Spiral Elements for Broad-Band Phased Arrays

In this paper, we present a numerical analysis of an infinite periodic array of planar spiral elements with octave bandwidth. For off-broadside scan the array is found to exhibit very narrow resonances, which are independent of scan angle. They occur when the spiral arms are multiples of half a wavelength, in which case the current forms a high amplitude standing wave along the spiral arms. The resonances are conveniently suppressed by making the arms unequally long. We also discuss the equivalent 3-port for this nonsymmetrical array element and evaluate the element polarization performance.