A Very Low-Profile, Omnidirectional, Ultrawideband Antenna

We present a compact, ultra-low-profile, ultrawideband (UWB) antenna with monopole-like radiation characteristics. The antenna is composed of two bent diamond-shaped loops that are fed in parallel and loaded with a common capacitive top hat to reduce their lowest frequencies of operation. A series capacitor is used at the feeding terminals of one of the loops to create an additional resonance below the lowest frequency of operation of one of the loops and, hence, further reduce the lowest frequency of operation of the antenna. At the lowest frequency of operation, the proposed antenna has electrical dimensions of L × W × H = 0.22λ_min × 0.22λ_min × 0.033λ_min, where λ_min is the free-space wavelength. The antenna demonstrates a VSWR lower than 3:1 and consistent monopole-like, omnidirectional radiation patterns over a 4:1 bandwidth. A prototype of the antenna with physical dimensions of 7 × 7 × 1 cm³, which operates in the 1.0-4.0-GHz frequency range, is fabricated and characterized using a near-field system.