Abstract: Application device (1), for example, a metal needle or a plastic catheter that has in the end portion (2) of the free end a gradually increasing thickness to form substantially a chute guide (3) which ends at a side opening (4) made on the application device (1). This way, an interstitial antenna (10) is obtained formed by a co-axial tube having an external conductor (7), by a dielectric layer (9) and by a central conductor (8) embedded in the dielectric layer (9) that insulates it from the external conductor (7). The antenna (10) can be put in a target tissue, along an actuation direction forming an angle a with the introduction direction. This way, the antenna (10) achieves an actual surface isothermal, i.e. a mass of tissue actually coagulated within a curved surface (13), since the tip of the application device (1) is connected electrically with the external conductor (7) and increases its area of action.