a mobile two - way communication antenna is mounted on a metal plate which is pivotally secured to a housing so as to pivot about a horizontal axis . an electric reversible motor drives an elongated screw which in turn moves a cam surface linearly . the cam surface coacts with the metal mounting plate for moving the plate and the antenna between a horizontal and a vertical position . horizontally extending ground plane elements are provided in applications where the housing is nonconducting . these elements contact the metal mounting plate when the plate is in its full upright horizontal position .