A radio frequency transmission arrangement comprises a ground plate (8) having first and second opposite sides and a boss protruding from said second side of the ground plate (8), a first transmission line comprising a first elongate conductor (1) passing from the first side of the ground plate through an aperture (3) in the ground plate and the boss, and a second transmission line comprising a second elongate conductor (2) and a ground plane (6), the first elongate conductor (1) passing through the ground plane (6) to connect to the second elongate conductor. The boss has an end surface (4) disposed in a substantially parallel relationship with the ground plane (6) of the second transmission line, and there is a gap between the end surface of the boss and the ground plane.