This article describes a novel antenna design capable of omnidirectional steering of a THz fan beam in the WR3.4 band from 220–330 GHz with gimbal-controlled annular aperture excitation. The hybrid spatial and waveguide optics approach is based on a cylindrically symmetric structure. A gimbal reflector converts a TE11 mode from a circular waveguide into a guided wave toward a narrowing taper to radiate into free space as a fan beam. Design parameters are optimized to balance beam collimation, fast steering and size. Continuous and agile beam steering capabilities support real-time tracking of users and devices.