The 21-4 antenna is a linearly polarized, monopole antenna with wide band frequency characteristics intended for general communication and homing applications. Being of semi-rigid form, it is suitable for external fitting upon helicopters and fixed wing aircraft with a performance up to 350 kts EAS, and other similar applications. Electrical design consists of a single broad band radiating element coupled via matching circuitry to a single coaxial connector. The 21-4 consists of a tapered glass fibre sleeve supporting a light aluminium alloy radiating element. The element assembly is fitted to a metal body carrying the coaxial connector. This arrangement allows the antenna to be installed by single hole fixing and allows orientation of the coaxial connector through 360°. The antenna can be mounted true or raked using the angled spacers provided.