Spiral antennas over a ground plane

One technique for achieving a balanced feed is the so-called infinite balun used by J.D. Dyson (1959). A coaxial cable is placed along one arm of the spiral and the center conductor of the cable is connected to the other arm at the feed point. This structure is, however, somewhat difficult to build, and an easier implementation of the infinite balun is described in a brochure of the Ball Corporation (1983). A microstrip feed line connected to one arm of the spiral at the feed point is etched on one side of a thin substrate. The other arm of the spiral is etched on the other side of the substrate and serves as the ground plane for the microstrip line. The microstrip line can be tapered to provide an impedance transformation, if desired. Several one- and two-arm geometries were modeled using an integral equation formulation with patch basis functions. The mesh for a one-arm conical spiral, including its image and the triangular fins that provide the transition to a voltage generator, is shown together with the mesh used to model a planar, two-arm, one-turn spiral operated over a ground plane.<>