A rotary joint comprising: a first part; and a second part that rotates relative to the first part about an axis of rotation; wherein the first part has a first electrical connection annular portion; wherein the second part has a second electrical connection annular portion; wherein the electrical connection annular portions (26, 36) make contactless electrical connection with one another; wherein the electrical connection annular portions (26, 36) together define and surround a core region, in which said contactless electrical connection between the electrical connection annular portions (26, 36) is not made; wherein the core region includes the axis of rotation; wherein the first electrical connection annular portion includes: a first annular waveguide structure; and a first feed electrically coupled to the first annular waveguide structure; and wherein the second electrical connection annular portion includes: a second annular waveguide structure; and a second feed electrically coupled to the second annular waveguide structure; wherein the annular waveguide structures (22, 32) define respective annular gaps (52, 54) within the annular waveguide structures (22, 32), and an axial gap therebetween; and wherein the first feed is operatively coupled to the first annular waveguide structure to produce a transverse electromagnetic 'TEM' wave that propagates from the annular gap of the first annular waveguide structure, across the axial gap, to the annular gap of the second annular waveguide structure; wherein the feeds (24, 34) each include transmission lines that are between a pair of ground planes (92, 94); wherein the transmission lines include fingers (104, 108) that span a gap in one of the ground planes, the gap being aligned with one of the annular gaps (52, 54); and wherein the TEM wave is produced in the gap in one of the ground planes; the rotary joint further comprising one or more bearings allowing relative rotation between the first annular waveguide structure and the second annular waveguide structure. a first part; and a second part that rotates relative to the first part about an axis of rotation; wherein the first part has a first electrical connection annular portion; wherein the second part has a second electrical connection annular portion; wherein the electrical connection annular portions (26, 36) make contactless electrical connection with one another; wherein the electrical connection annular portions (26, 36) together define and surround a core region, in which said contactless electrical connection between the electrical connection annular portions (26, 36) is not made; wherein the core region includes the axis of rotation; wherein the first electrical connection annular portion includes: a first annular waveguide structure; and a first feed electrically coupled to the first annular waveguide structure; and a first annular waveguide structure; and a first feed electrically coupled to the first annular waveguide structure; and wherein the second electrical connection annular portion includes: a second annular waveguide structure; and a second feed electrically coupled to the second annular waveguide structure; a second annular waveguide structure; and a second feed electrically coupled to the second annular waveguide structure; wherein the annular waveguide structures (22, 32) define respective annular gaps (52, 54) within the annular waveguide structures (22, 32), and an axial gap therebetween; and wherein the first feed is operatively coupled to the first annular waveguide structure to produce a transverse electromagnetic 'TEM' wave that propagates from the annular gap of the first annular waveguide structure, across the axial gap, to the annular gap of the second annular waveguide structure; wherein the feeds (24, 34) each include transmission lines that are between a pair of ground planes (92, 94); wherein the transmission lines include fingers (104, 108) that span a gap in one of the ground planes, the gap being aligned with one of the annular gaps (52, 54); and wherein the TEM wave is produced in the gap in one of the ground planes; the rotary joint further comprising one or more bearings allowing relative rotation between the first annular waveguide structure and the second annular waveguide structure.