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A quality assurance approach to qualifying target components before and after assembly requires special tools to check specific electromagnetic areas of concern. Of particular concern are the reflection characteristics of multilayered assemblies that may be damaged or have altered performance due to construction practices. This paper describes a methodology for measuring construction for measuring construction or materials problems across a broad frequency range. Bistatic and transmissive test configurations are used to determine specific materials characteristics, including conductivity/resistivity properties. The test methods discussed here and non-destructive, so they may be used on any configuration of interest. The test results that will be presented include Ku-band transmissive measurement of resistive materials for uniformity and general performance. Bistatic measurement results at Ku-band are presented as a technique for isolating problems under or near the surface.
General guidelines for using software gating are presented. Examples which demonstrate both proper and improper use of gating are presented. The effects of RAM materials on the time domain signature and the selection of the gate parameters are discussed. A brief review of the general theory of high resolution RCS measurements is presented.
A three-layer sandwich structure consisting of a plastic film-to-foam lamination is presented as a low-RCS alternative to structural foam. Structural foam with 1-2 lb/ft density is commonly used as a low-RCS material. However, its RCS per unit load per unit volume is not as low as that of a composite foam structure. Equations relating mechanical strength and RCS are simultaneously solved for maximum mechanical strength and minimum RCS in the limit of Rayleigh and resonance region material thicknesses. A result is that a three-layer foam sandwich beam can have superior mechanical strength compared with an identical all-foam beam and a reduced RCS. Specific results for an optimized sandwich with mechanical strength equal to that of a homogenous beam and minimum RCS are presented. Experimental data quantify mechanical strength and RCS for several foam-mylar sandwiches.
A novel low-cost automatic system is described to measure both the complex permittivity and permeability of solid materials at 2 to 18 GHz. It is particularly useful for evaluating the frequency dependence of radar absorbing materials (RAM). The RF and the mechanical setups are described, including the computer algorithm and the measurement procedure. The results and the experimental errors of three materials are presented, which agree with results that were obtained by other methods, while the cost of putting up the system is considerably lower than any comparable alternative.
The intrinsic electromagnetic properties of bulk materials must be accurately measured to skillfully apply them in new designs. This paper surveys a variety of measurement methods based on RF/microwave network analyzers, and discussed the strengths and limitations of each. This includes recent enhancements to the popular “transmission-line-method”, plus a new open-ended coaxial probe for making permittivity measurements easier and more convenient.
The Hughes Aircraft Company conducted a study to characterize the backscattering performance of wedge shaped anechoic absorbers for use in treating the sidewall regions of RCS chambers. ISAR imaging techniques were utilized to obtain a diagnostic results at near-grazing incidence angles which were not possible with conventional testing methods. These techniques allowed for separation and identification of individual scattering sources from each of the evaluated samples. As a result, the backscattering from an entire wall of absorber can be simulated by evaluating only a few samples. Absorber performance data was collected over frequencies from 2 to 40 GHz. Results from this study clearly show that differences in absorber fabrication methods have a significant impact on the performance of the materials. Various approaches for impregnating, loading, and cutting the absorber have also been evaluated. Gaps, formed during installation, at the joint between two pieces of material are shown to significantly degrade performance, whereas, offsets and glue lines are shown to have less of an effect, provided the absorbers are uniformly loaded.
Mechanical and environmental considerations for outdoor operation of an inflatable column are discussed in the context of a 30-ft-high column. The column is designed to support a 900-lb load in a 30-knot wind. Column RCS is less than -40 dBsm below 1 GHz for both horizontally and vertically polarized illumination. Designs using Mylar and Teflon-coated Kevlar as skin materials are compared. The primary concerns are wind loading, pressure regulation, and solar heating. Wind effects include static loading, gusting, and vortex shedding. In addition, wind-driven particulates, such as sand or stones propelled by passing vehicles can puncture the column. A pneumatic control system maintains a constant internal support pressure in the presence of leaks or pressure fluctuations due to changes in solar illumination.
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.
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 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.
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.
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.
Texas Instruments' antenna test range complex consists of 15 new indoor ranges at the McKinney site. To meet projected business requirements, Texas Instruments initiated an aggressive antenna test range expansion and upgrade program in 1986. Construction of the new test range facility at McKinney is phase 1 of the plan which will be completed in the first quarter 1988. Phase 2, the construction of a new outdoor facility, will be completed in 1991. When completed the new facilities will be equipped with the latest technology in instrumentation and materials. A staff of antenna measurement experts maintain the ranges; they are equipped to make very short turn-around-time modifications to the range to meet special measurement requirements.
Data are presented on the normal reflectivity of several commercially available microwave absorbing materials at frequencies of 8-18, 35 and 95 GHz. The materials tested were mostly of the carbon-loaded urethane foam type with non-pyramidal surfaces, ranging in thickness from 1/4" to 1". All testing was performed in the Millimetre-wave Scattering Range of the Boeing Military Airplane Co (BMAC) of Seattle, WA. * Current affiliation; this work not performed at NBS.
This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.
The recent activity and study of the compact range has been increasing the past few years. Both radar cross section (RCS) and antenna measurements have been conducted in the compact range. Important research and analytical investigation has also been done in the design and construction of the reflectors so characteristic of these types of ranges. Edge diffraction from the reflector has been studied and characterized by methods of geometrical optics, geometrical theory of diffraction, physical optics and physical theory of diffraction. Treatment of edge diffraction effects on the reflector have included serrations, rolled edges, and absorbing materials. The primary goal is to obtain as perfect a plane wave as possible in the enclosed chamber with reduction of edge diffraction from the reflector.
For many years, rain impact and erosion failures to aircraft radomes have been a recurring problem. Impingement upon rain droplets by aircraft traveling at velocities of 300 mph, or greater, may be destructful to radomes and jeopardize the function of associated antennas, unless sufficient rain erosion resistant materials are employed in the construction. Changes to the surface of a radome due to rain erosion, such as porosity and structural failure, will affect electrical performance. Other material properties that must be considered besides rain erosion are dielectric constant and lost tangent.
Measurement of complex permittivity (er) and permeability (µr), both vector quantities of absorptive materials, has gained increasing importance with expanding use of the RF and microwave spectrum, particularly in communications and electromagnetic countermeasure applications. In addition, the network analyzer has seen increasing use in non-destructive measurements to determine the chemical composition of a sample dielectric material. The method described here is suited for the measurement of complex permittivity and permeability of ansorptive materials. These measurements have been made for years using numerous methods. A conventional technique involves a two-step process using a slotted line or network analyzer. First, the sample is backed up by a short circuit and the input impedance is measured. Next, the short circuit is moved ¼ ? from the sample to simulate an open circuit termination (where ? is the incident signal wavelength), and a second measurement is made. The results of these two measurements are used to solve simultaneous equations for er and µr. This procedure is repeated for each frequency of interest. Uncertainties in the measurement include test set-up frequency response, mismatch, and directivity errors, as well as the uncertainty in the physical position of the short circuit.