Design and analysis off a six-port receiver for use in near-field antenna measurements

Abstract

The configuration, analysis and calibration of a six-port based receiver for near-field antenna measurements is described. The receiver operates in an approximate manner in which the transmission coefficient is obtained from a single measurement. Design rules are given which keep the operational errors of the device smaller than those introduced by the approximations inherent in the near-field measurement theory. Tests show that the receiver is accurate and stable when operated in a dynamic range of approximately 50 dB.The suitability of the six-port receiver in nearfield measurement systems is demonstrated through the measurement of radiation patterns of horn antennas. IntroductionWith the increasing development of new antenna systems there is a growing need for fast and accurate measurements of the radiation patterns of newly developed antennas. The two most popular antenna radiationpattern measurement techniques are the far-field range and the more recent near-field range with its associated near-to far-field zone transformation [1-3]. Far-fields measurements have the advantage that the required fields are measured directly. The results can, however, be influenced by external factors including the surrounding terrain and the imposed weather conditions. By contrast near-field systems feature a controlled environment but usually require a computer for scan control, data storage and finally post processing of results. A common requirement of both near-field and far-field ranges is the need for a stable combination of RF source and receiver. To date, phase-locked heterodyne receivers have been extensively employed in antenna pattern measurement applications. These devices are, however, based on complicated and expensive electronics and therefore are a major investment cost in any antenna measurement system.Recent work with new measurement devices based on six-port techniques [4][5][6][7] have indicated that complex signal ratios (including magnitude and phase information) can be determined with suitably designed six-port network analysers. Such networks feature low cost, good accuracy and stability and can easily be Paper 5637H (Ell), first designed to operate in a dynamic range of 50 dB. The six-port network analysers are therefore a potentially attractive alternative to conventional receivers used in near-field measurements. They may also be suited to farfield ranges if a 50 dB dynamic range is sufficient.During antenna testing it is necessary to measure the characteristics of two-port networks formed by the connection of the test and probe antennas, respectively. A number of six-port network configurations have already been devised to measure the full set of scattering parameters of a two-port network. Single as well as dual sixport network devices have been described [5][6][7][8]. In general, the single six-port configurations require the two ports of the device under test to be reversed during measurements and are therefore inconvenient for antenna tests. Dual six-port networks overcome this drawback but at t...

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