Dual-polarization monopulse antennas utilizing conical arrays of log-periodic antennas

Abstract

The antenna system consists of N log-periodic monopole antennas fed against a central conducting cone and a feed network comprised of a multiplicity of magic-Tls and quadrature hybrids. The antenna elements are placed on the cone in a rotationally symmetric manner such that a rotation about the cone axis of 2n/N radians leaves the antenna unchanged. With the proper feed for both left and right circular polarizations simultaneously.network, rotationally symmetric sum and difference patterns may be produced Figure 1 illustrates a 2-12 GHz monopuke antenna dish feed utilizing eight log-periodic elements, eight tapered-line magic-T's and hvo eleven-section symmetrical hybrids. A close-up of the antenna array, which makes use of log-periodic parasitic monopole antennas, is shown in Figure 2.Approximate pattern characteristics of the conical array may be determined kom an analysis of the circular array of N dipoles illustrated in Figure 3.Because of the symmetry of the antenna, i t is most convenient to work with the "normal modes" of excitation of the array. Consequently, let the dipole currents be given bywhere n = 0, . . N -1 denotes the dipole number and m = 0, . . N -1 excitation amplitude of the mlth normal mode. Since the array normal denotes the normal mode number. The coefficient bm represents the venient to work with left and right circularly polarized components rather modes tend to produce circular polarization near the z axis, it will be conand right circular components of the electric field for the mlth mode are than linear components of the electric field. It may be shown that the left given by