A loss button for a traveling wave tube. The front and rear planar surfaces of the ceramic pill-shaped button body are of coated metallic composition. The periphery is uncoated to permit r.f. energy to enter the button through thee cavity of the planar, metallic spacer into which the button is inserted. By coating the body surfaces, the button itself acts as a resonant cavity for attenuation of a predetermined bandwidth so that the dimensional changes that occur during heating to the surrounding and adjacent metallic elements, including the cavity in the spacer for receiving the button and the adjacent braze washers, will not affect the attenuation band of the tube.