RECENTLY, IT WAS PROVEN that the phase of signals in the microwave and millimeter-wave bands can be modulated by exploiting the physical response of liquid-crystal molecules to an applied electric field. Several prototype devices have emerged that are based on liquid-crystal substrates with tunable permittivity. For example, prototype reflectarray structures that operate at frequencies above 100 GHz have been designed by Wenfei Hu, Robert Cahill, Raymond Dickie, and Vincent Fusco from Ireland's Institute of Electronics, Communication, and Information Technology, Queen's University of Belfast. This work was performed in conjunction with Jos A. Encinar from the Universidad Politecnica de Madrid, Harold Gamble from the Northern Ireland Semiconductor Research Centre, and Norman Grant from the EADS Astrium Earth Observation Navigation and Science Directorate, Portsmouth, UK.
This computer model consisted of two arrays comprised of equally sized elements, which were constructed on a 15-m-thick tunable liquid-crystal layer. The arrays were designed to operate at center frequencies of 102 and 130 GHz. To fabricate the grounded periodic structures, the researchers employed micromachining processes based on the metallization of quartz/silicon wafers as well as an industry-compatible, liquid-crystal-display (LCD) packaging technique. By applying a low-frequency alternating-current (AC) bias voltage of 10 V, they were able to obtain a 165-deg. phase shift with a loss of 4.5 to 6.4 dB at 102 GHz. At 130 GHz, the researchers achieved a 130-deg. phase shift with a loss variation between 4.3 and 7 dB.
The loss and the phase of the reflected signals were measured using a quasi-optical testbench. The reflectarray was inserted at the beam waist of the imaged Gaussian beam, thereby eliminating some of the problems associated with traditional free-space characterization at these frequencies. The scattering parameters (S-parameters) were computed by modeling the structure as an infinite array of identical-size patch elements using a unit cell with appropriate boundary conditions. See "Design and Measurement of Reconfigurable Millimeter-Wave Reflectarray Cells with Nematic Liquid Crystal," IEEE Transactions On Antennas And Propagation, October 2008, p. 3112.