1991 Technical Papers

A Hilbert Transform Based Receiver Post Processor

Author: Dan Slater
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

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.


A New Wideband Dual Linear Feed for Prime Focus Compact Ranges

Authors: Ray Lewis, James H. Cook, Jr.
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

Performance trade-offs are Investigated between the use of clustered waveguide bandwidth feeds and the use of one multioctave 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.


Applications of Portable Near-Field Antenna Measurement Systems

Author: Greg Hindman
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flightline testing, anechoic chamber quiet zone imaging, and EMI testing.

Many of NSI's near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.


Measurement Receiver Error Analysis for Rapidly Varying Input Signals

Author: 0. M. Caldwell
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system.

This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas.

Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.


Measurement System Performance Considerations for Planar Near-Field Scanning Applications

Author: J. H. Pape
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

This paper describes measurement system performance parameters that were considered during the design phase of a planar near-field measurement range for Spar Aerospace Limited. All aspects of the planar near-field measurement system are addressed. These include; instrument selection, scanner interface hardware, system controller/computer hardware, software for data collection, near-field to far-field transformation, data analysis, networking and system configuration. The Scientific-Atlanta Model 2095 Microwave Measurement System with its near-field options is used as the basis for meeting the Spar requirement. The various data collection parameters of the Model 2095 are described with special emphasis on how the factors relate to near-field requirements such as fixed grid sampling. Examples of typical test scenarios are presented as an aid in exploring detailed data collection system timing.


Near-Field Measurement Experience at Scientific-Atlanta

Author: Doren W. Hess
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific-Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.


Probe - Correction Coefficients Derived From Near-Field Measurements

Author: Gregory F. Masters
Publication: AMTA 1991
Copyright Owner: NSI-MI Technologies

Probe correction is necessary in near-field measurements to compensate for non-ideal probes. Probe compensation requires that the probe’s far-field pattern be known. In many cases direct far-field measurements are undesirable, either because they require dismantling the probe from the near-field range set-up or because a far-field range is not available. This paper presents a unique method of deriving probe-correction coefficients by measuring a probe on a near-field range with an “identical” probe and taking the square root of the transformed far-field. This technique, known as the “Probe-square-root” method can be thought of as self-compensation. Far-field compensations are given to show that this technique is accurate.

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