AMTA Paper Archive
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An Automated Precision Microwave Vector Ratio Measurement Receiver Offers Solutions for Sophisticated Antenna Measurement Problems
This paper describes a new, automated, microprocessor controlled, dual-channel microwave vector ratio measurement receiver for the frequency range 10 MHz to 18 GHz. It provides a greater than 120 dB dynamic range and resolutions of 0.001 dB and 0.1 degree. Primarily designed as an attenuator and Signal Generator Calibrator, it offers solutions to antenna measurement problems where high accuracies and/or wide dynamic measurement ranges are required such as for broadband cross-polarization measurements on radar tracking antennas, highly accurate gain measurements on low-loss reflector antennas, frequency domain characteristics measurements on wide-band antennas with resulting data suitable for on-line computer conversion to time domain transient response and dispersion characteristics data and wideband near field scanning measurements for computing far field performances. The measurement data in the instrument is obtained in digital form and available over an IEEE-488 bus interface to an outside computer. Measurement times are automatically optimized by the built-in microprocessor with respect to signal/noise ratio errors in response to the measurement signal level and the chosen resolution. Complete digital measurement data amplitude of both channels and phase, is updated every 5 milliseconds.
Measurement of surface coupled antennas for subsurface radar probing
The experimental techniques presented here can be used to obtain the approximate time domain transfer function and pattern of underground radar antennas. These techniques provide an easy approach to obtaining relative antennas performance. The experimental setup which is used to perform these experiments consists of slanted hollow plastic pipes bored in the ground, the receiver unit, transmitter unit, controller and processor units etc. A buried antennas is used to transmit to a test antenna on the ground surface. The data obtained from two separate test antennas are presented and compared.
Using the HP 8510 network analyzer to measure the radiation patterm of a dipole antenna using time domain and gating to remove the effects of ground clutter
A classical problem encountered when measuring the far-field radiation pattern of an antenna in a medium-distance range is the degradation that occurs when undesirable reflections (from the ground or nearby objects) are present. To reduce this problem, the source and test antennas are often installed on towers to remove them from the reflective objects, RF absorptive materials are used to reduce the magnitude of the reflected signals, and often the reflective objects in the range are adjusted in order to null out the reflections and “clean up” the range. These solutions are often limited in their effectiveness and can be prohibitively expensive to implement.
Long range antenna measurements with the HP 8510 using harmonic mixers
This paper addresses two major issues that impact long-range outdoor antenna measurements with the HP 8510 network analyzer: using a radiated reference signal to provide phaselock reference, and using harmonic mixers with a phase locked local oscillator (LO). The measurements were made at microwave frequencies on a 700 ft outdoor antenna range using a reference antenna in antenna test configurations with the HP 8511A frequency converter and with a harmonic mixer configuration using the new HP 8510 "Remote Phaselock" option developed by Hewlett-Packard. In addition to CW antenna patterns, the use of time domain and gating to reduce the effects of ground reflections was investigated. Measurement considerations and results are discussed. The favorable outcome of this investigation is applicable to a broad variety of antenna measurements.
An Automated antenna measurement system using the HP8510
An automated antenna measurement system using the HP8510 is described. The system controls the HP8510, associated signal source, and antenna positioner, to provide a fully integrated, automated test facility. Automation speeds and enhances testing by implementing the following features: - Multiple frequency pattern measurements in a single cut of the pedestal. - Patterns with rotating linear polarization - Automatic pedestal control - Storage and presentation of fully documented test data. - Storage and recall of test routines These features complement the premier microwave receiver available today, the HP8510 which offers: - Continuous frequency coverage from .045 to 26.5 GHz - Unparalleled measurement accuracy - 80 dB dynamic range - Time domain gating These features are integrated through software developed using modern software management techniques to form a system which is state of the art in measurement performance, operator interface, and software life cycle supportability.
A Broadband, inexpensive, KA-band pulsed radar system
There has been much interest recently in Ka-band scattering measurements. Although Ka-band components are steadily improving, one is presently limited to narrow bandwidths (2 GHz) for higher power (more than 100 milliwatts) applications. If the whole wavelength bandwidth was useable, one could scan the target in frequency, transform to the time domain and simulate a very narrow pulse illuminating the target. With such a system, one could identify scattering centers separated by just an inch or so. * This work was supported by the National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG 1613 and Sandia National Laboratories under Contract No. 58-3465.
Time domain gating in RCS measurements
Gating is a widely used technique of improving RCS measurements. However, the exact type of gating used has a dramatic effect on such parameters as dynamic range and clutter rejection. Time Domain Gating offers significant advantages over software gating as used in some network and spectrum analyzers. This paper explores a technique used by Scientific-Atlanta in CW and FMCW RCS measurements. With the adaptation of an external computer controlled hardware gating unit, existing RCS and antenna systems can be retrofitted for significant performance improvements.
Far-field pattern measurements and time domain analysis of reflector antennas in the compact range
The direct far field pattern measurement of an aperture antenna becomes more difficult as the size of the aperture increases. Recent measurements on reflector antennas with 2D2/? =1500’ at The Ohio State University ElectroScience Laboratory have demonstrated the usefulness of the compact range in obtaining the complete far field pattern of antennas with large far field distances.
Measured performance of the Harris family of compact ranges
This paper reports the quiet zone characteristics of the Harris family of compact ranges. Field probe measurements of systems having quiet zones of 3, 6, and 40 feet are presented. The quiet zones were characterized using a two way measurement with a trihedral corner reflector target. One way CW field probe measurements with an open ended waveguide are also presented for the Model 1606 range. A Discrete Fourier Transform (DFT) is imbedded in the test set software and provides an angle domain signature of extraneous signals illuminating the quiet zone. The two way range transfer functions of the Model 1603 and 1606 ranges are verified using calibrated spherical targets with the HP-8510 network analyzer operating as a time domain reflectometer.
Applications of autoregressive spectral analysis to high resolution time domain RCS transformations
Modern analysis techniques of radar scattering data or radar cross section (RCS) data often include transformation to the time domain for the purpose of understanding the specific scattering mechanisms involved or to isolate or identify specific scattering points. The classic technique is to transform from the frequency domain to the time domain using an inverse (Fast) Fourier Transform (IFFT). Often, however, the scattering centers are too close together to resolve or the requirement for accuracy in the measurement of the differential time delay is too high given the IFFT inverse bandwidth. This paper presents a technique for determining the time domain response of a radar target by processing the data using modern autoregressive (AR) spectral analysis. In this technique, the scattering from a radar target in the high frequency regime is shown to be autoregressive. This paper will show examples using the maximum entropy method (MEM) of Burg.
The Effect of instrumentation VSWR on compact range ringdown performance
Analysis and measurement activities to quantify compact range feed/subreflector time domain response are described in this paper. Reflection properties of various components are quantified and their interaction studied. Results indicate that although the feed/subreflector interaction is a factor, reverberation is dominated by instrumentation interaction particularly in the case of small compact ranges.
Practical transient analysis
Transient signature representation of scattered fields and their interpretation have become common in downrange and crossrange scattering center identification. A review of the basic concepts for one dimensional transient analysis is presented. The topics included are the frequency-time domain dual representation, general characteristics of transient signatures and temporal mechanism extraction.
High performance hardware gate improves compact range performance
Comparative measurements have been made in a compact range to determine the performance improvements that can be achieved when adding a hardware gate to a CW-based measurement system. Starting with conventional stepped frequency CW measurements made in the time domain mode, high resolution downrange data was collected to determine the background levels of the compact range. This was followed by comparative measurements under the same conditions adding a narrow pulsed hardware gate to reject inter-horn coupling and high returns from the compact reflector. A second mode of comparison was examined by collecting aspect data with a specific range gate fixed about the target. Software gated measurements required more points to insure alias free operation, while the hardware gated measurements allowed fewer points which reduced measurement time without sacrificing any accuracy. Finally, imaging measurements were made with both software and hardware gating to compare the measurement time and accuracy
Near-field antenna testing using the Hewlett Packard 8510 automated network analyzer
Near-Field antenna measurements were made using a Hewlett Packard 8510 automated network analyzer. This system features measurement sensitivity better than -90 dBm at measurement speeds of one data point per millisecond in the fast data acquisition mode. The system was configured using external, even harmonic mixers and a fiber optic distributed local oscillator signal. Additionally, the time domain capability of the HP 8510, made it possible to generate far-field diagnostic results immediately after data acquisition without the use of an external computer.
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.
Microwave absorber performance analyses from PMM calculations and RCS measurements
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
Evaluation of compact range design using the diffuse surface ray trace model
A code based n Geometric Optics, but applicable to diffuse surface scattering, it is evaluated for prediction of downrange high range resolution (HRR) plots of signatures generated in a compact range. A description of the technique is given, including physical justification, underlying assumptions, and flexibility of implementation. Data collected at the Hughes Compact Range will be presented in support of the analysis. Usefulness of this code in generating tradeoffs for compact range designs is demonstrated. Variations in the performance of the compact ranges are shown as a function of various range design parameters, including horn performance, chamber length, and target/wall interaction. Results are analyzed and presented in space and time domains.
Imaging stray signals in a compact range using a diagonal flat plate
Accurate scattering and antenna measurements require excellent plane wave purity in the target zone; however all measurement systems are contaminated by various stray signals which result in measurement errors. In this paper, a technique of evaluating the stray signal sources in a compact range using a diagonal plat plate as a test target is presented. The scattering cross section of the diagonal flat plate as a function of frequency and angle of rotation is first measured. Then the time domain response for each projection angle is processed to obtain a two dimensional ISAR image of the plate as well as the stray signals. From the stray signal images, the location and relative strength of the stray signals can be determined. Experimental results from the OSU/ESL Compact Range Facility are presented to demonstrate this stray signal imaging technique.
Real-time 3D image processing techniques
Based on the complexity of the scattering mechanisms associated with a real-world target, it is obvious that measurement diagnostic tools are extremely helpful. On technique that has found great success in this regard is the conventional ISAR or down range/cross range image. However, the results are basically two-dimensional, which limits the usefulness of the data in that most real-world targets have significant three-dimensional features. A very efficient class of 3D image algorithms has been developed which are based on various time domain look angles relative to the target . It has been shown that one can use multiple feed antennas in a compact range to collect this data and then process it directly to obtain a 3D image of the target. This can be done very rapidly, say every 10 seconds, using an approximate solution, or in 10 minutes using a 3D ISAR approach. The system design and techniques used to implement this system are presented in this paper.
A Comparative study of MUSIC and ESPRIT methods in scattering analysis
MUSIC and ESPIRIT are two popular eigen analysis based super-resolution estimation techniques. The use of these techniques in scattering analysis is of interest in this paper. A comparative performance evaluation of these techniques is reported based on the time domain study of the RSC of various targets.
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