Abstract: A waveguide structure including two parallels electrically conducting ground planes (1,2), each of which includes at least one row of spaced apart electrically conducting posts (3). The rows of posts are arranged substantially parallel to one another and the space bounded by the plates and posts defines a guided wave region (4) along which electromagnetic radiation may propagate. The posts are connected to only one of the planes so that there is no physical connection between the two ground planes (1,2). Actuating means may be connected to one or both of the ground planes to cause relative movement there between to thereby alter the electrical response of the waveguide. The direction of the relative movement may such that the distance between the rows of posts (3) is changed and/or the distance between the ground planes (1,2) is changed. Various device may utilize the described waveguide construction, including reconfigurable waveguide filters and antenna structures e.g. slotted waveguide arrays.