and data from a specific design was reported. This paper describes the extension of the vertical vertex serrated edge concept to other reflectors that serve a wider range of application. Two new 12 ft focal length reflectors have been built that possess 3 ft and 6 ft diameter symmetric test zones. We describe the electromagnetic considerations and the mechanical design approach that has been used for these reflectors. We demonstrate the performance with field probe data showing the excellent surface accuracy of these units.
An image processing method which uses the bistatic scattered fields of a target obtained in a compact range is presented in this paper. The transmitting and receiving antennas can be either two compact ranges or one compact range and a horn antenna. The compact range reflector can be either focussed or defocussed so that a near field situation can be simulated. The bistatic scattered fields are collected as a function of frequency and the angle of rotation of the target. Then they are processed coherently to determine the cross-range and down-range scattering centers of the target. Experimental results are presented to validate this image processing technique.
The scattering characteristics of a moving object are different than those of a stationary one. A common property of a moving object is the doppler frequency shift. For objects with rotating propeller blades, another property is the frequency shift from the periodic rotation of the propeller blades. This frequency shift is due to an amplitude modulation of the carrier frequency. Two techniques are presented involving the use of amplitude modulation. The first technique is for the observation of such modulation due to slowly rotating structures in a compact range. These results can be scaled to obtain results for this structure rotating at any speed. This is a great advantage since the model does not have to be designed to rotate propellers at high RPM's. The second technique utilizes the power spectrum density of the dynamic scattering signature of the rotating propellers to reduce undesirable clutter in the measurement.
Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.
The MUSIC (Multiple Signal Characterization) algorithm uses an eigenvector decomposition of measured data to classify signals in the presence of noise. It has been used for the angular classification of multiple radar signal emitters and ISAR imaging. Interest has grown in stray signal analysis in anechoic chambers. This paper will discuss the modification and use of the MUSIC algorithm for the decomposition of field probe data to angular spectrum. A brief discussion of the MUSIC algorithm theory will be presented. Modifications required for use in compact range angular spectrum analysis will be discussed in detail. Requirements on field probe measurements will be presented as well as their effects on the implementation of the algorithm. Both one way and two way measurements are considered for their relationship to the array manifold. Finally, some experimental validation generated on the Boeing range will be presented.
A major concern for any user of a compact range for RCS or antenna measurements is the quality of the wavefront over the quiet zone and background chamber levels at the desired frequency band. Amplitude and phase ripple in the quiet zone is an indicator of how well the electromagnetic energy is collimated coherently by the reflector system. The amount of ripple depends on the reflector system, reflector edge treatment to reduce diffraction, frequency band and chamber interactions. Edge treatment techniques such as serrations on the reflector edge helps to reduce diffraction of unwanted energy into the quiet zone. Constructive and destructive interference of diffracted of energy in the quiet zone causes the amplitude and phase ripple. The goal is to reduce the ripple to a minimal amount. Previous studies by the author have compared two-way and angle transform field scanning techniques. The results strongly indicate that both techniques provide good agreement. The two-way method has the disadvantage of strong dependence on the scanning target directivity. A directive target will tend to disregard diffraction from the reflector edges because of its low sidelobes. Its advantage is that there is no need for external mixing equipment for the microwave receiver. The angle transform is simple in configuration consisting of a narrow flat plane or bar mounted on an azimuth positioner and rotated. The disadvantage is a summing of energy in the zero-doppler cell yielding an artifact ripple. Both of these methods also depend upon software gating algorithms including gate shape and width which directly influence the amplitude and phase ripple. The aim of this study is to compare the two-way, one-way field scanning techniques and the angle transform method. Can comparisons be made between the methods? Can a fairly good agreement be made? Are multi-path considerations addressed in one-way scan techniques? Hughes Aircraft will use one of the compact ranges at the Antenna Test Facility of Motorola GEG at Scottsdale, Arizona with the March Microwave (Vokurka) dual reflector system. Field scans will be measured using both the two-way and one-way techniques. The two-way method will use the 8 cm diameter disk as the scanning target, mounted on a horizontally traversed scanner. The one-way method will use a standard gain horn mounted on the same scanner. The angle transform method will use an 8 ft narrow flat plate rotated in the quiet zone. The field scans will be measured and studied at 10 GHz.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. The planewave spectral (PWS) probe technique is one proposed method for identifying the location and magnitude of range scattering. This paper presents the results of a PWS probe of a compact range. The interpretation of the PWS plots is discussed in comparison with the range geometry. Nine separate scattering centers are identified. The meaningfulness of the PWS picture was tested by introducing a known dipole source.
Georgia Tech Research Institute has designed, developed and installed a large outdoor compact range for the U.S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. Some of the unique hardware and software developed as part of the instrumentation and computer control tasks for the compact range are described.
An accurate and efficient method to compute the scattered fields in the target zone of a compact range main reflector is presented in this paper. This method is valid for reflectors of arbitrary rim shape with convex rolled edge terminations. The method is based on the uniform geometrical theory of diffraction (UTD) where the diffraction coefficients are obtained numerically using a procedure involving a physical optics line integration. Results obtained using the numerical UTD (NUTD) are compared to those obtained using UTD and corrected physical optics surface integration solutions for reflectors with both unblended and blended rolled edges. It is shown that the results are in good agreement. In addition, the NUTD is much more efficient than the traditional physical optics surface integration and provides diagnostic information on the effects of individual scattering mechanisms.
A Physical optics (PO) analysis of serrated edge reflectors is presented. It is shown that to obtain the true scattered fields in the target zone, one should use PTD (physical theory of diffraction) along with the PO solution. Using PTD, scattered fields of various serrated edge reflectors are presented. From these scattered fields, one can see that by proper design of the serrations, the edge diffracted fields can be reduced in the target zone. The edge diffracted fields, however, still may be too large for certain applications.
A novel technique is presented for the determination of the quiet zone field distribution of compact range antennas with serrated edges. The main reflector has a linearly serrated rim, so that the rim projection onto the reflector aperture plane is an arbitrary polygon. Additionally, the reflector aperture field is uniform in both amplitude and phase, and can therefore be expressed as a window function. The plane wave spectrum of the aperture field can then be obtained in closed form. Next, the spectrum is expressed at a plane in the quiet zone and the field is obtained by implementing an inverse fast Fourier transform (FFT) algorithm. Quiet zone field distributions are computed for various serrated rim configurations.
This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.
Images of an open box, closed box, and open and closed box on a ground plane were taken at the Hughes/Motorola Compact Range. Comparison of these images show the effect of multiple reflections in the image of an open box. A simple analytic/computer model was developed to interpret these multiple images. Data and analysis are presented on the various mechanisms that come into play in scattering from the open/closed box and the ISAR images generated as a function of the viewing angle for the box.
Serrations are used on Compact Antenna Test Range reflectors to reduce the effects of edge diffraction. It has been found that the traditional triangular shape for these serrations is not optimal and that more continuous shapes should perform better. To verify this, RCS measurements were performed on test targets consisting of strip reflectors terminated by end sections of various shapes. The RCS vs. angle data were corrected for the field irregularities caused by the measurement range and then converted to the induced current distributions on the targets, from which the fields in front of the targets were calculated using Physical Optics. These fields are equivalent to the test-zone fields of an actual Compact Range. The results are compared with theoretical data. The agreement is good.
To fulfill the future demand of highly accurate antenna-, RCS- and payload testing, MBB built a new antenna test centre at Ottobrunn (Ref. 1). This paper describes the development and qualification of the large, dual reflector Compact Range (CR) which has a plane wave zone of 5.5 x 5.0 x 6.0 m (w x h x d). It starts with the results of a detailed electrical trade-off study between different CR-concepts, followed by some mechanical/thermal construction aspects of the large, highly accurate reflectors. Finally, some qualification results are shown, covering the frequency range from 3.5 GHz up to 200 GHz (lowest frequency of operation approx. 2 GHz). The achieved plane wave performance (amplitude ripple ±5o, phase ripple ±5o, cross-polarization isolation > 40 dB) verifies the high quality overall system design.
To perform antenna measurements, it is necessary that the entire antenna structure is illuminated by a uniform plane wave. Since almost all sources radiate spherical waves, plane wave field configurations can be achieved locally only at very large distances from the source. The proliferation of compact range designs have reduced the distance required to achieve nearly plane wave field configurations to distances which can be satisfied by indoor facilities. While most compact ranges have been designed to create a nearly plane wave field configuration, at Arizona State University an operational compact range exists which creates a nearly cylindrical wave field structure. The pattern measured under cylindrical wave illumination is transformed, using analytical and numerical methods, to obtain the plane wave response of the antenna system. Measurements have been performed, using the cylindrical wave compact range, of a 15 GHz axial waveguide antenna on a 1/10 scale Advanced Attack Helicopter model. The measurements were then transformed and compared with those made of the same antenna system in a plane wave compact range facility.
In the past, models suspended indoors for radar cross-section measurements have weighed up to several hundred pounds, suspended on the order of 20' or less from the ground, and measured statically or rotated for great circle cuts. Under these circumstances it has been acceptable to choose the best string configuration from a signature point of view and simply wait for the model to reach a visually calm state before testing. However, indoor ranges are now requiring suspension of models weighing several thousand pounds 40' or more above the floor. In addition, the demand for imaging data during model conics requires both precise dynamic control and model stability. This work discusses techniques developed at Boeing's 9-77 Range in Seattle, to achieve model stability during suspension and manipulation. In addition, techniques to determine spring and damping constants of suspension systems for individual models are addressed.
Many targets today exhibit radar cross sections sensitive to the angular orientation of the target. While some of these targets have prominent scattering centers which can be exploited to obtain a relative positional reference, many targets unfortunately do not. In addition, many complex targets have a highly directional scattering behavior requiring careful alignment to the incident planar field. This need for accurate positioning has prompted the development of laser alignment techniques for the compact range. One such system has been under development at the ElectroScience Laboratory, and the designs and results of the first prototype are presented here. Performance goals and design criteria are discussed, and future improvements are considered. In addition, similar systems for feed and pedestal location reference systems are presented.