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 (f_c) of the host SIW or a stopband will appear above f_c 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) f_c of the host SIWs. Both cases are considered in this paper. First, an SIW with f_c=7 GHz is loaded by five VLESRRs showing a tunable passband below the cutoff from 1.5 to 4 GHz. Second, an SIW with f_c=1.9 GHz and five VLESRRs is discussed where a tunable stopband is achieved above f_c 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.