An apparatus includes a measurement structure for performing measurements of an RF device. The measurement structure includes an aperture in a conductive surface of the RF device and a conductive projecting region projecting into the aperture from a conductive perimeter of the aperture and electrically connected to that conductive perimeter. The aperture has a similar width in all dimensions. A combined shape of the aperture and the conductive projecting region does not possess even rotational symmetry about a point where a signal conductor will be placed on the conductive projecting region in order to conduct RF energy between the measurement structure and an external measurement instrument for performing the measurements. The measurement structure may be used for performing measurements of a multimode resonator, the measurements comprising one or more of resonant frequencies and quality factors of resonant modes of the resonator.