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Obtaining the tangential electric fields on a planar near field surface has many important applications. The need for this information may not, however, justify the expense of a planar near field test range. A technique has been developed to obtain planar near field data from spherical near field measurements. Spherical near field measurements can be performed on practically any range that employs a roll over azimuth positioner and a phase/amplitude receiver.
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
Analysis and measurement activities to quantify compact range feed/subreflector time domain response are described in this paper. Reflection properties of various components are quantified and their interaction studied. Results indicate that although the feed/subreflector interaction is a factor, reverberation is dominated by instrumentation interaction particularly in the case of small compact ranges.
Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period off time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.
Due to the limited size of a compact range, an antenna with low sidelobes, broad bandwidth, broad beam, small physical signature, low scattering level and reasonably high power handling are required. Historically, slot line antennas are circuit board type antennas noted for their thin cross-section, low cost of fabrication, scalability and high package density in array applications. A broadband version, fed by a microstrip line (and therefore easily connected to microstrip transceiver circuits etched on the same circuit board) is described in this paper. Test models with different shapes and using different dielectric materials were built and tested. The measured VSWR, radiation and scattering patterns of the various antenna designs are presented.
An argon laser source in conjunction with an interferometric fringe generation technique allowed projection of high contrast fringes on to the surface of an antenna over a height of 20 to 60 feet. The projected beam, located at the base of the antenna, made an angle of ~ 15 degrees with the surface. The viewer was placed near the central axis of the antenna ~ 80 feet away where the illuminating antenna surface was imaged on a Ronchi grating. A low light level video camera viewed the moire contours through the Ronchi grating. The spacing between two contours represented a surface height variation of ~ .050". Panel edge misalignments of .005" were readily discernible. Applications of this technique are illustrated with photographs.
The construction of a large reflecting surface is invariably a compromise between the technical requirements and what is economically achievable. During the past three years, the compact range team at Harris has learned a great deal about this process. While aligning and testing the Harris Model 1630/1640 Compact Ranges, we have gone through a long learning experience. This paper presents some of the results of that experience.
The effects of metallic tapes which are used to cover gaps in a compact range reflector are studied in this paper. To study these effects, the normalized tape scattered fields are computed in the target zone. A method of moments technique is used to compute the tape scattered fields. It is shown that the tape scattered fields are directly proportional to the thickness and width of the tape and are inversely proportional to the square root of the distance from the tape. Using the computed results, an empirical formula for the tape scattered field is developed. One can use the empirical formula to compute the highest frequency for which a given size tape can be used.
After having delivered a model 1630 and a model 1640 compact range plus a number of smaller 1606 and 1603 ranges, Harris has improved their product to meet the demanding needs for operating frequencies of 35 GHz and higher. In characterizing the two large ranges, it was discovered that the surface accuracy as originally optimized would not support the highest operating frequency. Achieving the required surface accuracy required additional surface measurement data in combination with RF contour plots and was very time consuming. From those lessons learned, several features have been incorporated into the next generation of compact ranges that make more accurate reflector surfaces easily achievable. The features include optimally located adjustment mechanisms, additional targets on each panel, software for best fitting the panel surface to minimize steps, techniques for eliminating panel steps in place, and gravity bias setting of panels.
An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
Present day manufacturing techniques often employ composite materials in the fabrication of many structures. Graphite is one common material used to form structurally strong fibers for use in a resin binder. The material characteristics of graphite composites naturally differ from those of metallic materials. An interesting characteristic is the smoothness or roughness of composite materials as examined from an electromagnetic viewpoint. Radar backscatter measurements of several different planar panels were performed near grazing incidence to compare their scattering characteristics against a smooth metallic surface. These results show the "electrical" smoothness of the surfaces in terms of fabrication and material dependencies.
Low RCS signatures require verification of test body conductivity and material performance. A miniaturized radar system with a unique horn antenna was designed for the detection of conductivity gaps and material imperfections in radar absorbing material. The antenna system has a small aperture and low VSWR permitting direct placement against a surface for localization of electromagnetic phenomena. Test results indicate that test body construction gaps and material imperfections are readily detectable using the test system in either a handheld or robotic-type configuration. Preliminary results also indicate delaminations, conductive panel penetration, and structural component steps will be detectable.
Measurements of the microwave reflectivity of materials is often performed with complex test setups using probes attached to a vector network analyzer. The lack of portability of these systems prevents the user from measuring reflective properties of surfaces that are not easily moved to an appropriate test facility. This paper describe a small, hand held microwave reflectometer which is designed to perform rapid reflectivity measurements in the field. The reflectometer consists of a tuneable Ku band source, a dual polarization sampling horn, a pair of crystal detectors, and a battery powered microcomputer.
A technique is presented for performing vector error corrected measurements of reflection coefficient using an antenna measurement receiver, the Scientific-Atlanta Model 1783, in an automated system. The technique uses an open-short-load calibration implemented in software in the system controller. The technique is simple and accurate. The equations for the measurement are derived and results as compared to the HP 8510B network analyzer are presented.
Antenna measurement accuracy is often not addressed in a rigorous manner. A methodology for projecting antenna measurement accuracy is described together with some of the error components that limit measurement accuracy. Antenna measurement accuracy is approached through an error budget projection, which requires the first and second order statistics of the individual error sources. Typical error sources are described along with methods of obtaining the statistics required for the error budget.
A microwave holographic analysis system is shown to have successfully resolved the surface deformations on an 8' symmetric Cassegrain reflector antenna known to have significant surface deformation problems. The technique is based on the Fourier transform relationship between the aperture field of an antenna and its radiated far-zone field. A signal processing technique dubbed "pattern simulation and subtraction" is discussed that increases the resolution in the transformed aperture domain by removing unwanted signals from the aperture distribution. Measurements taken on the Cassegrain reflector at 11 GHz in the OSU-ESL Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Surface deformation profiles generated by this system were then compared to an optical measurement of the main reflector surface. Excellent agreement was obtained with a worst case deviation in the adjusted profiles being 0.05 ?.
An overview of results are presented for far field pattern, antenna gain and antenna temperature measurements of reflector antennas in several frequency bands. The pattern and gain measurements were taken in the compact range at The Ohio State University. The dynamic range available, which gives the ability to take a full 360 degree pattern, and the relatively high speed at which data is collected, are major advantages for pattern and gain measurements in the compact range. In a series of related measurements an 8-foot diameter Cassegrain reflector was used for antenna temperature measurements under clear weather conditions in an outdoor environment.
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
A new measurement technique that is used to measure the polarization properties of dual port, circularly polarized antennas is described. A three antenna technique is used, and high accuracy results are obtained for all three antennas without assuming ideal or identical properties. This technique eliminates the need for a rotating linear antenna, reduces the setup time when gain measurements are also performed, and reduces errors for antennas with low axial ratios.
Compact ranges are special facilities, requiring a huge anechoic chamber and a large RF reflector to test a full size aircraft. These facilities are expensive and fixed structures, consequently they remain essentially research and design tools. However, as more and more aircraft are being made from composite materials, manufactures with high production volumes may be justified in having a compact range for purposes of quality control. The RF characteristics of these aircraft will change during their useful life cycle. The high cost of compact ranges will deprive most service and maintenance centers from owning one of these unique facilities, and force them to compromise the RF specifications of those aircraft in service. There is a definite need for a low cost and portable compact range. We present the design concept for such a range, whose reflector is divided into several identical pieces while the measurement is done sequentially. The edge effects of the portable reflector will be discussed.