Abstract: A structure for use in an antenna or as a broadband electromagnetic field simulator, having an open horn TEM waveguide coupled to launch energy in a strip line formed between a forwardly extending plate section and a ground plane. When used as a simulator the test volume is located outside the horn between the forwardly extending plate section and the ground plane. The plate section which functions as a radiating element initially extends parallel to the ground plane and then curves downwardly to terminate adjacent it. In alternative embodiments the plate section extends upwardly, initially with an exponential configuration. In a simulator embodiment it may then terminate at a vertical, grounded wall faced with anechoic material. The horn contains a flat conductor plate positioned parallel to and spaced from the top edge defining an asymmetrical transmission line within the horn.