Screened printed monopole spiral antenna for mobile handsets

Spiral antenna elements have been shown to have useful wide bandwidth and circular polarisation properties. A 'Curl' antenna design has been proposed in the literature where a single spiral arm parallel to and above a groundplane is fed through this groundplane via a single wire feed. Such an arrangement provides screening at the back of the groundplane, and also obviates the need for a balun since the whole antenna is essentially a monopole fed against an earth plane, so that an unbalanced coaxial feed may be used. In this contribution the Curl antenna design has been extended to a spiral arm printed on a grounded dielectric substrate, and fed via a wire through the dielectric. In a practical deployment scenario therefore, such a spiral could be printed on the back of a phone with some suitable intervening dielectric grounded by the transceiver casing, which would provide screening of the user's head from the active element. In order to achieve wide bandwidth and good circular polarisation from this printed spiral, the current distribution along the spiral arm must be essentially that of a travelling wave, and this is dependent on parameters such as substrate permittivity and thickness, and also the spiral function and arm length