The ornamental design for a radiantheater, as shown and described.
FIG. 1 is an isometric view of an embodiment of the radiant heater according to the invention, showing the front glass as curved (left to right) with a straight glass retainer, and with a sail;
FIG. 2 is a left end view of the radiant heater in FIG. 1, the right end view being a mirror image thereof;
FIG. 3 is a front face view of the heater element of FIG. 1;
FIG. 4 is a top view of the radiant heater of FIG. 1;
FIG. 5 is a bottom view of the radiant heater of FIG. 1; and,
FIG. 6 is a front elevation view of the radiant heater of FIG. 1.