A transmission line segment coupler is for coupling together first and second coaxial transmission line segments each including an inner tubular conductor and an outer tubular conductor surrounding the inner tubular conductor and a dielectric therebetween. The coupler apparatus includes an outer tubular bearing body to be positioned within adjacent open ends of the inner tubular conductors of the first and second coaxial transmission line segments, and an inner tubular bearing body configured to slidably move within the outer tubular bearing body to define a linear bearing therewith. The inner tubular bearing body is configured to define a fluid passageway in communication with the adjacent open ends of the inner tubular conductors of the first and second coaxial transmission line segments.