that is adjacent to the second cover surface 220 in the longitudinal direction 201 the inner gap 330 may be made very narrow for example the width of the inner gap 330 in the longitudinal direction 201 may be small compared with a length of the cavity 200 in the longitudinal direction 201 and small compared with a wavelength of a radio frequency oscillation mode that may be excited in the rf resonator 100 an electrically conductive outer coating 400 is arranged on the outer side 250 of the cavity 200 the outer coating 400 may consist of a metal for example the outer coating 400 includes a first outer coating 410 and a second outer coating 420 an outer gap 430 is arranged between the first outer coating 410 and the second outer coating 420 in the region of the outer gap 430 no electrically conductive coating is provided on the outer side 250 of the cavity 200 the outer gap 430 electrically insulates the first outer coating 410 and the second outer coating 420 from one another fig2 shows a section through a portion of the lateral surface 230 of the cavity 200 of the rf resonator 100 in the region of the inner gap 330 and of the outer gap 430 the outer gap 430 is situated at the.