CALCULATION OF ANTENNA PERFORMANCE USING A HYBRID TECHNIQUE WHICH COMBINES THE MOMENT METHOD WITH AN ASYMPTOTIC CURRENT

Abstract

Previously, this hybrid technique, a hybrid theory of diffraction, which combines the moment method with an asymptotic current, was used by Kim and Thiele to solve several 2-dimensional and 3dimensional scattering problems when the incident field was a plane wave. In this paper the method is extended to problems in which the incident field is the near field of an antenna and also extended to calculate antenna impedance. As a sample problem, a monopole at the center of a circular disk is considered. Since the general procedure is an iterative one, the starting point in this problem is the assumption of the current distribution on the monopole. This current distribution generates the initial incident field which in turn gives the initial value of the asymptotic current on the circular disk (which is similar to the geometrical optics current). The current in the asymptotic region (i.e., away from the edge of the disk), which is dominated by the optics type current on the entire surface of the scatterer, is solved by an iterative method to give the approximate surface current using the magnetic field integral equation. The difference between the approximate and true surface currents is calculated from the moment method current, which exists near shadow boundaries and/or sharp discontinuities in geometry. That is, the moment method current induces a current in the asymptotic region which is the difference between the optics type current and the true current. The impedance may be calculated by using the infinite ground plane value plus the change caused by the finite extent of the ground plane through the use of equivalent currents. Results are shown for both the impedance and the radiation patterns. These results are compared with other known results and the agreement is seen to be very good for both large (e.g., ka > 6) and small (e.g., ka < 3) ground planes. [Vol. 6, No. 1, pp. 75-95 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]