The 21-40-19 is an ultra wideband, airborne ground plane dependant antenna, designed for use in SIGINT type applications where a number of them can be used to provide a DF array. The antenna structure comprises a contoured monocone with careful element shaping to optimize radiation diagram and avoid deep nulls in the elevation pattern throughout the band.

Construction is from high strength fibre glass with a single stud for attachment to the airframe and the whole structure is filled with closed cell foam for long term reliability in world wide condition of service.