Understanding Leaky-Wave Structures: A Special Form of Guided-Wave Structure

Guided-wave structures are the foundation for the design and development of RF and microwave circuits and systems. Whether it is planar or nonplanar, periodic or straight, a guided-wave structure, which consists of metallic and/or dielectric composite building blocks, is used to support signal propagation and processing. Except for free-space propagation, diffraction and scattering in an open medium or space, microwave energy is usually transferred by means of specially designed guided-wave structures with modal propagation behavior that are fundamentally characterized by a propagation constant and transmission loss with respect to specific guided-wave modes. Those modes are structure and frequency dependent, and satisfy the boundary conditions of a guided-wave structure. Guided-wave structures can be divided into two kinds, open and closed. The completely closed structures are known to support only the wave guided therein, while the open structure, including semi-open or partially open structures, are subject to a potential gradual wave leakage along the propagation path. Wave leakage is always related to wave guidance, which is very much dependent on a number of factors including structural geometry, fill materials, operating frequency and guided mode. Such leakage can be used positively to develop a leaky-wave structure called a leaky-wave antenna, which enjoys some distinctive properties such as beam-scanning with frequency. Leaky-wave radiating structures are easily fabricated at millimeter wave frequencies compared with other antennas. Although leaky-wave and guided-wave structures have similar characteristics and can be designed by common methods, the simulation and design of leaky-wave structures are usually much more complicated. This is because leaky-wave structures are not closed and their attenuation constant related to leakage needs to be considered. In this article, we focus on the presentation of basic operating principles and special features of positive leaky-wave structures (leaky-wave antennas) ranging from straight to periodic geometries.