An electrosurgical vessel sealer comprising: an instrument shaft comprising a coaxial transmission line for conveying microwave electromagnetic energy; a distal end assembly arranged at a distal end of the instrument shaft to receive the microwave EM energy from the instrument shaft, the distal end assembly comprising: a pair of jaws that are movable relative to each other to open and close a gap between opposing inner surfaces thereof; and a blade for cutting through biological tissue, wherein the pair of jaws comprise an energy delivery structure arranged to emit the microwave EM energy into the gap between the opposing inner surfaces, wherein the energy delivery structure comprises a coplanar microstrip antenna mounted on the inner surface of one or both of the pair of jaws, the coplanar microstrip antenna being arranged to confine an emitted microwave field substantially within a region between the pair of jaws, wherein the blade is slidably disposed within the distal end assembly to be movable through the region between the pair of jaws, wherein the coplanar microstrip antenna comprises: a planar dielectric substrate having a top surface that is exposed at the gap between the opposing inner surfaces, and an under surface on an opposite side of the planar dielectric substrate from the top surface; a ground conductor layer on the under surface; a ground conductive strip on the top surface and electrically connected to the ground conductor layer; and an active conductive strip on the top surface, the active conductive strip being spaced from the ground conductive strip, and wherein the active conductive strip and the ground conductive strip are positioned to have a uniform closest spacing within the region between the pair of jaws. an instrument shaft comprising a coaxial transmission line for conveying microwave electromagnetic energy; a distal end assembly arranged at a distal end of the instrument shaft to receive the microwave EM energy from the instrument shaft, the distal end assembly comprising: a pair of jaws that are movable relative to each other to open and close a gap between opposing inner surfaces thereof; and a blade for cutting through biological tissue, a pair of jaws that are movable relative to each other to open and close a gap between opposing inner surfaces thereof; and a blade for cutting through biological tissue, wherein the pair of jaws comprise an energy delivery structure arranged to emit the microwave EM energy into the gap between the opposing inner surfaces, wherein the energy delivery structure comprises a coplanar microstrip antenna mounted on the inner surface of one or both of the pair of jaws, the coplanar microstrip antenna being arranged to confine an emitted microwave field substantially within a region between the pair of jaws, wherein the blade is slidably disposed within the distal end assembly to be movable through the region between the pair of jaws, wherein the coplanar microstrip antenna comprises: a planar dielectric substrate having a top surface that is exposed at the gap between the opposing inner surfaces, and an under surface on an opposite side of the planar dielectric substrate from the top surface; a ground conductor layer on the under surface; a ground conductive strip on the top surface and electrically connected to the ground conductor layer; and an active conductive strip on the top surface, the active conductive strip being spaced from the ground conductive strip, and a planar dielectric substrate having a top surface that is exposed at the gap between the opposing inner surfaces, and an under surface on an opposite side of the planar dielectric substrate from the top surface; a ground conductor layer on the under surface; a ground conductive strip on the top surface and electrically connected to the ground conductor layer; and an active conductive strip on the top surface, the active conductive strip being spaced from the ground conductive strip, and wherein the active conductive strip and the ground conductive strip are positioned to have a uniform closest spacing within the region between the pair of jaws.