Bismuth niobate glass-ceramics for dielectrically-loaded microwave antenna applications

Dielectrically-loaded quadrifillar-helix (D-LQH) antennas are used in applications such as global positioning systems and satellite radio where high gain is required for direct communication with satellites. The current generation of D-LQH antenna are made from hollow cylinders of medium permittivity (15??,??????r????80), high quality factor (Qf), temperature stable ceramics coated with a helical Cu metallisation. To reduce the processing costs of the ceramic core, a castable bismuth niobate glass-ceramics (30% Bi2O3, 30% Nb2O5, 30% B2O3 and 10% SiO2, in mol%) has been developed. On crystallisation at 600??C, a nano-dendritic, pyrochlore-structured phase evolves with ????r = 41 and Qf = 226. This phase is unstable at >700??C and is initially replaced by triclinic-BiNbO4 (BN) (700 ?????? 900??C) and then by orthorhombic BN (>900??C). Optimum properties for antenna core applications occur at 920??C heat treatment with ????r = 15, Qf = 8000 GHz at which point the temperature ! coefficient of resonant frequency is ??60 MK-1.