A coaxial tissue ablation probe (900, 910, 920) for delivering microwave radiation into biological tissue to ablate that tissue. One or more baluns (930) in the form of a conducting material in conductive contact with the outer conductor (920) of the probe are formed by spraying dielectric material (932) between the balun conductor and the outer conductor. The length of each balun is one quarter wavelength or an odd multiple thereof in order to minimise the return current along the outer conductor.