Liquid-Crystal Reconfigurable Coupler for Millimeter-Wave Applications

A substrate-integrated waveguide (SIW) coupler whose coupling ratio is controllable at millimeter-wave frequency is presented. To realize this unique property, the SIW coupler is loaded with a tunable liquid crystal (LC) in the central region. To increase, translate, and shift the tuning range of the LC to the required permittivity, an artificial dielectric slab (ADS) is introduced and validated. A new approach to locally increase the permittivity of the LC is presented, wherein the relative permittivity is increased over the desired band. Initially, the variation in the relative permittivity of the LC is 2.46–3.5; the slab offers an increment in the variation, from 8.82 to 14.7. Varying the permittivity of the slab in the central region enables coupling ratio control. Based on the investigation of the proposed model, the design requirements and physical parameters to achieve a wide range of reconfigurability are obtained at the millimeter-waveband. For experimental/demonstration purposes, two reconfigurable coupler prototypes with angles of 15° and 55° between the arms are fabricated and characterized. The results demonstrate a tunable operating frequency to a certain extent with a controllable coupling ratio between output ports from 0.5 to 15 dB. These results show very good agreement with the values determined via simulation.