Spiral element for wide-band phased arrays

There is great interest in wide-band phased arrays for multi-function radar and communication systems. Planar, two-dimensional elements offer a simple geometry, potentially simple manufacturing and flush, conformal mounting. However, apart from a recent wide-band patch array (Munk, B. et al., IEEE S-AP Int. Symp., 2003), few designs have been published. This paper presents a numerical analysis of a planar spiral element for an octave bandwidth phased array. It is a thin planar 'printed circuit' array element for circular polarization, which performs well over one octave. Its depth is only 1/8 of the wavelength at the low frequency end. Analysis indicates that low frequency performance could be improved by tighter element spacing. However, this increases the number of electronic controls, and is an expensive approach toward wide-band scanning. The design of the spirals appears to be quite robust with respect to number of turns, line length and line width. The results shown are generic in the sense that, so far we, have made no attempt to optimize the spiral parameters or to improve performance with the aid of various dielectric superstrates and substrates, as used by Munk et al.