A body passage or cavity insertable applicator apparatus for electromagnetic radiation (EMR) systems, particularly for medical hyperthermia use, comprises an elongate applicator having coaxial inner and outer conductors electrically conducted at an EMR input end to a conventional coaxial transmission line for transmitting high frequency EMR from a source to the applicator. The applicator outer conductor is longitudinally split on opposite sides to form first and second outer conductor segments, the inner conductor being electrically connected, at an applicator termination end to one of such segments. A dielectric media is disposed between the applicator inner and outer conductors, and the outer conductor and termination end are covered by a dielectric sheath. A substantially uniform, external electric tissue heating field is obtained along substantially the entire length of the applicator by exponentially increasing thickness of the dielectric sheath towards the termination end and making thickness of the sheath over the termination end equal to at least half the outer diameter of the applicator. Cooling means comprising a flexible envelope disposed around the applicator and cooling fluid flow means are provided for tissue surface cooling, the cooling fluid in the envelope also enhancing dielectric field coupling into tissue being irradiated. Several applicator variations are described, including an applicator in which the outer conductor segments are helically wrapped to enhance flexiblity.