TO AID in detecting and tracking large metallic objects in a warehouse setting, the University of Michigan's Jacquelyn A. Vitaz, Amelia M. Buerkle, and Kamal Sarabandi designed a planar Van Atta array that is retro-reflective in one plane. First, the team designed a series-fed, grounded, coplanar waveguide patch antenna with minimal cross coupling among elements. The engineers then incorporated a tilted beam in the linear series-fed array, effectively isolating the large radar cross section (RCS) of the planar array structure at boresight from the desired modulated, retro-reflective RCS.
Thanks to the high-speed PIN switches that are incorporated into the array structure, the RF tag is uniquely identified. That tag is based on a planar Van Atta retro-reflector. In a Van Atta array configuration, individual linear elements are connected in pairs according to their relative distance from the array center. Because they are linked with lines of equivalent phase length, incident energy re-radiates back in the direction of incidence.
To increase the RCS and provide a narrow beam in the y-direction, linear series-fed patch arrays were used as the retro-reflector's radiating elements. By using grounded coplanar waveguide feed lines for the feed network, the researchers also minimized radiation and substrate loss while allowing for maximum energy transfer. Amplitude modulation (AM) was used in the interconnecting lines, allowing the retro-reflected signal to relay information from the target to the source.
The team placed a switch within the line connecting each antenna pair, which guides the signal to either a matched load or the transmit antenna. The retro-reflective signal can therefore be turned on or off. See "Tracking of Metallic Objects Using a Retro-Reflective Array at 26 GHz," IEEE Transactions On Antennas And Propagation, Nov. 2010, p. 3539.