Characterization of the radiation pattern of antennas using time-domain spherical-multipole and moment expansion

Abstract

This paper presents results of a time-domain spherical-multipole near-to-far-field transformation together with a moment expansion applied to antenna radiation patterns. The equivalence principle is applied to the tangential fields all over the FDTD/WP-PML spatial grid and the use of the spherical-multipole expansion leads to the near-to-far-field transformation, and energy and power norms of temporal signals are used to obtain the antenna radiation patterns for transient and steady-state responses. Such approach is employed to obtain UWB antenna radiated fields and radiation patterns directly in time-domain, it being more convenient to perform an unified characterization in time and frequency domains. Index Terms-Finite-difference time-domain method, time-domain spherical multipole, time-domain moment expansion. I. INTRODUCTION The finite-difference time-domain (FDTD) method in addition to absorbing boundary conditions and perfect matched layers (PML) has been successfully used in analysis of radiation and scattering problems. FDTD is very efficient when making near-field calculations, while for the far field it is necessary to implement an additional method to perform a near-to-far-field (NF-FF) transformation. The first technique used to perform the NF-FF transformation could only be applied for a single frequency analysis [1], [2]. In order to obtain a larger frequency bandwidth response, a broadband excitation followed by discrete Fourier transform was performed [1], [3]. Recently, methods involving a recursive addition of tangential fields contribution over a virtual equivalence surface have been used to perform the same task [4]-[7]. Conventional time-domain NF-FF transformation techniques [1], [3], where time-domain far fields can be directly obtained, are based on the retarded potential method and as a consequence, a new integration over the near-field is necessary for each observation point [5], [9]. Therefore, when broadband results are required at a large number of observation angles, as is the case with radiation patterns, these techniques require a large number of computational resources [3]. Recently a time-domain spherical-multipole near-to-far