Compact Dual Band Ring Coupler Using Miniaturized Metamaterial Left-Handed Impedance Inverters

A compact 3-dB dual band ring coupler is designed, fabricated and tested. The coupler consists of three identical miniaturized Left-Handed (LH) impedance inverter transmission lines (TLs) and a dual one. The general principle of using LH TLs' duality to reduce the size of a dual band ring coupler is explained in detail, showing the equivalent lumped element circuits and the designing relations. A numerical example is considered for the duality analysis and the results are used to obtain a size reduction for both the coupler's dimensions and the number of utilized components. Other arbitrary coupling levels (1 dB and 5 dB) and frequency ratios are investigated and the results of simulations show very good results, proving the validity of the general principle of using LH transmission line's duality. The 3-dB coupler is implemented in microstrip technology and using high-quality lumped components. It works at two arbitrary frequencies, 1.1 GHz and 2.5 GHz, respectively with similar performances. The overall dimension of the proposed coupler is 0.07λ×0.07λ for a wavelength, λ computed at 1.1 GHz. The measured results are in good agreement with the predicted ones for both frequencies, having the return loss and isolation loss better than 33 dB, whereas keeping the insertion and coupling losses at around 3.5 dB in both frequency bands. The phase imbalance at the output ports is less than 1.5°, when the difference and sum ports are excited.