Analysis of impedance resonances in a spiral-element array

Recently, Steyskal et al. [1] introduced a planar spiral radiating element for use in a broad-band phased array. A numerical analysis using the finite-difference time-domain (FDTD) showed that an infinite array consisting of this element was well matched to a 280 Omega source from 5 GHz to at least 15 GHz when the array was excited for broadside scan. However, sharp resonances appeared in the input impedance when the array was scanned off-broadside. The resonances appeared at frequencies where the length of spiral arms was a multiple of approximately one-half the excitation wavelength. Furthermore, it was shown that the resonances could be eliminated by ldquospoilingrdquo the symmetry of the spiral elements by extending one arm slightly through a partial turn. No explanation was given for this effect. Here, the numerical analysis is repeated using a different computational electromagnetics technique that allows finite-extent arrays to be considered, providing additional insight into the resonance mechanism. The spiral element is then modeled as a loaded transmission line, revealing the source of the resonances.