Compact tri-band rat-race coupler based on novel metamaterial transmission line

A compact tri-band rat-race coupler is proposed in this paper by using a novel metamaterial (MTM) transmission line (TL). The novel MTM TL has two frequency points with — 90-degree shift and one frequency points with +90-degree shift in the passbands as well as the nonlinear dispersion. Taking advantage of these features, a compact tri-band rat-race coupler operating at 2.20 GHz, 3.38 GHz and 7.30 GHz is designed, fabricated and measured. The measured and simulated results are in good agreement, indicating good performances for in-phase operation and anti-phase operation at the three operating bands. Moreover, compared with the conventional rat-race coupler, this one not only realizes triple-band operation, but also realizes 64.6% area reduction.