A radio frequency transmission arrangement comprising: a ground plate having first and second opposite sides and an aperture (14, 15a, 15b) formed in the ground plate and passing through the ground plate from the first side to the second side, the aperture comprising a parallel-sided slot, the slot having an elongate cross-section in the plane of the first side of the ground plate, the cross-section having substantially parallel sides extending along the length of the cross section, and the slot having a width which is the distance between the parallel sides of the cross-section of the slot; a first transmission line comprising a first elongate conductor (12;21) disposed on the first side of the ground plate in a substantially parallel relationship with the first side of the ground plate, the first transmission line having an end terminated with a first termination stub (11;20); and a second transmission line comprising a second elongate conductor disposed on the second side of the ground plate in a substantially parallel relationship with the second side of the ground plate, the second transmission line having an end terminated with a second termination stub, wherein 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, wherein the thickness of the ground plate is greater than a width of the slot, wherein the aperture (14, 15a, 15b) formed in the ground plate is partially filled with a solid dielectric material in the ground plate and partially filled with air in the ground plate, wherein the aperture comprises a termination cavity (15a, 15b) at each end of the slot, and wherein each termination cavity (15a, 15b) is filled with the solid dielectric material and the slot is filled with air. a ground plate having first and second opposite sides and an aperture (14, 15a, 15b) formed in the ground plate and passing through the ground plate from the first side to the second side, the aperture comprising a parallel-sided slot, the slot having an elongate cross-section in the plane of the first side of the ground plate, the cross-section having substantially parallel sides extending along the length of the cross section, and the slot having a width which is the distance between the parallel sides of the cross-section of the slot; a first transmission line comprising a first elongate conductor (12;21) disposed on the first side of the ground plate in a substantially parallel relationship with the first side of the ground plate, the first transmission line having an end terminated with a first termination stub (11;20); and a second transmission line comprising a second elongate conductor disposed on the second side of the ground plate in a substantially parallel relationship with the second side of the ground plate, the second transmission line having an end terminated with a second termination stub, wherein 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, wherein the thickness of the ground plate is greater than a width of the slot, wherein the aperture (14, 15a, 15b) formed in the ground plate is partially filled with a solid dielectric material in the ground plate and partially filled with air in the ground plate, wherein the aperture comprises a termination cavity (15a, 15b) at each end of the slot, and wherein each termination cavity (15a, 15b) is filled with the solid dielectric material and the slot is filled with air.