A microwave ablation system for ablating a target tissue in a patient comprising: a microwave power source for generating microwave energy; and a microwave ablation catheter comprising: an elongate tubular body comprising a flexible proximal section and a distal section; an elongate antenna disposed within the distal section, and comprising a distal tip; a transmission line in electrical communication with the power source and extending through the body to the antenna and for transmitting the microwave energy between the power source and the antenna, the transmission line comprising a coaxial cable comprising an inner conductor electrically insulated from an outer conductor; a liquid inflow passageway and a liquid outflow passageway for transporting a liquid through the tubular body to and from the distal section of the catheter, and wherein the liquid inflow passageway and liquid outflow passageway are thermally conductive such that heat is absorbed by the liquid as the liquid is circulated therethrough; and wherein the distal section of the catheter comprises a liquid barrier proximally spaced from the distal tip of the antenna prohibiting the liquid from passing therethrough and such that the liquid absorbs a first amount of the microwave energy emitted from the antenna when the microwave energy is transmitted to the antenna from the power source and the liquid is circulated through the catheter, further wherein the antenna is a monopole formed by an exposed portion of the inner conductor extending beyond the outer conductor, wherein liquid is circulated around the proximal part of the monopole, whereas the distal part of the monopole is encapsulated in a low-loss dielectric material. a microwave power source for generating microwave energy; and a microwave ablation catheter comprising: an elongate tubular body comprising a flexible proximal section and a distal section; an elongate antenna disposed within the distal section, and comprising a distal tip; a transmission line in electrical communication with the power source and extending through the body to the antenna and for transmitting the microwave energy between the power source and the antenna, the transmission line comprising a coaxial cable comprising an inner conductor electrically insulated from an outer conductor; a liquid inflow passageway and a liquid outflow passageway for transporting a liquid through the tubular body to and from the distal section of the catheter, and wherein the liquid inflow passageway and liquid outflow passageway are thermally conductive such that heat is absorbed by the liquid as the liquid is circulated therethrough; and an elongate tubular body comprising a flexible proximal section and a distal section; an elongate antenna disposed within the distal section, and comprising a distal tip; a transmission line in electrical communication with the power source and extending through the body to the antenna and for transmitting the microwave energy between the power source and the antenna, the transmission line comprising a coaxial cable comprising an inner conductor electrically insulated from an outer conductor; a liquid inflow passageway and a liquid outflow passageway for transporting a liquid through the tubular body to and from the distal section of the catheter, and wherein the liquid inflow passageway and liquid outflow passageway are thermally conductive such that heat is absorbed by the liquid as the liquid is circulated therethrough; and wherein the distal section of the catheter comprises a liquid barrier proximally spaced from the distal tip of the antenna prohibiting the liquid from passing therethrough and such that the liquid absorbs a first amount of the microwave energy emitted from the antenna when the microwave energy is transmitted to the antenna from the power source and the liquid is circulated through the catheter, further wherein the antenna is a monopole formed by an exposed portion of the inner conductor extending beyond the outer conductor, wherein liquid is circulated around the proximal part of the monopole, whereas the distal part of the monopole is encapsulated in a low-loss dielectric material.