Additively Manufactured Luneburg Retroreflector Figure 1. Propagation of power through Luneburg lens retroreflector. Figure 4. Illustration of the FDM layer-by-layer printing process of the Luneburg lens retroreflector using space filling curves. Figure 2. Space-filling geometry used for generating spatially varying permittivities. By varying the number of turns the local volume fraction of printed material, and thus its effective permittivity, is controlled. Unit cells are stacked in rows and columns such that a layer is spatially graded using a single continuous curve. Figure 5. View of the copper tape placed over the spherical end cap of the reflector. The final lens was 120 mm diameter with its unit cells specifically tailored for operation over frequencies that range from 8-30 GHz. Upon completion of the AM process, a set of copper tape strips were overlapped onto the rear hemisphere of the lens to create a simple end-cap reflecting surface. The device and end-cap structure can be seen in Figure 5. Figure 3. Free-space focused beam measurement system. lens is illustrated in Figure 4. As shown in the figure, a low permittivity ð"r < 1:1Þ support base was added to enable the fabrication of the lens in a single print run. It is very challenging to FDM print a spherical geometry without support. The low permittivity base serves this purpose. It was found through simulations and experiments that the low permittivity base did not greatly affect the EM properties of the lens. 22 RCS MODELING METHODOLOGY High-frequency structure simulator (HFSS) was used for electromagnetic modeling and simulations. HFSS predicted a full 360 RCS measurements for the fabricated lens at the various frequency bands. The simulated model IEEE A&E SYSTEMS MAGAZINE NOVEMBER 2019