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Antenna testing is generally predicated upon using a Standard Gain Antenna co-located with the Antenna Under Test (AUT). At HF/VHF/UHF frequencies Standard Gain Antennas (Horns) are too large for co-location on the Antenna Under Test's (AUT's) rotating platform. Co-location is desirable for maintaining equal range lengths and equality in the environment; a prime source of multipath effects. In the HF/VHF/UHF frequencies bands the Log-Periodic is quite often employed as the "Source Antenna" but not necessarily the "Reference Antenna". Dipoles, monopoles above a large ground plane and horn antennas are often chosen as the Reference Antenna. The Log-Periodic Antenna, although also large, has pattern characteristics similar to the Standard Gain Horn's, has a superior and flatter bandwidth and is considerably lighter in weight. This paper will discuss a technique for using a Log-Periodic Antenna as a Standard Gain Antenna when co-location with the Antenna Under Test is not feasible.
This paper will address the issue of estimating and measuring the RCS of simple objects on a finite sized ground plane. RCS measurements of a one inch diameter hemisphere on a ground plane were collected at X-band and are shown to compare favorably with two different models of a hemisphere on a finite pc ground plane; a simple Geometric Optics (GO) model, and a EM Body of Revolution (BOR) model. The beauty of the GO model is borne out due to the insight which is gained in understanding the scattering mechanisms taking place. With the addition of a Physical Optics traveling wave component for the ground plane, the two models can be brought into good agreement with the measured data. Measurements were also conducted for a cylinder, cone and bicone whose results are also presented.
The measurements of an antenna at FM frequencies integrated in the bodywork of a terrestrial vehicle is a extremely (sic) delicated (sic) problem that will be larger if a ground plane must be simulated. An algorithm based on two measurements (magnitude and phase of the field components E() and E (1) on a scale model made in an anechoic chamber, has been developed to solve this problem. These measurements correspond to the value of the desired conical cut (only a narrow range of angles above the horizon is significant), and the associated cut needed to measure the specular reflection on the simulated ground plane.
A plasma-absorber system consists of a membrane that confines a collusional gas at atmospheric pressure and an ionization source. The ionization source generates a dense plasma that tenuous near the confinement membrane. An electromagnetic wave propagating through this plasma is attenuated. The mechanism for absorption is momentum transfer among electrons, driven by an incident wave, and a gas. Because the momentum-transfer collision rate, v, at atmospheric pressure can be as high as 870 x 10^9 s-1, the 3-dB bandwidth for absorption (~v/20) is approximately 40 GHz. The plasma thickness between the source and confinement membrane is approximately one wavelength at the lowest frequency. The magnitude of absorption depends on this thickness, the maximum electron number density, and the electron density gradient. A smooth gradient reduces reflections. This paper discusses a theoretical model that predicts general absorption and reflectivity phenomena. Experiments have quantified 40-dB performance at VHF using a 4-mil polyethylene vessel, and at X-band using a 2-mil Mylar inflatable support system. Applications to precision RCS measurements include reduction of backwall reflections and target interaction with the ground plane, and a shutter for reference targets.
To characterize a material's electrical performance or to understand a material's affects (sic) on electromagnetic systems, the constitutive parameters (e, u) of the material must be determined accurately. Materials with high dielectric constants, high loss tangents, or which are layered or complex (e.g., frequency selective surfaces, radomes, radar absorbing material, etc.) are difficult to measure and analyze. For example, germanium is an infrared window substrate in high performance aircraft. The germanium is doped to 1 - 4 ()-cm to raise the maximum operating temperature and to provide electromagnetic shielding. The material is very brittle. The standard methods (coaxial, waveguide, and cavity) are difficult to use. The brittle germanium pieces cannot be made thin enough or have a center conductor hole inserted making coaxial donuts are nearly impossible to fabricate and use. Usable waveguide samples absorb the transmitted energy needed in standard waveguide tests. The brittle sample cannot be made thin enough for X-band measurements or above. The sample, having a high dielectric constant, and having a high conductivity, reduces the Q of resonance techniques difficult and not repeatable. This paper discusses our methodology and shows comparisons with calculations. The technique is based on reflection measurements against a ground plane standard. This technique requires more measurements than other techniques, but the results are numerically more stable.
Two measurement systems for Radar Cross Section (RCS) measurements are described. One system employs propagation over a ground plane whereas the other system employs free space propagation in an anechoic chamber for target illumination. A comparison of measured data for different targets over a wide range of frequencies is presented. The measured data is also compared to RCS data computed using the Numerical Electromagnetics Code (NEC) computer program. The results may be useful for evaluating radar systems operating in the HF band of frequencies.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.
Recently an antenna pattern measurement technique has been developed which eliminates the effects of the finite ground plane on which the test antenna is mounted. The scattered fields from the edge of the ground plane can often cause perturbations in the total fields, and thus, result in significant differences in the measured patterns as compared the theoretical predictions. This technique consists of the measurement of the edge diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (obtained assuming an infinite ground plane) and “corrected” experimental antenna patterns. Experimental results are given from an open-ended waveguide opening into both circular and square ground planes.
A new approach has been developed to achieve an octave bandwidth, reduced size feed fot compact range reflectors. It can provide highly isolate, orthogonal polarizations with a minimal size, suitable for operation at frequencies down to 500 MHz and below. Its construction is relatively simple, with only a few specific dimensions. The beam-width is compatible with compact range reflector feed requirements. The method uses crossed dipoles over a small circular ground plane, with a rim to equalize the E- and H- plane patterns. Parasitic elements are employed to extend the bandwidth with matching provided via a section built into the feed line. The design was optimized using the Numerical Electromagnetics Code (NEC) computer program.
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.
Near-field testing of a very low sidelobe, L-band, 32-element, linear phased array antenna was conducted. The purpose was to evaluate testing and calibration techniques which may be applicable to a much larger, space borne phased array antenna. Very low sidelobe performance in a relatively small array was achieved by use of high precision transmit/receive modules. These modules employ 12-bit voltage controlled attenuators and phase shifters operating at an intermediate frequency (IF) rather than at RF. Three array calibration techniques are discussed. One technique calibrates the array by means of a movable near-field probe. Another method is based on mutual coupling measurements. The last technique uses a fixed near-field source. The first two calibration methods yield substantially the same results. Module insertion attenuation and phase can be set to 0.02 dB and 0.2 degrees, respectively. Near-field measurement derived antenna patterns were used to demonstrate better than -20 dBi sidelobe performance for the phased array. Application of increasing Taylor array tapers showed the limitations of the measurement systems to be below the -35 dBi sidelobe level. The effects of array ground plane distortion and other array degradations are illustrated.
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.
Versatile test bodies are extremely useful for RCS measurement facilities for many reasons, some of which are listed below: 1) evaluate the performance achievable for a given measurement facility 2) measure the RCS of components normally mounted on a ground plane, and 3) terminate a target pedestal in order to measure its cross-section since most pedestals are designed to attach directly to a target. In order to perform all of these functions a versatile test body should have flat sections to mount components efficiently, it should have a known smooth cross-section with angle of incidence from very low values to large ones, it should not use absorber that could attenuate the signal meant to illuminate the component pieces being tested, etc. Several such test bodies have been studied, some of which will be described.
The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.
This paper examines the development tests performed at Rockwell International in Anaheim, CA on VHF meandering monopole and dipole antennas which are part of the Global Positioning System satellite. The development tests included numerous impedance measurements of individual antennas configured first in their operational positions on a full-scale mockup of the GPS satellite spacecraft and second while mounted on an indoor ground plane. The initial measurements of antennas positioned on the mockup required the mockup to be located in an exceptionally large, obstruction-free environment because of the low operating frequencies (large wavelengths) of the antennas under test, and in our case a suitable environment was an empty parking lot approximately one-half mile away from the necessary test equipment. This situation necessitated frequent transportation of fragile test equipment to and from the test site which was both impractical and time-consuming. To avoid this situation when production units are to be tested later this year, a ten-foot diameter ground plane was constructed in order to perform the antenna parameter measurements indoors, which presents a very reflective environment. To minimize and theoretically eliminate the effects of these reflections on our measurements, the time-domain gating (time filter) feature of the HP 8510 Network Analyzer was utilized at the indoor test site. The gating function removes any time-domain responses outside of the gate span, the span in this case being the radius of the ground plane. When the time-domain response is Fourier-transformed back to the frequency domain, the effect of the unwanted (gated) responses is eliminated in impedance measurements. While the gated, ground plane parameter measurements will not yield the same values as those measured on the mockup, they can be used to establish an impedance, VSWR, or return loss standard from a known “good” antenna against which production antennas can be compared to determine electrical failures.
This paper discusses some of the more unique problems involved in the performance of measurements on a ground plane type of antenna range generally required for the study and design of multiple antenna shipboard systems. The discussion concentrates on the installation and use of a computer automated antenna analyzer system on this type of range. The methods and results of various range calibration measurements are presented with emphasis on the use of the system’s computerized capability to perform measurements, analyze data, and produce various graphic output formats. The test results obtained from a pair of monopole antennas mounted on a simplified model ship hull are also presented and discussed.