A compact and wideband rat-race coupler using two-section ring and artificial transmission lines

A compact and wideband microstrip rat-race coupler employing two-section ring and artificial transmission lines (ATLs) is reported in this paper. The bandwidth is highly improved by using a two-section rat-race coupler. The physical size of the planar circuit is reduced based on the substitution of the microstrip line with an ATL. The area of the proposed design footprint is 35% of the area of the conventional similar design. The proposed coupler operates over a frequency range of 0.9 GHz to 1.85 GHz. In the desired band, the maximum measured amplitude imbalance is 0.6 dB, the phase variation is +/- 7 degrees, and the measured return loss is greater than 15 dB with isolation better than 20 dB. The prototype has been fabricated and measured, and the results are compared with the simulated data to validate the theory.