The design, development and characterisation of an inclined waveguide slot-based circularly polarised (CP) omnidirectional antenna at Ka-band frequency are described. The antenna consists of inclined slots on a circular waveguide and a coaxial feeding probe. All the radiating slots are fed by the TM₀₁ mode of the circular waveguide to generate the omnidirectional pattern. Inclination of slots has been used to generate circular polarisation. The designed antenna has omni coverage in the azimuthal plane and −10° to +45° coverage in the elevation plane. The antenna has been simulated on finite element method-based ANSYS high-frequency structure simulator (HFSS) EM simulator software using proper boundary conditions. The proposed antenna has the advantages of being robust, having wide coverage, being easy to fabricate (being in waveguide configuration) and is suitable for mass production. The measured axial ratio lies well below 2.8 over the horizontal plane. A voltage standing wave ratio of 2.0:1 has been achieved over the frequency band of 37.9–38.55 GHz and the minimum omni gain of the antenna is 0 dBi over the frequency band as well as over the coverage.