Tunable phase shifter employing variable odd-mode impedance of short-section parallel-coupled microstrip lines

A complete design method for a tunable phase shifter that employs a short section of parallel-coupled microstrip lines is presented. In the proposed method, the variation in the phase is achieved by changing the odd-mode impedance of parallel-coupled microstrip lines using a varactor diode that is connected between them. A derived theoretical model shows that a unit-cell phase shifter of around one-tenth of the guided wavelength can be utilised to achieve a continuously tunable phase range in excess of 90° depending on the required bandwidth and acceptable insertion loss. The proposed method shows that in order to achieve the required tunable phase range across a wideband, high even-mode impedance is needed. A slotted ground structure is utilised underneath the coupled structure to realise that target. The proposed method is validated by building a phase shifter that has a very small length of around one-twentieth of the guided wavelength with 45° tunable phase range and about 1°dB insertion loss across the band from 2 to 2.5°GHz.