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A reflector antenna has been designed for the RCS measurements. The antenna is dual linearly polarized and exhibits constant beamwidth over an octave bandwidth. The design principle has been to keep the effective antenna aperture constant in terms of the wavelength over an octave bandwidth. The theoretical design lead into the choice of the antenna and the feed. The reflector was an offset parabolic reflector. The feed was a ring-loaded conical corrugated horn. The measurement results of the designed reflector antenna showed very good agreement with the computer results. The V- and H- polarization characteristics of the antenna are almost identical.
The HP-85330A multi-channel, multi-even system controller was designed to take advantage of the speed, performance and flexibility of automated HP 8530A microwave receiver antenna and RCS measurement systems. In its simplest form, the HP85330A is a VXI mainframe and card that controls high-speed, high-isolation solid-state microwave switch modules. Using the FAST data modes and internal data buffers of the HP 8530A, the measurement system is allowed to run at the maximum speed and performance specification. It accomplishes this through hardware control of the triggering and timing of the HP 8530A microwave receiver, HP8360 series sources, positioner controllers, and external switch modules. For outdoor ranges, the HP 85330A is capable of hardware handshaking with other HP 85330As through balanced twisted-pair wires. Accompanying the HP85330A controller are the HP 85331A and HP 85332A SP2T and SP4T external switch modules. To facilitate remoting the switches, communication between the HP 85330A system interface and 85331/2A is by parallel, twisted pair balanced lines. The lines are capable of distances of 40 meters. The switch modules are individually addressable or can be cascaded to form complex switch trees. Actual measurement throughout data is presented.
As part of the Phalanx weapons system refurbishment program, Thorn EMI Electronics was required to perform a sequence of monopulse feed-pattern measurements at microwave frequencies and receiver-pattern measurements at IF frequencies. The feed-pattern measurements were accomplished using a HP 85301A antenna measurement system. Measurement of the receiver IF patterns, however, required a more novel approach. Because of dual downconversion within the Phalanx receiver, adaptation of the HP 85301A system was required to allow for this frequency translation characteristic. Reconfiguration time between the feed RF and the receiver IF test systems was to be kept to a minimum. The antenna measurement system is described in this paper and IF patterns are presented. The results show that the performance of the test system is not compromised in any way.
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.
For the last few years, the Periodic Moment Method (PMM) has been used to analyze the scattered fields from an infinite absorber wall. Using this approach the absorbe4r can have different periodicities in the x and y directions, as well as arbitrary shapes and any dielectic (sic) distribution. This makes this analysis method very general such that it can treat any conventional wedge or pyramidal designs. Plus, it has been used to develop new ones, which is the subject of this paper. Traditionally there have been chamber uses for both wedge and pyramidal absorber (sic). In a normal RCS range, one uses pyramidal material in forward sector around the feed, wedge absorber through the target zone and pyramids on the backwall. Using this approach, one takes advantage of the basic features of the two types of absorber. To improve wedge material, one is interested in reducing its normal incidence reflection coefficient because the long straight edge is a rather large scatterer. Through the use of the PMM analysis, curved and serrated wedge absorber designs have been developed and tested. Both show significant improvement relative to conventional material. As for the pyramidal model, one would hope to improve its size requirements especially for lower frequencies. Recall that two wavelengths at 100 MHz is twenty feet. By placing twenty foot material throughout a chamber, one greatly restricts the size of the room. Again, the PMM analysis has been used to develop a new curved pyramid design which can perform as well as a conventional pyramid twice its size. Thus, one could use curved pyramids that are ten feet at 100 MHz and achieve the same performance as the commercially available twenty-foot material.
Most of the present available pyramid absorber lined anechoic chambers do not meet the new stringent requirements (plus or minus 4 dB criterion of CISPR) for EMC measurements at lower frequencies, say below 100 MHz, due to poor absorber efficiency. In this paper the actual field configuration in those chambers at these critical frequencies is numerically computed for extracting frequency dependent correcting relations for EMC measurements. To this end a finite difference formulation in frequency domain is used. The absorbers are modeled as planar dielectric layers. Examples of computed field configurations are presented and compared with measurement values. The results show the frequency response of the electrical field configuration with respect to the position of device under test, the test antenna, as well as the effect of chamber asymmetries.
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.
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.
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.
Performance trade-offs are investigated between the use of clustered waveguide bandwidth feeds and the use of one multi-octave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
A comparative analysis of different geometries of dual compact antenna test ranges is done looking at the cross-polarization level and the scanning capability of the system. The analysis is based on a very simple and quick computation of the fields over the main refector [sic] projected aperture.
As millimeter wave antenna systems become increasingly popular, engineers are challenged to develop effective methods for testing them. A practical method of designing a millimeter wave antenna measurement instrumentation system is presented in which frequency range, accuracy, dynamic range, and speed are considered.
This paper describes a software based receiver post processor that corrects circularity and gain errors in coherent receivers. The receiver post processor additionally provides range gating capabilities, signal quality estimation, mixer non-linearity detection and various display functions. This paper will concentrate primarily on the identification of circularity errors by the receiver post processor.
The radiation patterns of a low (40dB) sidelobe antenna have been measured on a variety of antenna test ranges including Near Field, Far Field and Compact versions. Originally intended to validate new Near Field Ranges, some of the early results will be presented and the variations examined. The need for some form of range validation is shown. There is also some explanation of the fundamental effects that various ranges have on results.
A method for measuring G/T of small gain active antennas has been developed. The measurement can be carried out inside an anechoic chamber with well controlled environment. The method has been validated by measurement of a simulated active antenna, whose G/T has been computed from the parameters measured by classical procedures.
Knowledge of the field character in a range is essential to the understanding of measurements performed. Field probe systems are commonplace for small compact ranges. Outdoor ranges have their systems and methods. A large compact range has unique needs. Available systems are not only fairly expensive, but normally time consuming to install. The McDonnell Douglas Technologies facility implemented a probe system designed to meet the particular needs of the facility.
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.
The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.
This paper describes the target handling system that was developed for use in the compact range facility at the University of Pretoria. The system was locally designed and manufactured and comprises of a lift platform, RCS pylon and utility trolley. The pylon utilises a unique design approach resulting in a structure with very high stiffness and surface finish.
This paper describes the technique and advancement of diagnostic radar imaging technology by comparing past SAR and ISAR techniques to the more recent advancement of Autofocus SAR techniques. This recent advancement has meant the relaxation of the stringent mechanical stability requirements needed to produce high quality, high dynamic range, calibrated RCS images.