Design of a Sub-6 GHz Dielectric Resonator Antenna with Novel Temperature-Stabilized (Sm 1- x Bi x )NbO 4 ( x = 0-0.15) Microwave Dielectric Ceramics.

Microwave dielectric ceramics exhibiting a low dielectric constant ( ε _r ), high quality factor ( Q × f ), and thermal stability, specifically in an ultrawide temperature range (from -40 to +120 °C), have attracted much attention. In addition, the development of 5G communication has caused an urgent demand for electronic devices, such as dielectric resonant antennas. Hence, the feasibility of optimizing the dielectric properties of the SmNbO ₄ (SN) ceramics by substituting Bi ³⁺ ions at the A site was studied. The permittivity principally hinges on the contribution of Sm/Bi-O to phonon absorption in the microwave range, while the reduced sintering temperature results in a smaller grain size and slightly lower Q × f value. The expanded and distorted crystal cell indicates that Bi ³⁺ doping effectively regulates the temperature coefficient of resonant frequency (TCF) by adjusting the strains (causing the distorted monoclinic structure) of monoclinic fergusonite besides correlating with the permittivity. Moreover, a larger A-site radius facilitates the acquisition of near-zero TCF values. Notably, the (Sm _0.875 Bi _0.125 )NbO ₄ (SB _0.125 N) ceramic with ε _r ≈ 21.9, Q × f ≈ 38 300 GHz (at ∼8.0 GHz), and two different near-zero TCF values of -9.0 (from -40 to +60 °C) and -6.6 ppm/°C (from +60 to +120 °C), respectively, were obtained in the microwave band. A simultaneous increase in the phase transition temperature ( T _c ) and coefficients of thermal expansion (CTEs) by A-site substitution provides the possibility for promising thermal barrier coating (TBC) materials. Then, a cylindrical dielectric resonator antenna (CDRA) with a resonance at 4.86 GHz and bandwidth of 870 MHz was fabricated by the SB _0.125 N specimen. The exceptional performance shows that the SB _0.125 N material is a possible candidate for the sub-6 GHz antenna owing to the advantages of low loss and stable temperature.