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.