Radiofrequency (RF) heating applicator, located at the distal end of a coaxial line catheter, produces deeper and more uniform heat dissipation. The active applicator element is a conductor helix fed via the coaxial line. The applicator has provisions for interception of intracardiac electrogram signal. A cardiac ablation system using the above catheter, ablates cardiac tissue responsible for ventricular tachycardia. The ablation system provides means to monitor intracardiac electrograms and to control the RF power. A variation of the helical applicator can be used in a hyperthermia system for treatment of malignant tumors.