Analysis and Feeding of a Spiral Element Used in a Planar Array

The operation of a previously introduced planar array consisting of square spiral radiating elements has been analyzed by both numerical simulation and a transmission line model. The numerical results show that previously unexplained narrow resonances in the active impedance of the spiral elements result from standing waves excited on the elements through coupling from other elements. The transmission line analysis confirms that the standing waves can only be established when the spiral arm lengths are simultaneously multiples of one half the operating wavelength. The standing wave is excited only by asymmetric coupling, so the resonances are strongly established only for off-broadside scan. A method to feed the unbalanced load presented by an asymmetric spiral that is immune to the narrow resonances is proposed.