Terahertz Near-Field Focusing Using a 3-D Printed Cassegrain Configuration for Backscattered Side-Channel Detection

This paper presents the use of terahertz near-field focusing for backscatter side-channel detection. Near-field focusing is done by using a Cassegrain reflector configuration. The focuser is designed to produce the focused beam 28 cm away from the antenna aperture. The focusing is done in the near-field region by axially moving the subreflector from the focal point. It is observed that the subreflector position has to shift approximately 11 wavelengths along the axis to create the focus at the required location. The focused antenna gain is 46 dBi, while the 3 dB focus width and depth of the designed antenna is similar to 4 mm and 10 cm, respectively. It is found that the focal plane position is sensitive to the subreflector shifts and it is observed that 1 mm change in the subreflector position can shift the focal plane by similar to 2 cm. The simulations are compared with the measurement results of a fabricated prototype and good agreement is observed. The antenna is fabricated by using 3-D printing technology, which allows rapid prototyping. Finally, we have demonstrated the detection of backscatter side channel from the board placed at 28 cm away from the designed antenna. The received power level of the backscatter signal increases by 6 dB as compared to horn antenna.