Reconfigurable HMSIW Quadrature Coupler

This letter, for the first time, presents a half-mode substrate integrated waveguide (HMSIW) quadrature coupler with tunable coupling ratio. Two HMSIW transmission lines with three fixed capacitors are parallelly arranged to construct the four-port coupler. Then, two tunable capacitors loaded at the center of the HMSIW transmission lines are used to tune the coupling ratio. The phase imbalance variations versus the design parameters are analyzed by closed-form design equations. For verification, a reconfigurable HMSIW coupler is designed and measured. The measured coupling ratio can be tuned from 7.1 to -3 dB with the phase imbalance smaller than 5°. The 15-dB return loss and isolation bandwidth are better than 300 MHz. The coupler size is 0.28\,\,\lambda \text{g}\,\,\times 0.42\,\,\lambda \text{g}.