Vialess composite right/left-handed stripline and its applications for broadband 3-dB and tunable couplers

Vialess shielded composite right/left-handed (CRLH) stripline is proposed. The structure preserves the CRLH properties while preventing undesired radiations and minimizing electromagnetic interference (EMI) coupling from the surrounding noisy environments. The vialess topology also allows a simple realization option. As an application example, a 3-dB directional coupler and a novel broadband tunable directional coupler are designed and fabricated. For the 3-dB coupler, both simulated and measured results show high coupling level over 33 % operation bandwidth. For the broadband tunable coupler, a single varactor is used to tune the coupling level between the two output ports. Simulation results based on the ideal variable capacitor show the amplitude difference between the two outputs ports can be tuned from 0 to 6 dB over the percent bandwidth of around 32 %. Fabricated circuit shows the tuning capability, verifying the feasibility of designing broadband tunable coupler based on CRLH stripline. The proposed structure may be applicable for high frequency microwave waveguides requiring broadband dispersive characteristics of the CRLH structure.