Application of a near-field transform algorithm to antenna coupling using the FDTD method

The finite-difference time-domain (FDTD) method is useful in many antenna applications because it facilitates modelling of complex structures and is capable of characterising antenna performance over a wide-frequency band. However, for antenna arrays of even modest size, the excessive computer storage and run-time may become prohibitive for design and optimisation purposes. Some work has been done with time-domain transform methods for computing weak coupling between antennas (Page et al. 1996). In this paper, an algorithm is developed using the Kirchhoff near-field to near-field transformation (Shlager and Smith, 1995) for the one-way coupling between antenna elements. This coupling representation is then applied to arrays in a manner analogous to an element-by-element analysis in the frequency domain, except that in the method described here the coupled currents at the antenna-feed ports are added in the time-domain so as to permit the introduction of true-time delays for wide-band beam steering. The advantages of the transform method over a full FDTD method are smaller computer storage and a reduction in late-time FDTD effects associated with reflections from the absorbing boundary on the outer surface of the computational domain and also from numerical dispersion and instabilities.