Abstract: A UHF antenna has an electrically insulative cylindrical core of a solid material having a relative dielectric constant greater than 5, and a three-dimensional antenna element structure disposed on or adjacent the outer cylindrical surface of the core. The antenna element structure is coupled to a coaxial feeder passing axially through the core. To reduce the effect of unwanted resonant modes associated with the resonant length of the feeder inside the core, the core is spaced from the outer conductor of the feeder by an intervening layer of insulative material having a relative dielectric constant which is much lower that that of the core material.

Abstract: A frequency selective surface includes at least one frequency selective layer (1) made up of an array of electrically conductive elements (2, 2a, 2b), at least one frequency selective layer (3) having an array of non-conductive apertures (4) therethrough overlaying the element layer (1) and a dielectric layer separating the two layers (1, 3). The element layer (1) is complementary in plan view to the aperture layer (3). The element layer (1) and the aperture layer (3) are rotated through 90 degrees in plan with respect to each other and substantially parallel to one another and the element array and the aperture array have the same periodicity.