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Reflection coefficient measurement system using a semi-open cavity


A.G. Romanov, Yu.I. Choni

Carbon fiber constructions are widely used in modern antennas of aircrafts and spacecrafts. Its production involves a number of technological factors, such as the structure of the carbon fabric and the distribution of the compound, which are difficult to be controlled. As applied to reflector antennas the most important functional characteristic of their material is the reflection coefficient (RC). Thus, there is a need for highly accurate RC measurement. Accuracy of the direct measurements is limited at least by instrumental error which is no less than ± 0,2 dB for wideband vector signal analyzers of such a prestigious manufacturers as Rhode Schwarz (Germany), Agilent Technologies (US) and «MIRKAN» (RF). This causes, accuracy at the level of ± 2,5% in amplitude or ± 5% in power, that is often not acceptable. A method to improve the accuracy of RC measurement on the basis of a specific semi-open cavity formed by a parabolic meniscus with a feed at the top of the parabola and the test sample, located at half the focal length and oriented perpendicularly to the axis. Is investigated describing process of multiple reflections of electromagnetic waves in terms of geometrical optics, it can be shown that the structure of the reflected field at the input of the feed is mainly due to prevailing contribution of spherical waves converging to the top of the parabolic after 3n-reflection from the sample. Several etalons which are reflectors with exactly known RC should be used to calibrate the measuring system. RC of the tested sample is calculated by processing the frequency dependence S11(f) of the reflected wave at the feed input. The results of simulation and test experiments are discussed.

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