PLANAR NEAR-FIELD TO FAR-FIELD TRANSFORMATION USING AN EQUIVALENT MAGNETIC CURRENT APPROACH

An alternate method is presented for computing far-field antenna patterns from near-field measurements. The method utilizes near-field data to determine equivalent magnetic current sources over a fictitious planar surface that encompasses the antenna, and these currents are used to ascertain the far fields. Under certain approximations, the currents should produce the correct far fields in all regions in front of the antenna regardless of the geometry over which the near-field measurements are made. An electric field integral equation (EFIE) is developed to relate the near fields to the equivalent magnetic currents. Method of moments (MOM) procedure is used to transform the integral equation into a matrix one. The matrix equation is solved using the conjugate gradient method (CGM), and in the case of a rectangular matrix, a least-squares solution for the currents is found without explicitly computing the normal form of the equation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions by exploiting the block Toeplitz structure of the matrix and using CGM and fast Fourier transform (CGFFT), thereby drastically reducing computation time and storage requirements. Numerical results are presented for several antenna configurations by extrapolating the far fields using synthetic and experimental near-field data.