There is provided a method of producing a device (10, 20) for manipulating a magnetic field of RF radiation from one or more RF antenna (53) in an MR system (50). The method comprises: determining a target resonant RF frequency of the device (10, 20) based on at least one characteristic of the one or more RF antenna (53); determining a design of the device (10, 20) to provide the device (10, 20) with the determined target resonant RF frequency; and making the device (10, 20) in accordance with the design. The device (10, 20) comprises: a plurality of conductive elements (12, 22) arranged in an array (14), wherein the array is arranged to redistribute energy between electric and magnetic fields of the RF radiation at a resonant RF frequency when receiving the RF radiation, the RF radiation having an RF wavelength greater than a respective dimension of each conductive element (12, 22); and a dielectric material (16, 26), wherein the dielectric material (16, 26) has a dielectric permittivity and a loss coefficient.