A scanning antenna with a waveguide assembly for the transmission and/or reception of an electromagnetic signal, the antenna (10,50,60,80) including an antenna element (12,52,62,82) with an evanescent coupling portion with a selectively variable coupling geometry, the waveguide assembly comprising: a transmission line (14,54,64,84) through which an electromagnetic signal is configured to be transmitted, wherein the transmission line defines an axis, and wherein the transmission line (14,54,64,84) is configured to be located adjacent the evanescent coupling portion of the antenna element (12,52,62,82) so as to permit evanescent coupling of the electromagnetic signal between the transmission line (14,54,64,84) and the antenna element (12,52,62,82); first and second substantially parallel waveguide plates (16,56,66,86,130) disposed on opposite sides of the transmission line (14,54,64,84), each of the plates (16,56,66,86,130) defining a plane that is substantially parallel to the axis defined by the transmission line (14,54,64,84), each of the plates (16,56,66,86,130) having a proximal end adjacent the antenna element (12,52,62,82) and a distal end remote from the antenna element (12,52,62,82); whereby the transmission line (14, 54, 64, 84) is configured such that the electromagnetic signal coupled between the transmission line (14,54,64,84) and the antenna element (12,52,62,82) propagates as a beam that is substantially confined to a space defined between the first and second plates (16,56,66,86,130), wherein the beam is in a plane that is substantially parallel to the planes defined by the first and second plates (16,56,66,86,130), wherein the antenna element (12,52,62,82) comprises a diffraction grating having a controllably variable grating period, and wherein the antenna element comprises a rotating drum having a surface defining the diffraction grating, and wherein the controllably variable grating period is provided by a plurality of diffraction gratings of different grating periods formed on the surface of the drum. a transmission line (14,54,64,84) through which an electromagnetic signal is configured to be transmitted, wherein the transmission line defines an axis, and wherein the transmission line (14,54,64,84) is configured to be located adjacent the evanescent coupling portion of the antenna element (12,52,62,82) so as to permit evanescent coupling of the electromagnetic signal between the transmission line (14,54,64,84) and the antenna element (12,52,62,82); first and second substantially parallel waveguide plates (16,56,66,86,130) disposed on opposite sides of the transmission line (14,54,64,84), each of the plates (16,56,66,86,130) defining a plane that is substantially parallel to the axis defined by the transmission line (14,54,64,84), each of the plates (16,56,66,86,130) having a proximal end adjacent the antenna element (12,52,62,82) and a distal end remote from the antenna element (12,52,62,82); whereby the transmission line (14, 54, 64, 84) is configured such that the electromagnetic signal coupled between the transmission line (14,54,64,84) and the antenna element (12,52,62,82) propagates as a beam that is substantially confined to a space defined between the first and second plates (16,56,66,86,130), wherein the beam is in a plane that is substantially parallel to the planes defined by the first and second plates (16,56,66,86,130), wherein the antenna element (12,52,62,82) comprises a diffraction grating having a controllably variable grating period, and wherein the antenna element comprises a rotating drum having a surface defining the diffraction grating, and wherein the controllably variable grating period is provided by a plurality of diffraction gratings of different grating periods formed on the surface of the drum.