Helical Antennas

Abstract The helical antenna operates in several different modes. At lower frequencies, thenormalmode predominates, with a beam pattern similar to that of a dipole along the helix axis. As frequency increases and helix circumference approaches one wavelength, two new modes appear. Theaxialmode radiates along the forward direction of the helix axis. It is broadband and circularly polarized. Thebackfiremode radiates along the backward direction of the helix axis, scanning forward in a conical beam as frequency increases. The quadrifilar helices, along with other multifilar helices, show many of the characteristics of the monofilar helix with some important differences. All three modes are exhibited. The bandwidth of theaxialmode is increased roughly in proportion to the number of wires. Thebackfirequadrifilar helix is useful for mobile communications. The radiation theory of the helix antennas is outlined. Numerous applications of the different modes are presented. Recent work on human proximity effects is noted.