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A.R. Howland (The Howland Company, Inc.),T.J. Lyon (The Howland Company, Inc.), November 1986
This paper describes specially constructed instrumentation and positioning systems used in evaluating RF absorber, discusses measurement techniques, and presents data and conclusions from current programs. The selected absorbers which were evaluated are typical of those used in anechoic chambers and terminated ranges for antenna, radome and RCS testing.
W.D. Burnside (The Ohio State University ElectroScience Laboratory),I.J. Gupta (The Ohio State University ElectroScience Laboratory),
J. Clerici (The Ohio State University ElectroScience Laboratory),
R.C. Rudduck (The Ohio State University ElectroScience Laboratory), November 1986
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.
M.S.A. Sanad (University of Manitoba),L. Shafai (University of Manitoba), November 1986
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.
M.S.A. Sanad (University of Manitoba),L. Shafai (University of Manitoba), November 1986
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.
Aerojet ElectroSystems of Azusa, California, has a commitment to design and test antenna systems in the 149-184 GHz frequency range. Many problems have arisen in attempting to design and implement a feasible system to make antenna measurements. This paper will discuss the problems that have been encountered, the solutions that have been implemented, and future considerations. Topics discussed will include availability of equipment, source stabilization, signal detection, and range design considerations.
J.M. Ralston (System Planning Corporation), November 1986
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.
G. Ratte (Laval University),G.Y. Delisle (Laval University),
M. Lecours (Laval University), November 1986
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.
W.D. Burnside (The Ohio State University ElectroScience Laboratory),D. Jones (The Ohio State University ElectroScience Laboratory),
M. Gilreath (National Aeronautics and Space Administration),
P. Bohley (The Ohio State University ElectroScience Laboratory), November 1986
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.
J.L. Bradberry (Scientific-Atlanta), November 1986
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.
A. Lai (The Ohio State University ElectroScience Laboratory),W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1986
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.
A. Dominek (The Ohio State University),H. Shamansky (The Ohio State University),
R. Barger (NASA Langley Research Center),
R. Wood (NASA Langley Research Center), November 1986
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.
A. Dominek (The Ohio State University),L., Jr. Peters (The Ohio State University),
W.D. Burnside (The Ohio State University), November 1986
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.
J.K. Conn (Harris Corporation),M.L. Foster (Harris Corporation), November 1986
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.
S.H. Lim (Andrew Antenna Company Ltd.),R. Boyko (Andrew Antenna Company Ltd.), November 1986
This paper describes the mechanical as well as electrical measurement of doubly curved reflector antennas. The techniques developed for measurement of the new Canadian RAMP Primary Surveillance Radar antenna are described. Instead of a conventional full size template fixture to measure the antenna contour accuracy, an optical twin-theodolite method is used. The problems of the method are discussed and a new simplified analysis for calculating reflector error of doubly curved antennas is presented. Reflector errors are calculated and displayed concurrent with the actual measurements. The measurement of primary and secondary patterns for such antennas are described. Included are brief descriptions of the improved Andrew pattern test range and anechoic chamber facilities.
J.G. DuMoulin (David Florida Laboratory),F. Buckles (David Florida Laboratory),
H. Raine (David Florida Laboratory),
P. Charron (David Florida Laboratory), November 1986
Intermodulation products are generated when multiple frequency signals are applied to a circuit element having a non-linear input/output characteristic. The space community first became aware of the Passive Intermodulation (PIM) problem due to the difficulties encountered by the Fleetsatcom program.
This paper describes the design philosophy and construction of an "UHF Test Set" to satisfy the test requirements of the M-Sat program.
Predicted response curves of the IM characteristics of HPA's, ferrite isolator and passive hardware are presented as tools for the filter designer interested in the test set design, with the required rejection values for the various filters to be used.
W.D. Burnside (The Ohio State University ElectroScience Laboratory),A. Dominek (The Ohio State University ElectroScience Laboratory),
M. Gilreath (NASA/Langley Research Center),
R.C. Rudduck (The Ohio State University ElectroScience Laboratory),
T-H. Lee (The Ohio State University ElectroScience Laboratory), November 1986
The potential for an aircraft to fly directly through a ground station-to-satellite link becomes more significant if the link is located closer to an airport, obviously. Because this situation is much more likely near airports, it is appropriate to examine the effects of such an encounter.
M. Cuchanski (RCA/Astro Electronics),C. Renton (RCA/Astro Electronics),
D. Wozniak (RCA/Astro Electronics), November 1986
This paper describes the computer system interfaces and hardware additions which provide engineers enhanced capabilities and greater flexibility from their antenna measurement systems. RCA Astro-Electronics has built three outdoor antenna ranges, each equipped with Scientific-Atlanta Antenna Analyzers. Two Series 2020 and one Series 2080 Analyzer are used to perform data acquisition and preliminary data processing for the three antenna ranges. Both S-A 2020 system computers have direct interface capabilities with two remote computer systems, which are primarily used for antenna design and analysis. The S-A 2080 system is interfaced indirectly, via one of the S-A 2020 range computers. Direct line and modem interfaces provide user access to remote computer software and allow up and downloading of measured or computed data files. Using RCA software resident in each range computer, measured data files are unpacked, reformatted and downloaded for off-line processing. This process accelerates test schedules and allows analysis software to process data files from three antenna ranges in a single data format. Other enhanced system features include access to remote analysis software, requiring large disc storage space, for real-time evaluation at the antenna analyzer location.
A.D. Ergene (General Dynamics Convair Division), November 1986
Theory, design, and test results of a conformal test coupler that can be mounted on the exterior of a vehicle for direct on site measurements of a fuselage mounted L-band antenna are presented.
When there is a requirement to test vehicle instrumentation for radiated power, signal format, etc., a desired method is to couple the test equipment directly to the dedicated antenna on the vehicle. Cavity test couplers have been traditionally employed for direct measurements at the antenna under test. However, a low-profile conformal cavity has poor performance when there is no match between the energy radiated by the antenna and the received energy in the cavity. To suppress unwanted resonances and a high Standing Wave Ratio, such mismatched cavities are loaded heavily with absorber material inside, and in operation exhibit high sensitivity to surface contact and high insertion loss, yielding nonrepeatable measurements.
The coupler presented here is a nonresonant cavity that supports a TEM mode compatible with the radiation from the vehicle antenna and avoids spurious resonance spikes. It exhibits extremely low insertion loss and is not sensitive to mounting misalignment. A circumferential microstrip radiator with multiple feed points and a matching network on the back side of the same substrate is wrapped around the inside of a top-hat cylindrical aluminum container. The particular test cavity was designed for the vertically polarized L-band IFF antenna on the cruise missile; however, the same principle makes testing of other fuselage-mounted antennas easier and more reliable.
The paper describes a simple feed horn designed to illuminate an antenna test range used to measure broad bandwidth antenna patterns with rotating linear polarization. Principal requirements of the feed are equal E and H plane beamwidths with minimal sidelobes in all planes. These characteristics are required to avoid undesirable pattern modulation caused by varying specular scatter and unequal beamwidth vs rotation angle. A survey of pyramidal, conical, and diagonal feed horn patterns revealed that each configuration has high sidelobes in at least one plane making it undesirable for the intended application. Both the pyramidal and conical horns have high side lobes in the E plane. The diagonal horn has very good sidelobe characteristics in the principal planes, but has 13 to 16 dB sidelobes in the diagonal plane.
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