A microwave antenna assembly is disclosed. The antenna assembly includes an elongated member defining a longitudinal axis and having proximal and distal ends. The antenna assembly also includes an outer conductor and an inner conductor each disposed within the elongated member and extending along the longitudinal axis. A portion of the inner conductor is deployable relative to the outer conductor such that the antenna assembly may transition from a first configuration to a second configuration. The antenna assembly also includes an expandable sheath at least partially disposed about a distal portion of the inner conductor and defining at one or more lumens configured to couple to a supply of dielectric material used to regulate the expansion of the expandable sheath.