An improved radio-frequency catheter system for ablating biological tissues of a body vessel in a patient including a catheter, a deployable antenna guide disposed at the distal portion of the catheter and a radio-frequency (“RF”) antenna mounted on the antenna guide. The RF antenna includes a helical coil which defines an axial passageway to accommodate the antenna guide, and is adapted to receive and transmit RF energy for tissue ablation. Upon deployment, the antenna guide acquires a loop configuration which establishes line contact with the body vessel conformable to its internal contour to prescribe the precise and affixed tissue ablation pathway despite body vessel movements. The RF antenna is carried by the antenna guide to be deployed along the established tissue ablation pathway. Alignment of the loop with the desired tissue ablation pathway is facilitated with the use of radio-opaque markers and intracardiac electrodes mounted along the antenna guide. The catheter system is provided with steering mechanism for navigation through the body vessel passageways.