Ultrawideband Hemispherical Helical Antennas

A compact ultrawideband antenna with hemispherical helical geometry is proposed and investigated both theoretically and experimentally. The radiating element of this antenna consists of a tapered metallic strip that assumes a non-conformal orientation relative to the hemispherical surface in order to yield maximum bandwidth. The number of turns is about four and half with a constant spacing between them. The antenna is fed by a coaxial cable with the inner conductor connected to the radiating strip through a matching section, and its outer conductor connected to a ground plane. Radiation properties of the proposed hemispherical helical antenna, including far-field patterns, axial ratio, directivity, input impedance and voltage standing wave ratio (VSWR), are studied numerically and evaluated experimentally. Simulation and measured results are in good agreement. This design provides a maximum directivity of 9±1 dB, a VSWR ≤ 2 (relative to a 50 Ω reference impedance), and nearly equal E- and H-plane far-field patterns with high degree of axial symmetry over a bandwidth of more than 50%. Also, over a bandwidth of about 24% the axial ratio remains below 3 dB. The compact size and ultrawideband performance of this antenna make it advantageous for high speed wireless communication systems and avionics.