Impedance of a monopole on the base of a large cone

This paper is concerned with the determination of the driving-point impedance of a monopole which is located at the center of the base of a perfectly conducting, right circular cone. The cone is assumed to be sufficiently large that it is valid to apply ray-optical theory. It is shown that the field which is reflected back to the monopole from the edge of the base may be pictured as arising from an equivalent magnetic current located at the base edge, the magnitude of which is determined by wedge-diffraction principles. Use of the induced electromotive-force (EMF) method then allows estimation of the change in the monopole impedance from its value on a ground plane of infinite extent. Proper account is taken of the edge curvature and of the more significant multiply diffracted rays. In the degenerate case in which the cone becomes an infinitely thin disk, the general result is shown to reduce to that obtained by a completely independent method. For this particular case, the theory is also compared with published experimental data and the agreement shown to be good for disks with a diameter exceeding one wavelength.