a cylinder-shaped dielectric member having an outer surface and an inner surface, the inner surface of the dielectric member defining a bore along the longitudinal axis;

a first conductive member mounted around the outer surface of the dielectric member, the first conductive member extending along the longitudinal axis thereby having a first end and a second end, the first conductive member having a continuous inner surface from the first end to the second end, a cross-section of an outer surface of the first conductive member being orthogonal to the longitudinal axis and defining a first perimeter, a cross-section of an inner surface of the outer conductor of the coaxial transmission line being orthogonal to the longitudinal axis and defining a second perimeter that is larger than the first perimeter; and

a plurality of contact members mounted on the outer surface of the first conductive member, the plurality of contact members being spaced apart around the first perimeter;

wherein when the apparatus is positioned in an annulus defined by the inner conductor and the outer conductor of the coaxial transmission line:

the plurality of contact members is in electrical contact with the inner surface of the outer conductor;

spacing between the plurality of contact members around the first perimeter provides clearance between the outer surface of the first conductive member and the inner surface of the outer conductor, the clearance extending from the first end to the second end of the first conductive member; and

the dielectric member and the first conductive member extend along only a portion of the length of the outer conductor.