Compact All-metal In-line Combline Coaxial Cavity Diplexer

This article describes the design of an all-metal combline coaxial cavity diplexer. The device is based on a Y-shaped star-resonant junction which allows to achieve a compact design by positioning the two channels in an in-line and side-by-side arrangement. The channels share the same geometry and are tuned to resonance using screws. The device was designed using the coupling matrix method. For verification, a combline cavity diplexer was manufactured and tested for E1 Galileo (1559-1591 GHz) and Iridium (1606-1638 GHz) applications with fractional bandwidth equaling 2% for both channels. The order and the return loss of each channel are 5 and 19.4 dB, respectively. The volume is 154.1 × 36 × 27.8 mm³, corresponding to a normalized volume of 0.810 × 0.189 × 0.146 λ³. The normalized volume per resonator is as low as 0.0047 λ³, while isolation is better than 55 dB. The ratio between the unloaded quality factor and the normalized volume per resonator is as high as 19.8 × 10⁴ λ⁻³. The design is very easy to manufacture, since it is all-metal and has a simple geometry.