Monolithic antenna on GaAs to be used in miniature active sensors is presented. After a first study of technological processes and the characterisation of various substrates in K-band, the feasibility of an antenna, operating in K-band, electromagnetically-coupled and integrated on GaAs by monolithical realisation, has been demonstrated. A low profile (1mm) and wide bandwidth (2.3 GHz around 25 GHz) antenna is obtained.