Narrow band resonances in planar array of spiral antennas without ground plane

Steyskal noted that, for off broadside scanning, some “resonances” can appear in a uniform spaced array of two-arm spiral antennas [1]. These resonances are related to the spiral arm´s length. West also showed that these resonances are related to the coupling between the spirals [2]. Those papers concerned a planar array of square spiral antennas over a ground plane. The present work extends these findings. It will be shown that these resonances are also present when no ground plane is used and in any type of planar spiral, including Archimedean, and logarithmic spiral. An explanation of this phenomenon, using a double transmission line model, is presented.