Multi-service dual-mode spiral antenna for conformal integration into vehicle roofs

This paper presents a planar four-arm spiral designed for the radio services AMPS, GSM900, DAB, GPS, GSM1800, GSM1900 and UMTS, intended for integration into cars. The different patterns for terrestrial and satellite services are obtained by feeding the four-arm spiral with two different modes. The dual mode feeding is performed with a coplanar waveguide transmission line. The two radiation modes are generated by exciting the even and odd modes on the coplanar transmission line. Both radiation patterns can be excited simultaneously. The spiral is backed with a reflecting cavity in order to suppress back radiation and to conserve the radiated energy. No absorbing material is used. Calculations of a spiral with a ground plane structure based on frequency selective surfaces show the possibility of further reducing the height of the antenna.