The computation of antenna electromagnetic fields at nonasymptotic distances from cylindrical near-field measurements

A technique for the determination of the electric field at arbitrary points in the space surrounding an antenna is described. The cylindrical modal expansion formulation of the electric field can be evaluated using A.C. Ludwig´s (1968) algorithm after the factorization of the integrands into slowly and rapidly changing factors. The latter is achieved by factorizing the Hankel functions in the integrands using an asymptotic approximation. The modal coefficients of an E-plane sectoral horn were computed from cylindrical near-field measurements at 3 GHz. Using the method described, the copolarized electric field was computed in the E-plane at distances of 1 and 10 m from the antenna. It is concluded that the method can be used successfully to compute the electric field in the radiating near field and further.<>