A horn antenna having dual-band and dual-polarisation operation is proposed. To implement a small-sized structure providing circular polarisations in the Ka- and K-band, the antenna is designed using the horn radiator on the basis of a conical helix which is exited by two ports for TX and RX in both ends. Through a design optimisation process, a compact dual-band and dual-polarised horn antenna having high gain and high isolation performance was realised.