AMTA Paper Archive
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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.
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.
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.
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.
Application of genetic algorithms to the optimisation of wideband Jaumann radar absorbers for normal and oblique incidence
The design of wide-band, multi-layer radar absorbing materials involves the solution of what is essentially an N-dimensional optimization problem. Genetic algorithms appear to offer significant advantages over conventional optimization techniques for this type of problem due to their robustness and independence of performance function derivatives. To illustrate their use, the paper considers the optimum design of wideband, multi-layer, Jaumann radar absorbers for normal and oblique incidence.
Progress in adaptive radar absorbing materials
Possible mechanisms and structures for realising a dynamically adaptive radar absorbing material (DARAM) are discussed and their potential evaluated through computer simulation. Some pointers towards practical implementation are outlined and measured results for large-area DARAM panels operating over I and J bands are shown.
Inverse synthetic aperture imaging studies of a ship at X-band
Techniques for the X-band inverse synthetic aperture radar (ISAR) imaging of a naval ship at sea are presented. We show that the longer the observation time (and thus the angle span), the better the image until a limit based on the pitch roll and yaw motion of the ship is reached. A Fourier transformation ISAR algorithm will be shown and a modified hybrid algorithm will be demonstrated using autoregressive spectral estimation. A hybrid algorithm based on data extrapolation obtained using FBLP coefficients will be demonstrated. Specific motion compensation tradeoffs will also be discussed.
Architecture and algorithms for real-time ISAR imaging of dynamic targets
The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.
Interferometric techniques for discriminating multipath in ground to ground radar diagnostics with minimal constraints on collection geometry
Due to inherent cost, safety and logistical advan tages over dynamic measurements, Ground-to-Ground (G2G, aircraft and radar on tarmac) diagnostic radar measurements may be the preferred method of assessing aircraft RCS for signature maintenance. However, some challenging complications can occur when interpreting SAR imagery from these systems. For example, the effect of ground induced multi-path often results in the measurement of a significantly different image based RCS than would have been obtained by a comparable Ground-to-Air (G2A) or Air-to-Air (A2A) system. Although conventional 2-D SAR images are useful in determining the physical source (down-range/cross range) of scatterers, it is difficult at best to deduce whether an image pixel is a result of direct (desired) or ground induced multi-path (undesired) scattering. ERIM and MRC recently completed an experiment testing the utility of collecting and processing interfero metric (2-antenna) SAR radar data. This effort produced not only high resolution SAR imagery, but also a com panion data set, derived from interferometric phase, which helps to isolate the source (direct or multi-path) of all scattering within the SAR image. Additionally, the data set gives a measure of the physical height of direct scatterers on the target. This paper outlines the experiment performed on a RCS enhanced F-4 aircraft using a van mounted radar. Conventional high resolution imagery (down-range/ cross-range/intensity) will be shown along with down range/height/intensity and cross-range/height/intensity images. The paper will also describe the processing pro cedure and present analysis on the interferometric results. The unique motion compensation processing technique combining prominent point and motion mea surement instrumentation data, eliminates the need for a tightly controlled collection path (e.g. bulky rail sys tems). This allows data to be collected with the van driven somewhat arbitrarily around the target with side mounted antennas taking measurements at desired aspects.
INTA's free space NRL arch system and calibration for absorber material characterization
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.
Convenient, multi-platform, boresight mounting scheme for compact range, A
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
On the use of lens antennas in the free-space method for measuring dielectric properties of materials
In this paper we present an improved theoretical modelling for the free space technique for measuring the complex permittivity of materials at microwave frequencies. The theory was developed for a transmission set-up with two identical pyramidal horn antennas. By performing a spectral decomposition of the aperture fields, the new model takes the effect of the non plane wave character into account when the sample is not placed in the far field of the transmitting antenna. With the use of the new theoretical model it becomes possible to place the sample much closer to the antennas without infringing the theoretical assumptions since no plane wave incidence is needed. In this way the transversal dimensions of the sample can be reduced significantly. The validity of the new theoretical model was verified by measurements on many dielectric (Plexiglas, polystyrene,…) and lossy materials. A comparison was made with the values obtained when the usual plane wave theory is used.
Polarimetric calibration of anisotropic materials measurements
Because the incident wave on an anisotropic material is likely to be depolarized, a complete characterization of such a media requires to measure its whole scattering matrix, which afterwards complicates the calibration process. A suitable technic is the Wiesbeck calibration method . In this paper, we apply this method to two configurations, the reflection configuration and the transmission configuration, and obtain very good agreements between theoretical and experimental results.
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