A multifilar helical antenna on a sphere and the limits of broadbandness

Abstract

An antenna in form of a dense multifilar helix of thin metal ribbons on a spherical surface with lower order transverse-electric and transverse-magnetic modes is considered. Averaged boundary conditions taking into account the capacitances between the edges of ribbons are formulated. Formulas for calculating the angle of inclination of ribbons and the required inter-edge capacitance from a given resonance frequency and approximate ellipticity coefficient are obtained. The inclination of the axes of the polarization ellipse and an improved ellipticity coefficient are determined. The fractional bandwidth of the antenna is expressed via the improved limits of the fractional bandwidth of linearly polarized antennas. The limits of the fractional bandwidth of circularly polarized antennas are found. It is established that the maximum fractional bandwidth is reached if the ellipticity coefficient is slightly different from unity.