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This paper reports on the measurements portion of an ongoing research program at the Georgia Institute of Technology into the design, analysis and measurement technologies of radomes. Specifically this paper reports on a technique for the near-field measurement of radome performance. The motivation for the development of the near-field measurement technique for radomes is to identify the types of interactions which take place between the radome and the transmitted electromagnetic field. It is postulated that such phenomena as coupling to the radome wall, tip scattering, internal reflections and bulkhead reflection would be easier to identify through near-field measurement than far-field measurement. * This work was supported by the Joint Services Electronics Program and Northrop Corporation
Three measurements commonly used in the absorber industry include absorber testing in NRL arches, testing absorber in waveguides, and testing performance of anechoic chambers. These measurements are closely related. All are looking for the size of one E field vector in the presence of several other E fields of variable amplitudes and phases. The information is extracted from the behavior of the sum as a function of some physical position change or frequency change. Computer controlled, synthesized sources and receivers have had two effects on the way these measurements may be taken and interpreted. First, the data are now available as a series of numbers in a computer instead of a series of lines on a piece of paper. Precise and elegant processing is available to extract the information from the data. Secondly, since frequency changes are made rapidly with this type of instrumentation, and precise position changes are made slowly, the data may be taken for many frequencies at each physical position, this makes it possible to extract additional information from the observed data changes as a joint function of frequency and position. These changes are spread throughout the block of data for signal amplitude vs position and frequency.
This paper presents the results of a study conducted to determine the effects of reflector surface errors on compact range performance. The study addressed only the reflector surface accuracy and not edge scattering, reflector illumination or reflector size. The study showed that low spatial frequency sinusoidal surface errors are significant contributors to amplitude ripple in the quiet zone field. Simple equations are presented for estimation of quiet zone amplitude ripple due to reflector surface errors. The study also presents measured surface error for two manufactures of reflector panels. The spectral (plane wave) components of the reflected field are displayed for a compact range reflector composed of a collection of these panels. *This work supported by the U. S. Army Electronic Proving Ground, Ft. Huachuca, AZ and the Joint Services Electronics program
Prime focus fed paraboloidal reflector compact ranges are used to provide plane wave illumination indoors at small range lengths for antenna and radar cross-section measurements. The "quiet zone", which is the region of space within which a uniform plane wave is created, has previously been limited to a small fraction of the reflector size. A typical quiet zone might be six feet by four feet for a ten foot radius reflector.
A compact range is a facility used for the measurement of antenna and target scattering parameters. It offers many advantages over other types of ranges, and consequently a lot of effort is being directed towards the improvement of compact range performance. This discussion focusses on the reduction of diffracted fields from the termination of the parabolic main reflector. *This work was supported in part by the National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
In this paper a theoretical study is reported of the electromagnetic performance of the new Scientific-Atlanta compact range reflector system. The reflector consists of a 15-foot semicircular parabolic reflector with a 5-foot blended rolled edge added to the circular section and skirt mounted on the base. The performance of this system is examined in terms of its probed near field data at the center (36-feet) and back end (50-feet) of the target zone. The calculated results are for the three-dimensional reflector and include the skirt and blended rolled edge diffracted field as well as the aperture blockage scattering caused by the feed and associated feed/mount structure. The potential target zone size based on these parameters is presented as a function of frequency and desired ripple level requirement. *This work was supported by the National Aeronautic and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG-1613 with The Ohio State University Research Foundation.
Measurements of the Electromagnetic Field in the quiet zone of Scientific-Atlanta's Model 5753 Compact Range are presented. The Model 5753 is believed to be the largest high frequency compact range yet built and measurements demonstrate a quiet zone exceeding 8 ft. high by 12 ft. wide. Both field probe measurements and pattern comparison measurements are presented in the operating frequency range of 1-94 GHz.
The geometry of a dual parabolic cylindrical reflector system and its projection on the plane of symmetry are shown in Fig. 1. It consists of a point source f and two parabolic cylindrical reflectors S1 and S2 with curvatures in two orthogonal planes and of focal lengths F1 and F2, respectively. Alpha is the angle between the generator of the sub-reflector and the main beam direction. It is considered positive if the generator of S1 rotates towards the main reflector and negative if it rotates in the other direction. The feed orientation is specified by the angle gamma which is the angle between the feed axis and the normal from the feed point f to S1. The feed angle is 2f , which is the angle subtended by the sub-reflector in the principal planes. The sub-reflector geometry is selected such that it subtends equal angles from the feed in two orthogonal planes. The main reflector geometry is selected to intercept all reflected rays from the sub-reflector. The projected aperture of the main reflector is rectangular in shape, the sides of which are denoted as A and B. The ratio between these aperture sides is given by [1]. The separation between the two reflectors may be increased by any value delta which results in reducing the aperture dimensions. The feed radiation pattern is assumed to be rotationally symmetric and its electric field distribution in the feed coordinates is represented by cos??. If the feed is vertically polarized in the asymmetric plane (along y-axis), the y and x-components of the aperture field are the co-polar and cross-polar components, respectively. The feed may also be horizontally polarized along the unit vector [sin (?+a) i + cos (?+a) k] in the symmetric plane. In this case the co-polar and cross-polar components of the aperture field are the opposite of the above case.
Recently there has been an increasing interest in the compact ranges for antenna measurements. Most of the early attempts used lenses, but recently reflectors have become more acceptable [1]. Both dual cylindrical parabolas and symmetric or offset paraboloidal reflectors have been used as compact ranges. In this paper, the performance of both systems is studied and some of their advantages and shortcomings are presented. For both systems the aperture field distributions, under varying conditions have been determined and compared.
Flam & Russell, Inc. has developed a short pulse radar cross section measurement system (Model 8101) which operates from VHF up to L band. This paper describes operation of the system, with emphasis given to the design considerations necessary to minimize susceptibility to a number of problems that have imposed serious limitations on achievable sensitivity at lower frequencies in pulsed RCS outdoor measurement systems. These problems have been, to a great extent, solved in the current system design. The system has been designed for use in outdoor range facilities with a variety of target sizes. A w ideband, high power transmitter is capable of producing pulses 50-350 nanoseconds wide at peak levels of up to several kilowatts. A phase coherent wide bandwidth receiver provides amplitude and phase information at video for sampling. A maximum of four independently located range gates may be selected and set with a resolution of one nanosecond. The data collection system features a three-tier processor structure for dedicated position data processing, target data processing, and system I/O and control, respectively. A real time display of RCS versus position coordinate is available to the operator, as well as a real time indication of the presence of radio frequency interference (RFI). A 60 foot reflector antenna equipped with a duo polarized feed provides full scattering matrix capability with 30 dB of polarization isolation and better than 50 dB of "ghost" suppression. Careful antenna structure and transmission line design has eliminated reverberation or "pulse ringing" problems. A radar "figure of merit" (ratio of peak transmitted power to receiver noise floor for the required pulse bandwidth) of better than 150 dB has been achieved.
In this paper we consider those factors having primary impact on submicrowave RCS measurements in outdoor (ground-bounce range) environments, including: 1. The target illumination problem, reflecting fundamental limits on antenna size and height 2. Measurement sensitivity as limited by thermal noise and radar frequency interference (RFI) 3. Antenna selection at VHF frequencies 4. Ground-bounce effects near Brewster's angle. 5. Clutter (due to either terrain or target support) and clutter suppression techniques. Some improvements to basic RCS measurement range design are analyzed in detail, with emphasis on mobile (variable range) antenna/radar systems.
Prediction methods currently being developed for estimating the Radar Cross Section (RCS) of a complex target are based on the concurrent use of different numerical techniques each being employed in the region where it performs best. Since the high frequency techniques and the numerical methods used in the computation must deal with important rapid phase and amplitude fluctuations of the resultant scattered field, it is sometimes very difficult or impossible to know to what extent the computed solution is valid, unless measurements are available for comparison purposes.
There has been much interest recently in Ka-band scattering measurements. Although Ka-band components are steadily improving, one is presently limited to narrow bandwidths (2 GHz) for higher power (more than 100 milliwatts) applications. If the whole wavelength bandwidth was useable, one could scan the target in frequency, transform to the time domain and simulate a very narrow pulse illuminating the target. With such a system, one could identify scattering centers separated by just an inch or so. * This work was supported by the National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG 1613 and Sandia National Laboratories under Contract No. 58-3465.
Gating is a widely used technique of improving RCS measurements. However, the exact type of gating used has a dramatic effect on such parameters as dynamic range and clutter rejection. Time Domain Gating offers significant advantages over software gating as used in some network and spectrum analyzers. This paper explores a technique used by Scientific-Atlanta in CW and FMCW RCS measurements. With the adaptation of an external computer controlled hardware gating unit, existing RCS and antenna systems can be retrofitted for significant performance improvements.
The ogival target-support pedestal as shown in Figure 1 is claimed to have a low radar cross section (RCS); yet, it can handle very large and heavy structures. This paper attempts to find out whether this claim is true through analysis as well as measurements. The pedestal backscatter is just one aspect of this study. Another more serious issue is associated with the bistatic scattering by the pedestal which influences the target illumination. * This work was supported in part by the National Aeronautic and Space Administration, Langley Research Center, Hampton, Virginia, under Grant NSG-1613 with The Ohio State University Research Foundation.
The advent of improved compact ranges has promoted the development of a test body, named the almond, to facilitate the measurement of scattered fields from surface mounted structures. A test body should at least have the following three features: (1) provide a very small return itself over a large angular sector, (2) provide an uncorrupted and uniform field in the vicinity of the mounted structure and (3) have the capability to be connected to a low cross-section mount. The almond satisfies the first two requirements by shaping a smooth surface which is continuous in curvature except at its tip. The name almond is derived from its surface similarity to the almond nut. The surface shaping provides an angular sector where there is no specular component. Hence, only low level tip and creeping wave scattering mechanisms are present resulting in a large angular quiet zone. The third requirement is accomplished by properly mounting the almond to a low cross-section ogival pedestal. The mount entails a metal column between the almond and the pedestal covered with shaped absorbing foam. These contoured pieces hide the column and form a blended transition from the almond to the pedestal and yet allow an unobstructed rotation of the almond. Backscatter pattern and swept frequency measurements performed in our compact range illustrate the scattering performance of the almond as a test body. The almond body alone has a backscatter level of -55 dB/m(squared) in its quiet zone. Comparisons of measured hemisphere backscattered returns on the almond are made with those calculated of a hemisphere over an finite ground plane for both principal polarizations for a verification performance test. * This work was supported in part by the National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
This paper discusses some of the applications of high-resolution coherent radar image processing techniques in unimproved indoor facilities. The techniques are particularly useful in situations where traditional indoor range chambers are unavailable or impractical. Experiments in an 18-foot-high warehouse building have shown that useful measurements can be made at close quarters, in a high-clutter environment.
The total scattered field from a complex, electrically large target is comprised of several scattering mechanisms. Each individual scattering center has its unique scattering character as a function of target orientation and frequency. Hence it is desirable to isolate a scatting mechanism to acquire its dependencies. The extraction of a particular mechanism from other mechanisms can be done by knowing its spatial location. The spatial information is contained in the phase associated with each scattering mechanism. Thus, a "filter" can be constructed to numerically extract the scattering properties of a particular scattering center by recognizing its spatial location. This can be done as a function of angle and frequency from calculations or measurements. The filter can be realized either as a data smoothing process using special mean square error methods or as a gating procedure in a transformed space of the variable of interest to isolate a mechanism. Examples will be shown to demonstrate the properties as well as accuracies of both techniques. * This work was supported in part by the National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
In recent years many of the problems making RCS measurements on a compact range have been addressed [1,2,3]. Factors such as ripple and taper in the target zone have been analyzed and existance of lower level effects such as stray radiation in the chamber. This paper discusses this problem and the way it was addressed in the design of the Harris Model 1606 Compact Range shown in Figure 1, 2 and 3. This range was designed to operate from 2 to 18 GHz with a six foot quiet zone with extension of the frequency range to 95 GHz possible.
Some large paraboloidal reflectors, such as radio telescopes, have a means of remotely adjusting the axial position of their feed. For these reflectors it takes little effort to measure a "focus curve", i.e. the intensity of radiation received as a function of the axial position of the feed. The purpose of this note is to point out how this curve can be interpreted to give information about the reflector surface.