A study of one- and two-dimensional (1D and 2D) uniform leaky-wave antennas (LWAs) is presented, with the aim of characterizing their fundamental radiation properties when the beam points at broadside. In particular, it is shown show to optimize radiation at broadside, and the general properties of the broadside radiation, including pattern bandwidth, are examined. Although directed at antennas, this work also has important applications in other areas such as the enhanced transmission through subwavelength apertures, where the phenomenon is due to leaky-wave radiation at broadside. The class of LWAs considered here consists of a grounded dielectric slab excited by either a horizontal electric line source (1D LWA) or a horizontal electric dipole source (2D LWA) inside the slab. At the air-slab interface a partially-reflecting surface (PRS) is assumed, which may take the form of a high permittivity layer, or a metallic plate with a periodic arrangement of holes, or some other structure that partially shields the slab, allowing radiation to occur. The PRS creates a leaky parallel-plate waveguide (PPW) region, which is excited by the source. The source launches the first higher-order (n=1) TE and/or TM PPW modes (depending on the source), which are leaky modes due to PRS. In this class of structures radiation occurs from the fast-wave nature of the PPW modes, and not from a space harmonic, and thus the structure is classified as a uniform type of LWA structure. The analysis of both the radiated far field and the modal properties of the structure is carried out by means of an transverse equivalent network in which the PRS is modelled as a constant shunt susceptance BS (this has been shown to accurately model a 2D periodic array of metal patches or slots in a metallic plane, as well as superstrates with high permittivity). It is then possible to obtain in a simple way a closed-form expression for the power density radiated in the far-field region. The analysis reveals that a fundamental condition for optimum antenna operation at broadside is the equality between the phase and attenuation constants of the leaky mode. Moreover, the same condition corresponds to the maximum level of radiation at broadside. At this point the beam is on the verge of splitting into two distinct peaks off broadside. For both lossless and lossy structures, design formulas have been obtained based on an approximate asymptotic expression for the leaky-mode propagation constant in the limit of large BS (i.e., small beamwidth). These formulas show that in the lossless case the maximum level of power density radiated at broadside increases indefinitely as BS increases, while in the lossy case an optimum value for the shunt susceptance BS exists. In addition, the antenna bandwidth for operation at broadside (defined as the frequency range over which the power density radiated at broadside is 3 dB or less lower than its maximum) has been characterized in terms of the location of the leaky poles in the complex plane, and a simple and useful approximate expression for the fractional bandwidth has been given in terms of the physical parameters of the antenna.
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