An Electrically Small, Vertically Polarized Ultrawideband Antenna With Monopole-Like Radiation Characteristics

We present an electrically small, low-profile, ultrawideband (UWB) antenna with monopole-like radiation characteristics. The antenna is composed of a monopole bow-tie antenna reactively loaded with a cascaded system of top hats, two shorting arms, and a ring slot cut into the ground plane. The reactive loads are used to introduce two additional resonances close to each other and below the lowest resonant frequency of the bow-tie. This results in a very compact, ultrawideband antenna that utilizes the available volume inside the Chu´s sphere rather efficiently. At the lowest frequency of operation, the proposed antenna has electrical dimensions of 0.085λ_min×0.189λ_min×0.189λ_min, where λ_min is the free-space wavelength. The antenna demonstrates a VSWR of 2.2:1 and consistent monopole-like, omnidirectional radiation patterns over a 5.5:1 bandwidth. A prototype of the antenna, which operates in the 0.6-3.2-GHz band, is fabricated and characterized using a multiprobe spherical near-field system.