Abstract: A monopulse spiral antenna system of the type having a minimum of three, interwound spiral arms for multimode, direction of arrival sensing, is disclosed in which the antenna arms are shaped and arranged in an overlapping configuration that allows the interarm impedance of the antenna to be adjusted, substantially independently of other electrical properties of the antenna, for matching of the antenna impedance of a mode forming network while preserving the broadband, directional capabilities of the antenna. Several different embodiments of the impedance adaptive antenna are disclosed including a preferred, eight-arm exponential spiral in which the arms are conductive strips transversely inclined relative to a plane formed by the spiral so that opposed and parallel surfaces of adjacent arm strips create a dominant interarm capacitance that in turn determines the overall input impedance of the antenna.

Abstract: To optimize antenna bandwidth and efficiency in broad-band antenna elements of the planar, multiarm spiral and log periodic types, a conically shaped ground plane characterized by progressively sized circumferential chokes, is arranged on a common axis with the axial center of the spiral or log periodic elements so that the electrical spacing between the excited regions of the log periodic of spiral elements and the ground plane maintains a constant one-quarter wavelength relationship. The progressively sized circumferential chokes on the ground plane cut off the flow of excessive radial currents along the ground plane surface to achieve an improved mix of excitation and reexcitation modes. In one embodiment, the circumferential chokes on the conical ground plane are partially shunted by shunting strips that electrically or capacitively bridge the choke walls to reestablish limited radial currents along the ground plane for sustaining certain desirable antenna modes.