An extensive parametric analysis of conductor-backed square Archimedean spiral antennas has been conducted numerically by the method of moments. The results show that, at a near quarter wavelength above the ground plane, the square spiral yields a 1:1.21 bandwidth of the boresight axial ratio less than 3 dB. It also demonstrates that the axial ratio of the square spiral is controlled not only by its outer perimeter but also by its arm length. Design charts on the selection among wire radius, arm spacing, and spiral ground-plane height are presented as general guidelines for the design of circularly polarized conductor-backed square Archimedean spiral antennas.