The 21-50-400 is an ultra wideband, monopole antenna suitable for airborne and vehicular use, 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 unique element design 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 aluminium alloy base plate and the whole structure is filled with closed cell foam for long term reliability in world wide conditions of service.