(a) Typical periodic LWA using a periodic arrangement of stubs along a microstrip line. (b) A representative brillouin (βp − k ₀ p) diagram of a periodic leaky-wave antenna. Grey shaded region is the slow-wave region, where the modes are strongly bound to the surface and the region within the cone bounded by straight βp = ±k ₀ p lines, indicated by thick black solid lines, is the leaky-wave region, where radiation occurs.