Higher-Order Leaky-Mode Bessel-Beam Launcher

We present a Bessel-beam launcher based on a leaky radial waveguide consisting of a capacitive sheet over a ground plane that supports higher-order leaky modes. A propagating Bessel beam is generated above the radiating waveguide. The Bessel beam is transverse-magnetic (TM) polarized with a vertical component of electric field that is a zeroth-order Bessel function of the first kind. A higher-order leaky-wave mode is used to reduce losses at millimeter waves and, at the same time, avoid the thin dielectric layers used in previously proposed lower order leakywave Bessel launchers. Closed-form design equations are provided for the proposed structure. In addition, the operating bandwidth of the launcher is defined using dispersion analysis. Near-field measurements of a prototype operating in the frequency range 38- 39.5 GHz validate the concept. The measured launcher generates a Bessel beam with a stable spot size of about 4.3 mm (0.57 λ) over a nondiffractive range of about 16.4 mm (2.2 λ), within about a 4% fractional bandwidth.