Figure 1. Basic design concept for planar-type sound antenna with curved reflection phase Φ(x).

(a) Conceptual figures of planar directional surface with symmetric phase gradient dΦ(x)/dx with respect to the origin. Such surfaces can be obtained using space elements with varying degrees of coiling positioned along the planar surface. The surface can then be used to form a distinct focal line located directly above and at a distance f from the origin, as shown in the figure. The structure is invariant in the z direction. (b) Conceptual schematic of parabolic antenna, having an identical focal line to the planar meta-surface.