A radio frequency transmission arrangement comprises a ground plate having an aperture comprising a slot with an elongate cross-section and substantially parallel sides, and a first and second transmission line. The thickness of the ground plate is greater than a width of the slot. The aperture is partially filled with a solid dielectric material and partially filled with air. The first transmission line comprises a first elongate conductor on a first side of the ground plate and has an end terminated with a first termination stub. The second transmission line comprises a second elongate conductor on the opposite side of the ground plate and has an end terminated with a second termination stub. The first transmission line is arranged to cross the slot at a point adjacent to the first termination stub, and the second transmission line is arranged to cross the slot at a point adjacent to the second termination stub.