A fluid sensor includes a core defining a fluid flow path, a confinement member located externally of the core, and a patch antenna located between the fluid flow path and the confinement member. The confinement member is configured to confine an electromagnetic field which extends into the fluid flow path. The patch antenna is configured to couple an electrical signal to and/or from the electromagnetic field. The fluid sensor may be configured for measuring the composition and/or flow rate of a fluid and, in particular but not exclusively, for measuring the composition and/or flow rate of mixtures of oil, water and gas.