The present invention provides a passive radio frequency (RF) shim resonator (144) for field homogenization of an RF field emitted by an RF antenna device (140) of a magnetic resonance (MR) imaging system (110), whereby the passive RF shim resonator (144) has a first resonating capability and a second resonating capability, and the passive RF shim resonator (144) comprises a switching device, whereby the switching device is adapted to switch between the first and the second resonating capability in accordance with a TX-mode and a RX-mode of the RF field emitted by the RF antenna device (140) of the MR imaging system (110). The present invention further provides a patient bed (142) or a patient mattress for use in a magnetic resonance imaging (MRI) system (110), whereby the patient bed (142) or the patient mattress comprises an above passive RF shim resonator (144). The present invention further provides a RF antenna device for generating and/or receiving a RF field for use in a MRI system (110), whereby the RF antenna device (140) comprises a coil housing and an above passive RF shim resonator (144), wherein the passive RF shim resonator (144) is located within the coil housing. The present invention also provides a magnetic resonance (MR) imaging system (110), comprising an above patient bed (142) or patient mattress or at least one above RF antenna device (140).