An improved radio-frequency based catheter system and method for ablating biological tissues within the body vessel of a patient comprises a radio-frequency ('RF') generator for selectively generatinghigh frequency energy in a deployable catheter having a RF co-axial transmission line with a hollow lumen, a RF antenna mounted on the distal portion of the catheter, and steering control lines in thelumen of the transmission line for steering catheter while being inserted into the body vessel and for deflecting the RF antenna to accommodate the contour of the body vessel where tissue ablation is needed.