A novel dual‐band waveguide slot array antenna with low cross‐polarisation is proposed. The main structure of the proposed antenna consists of two waveguides in which the top broad wall of the smaller waveguide, a WR‐28 Ka‐band waveguide, is connected to the top broad wall of the larger waveguide, a WR‐90 X‐band waveguide. The WR‐28 waveguide operating in the higher frequency band of the proposed antenna acts as a ridge inside the WR‐90 waveguide operating in the lower frequency band. For achieving a suitable side lobe level (SLL) and cross‐polarisation in the lower frequency band, the slots are placed close to the higher frequency waveguide and side ridges are placed beside each slot. A 2D graph is presented in which the normalised resonance conductance versus the offset of a side ridge from the outer edge of the lower frequency waveguide is shown, while the slot is located between the higher frequency waveguide and the side ridge. By applying proper coefficients to this graph, appropriate offsets from the outer edge of the lower frequency waveguide for the side ridges are obtained so that a low SLL can be realised.