A new multi-functional microwave antenna structure for use in biological applications in described. The antenna structure allows efficient propagation of Ku -band energy into biological tissue to perform in situ tissue characterization (measurement) and ablation or coagulation (treatment) of solid cancerous tumors and surrounding tissue. A calibration method that allows tissue characterization has been developed. The structure also supports a separate channel to allow material to be transported inside the structure without affecting the electromagnetic field propagation within the structure and the radiation pattern produced at the distal end. The structure is rigid and of small enough diameter to enable it to be introduced percutaneously into the human body under local anaesthesia.