Device for concentrating a magnetic field of radio frequency (RF) signals in an magnetic resonance (MR) system comprising a plurality of conductive elements 12 arranged in an array 14, wherein the array is arranged to redistribute energy between electric and magnetic fields of RF radiation, a plurality of semiconductor devices 22 each connected between two respective portions of the conductive elements； and a controller to control a bias voltage of each semiconductor device, thereby controlling the resonant frequency of the array and an associated method. Each semiconductor device may be coupled between pairs of the conductive elements, so the conductive elements of the pair are shorted when the respective semiconductor device is conducting. The device may include a conductive extension 52 in line with the conductive element 12 which extends its effective length. Each conductive element 12 may be a curved element comprising one or more of a split ring, a loop and a swiss roll (figs. 6a-6c, 61,67,63 respectively)； or be a curved wire medium； or be elongate with a length of approximately half the wavelength of the resonant frequency.
