Hybrid TM/TE Cylindrical Leaky Waves: Electromagnetic Radiation at Broadside from Radially Periodic Structures

The nature of broadside radiation from planar, radially periodic structures excited at the center is investigated in this article by considering an annular microstrip antenna operating in the microwave range. This kind of antenna, capable of radiating either a scanned conical pattern or a broadside pencil beam, is typically designed by enforcing that its linearized version supports a single TM leaky mode. However, a single TM cylindrical leaky wave is known to be incapable of radiating a directive broadside beam. To solve the conundrum, a generalized pencil-of-function analysis of the exciting field is performed. It is found that a nonnegligible TE component is present, which exhibits a radial decay different from the TM component but the same complex radial wavenumber. Based on the geometrical symmetries of the structure, an electromagnetic model for the surface currents excited on the annular microstrips is developed and validated by full-wave results for both the currents and the associated radiation patterns. The generality of the adopted approach sheds new light on the nature of the electromagnetic field radiated at broadside by the general class of annular, radially periodic structures, including novel terahertz antennas and plasmonic structures used for achieving extraordinary transmission at optical frequencies.