The coupling and mutual impedance between conical log-spiral antennas in simple arrays

The radiation characteristics of the conical logarithmic spiral antenna make it attractive as an element for circularly polarized arrays. The design of such arrays requires a knowledge of the radiation coupling and mutual impedance between these log-spiral elements as a function of rotation as well as element spacing, and for some array configurations a knowledge of the location of the phase center of the elements. A general consideration of the conical log-spiral antenna as a locally periodic, slow wave structure is outlined, and in so-doing a quasi-empirical formula for the location of the antenna phase center is developed. An investigation of the mutual impedance between the log-spiral antennas has shown it to be very low. Information on this impedance and on the coefficient of coupling between antennas is supplied for two different geometrical array configurations and for various antenna parameters. Pattern modification due to the presence of parasitic elements has been investigated. For element to element spacing of at least one half wavelength, this consists primarily of an increase in beamwidth. Radiation patterns for several arrays, including one 4 element conical array are shown. This latter array should be of interest as a primary antenna or as a feed for a large parabolic reflector in telemetry applications.