A coupler for coupling a flexible coaxial cable, a fluid cooling system and the outer sheath of a catheter, the coupler including a fluid coupler body having a fluid inlet, a fluid outlet, a bypass bulb, an outer sheath, and a fluid sealing system housed in the fluid coupler body. The fluid sealing system includes a distal sealing diaphragm and a proximal sealing diaphragm configured to form a fluid-tight seal with the fluid coupler body.