Array evaluation at millimeter wavelengths employing planar near-field scanning

The emerging deployment of radiating arrays at millimeter wavelengths introduces new complexity to the task of assaying array performance due to the small size of the array, to the fine tolerances imposed by short wavelength, and the potential inaccessibility of intermediate points in the RF system distributing power through the array. Ransom and Mittra (1972) and Lee et al. (1988) have proposed planar near-field scanning as a means of inferring the aperture field across a radiating array by way of standard holographic near field transformation. The system that we report here is implemented in the spirit of Ransom et al. and Lee et al. and provides a scheme that is complementary to the relatively elaborate electro-optical system of David et al. (see 1998 European Microwave Conference, Amsterdam, Netherlands) and of Yang et al. (see IEEE Trans. Microwave Theory Tech., vol.46, no.12, p.2338-43, 1998). The near-field system is limited, of course, by the fact that invisible spectrum data is lost in a scanning plane that is removed by a few wavelengths from the radiating aperture. On the other hand, as we demonstrate, the level of detail retained is adequate to provide assay of amplitude and phase of radiating elements of the order of ten percent in amplitude and fifteen degrees in phase. This level of accuracy can be quite useful in the initial adjustment of a radiating array or in assessing failures.