Examination of radiation from 2D periodic leaky-wave antennas

Abstract

Summary form only given. Two-dimensional (2D) leaky-wave antennas (LWAs) in the form of Fabry-Pérot resonant cavity antennas are known to produce highly directive beams of radiation, with broadside radiation usually the desired objective. These structures are often made using a periodic partially reflective surface, but nevertheless they operate as a quasi-uniform LWA, radiating from the fundamental space harmonic (Floquet mode). A cylindrical leaky wave that is a fast wave propagates outward from the source at the center of the structure, and this radiating wave does the beamforming. Such a structure is classified as a quasi-uniform LWA instead of a periodic LWA. For one-dimensional LWAs, where a guiding structure along a fixed direction is used, one-dimensional (1D) periodic LWAs are very common. For these structures radiation is from the n = -1 space harmonic with a phase constant β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sub> . Since β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sub> can be positive or negative, radiation can occur in either the forward direction or the backward direction. Radiation at broadside in the form of a fan beam is possible by optimizing the structure, with maximum radiation at broadside occurring when |β-1| = α. Usually, 2D LWAs do not operate as a periodic LWA and hence do not radiate from the n = -1 space harmonic, but in some cases one is forced to use such a structure. One example is the directive beaming effect that has been observed at optical frequencies, where a corrugated silver film with 2D corrugations is used to create directive beams at broadside by using a surface plasmon mode on the film that radiates from the corrugations, in effect creating a periodic leaky-wave antenna. This motivates the study of 2D periodic LWAs. The goal here is to explore the radiation patterns that are achievable using a 2D periodic leaky wave antenna, where the fundamental wave is a slow wave and radiation occurs from the n = -1 space harmonic. A canonical structure that is examined at microwave frequencies consists of a grounded dielectric slab with a periodic arrangement of rectangular conducting patches printed on the top surface, excited by a horizontal magnetic dipole at the center of the structure. A radially-propagating TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> surface-wave mode is used to form the beam (playing the role of the plasmon mode at optical frequencies). The antenna is optimized for maximum radiation at broadside, and the nature of the resulting beam is explored. An exploration of the fundamental physics of the radiating leaky mode is made, in order to study the role of the leaky mode in forming the beam and to explain the shape of the beam. The beam is usually elliptical in shape, being narrower in the E plane (the plane of excitation of the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> surface-wave mode).