AMTA Paper Archive
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Free space characterization of materials
A simple change to the HP8510C or HP8720C vector network analyzer block diagram coupled with the TRM (Thru Reflect Match) calibration leads to accurate measurements of the material properties of flat samples. Algorithms developed for transmission line measurements can also be used in free space measurements. A description of recent improvements in the transmission/reflection algorithms is reviewed. Free space measurement results based on the transmission/reflection algorithms found in the HP85071B materials measurement software package are presented.
GHz compact range for scale model RCS measurements, A
A 585 GHz compact range has been developed for obtaining full scale RCS measurements on scale model targets. The transceiver consists of two CW submillimeter-wave gas lasers along with two colled-InSb heterodyne mixers. Coherent detection has been implemented to maximize sensitivity and allow for a vector measurement capability. In addition, the target can be rapidly translated in range to generate a doppler modulation which is used to reject background signals during low-RCS measurements. Although most scaling has evolved to develop non-metallic materials with scaled dielectric properties as well as validation and test program, RCS measurements are made on scaled simple and complex shapes and compared with full-scale measurements and computer predictions. A description of the 585 GHz compact range along with measurement examples are presented in this paper.
Characterization and modelling of conducting polymer composites and their exploitation in microwave absorbing materials
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
Anechoic Chamber Specifications: A Guide
As many institutions and companies have constructed anechoic chambers in the past few years, there has been little work done to codify the specification requirements. Often chambers have been constructed from woefully inadequate specifications resulting in chambers that may be too costly, unable to meet the performance criteria, and in some cases, be unsafe. This paper shall present various model specifications and guidelines to properly specify a chamber complex. Compact ranges, tapered chambers, as well as traditional rectangular chambers will all be examined. How to specify absorbing materials and quiet zone sizes, as well as tradeoffs associated with them, will be discussed. Finally, a guide for coping with facility concerns such as civil, structural, RF shielding, HVAC, electrical, and fire protection will be presented. Examples of good specifications and inadequate specifications will be demonstrated and reviewed.
Measurement of Constitutive Parameters at Microwave Frequencies
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.
Surface resistivity measurements using a VHF ohms per square test system
Many aerospace applications specify materials with precise surface resistivities to satisfy electromagnetic requirements. Frequently, composite materials are physically protected by a layer of dielectric (such as Mylar) or by virtue of being imbedded in a dielectric (such as fiberglass). Conventional resistivity measurement techniques, such as four point probes and busbars, fail in measurement of surface resistivity due to the inability to establish direct electrical contact with the composite structure. This paper describes a new methodology for measuring resistivity of aerospace materials. Capacitive coupling is used in a VHF surface resistivity test system. This system measures ohms per square in a non-destructive, non-intrusive fashion using a capacitive coupled probe technique. Several probe designs have been fabricated and tested. Baseline measurements are presented that illustrate performance against surface resistance which vary from ten to two thousand ohms per square. The effects of different types and layers of resistive materials are also discussed.
Resistivity measurement techniques using a portable reflectometer
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.
Practical considerations for effective software gating in high resolution RCS measurements
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.
Low-RCS structural laminate materials for scattering measurements
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.
An Automatic system for measuring complex permittivity and permeability of solid materials at microwave frequencies
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.
Measuring high-frequency properties of materials
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.
Diagnostic evaluation of wedge absorbers for RCS chambers
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.
Design of an inflatable support for outdoor RCS measurements: mechanical and environmental considerations
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.
A Portable compact range
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.
Broad band antenna for compact range use
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.
Automated millimeter wave evaluation system for advanced materials and frequency selective surfaces
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.
Electromagnetic surface roughness for composite materials
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.
A Hand held microwave reflectometer
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.
Improving the performance of anechoic absorbers
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.
Characterization of materials at UHF and VHF
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.
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