An antenna formed by winding a conducting wire in the shape of a screw thread, producing a helix. Usually the helix is used with a ground plane and coaxial transmission line feed, the centre conductor of the transmission line being connected to the helix and the outer being connected to the ground plane.

The helical antenna can be used in a variety of modes, the most common two being normal (broadside), in which the radiation is linearly polarized, and axial (endfire), in which the radiation is circularly polarized (see polarization). The mode is dependent on the circumference and pitch angle of the helix. In axial mode, the antenna is particularly useful for space communications as it will still receive a component of a linearly polarized signal but is insensitive to the actual orientation of the polarization, something difficult to keep constant for satellites, etc. In normal mode the antenna is very common on hand-held mobile communications systems as it performs similarly to a monopole antenna with ground plane but can be significantly shorter.