Experimental Realization of Tunable Transmission Lines Based on Single-Layer SIWs Loaded by Embedded SRRs

Wave propagation in single layer substrate integrated waveguides (SIWs) loaded by embedded split-ring resonators (ESRRs) is discussed theoretically and experimentally. It is shown that the loaded SIW can support a backward-wave passband below the cutoff frequency (fc) of the host SIW or a stopband will appear above fc if the effective transversal permeability is negative. In addition, varactor-loaded ESSRs (VLESRRs) can show resonance frequency agility through a variable reverse bias voltage applied to the varactors. Next, VLESRRs are loaded in SIWs to realize transmission lines with either tunable passbands (stopbands) below (above) fc of the host SIWs. Both cases are considered in this paper. First, an SIW with fc=7 GHz is loaded by five VLESRRs showing a tunable passband below the cutoff from 1.5 to 4 GHz. Second, an SIW with fc=1.9 GHz and five VLESRRs is discussed where a tunable stopband is achieved above fc from 2 to 4 GHz. In both cases, the reverse-bias voltage applied to the varactors varies from 0 to 20 V. Finally, two fabricated prototypes are provided for each case to validate the analysis. The measured and simulated results are in good agreement.