This study was conducted to design a wideband septum polariser as a feeder of the antenna to generate right‐hand circular polarisation (RHCP) and left‐hand circular polarisation (LHCP) concurrently. The proposed septum polariser overcomes the conventional bandwidth limit and covers 40% bandwidth from 8 to 12 GHz. Moreover, a quad‐ridged horn is chosen as a radiator of the feeding network due to its waveguide structure, wide bandwidth, and dual polarisation. Therefore, the proposed structure consists of a quad‐ridged horn fed by a square waveguide, a waveguide transition, and a wideband septum polariser. The final design has an axial ratio <1.5 dB over the X‐band. This configuration can be used as a feed for reflector antennas due to the wideband and dual polarisation characteristics. Furthermore, the structure is highly recommended for high‐power applications because of its waveguide feature.