Satcom on-the-Move requires antenna pattern coverage to be as near to hemispherical as possible. The CAL 21-30-91M utilizes a pair of quadrature connected UHF “droopy dipoles” with geometry arranged to maintain radiation coverage within a 120° cone. It is constructed of rugged fiberglass/epoxy with aluminum alloy dipole elements and base housing. The antenna is foam filled for structural integrity and prevention of moisture ingress.

The 21-30-91M is designed for quick installation and deployment on ferrous roofed vehicles and shelters.

The magnetic mount is capable of retaining the antenna securely in position atop a vehicle at highway speeds up to 80 mph and cross country speeds up to 50 mph. If a more permanent fixing is required, the antenna can be fixed down with four no 10 bolts or similar via the 4 holes provided on the base plate.