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The possibility of using the quasi-optical waveguide modeling (QWM) method in the near millimeter and submillimeter regions of the electromagnetic spectrum for study of backward and forward scattering amplitude-phase characteristics of physical objects and their scale models is grounded in the article. The laboratory measuring quasi-optical installation has been carried out for realizing the method. The microwave part of the installation was built on the basis of a hollow dielectric waveguide and quasi-optical devices and transmission-line elements. The results of measurements of backward scattering characteristics and forward scattering characteristics of a number of standard objects made in the 4-mm wave range are presented.
We are developing a new indoor bistatic measurement technique for scale model targets. This procedure will collect far-field data at bistatic angles from 60° to nearly 180° and near-field data over a 10' high, 10' radius cylinder surrounding the target. A stationary parabolic reflector illuminates the target while a duplicate parabolic reflector, rotated to its bistatic position, acquires far-field data. The independent, concentrically mounted near-field scanner gathers comparison data. Most compact range reflectors employ shaped edges to avoid edge diffracted signals entering the measurement volume. We report results of using shaped absorber material over otherwise unmodified reflector edges to reduce diffraction. High-resolution 3D images of sample structures demonstrate the practicality of this approach.
ORBIT/FR has recently installed and qualified a combined microwave (2-18 GHz) and millimeter wave (92.5-95.5 GHz) RCS system in an existing compact range based chamber. The facility is used for scale model reflectivity measurements on a wide variety of targets. The system features a unique, high power hardware gating system at the millimeter wave band that contains an integrated compact range feed assembly specifically designed to optimize RCS performance. Changeover between the microwave and millimeter wave bands is possible by placement of the appropriate compact range feed assembly on the feed stand, with locating pins being utilized to assure repeatable performance of the feeds in the compact range system. The system utilizes the FR959 RCS Measurement Workstation and HP 8530/85330 "turbo" based receiver system. Appropriate upconversion and downconversion hardware is integrated into the millimeter wave gating system to allow a common set of HP 8360 series sources and the HP 8530 IF receiver to be utilized for operation in both bands. The system is capable of producing high quality ISAR images at the millimeter wave frequencies, as well as in the microwave band.
A fully-polarimetric compact range operating at 160 GHz has been developed for obtaining X-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, CW, X-band synthesizer driving dual X16 transmit multiplier chains and dual X16 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and a rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 40 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 20” quiet zone. A description of this 160 GHz compact range along with measurement examples are presented in this paper.
Both Near-field Antenna Measurement Technology and Beam Waveguide Antenna techology have been in existence for some time. This paper describes a measurement combining both of these technologies. During an internal study of beam waveguide implementation, a near-field antenna measurement was made of a development model. The model and techniques of measurement are described herein.
At high frequencies, the scattered fields from a radar target can be modeled as a sum of contri butions from a finite number of scattering centers. We use a parametric model based on the Geometric Theory of Diffraction (GTD) to estimate the location and type of scattering centers present in a frequency domain data set. The parameters of the model are estimated using a modified MUSIC algorithm that incorporates the GTD model. A new spatial smoothing algorithm is also introduced.
Automobile manufacturers have noticed the proliferation of after market antennas, primarily for cellular phones, defacing their otherwise stylish vehicle designs. Investigations are being made by the manufacturers to include antennas for communications requirements, such as cellular phone, personal communications service (PCS), global positioning system (GPS) and Intelligent Vehicle Highway System (IVHS), within their vehicle This paper presents the initial phase of an investigation undertaken within the Research Branch (ERB) of NASA Research Center (LaRC). The measurements, presented in this paper, were performed using a one-fourth scale model of a currently popular vehicle design. The bands of interest for this investigation include the cellular, GPS and FM broadcast frequencies. Comparisons of measured and computed patterns of commonly used antennas such as wire and microstrip patch antennas are presented.
Finite-Difference Time-Domain (FDTD) is prov ing to be a practical and accurate technique for an alyzing and predicting the performance of anten nas mounted on complex structures. As part of an effort to develop and validate an FDTD code, the impedance and radiation patterns of helicopter mounted loop antennas are predicted and compared to full-scale and 1:10 scale measurements. The input impedance and coupling of HF loop an tennas on the scale model helicopter are measured in the ElectroMagnetic Anechoic Chamber facility at Arizona State University. Although made difficult by the large mismatch between the highly reactive HF antennas and the instrumentation, the scaled impedance measurements agree well with the full scale measurements and predictions. In addition, ro tor blade modulation effects on the input impedance are examined.
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.
This paper will describe the requirement, design, implementation, and performance evaluation of MMWRCS measurement subsystems to be integrated with an existing RCS measurement system in the Sikorsky Compact Range in Bridgeport, CT. The subsystems will operate at V-band (58-62 GHz) and W-band (92-98 GHz). The requirements to test at V-band and W-band is driven by limitations of quiet zone physical volume. The Harris model 1606 reflector system produces a 6 foot diameter zone of virtual uniform amplitude and phase. Therefore scale models are fabricated for test. This translates to approximately 1/6 scale of contemporary Sikorsky Helicopter designs. Testing at 60 and 95 GHz will provide accurate simulated full scale RCS data at X and Ku-bands.
Mathematical techniques (calibration, background subtraction, software range gating, imaging, etc.) have become integral to the process of generating precision radar cross section measurements. The "reference target method" is a powerful RCS correction algorithm which yields plane wave illumination results from data acquired under an arbitrary but known illumination. This method is analogous to a two dimensional RCS calibration. Measurements of long bars (at X- and Ku-bands) and of a scale model aircraft (at C-band) were performed under the cylindrical wave illumination produced by March Microwave's Single-Plane Collimating Range (SPCR) at Arizona State University. The targets were also measured under the quasi-plane wave illumination produced by a March Microwave dual parabolic-cylinder CATR. The SPCR measurements were corrected using the reference target method. The corrected SPCR measurements are in good agreement with the CATR measurements.
The measurements of an antenna at FM frequencies integrated in the bodywork of a terrestrial vehicle is a extremely (sic) delicated (sic) problem that will be larger if a ground plane must be simulated. An algorithm based on two measurements (magnitude and phase of the field components E() and E (1) on a scale model made in an anechoic chamber, has been developed to solve this problem. These measurements correspond to the value of the desired conical cut (only a narrow range of angles above the horizon is significant), and the associated cut needed to measure the specular reflection on the simulated ground plane.
Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.
This paper describes the techniques applied to a fully automatic computer controlled, HP8510 based, range gated digital data acquisition system used to provide scale modeled large aperture synthesis, evaluation of aircraft blockage effects, array patterns, element cancellation ratios, as well as providing a large accurate data base for radar simulation exercises.
Scale modeling and its application to antenna measurements has been elaborated many years, though implemented in most cases with much simplification. The foundation for scaling originates from the linearity of Maxwell's equations. However, scaled-down modeling is incompatible with such formulation in authentic applications. The main essence in antenna measurements on models is the simulation of vectorial field configuration, which may be satisfied by so called geometrical scaling, as compared to absolute modeling ~ a quantitative class of modeling, which additionally comprises the simulation of power levels. A set of measurable quantities, adequate for such scaling, is listed. Attention is given to the principle of Electromagnetic Similarities related to geometrically homologous antennas. An algorithm for some distinctive scaling qualifications is developed, and physical aspects of scale modeling are discussed. The relative advantages of scale modeling in antenna measurements are examined, and some remarks for particular technical interests are also presented.
The AM Broadcast service area depends on the radiation pattern of the antenna employed. The approach used here to compute the service area requires the radiation pattern of the monopole antenna mounted on a perfectly conducting plane earth. The effects of the ionosphere and the finitely conducting earth can then be calculated and the service area determined. The use of the theoretical thin monopole radiation pattern for the determination of the actual service area is not very accurate. The best solution is to use the measured radiation pattern. But due to the large dimensions of the antenna it is more practical to use a scale model for the measurement.
A novel method for evaluating conductive coatings used for radar cross section (RCS) scale models is presented. The method involves the RCS measurement of a short circuited cavity whose interior is coated with the material under study. The dominant scattering from such a structure occurs from the cavity rim and surface walls internal to the cavity. The method of conductivity testing has excellent sensitivity due to the energy that couples in and out of the cavity. This energy undergoes many reflections with the interior walls and thus very small losses can be detected. Calculations and measurements are shown for several different types of coatings, including coatings of silver, copper, nickel and zinc.
A large, tapered anechoic chamber exists at the NASA Johnson Space Center (see Figure 1). This chamber has been used to test antennas mounted on full-size replicas of the Apollo moon lander. Also, antennas mounted on a scale model of the Space Shuttle have been tested in this facility. The chamber will have extensive utilization in the future for testing proposed Space Station antennas and other satellite antennas.
The RF characteristics of a four-element diagonal array configured to yield low sidelobes, dual circular polarization with low axial ratio and high front-to-back ratio are described. The array was designed for use as source antenna in a VHF test range, where the test antenna is nearly omnidirectional and ground multipath effects are a major problem. To achieve broadband performance, crossed open-sleeve dipoles were used as array elements. The array is capable of operation over a 1.66:1 band with a VSWR of <2:1. Experimental studies were made by means of scale model antennas in the 240 to 400 MHz band. The axial ratio is <1 dB, and the sidelobe/backlobe levels vary from –25 dB to –30 dB over the measurement frequency range.
Antennas are an integral part of the communications, navigation, EMC systems installed on aircraft. Aircraft, such as the Douglas DC-9, C-9A, C-9B, DC-10, KC-10A, A-3 and A-4, use approximately 20 antennas. These antennas operate from VLF to approximately 20 GHz. The radiation patterns of these antennas are affected by aircraft structure such as wings, vertical stabilizer, engines, and landing gear. Douglas Aircraft Company measures the radiation patterns of these antennas using scale model aircraft (and/or aircraft sections) to predict the performance of the associated system. This paper describes some of the scale model measurement techniques used by Douglas Aircraft Company to obtain scale model radiation pattern data.