Abstract: A flat plate or slab antenna (1) is fabricated from a number of sandwiched layers in which a number of arrayed individual antenna elements (3) are formed. The antenna elements (3) each include a horn (12) with a rectangular aperture (13) feeding (or fed by) individual rectangular waveguides (15). Two orthogonal probes (17,20) protrude into each waveguide (13), each of which is connected to respective beamforming networks. The network of first probes (17) operates at a first frequency while the network of second probes (20) operates at a frequency which is different from the first frequency. In the preferred embodiment, the edges (13) of the horn apertures (12) are parallel to the sides (6, 7, 8, 9) of the antenna slab while the walls of the rectangular waveguides are at 45° to the sides (6, 7, 8, 9) of the slab. The antenna (1) is able to receive and/or transmit two orthogonally linearly polarised signal at different frequencies and is therefore capable of full duplex operation.