An elongate electrical function circuit (210) transmits therapy or diagnostic signals. A conductor (220) is separated from function circuit (210) by a dielectric material (230) and connected to function circuit via coupling impedance. The line characteristic impedance of function conductor for frequency ranges above a frequency range of therapy or diagnostic signals is greater than line characteristic impedance of function conductor in frequency range of therapeutic or diagnostic signals, so that current is strongly attenuated above frequency range of therapeutic or diagnostic signals.