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The Rotman lens (sometimes called the Rotman-Turner lens, not to be confused with Canadian rockers Bachman-Turner Overdrive) is a type of beam-forming network. This BFN trick has been used in countless ways over the years. It might even be part of your spiffy car radar!
For the invention of the lens that bears his name and other great works, Walter Rotman is in the Microwave Hall of Fame.
This lens allows multiple antenna beams to be formed without the need for switches or phase shifters. Antenna elements are connected to the right side in the figure below, with beam ports connected to the left. You can think of the lens as a quasi-microstrip (or quasi-stripline) circuit where the beam ports are positioned such that constant phase shifts are achieved at the antenna ports. We all know what happens when antenna elements are fed at phases that vary linearly across a row, it behaves just like a phased array.
One noteworthy property of this lens is that even though there are many 50 ohm ports hanging off it, they they are isolated, in that they don't affect the loss (or noise figure) of adjacent beams. A well designed lens may have just 1 dB of loss.
Because the structure is more like a parallel-plate waveguide than a transmission line, linear simulators such as Microwave Office or ADS don't have models for the Rotman lens, and you will have to analyze it numerically like Rotman did, or use a 3D EM solver.
While we wait for someone to contribute more content on this topic, Below are images from Walter Rotman's original patent. Check out his original patent, here.
References
[1] W. Rotman and R.F. Turner, "Wide Angle Microwave Lens for Line Source Applications," IEEE Transaction on Antennas and Propagation, 11(6) pp. 623-632, 1963.
[2] US patent 3170158, Multiple Beam Radar Antenna System