Input impedance of an off-axis monopole on a circular disk

Several authors have calculated the input impedance of monopoles on finite ground planes using hybrid MM/GTD techniques [1]-[4]. However, in the case of a circular disk, only monopoles placed along the axis of the disk have been considered. This axis is then a caustic of the diffracted rays and the Equivalent Current Method (ECM) must therefore be used to solve the problem. We deal here with the more general situation of a monopole located at any point on the disk and oriented in an arbitrary direction , (Fig.1). The purpose of this paper is to compute the input impedance of the monopole off the axial caustic using a hybrid MM/UTD technique and to investigate the validity of the solution when the monopole is close to the edge of the disk.