An apparatus for a coaxial transmission line is provided. The apparatus can include a dielectric member having an inner surface defining a bore along a longitudinal axis of an inner conductor of the coaxial transmission line; and a first conductive member mounted axially around the dielectric member and extending along the longitudinal axis. A cross-section of an outer surface of the first conductive member can define a first perimeter. A cross-section of an inner surface of the outer conductor of the coaxial transmission line can define a second perimeter. The first perimeter can be smaller than the second perimeter and thereby provide clearance between a portion of the outer surface of the first conductive member and the inner surface of the outer conductor of the coaxial transmission line when the apparatus is positioned in an annulus defined by the inner conductor and the outer conductor of the coaxial transmission line.