An open waveguide antenna for a radio frequency identification reader includes a conductive annular waveguide concentric about an axis and configured for operation within an operating frequency band. A radiating slot is formed in at least one wall of the waveguide is also concentric about the axis. An odd-multiple of ports are electrically coupled to the annular waveguide, where the ports are equally spaced around the waveguide at a spacing between adjacent ports of one-half of a guided wavelength at a center frequency of the operating band. A second waveguide, smaller than the first, can also be incorporated. The second waveguide can have a different slot arrangement and fewer ports. The rectangular waveguides can operate in a non-transverse electromagnetic mode, and the ports can be individually driven to beamform the radiated signal of the antenna.