Aspects of antenna pattern estimation from planar near-fields

摘要

There are established procedures for determining the measurement uncertainty for certain pattern types (such as high or low directivity) for near-field measurement configurations [1-2-3]. This measurement uncertainly refers to the peak gain, rather than to the low directivity regions of a pattern which are seldom addressed. A very convenient configuration for pattern estimation is planar near-field sampling. The sampling density is governed by avoiding spatial aliasing of radiating waves. This paper discusses an experimental study of pattern estimation using planar near-field samples, including the effect of the sampling density on the far-fields. We use a standard professional-grade planar near-field system (NSI-200 V-5×5) to test a high-gain linearly polarized reflector antenna (10GHz 1.2m or 40 wavelength diameter) with an offset primary feed horn, and gain of about 40dB. Our account is from a typical user's viewpoint rather than from a manufacturer's viewpoint. We demonstrate that increasing the sampling density above the manufacturer's recommendation gives different far-field results for the pattern. Because the pattern is a transform of the near-field aperture, this suggests that the default sampling density of the near-field aperture is under-sampled or that the sampling is inaccurate. This highlights a grey area in the sampling requirements for the near-field region. We also demonstrate that although the accuracy of the peak gain is robust, the accuracy in low directivity regions of the main lobe is suspect.