The antenna of the MPA-4 target tracking radar used with the US Air Force <a href="https://en.wikipedia.org/wiki/MIM-3_Nike_Ajax" class="extiw" title="w:MIM-3 Nike Ajax">MIM-3 Nike Ajax</a> 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 <a href="https://en.wikipedia.org/wiki/Fresnel_lens" class="extiw" title="w:Fresnel lens">Fresnel lens</a>.  Microwaves from a <a href="https://en.wikipedia.org/wiki/feed_horn" class="extiw" title="w:feed horn">feed horn</a> below pass through the lens, which focuses them into a parallel beam which tracks the target aircraft.<br><br>
It functions similarly to a <a href="https://en.wikipedia.org/wiki/convex_lens" class="extiw" title="w:convex lens">convex</a> 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 <a href="https://en.wikipedia.org/wiki/waveguide_(electromagnetic)" class="extiw" title="w:waveguide (electromagnetic)">waveguides</a> 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 (<a href="https://en.wikipedia.org/wiki/phase_velocity" class="extiw" title="w:phase velocity">phase velocity</a>) of the microwaves, and thus have an <a href="https://en.wikipedia.org/wiki/index_of_refraction" class="extiw" title="w:index of refraction">index of refraction</a> 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 <a rel="nofollow" class="external text" href="http://ed-thelen.org/ifc_track.html#mtr">IFC-Tracking Radars page, Ed Thelen's Nike Missile website</a>
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<a href="//commons.wikimedia.org/wiki/Flickr" class="mw-redirect" title="Flickr">Flickr</a>: <a rel="nofollow" class="external text" href="https://www.flickr.com/photos/93452909@N00/2367040509">NIKE AJAX  Anti-Aircraft Missile Radar</a>