Antenna Spherical Coordinate Systems And Their Application In Combining Results From Different Antenna Orientations

Author: Allen C. Newell, Greg Hindman
Publication: ESA ESTEC Workshop on Antenna Measurements
Publication: European Space Agency

The results of theoretical calculations or measurements on antennas are generally given in terms of the vector components of the radiated electric field as a function of direction or position. Both the vector components and the direction parameters must be defined with respect to a spherical coordinate system fixed to the antenna. Along the principal planes there is no ambiguity about the terms such as vertical or horizontal component, but off the principal planes the definition of directions and vector components depends on how the spherical coordinate system is defined. This paper will define four different spherical coordinates that are commonly used in measurements and calculations, suggest a terminology that is useful to distinguish between them, and define the mathematical transformations between them. One important application of these concepts arises when comparing or combining measurement results from two antenna orientations. In this case, the axis of rotation dictates the preferred coordinate system and vector components. Measured results will be shown to illustrate the proper choice of coordinates for each situation.

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Copyright 1999 European Space Agency. Reprinted from 1999 ESA ESTEC Workshop on Antenna Measurements.

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Digital Receiver Technology for High-Speed Near-field Antenna Measurements

Authors: David S. Fooshe, Dan Slater
Publication: AMTA 1999
Copyright Owner: NSI-MI Technologies

High-speed receivers for near-field antenna and RCS measurements have traditionally been one-of-a-kind, expensive, difficult to interface and lacking in software support. Advances in digital signal processing, computer technology and software development now provide the means to economically solve these problems. NSI offers a high speed receiver subsystem, the Panther 6000 series, that allows multiplexed beam and frequency measurements at a rate of 80,000 independent amplitude and phase measurement points per second. The Panther 6000 receiver directly digitizes the 20 MHz IF test and reference input channels, and includes a high speed beam controller (HSBC) to sequence the measurement process. The HSBC receives an input trigger to initiate a measurement sequence of user-defined frequencies and beam or pol states.

NSI also offers a multi-channel all-digital receiver subsystem, the Panther 6500, to interface directly with Digital Beam Forming (DBF) antennas. The Panther 6500 allows up to 16 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The alldigital DBF receiver reduces the cost, complexity and performance limitations associated with conventional instrumentation in DBF antenna measurement applications.

All Panther series receivers are fully integrated with the NSI97 antenna measurement software and operate with existing microwave sources, mixers and IF distribution equipment.


Extending The Angular Coverage Of Planar Near-Field Mearsurements By Combining Patterns From Two Or More Antenna Orientations

Authors: Allen C. Newell, Greg Hindman
Publication: AMTA 1999
Copyright Owner: NSI-MI Technologies

The angular coverage of planar near-field measurements is limited by the size of the scan plane, and the “region of validity” is defined by the angle between the edge of the AUT and the edge of the scan plane. In some applications, results are required over a larger angular region than is possible with the available scanner. The angular coverage can be increased by rotating the antenna and repeating the measurement. The results of the two measurements are then combined. Successful combination depends on using both the coordinate system and vector components that are appropriate for the antenna rotation.

In general for a single antenna orientation, any coordinate system can be used with any vector components, but when combining or comparing patterns for two antenna rotations, the axis of rotation must be the polar axis and the vector components must correspond to that coordinate system. Measurements results will be used to demonstrate the proper choice of coordinates and components and to illustrate potential problems that may arise.


The Effect Of Measurement Geometry On Alignment Errors In Spherical Near-Field Measurements

Authors: Allen C. Newell, Greg Hindman
Publication: AMTA 1999
Copyright Owner: NSI-MI Technologies

Papers were presented at the last two AMTA meetings reporting on the effect of rotator system alignment on the results of spherical near-field measurements. When quantifying the effect of non-intersection errors on the AUT directivity, these two papers presented very different results. One AMTA paper1 and an earlier study at The Technical University of Denmark2 found that the directivity error was extremely sensitive to non-intersection errors while the other AMTA paper3 found a very small sensitivity. During the past year, scientists at the Technical University of Denmark, The National Institute of Standards and Technology, and Nearfield Systems Inc. have been working together to determine the reasons for these differences. It now appears that the major reason for the difference is due to the method used to acquire data on the sphere. Theta scans that pass through the pole, or equivalently, phi spans of 180 degrees, produce both plus and minus phase errors that tend to cancel in the on-axis direction. Theta scans that do not pass through the pole, or equivalently phi spans of 360 degrees, produce phase errors of the same sign over the sphere which are concentrated in the on-axis direction. Examples of measurements and recommendations for using this information in spherical measurements will be presented.

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