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New ultra-low loss, temperature-stable composite ceramics, (1- x) Mg 1.99 Ga 0.01 Si 0.99 Al 0.01 O 4 - x CaTiO 3 , have been developed with a view to expanding the portfolio of low-cost, MW dielectrics available for 5 G/6 G telecommunications, currently dominated by 95 wt%MgTiO 3 -5 wt%CaTiO 3 , (Mg,Ca)TiO 3 (relative permittivity, ε r ∼ 21, quality factor, Q × f ∼ 56,000 GHz and temperature coefficient of resonant frequency, τ f ∼ 0 ppm/°C). Compared with (Mg,Ca)TiO 3 , 89 wt%Mg 1.99 Ga 0.01 Si 0.99 Al 0.01 O 4 -11 wt%CaTiO 3 ceramics are also temperature stable (τ f = −2.72 ppm/°C) but have lower ε r (∼8.55) and a higher Q × f (∼ 72,900 GHz @ 12 GHz and 80,900 GHz @ 26 GHz), better suited to the higher frequencies earmarked for 6 G millimeter-wave (mm-wave) applications. To demonstrate its potential for 5 G/6 G telecommunications, a waveguide filter with novel negative coupling structure was fabricated based on simulated results. The filter had low insertion loss (0.43 dB) at the center frequency (4.8 GHz) and high selectivity with a roll-off rate of 576/375 dB/GHz. The S-parameters matched well at 25 °C and 85 °C, demonstrating that the filter had excellent temperature stability and was therefore suitable for future 5 G/6 G microwave and mm-wave communication devices. [ABSTRACT FROM AUTHOR]