Abstract: A quadrifilar helix antenna has four inter-twined helical antenna elements offset from one another by 90°. The elements are identical and each can be defined by an axial coefficient z, a radial coefficient r, and an angular coefficient &thgr;.

Abstract: A satellite telephone has a quadrifilar helix (QFH) antenna (1) having four inter-wound helical antenna elements (2). The antenna (1) can be rotated between a first extended position and a second folded position. A phase shifting arrangement (10) is coupled to the helical elements (2) for applying a first set of relative phase shifts (0,90,180,270 degrees) to signals applied to or received from the helical (2) elements when the antenna (1) is in said first position, and for applying a second set of relative phase shifts (0,270,180,90) when the antenna (1) is in the folded position. The antenna (1) is thus switched between end-fire and back-fire modes, optimising the spatial gain pattern of the antenna for both the extended and folded positions of the antenna (1).