Pattern and Gain Characterization Using Nonintrusive Very-Near-Field Electro-Optical Measurements Over Arbitrary Closed Surfaces

A nonintrusive near-field measurement technique for 3-D radiation pattern and gain characterization of antennas is presented. The method is of particular interest for low-frequency antennas for which anechoic chambers cannot be developed and far-field measurements are rather cumbersome. Nonintrusive, broadband measurements are performed using an extremely small all-dielectric electro-optical probe to measure the tangential electric fields of an antenna under test (AUT) at a very-near surface enclosing the antenna. Far-field radiation is computed from a new near-field to far-field transformation formulation using only the tangential components of the electric field over an arbitrary surface. This procedure employs reciprocity theorem and the excited electric current on the surface of a perfect electric conductor enclosure having the same geometry as the scanned surface and illuminated by a plane wave. In this way, a full spherical radiation pattern and gain of the AUT are easily computed without expensive computation and truncation errors. To demonstrate the proposed approach, a miniaturized low very high frequency antenna operating at 40 MHz with dimensions 0.013 lambda(0)x0.013 lambda(0) x 0.02 lambda(0) is utilized. The far-field results from our approach are shown to be in good agreement with those obtained from full-wave simulation and direct far-field measurement performed in an elevated outdoor range.