Development of an Omnidirectional and Low-VSWR Ultra Wideband Antenna

This paper reports an omnidirectional-in-azimuth and low voltage-standing-wave-ratio (VSWR) ultra-wideband (UWB) monopole antenna that we constructed as a prototype for the Federal Communications Commission-approved UWB frequency band (3.1–10.6 GHz). Most wideband antennas (e.g. double-ridged waveguide horn, monocone antenna) do not simultaneously meet omnidirectional and low-VSWR requirements, essential for some UWB measurement applications. The antenna consists of a circular ground plane and a teardrop that is defined as a combination of a finite cone and a sphere inscribed inside the cone at the cone's base. This antenna can be considered either as a rounded finite monocone antenna or as a simplified volcano smoke antenna. Assuming 50-Ω excitation, the optimum half-cone angle was searched to minimize the maximum VSWR between 3.1 and 10.6 GHz and found to be around 48°. A prototype antenna was constructed and its VSWR and radiation characteristics were measured. The prototype antenna demonstrated omnidirectional radiation and low VSWR (< 1.3) between 3 and 20 GHz. Performance comparison was made between this antenna and other representative wideband antennas: double-ridged-waveguide horn antenna, meander line antenna, and log-periodic dipole array antenna. The 3-antenna method was used to achieve high accuracy in the absolute gain measurements.