English: The antenna of the MPA-4 target tracking radar used with the US Air Force MIM-3 Nike Ajax anti-aircraft missile, the US's first anti-aircraft missile, in service from 1954 through the 1960s. It is a lens antenna, composed of a round lattice of parallel strips of metal of various thicknesses, in the form of a Fresnel lens. Microwaves from a feed horn below pass through the lens, which focuses them into a parallel beam which tracks the target aircraft.

It functions similarly to a convex optical lens, slowing the velocity of the waves passing through the center, while increasing the velocity of the waves through the periphery. The spaces between the strips act as waveguides of various widths. However in a converging optical lens the glass slows the speed of the waves, so the lens is made thicker in the center than the edges. In this microwave lens the waveguides actually increase the speed (phase velocity) of the microwaves, and thus have an index of refraction less than one, so to make a converging lens it must have a concave shape, thicker in the peripheral regions and thinner in the center. For more information see IFC-Tracking Radars page, Ed Thelen's Nike Missile website