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Currently many new compact range facilities are being constructed for making antenna pattern measurements indoors. Limited suppression of stray signals ~ due to range layout, confined surroundings and residual absorbing material reflectivity ~ represents a limitation on the accuracy of the measurements made in these facilities. Time-gating of the compact range signal appears to be a very attractive technique to reduce unwanted reflections. The authors have carried out an experimental investigation of time gating in a compact range. It is demonstrated that time-gating can improve the uniformity of the aperture field by removing the feed backlobe radiation; and, it is demonstrated that time-gating can remove the effects on a pattern of certain room reflections and of feed backlobes. When compared to conventional methods of reducing reflections based on placement of absorber, time gating appears equivalent. It does not appear however that time gating improves the conventional methods, except for measuring wide beamwidth antennas.
A technique to determine the radiation centers of large reflector antennas in a given direction is presented in this paper. Coherent processing is used to determine various radiation centers based on far zone pattern data of the antennas provided that adjacent centers are separated far enough so that their locations can be resolved. Numerical results for processing of two reflector antennas, a prime focus fed and a Cassegrainian, are presented to validate this technique. The diagnostic value of this technique for reflector antennas is demonstrated by processing the actual measured pattern and identifying some unexpected radiation centers. One can also use this technique to fine tune numerical pattern simulations of reflector antennas.
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
To construct an image of a complex target, increasingly smaller range cells are desired. To decrease range cell size and improve resolution the bandwidth must be increased. The bandwidth of RCS measurements utilizing an HP8510 based collection system is limited to a maximum of 801 frequency points. This paper will present a technique to extend the bandwidth by using off-line processing to overcome this hardware limitation. Fully focused ISAR images formed at millimeter wave frequencies, with the addition of eternal mixers, will be demonstrated. Bandwidths of 5, 10, and 14 GHz, measured from 7-17 GHz and 26-40 GHz will be shown. The comparison of these focused images, with 3.2 cm, 1.6 cm, and 1.1 cm resolution will illustrate a powerful engineering tool to analyze closely spaced scatterers.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
Conventional radar imaging requires large amounts of data over large bandwidths and angular sectors to produce the location of the dominant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency scan is the backscattered response of a target. A different plane wave illumination angle can be conveniently obtained by offsetting the feed horn from the focus of a compact range reflector without rotating the target. The two- and three-dimensional target information for the location of the dominant scattering centers is then obtained from the band limited impulse responses of these swept frequency scans.
We have reviewed the sampling-interval requirement associated with the algorithmic problem of extrapolating near-field radiation measurements to the far zone and concluded that the far-zone sampling rule (d?=?/D) works as well in the Fresnel portion of the near zone. In addition, we find that the angular window, W, over which the Fresnel-zone field must be measured is approximately W - 2D/R radians in width, where D is the nominal diameter of the antenna and R the range at which the near-field data are taken. This guideline is valid when one uses an integral extrapolation scheme, as opposed to a modal one, since the paraxial approximation gives some assurance that field contributions from points outside the sampling window will contribute negligibly to the far-zone amplitude. We have also looked at the sampling requirements associated with extrapolating near-field RCS measurements to the far zone and concluded that windowing techniques can reduce the magnitude of the bistatic scanning task dramatically.
It is common practice in antenna measurements to use a moderately directive source (or 'probe') antenna, to minimise the effect of reflections. The illumination of the test antenna then exhibits a degree of non-uniformity. A correction scheme has previously been proposed for spherical near-field measurements. This paper describes a new probe-correction algorithm that can be used in conjunction with spherical near-field or 'conventional' measurements. It is operable with a minimum amount of measured data (for either the test antenna or the probe). It may also be used for probe-correcting calibration measurements using a gain horn.
This paper reports on a research program at Georgia Tech to utilize spherical surface near-field measurements in the evaluation of the electromagnetic performance of radomes. Near-field measurements are performed on a spherical surface which encloses a non-spherical radome. A backward transform technique has been developed in which the spherical near-field measurements are used to determine the field on the outer surface of the non-spherical radome shape. This backward transformed field on the outer surface of the radome is compared to the backward transformed field at the same locations without the radome in place. Measured data is presented for a tangent ogive radome. The data shows the point by point phase characteristics of the radome wall. Several additional measurements are displayed showing the ability to detect small dielectric patches attached to the outer surface of the radome, simulataing radome defects.
NRL arches have been used to measure the reflection properties of material samples. The transmit and receive horns in the arch fixture are oriented to obtain the "specular" reflected field from the material sample. In actuality, the measured scattered field also has components which emanate from the edges of the material sample due to diffraction. This behavior is confirmed with moment method and geometrical theory of diffraction (GTD) calculations. Although edge diffraction has a different scattering behavior than does specular reflection, the GTD diffraction coefficients simulate specular scattering near the reflection shadow boundaries. These diffraction coefficients contain the material's specular properties by incorporating this information through the specular reflection coefficients for the material and geometry. Hence proper sample mounting is required to insure an adequate measurement of the material properties, since the edges dominate the measured scattered field.
Results of an experimental study of the interactions between a scattering target and the absorber-coated walls and ceiling of the OSU Compact Range Anechoic Room are reported. A 6 ft. square flat metal reflector was mounted in the quiet zone and oriented at selected angles non-orthogonal to the range symmetry axis. In theory, this target (when non-orthogonal) has a relatively low backscattering signature, and a strong planar bistatic scattering beam which can be pointed at several regions and absorber types in the room. By processing, the bistatic iteration terms can be separated form the plate backscatter, and frequency domain spectra and/or transient response signatures of the different mechanisms produced. Th paper will present calibration information on the actual performance of the bistatic scattering beam of the plate, and measurements of both backscattering and bistatic scattering of the absorber-coated walls in the ESL chamber. Suggested guidelines for use of this as a standard anechoic room diagnostic test will be discussed.
This paper presents a simple and straightforward technique which significantly improves the performance of some anechoic absorbing materials. The method is easily applied to existing absorbers and chambers and does not change the basic design of the material. The technique involves the proper placement of additional absorbing materials between the shaped structures of the absorber to reduce major scattering contributions. These scattering mechanisms are demonstrated in the paper with measured evaluation data for various absorber types and sizes. The effectiveness of the technique has been best realized for pyramidal shaped absorbers 24 inches and longer and for normal plane-wave incidence. Improvements in the absorber's reflectivity of up to 30 dB have been demonstrated. An example illustrating the method for the reduction of the backwall RCS level of a compact-range chamber is presented.
Three test methods have been developed and validated for characterizing materials at VHF and UHF in an indoor environment. The first method employs a resonant strip-line cavity for the independent determination of permittivity and permeability from .15-2 GHz. The planar field geometry and sample configuration permit evaluation of material anistropy. Measurements are taken on an Automatic Network Analyzer (HP 8510 ANA). The second method measures the reflection/transmission (R/T) of planar material samples at UHF. This is a free space measurement performed in an anechoic chamber. Data is taken from .2-2 GHz using two dual ridged horn antennas and the ANA. A calibration method has been developed for the ANA to correct for measurement errors. Off-set shorts and thru delays are used in this technique. The third technique evaluates reflection performance of materials from 150-250 MHz. This technique employs a custom designed corner reflector antenna. Only one such antenna is needed due to the calibration technique. These methods allow a synergistic approach to material development. Candidate material can be evaluated using the cavity or R/T systems. Material designs can then be tested on either the UHF and/or VHF systems.
A free-space RF absorber material (RAM) has been developed and optimized for frequencies above 30 GHz. It is particularly suited for use on equipment and fixtures for RCS, antenna, radiometric, and quasi-optical testing. The material has unique geometry which yields enhanced RF performance when compared with conventional wedge or pyramidal absorbers. Mechanically, the material is elastic, resists damage from flexing or repeated contact and is non-flammable and non-toxic. It offers advantages in size, durability, and mechanical uniformity over previously available products. Data describing RF and mechanical performance are presented.
This paper describes the measurement requirements of a phased array comprised of three sub-arrays and the test system built to measure it. To evaluate the performance of the array, it is necessary to measure the radiation patterns of all three outputs at various azimuth scan angles. Because the relative phase and amplitude between the elements is an important performance parameter, if data is to be taken "on the fly", then high speed measurements are required. In addition, when taking elevation patterns through the peak of the beam, which has been scanned in azimuth, the polarization of the antenna under test changes with elevation angle. Consequently, since the patterns are to be measured to matched polarization, the transmit antenna polarization must be varied as a function of elevation angle. To complicate matters, this is a non-linear relationship. The test system architecture and resultant performance capabilities are presented.
This paper describes recent advances in antenna measurement instrumentation for millimeter frequency applications. Application of a new, lightweight, programmable, ruggedized signal source at 40 and 60 GHz is outlined. An RF instrumentation system for millimeter frequency antenna range application is detailed. A millimeter-to-microwave converter is described which improves millimeter antenna range performance. System performance levels are predicted. Compact range configuration and operation at millimeter frequencies is detailed. Specific measurement examples are presented to demonstrate the measurement sensitivity which can be achieved.
Antenna and Radar Cross Section measurements require a large amount of data collection. Network Analyzers are often used to characterize these systems, and although these data ideally are collected automatically by computer it is not unusual for a single characterization to require many hours or even days to perform. We describe a technique for speeding up these measurements by at least an order of magnitude. Clearly making measurements in an hour that formerly took a day or making measurements in a day that formerly took two weeks is extremely appealing. The method we describe may be used for applications which require a large number of automatically performed measurements with sequentially swept frequencies, but which find lack of speed in tuning the network analyzer to be a limiting factor. Antenna, and Radar Cross Section measurements benefit substantially since frequency response measurements must be repeated many times to provide spatial characterization.
Recent advances in RCS measurement techniques, microwave hardware receiver technology and computer capability have drastically altered the price and performance considerations of turn key RCS measurement systems. Access to 'real time' data and processing improvements are a few of the issues addressed in lower cost and compute intensive configurations available in today's marketplace. This paper explores a systems approach to the wide variety of components configurable for 'state-of-the-art' RCS measurements. High performance, flexibility and productivity are emphasized.
The ISAR image is a domain that possesses many of the spatial physical characteristics of the target. Certain procedures can be performed in the image domain that are equivalent to physical operations on the target. These operations include the ability to modify the amplitude of the scatterers that are represented in the image and, after performing these modifications, subjecting the image to an inverse transformation that recovers RCS data of the whole body as a function of frequency and aspect angle. The RCS plots obtained by transforming the edited image are representative of similar modifications made to the physical body and are of value in eliminating the need for many model modifications and retests in low-observable model development. This paper describes, using simulated and actual target data, some of the procedures that can be fruitfully applied in this type of analysis.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.