A medical probe (24) includes a flexible insertion tube having a distal end (34) for insertion into a body cavity and having a proximal end. The probe further includes an electrode (35) attached to the distal end of the insertion tube and configured to make electrical contact with tissue in the body cavity. An electrical lead (72) runs through the insertion tube between the distal and proximal ends. In addition a coil (74) is electrically coupled between the electrode and the lead in the insertion tube so as to define a resonant circuit having a resonant frequency in a range between 1 MHz and 300 MHz.
