Abstract
This article presents a method to design compact rat race couplers with improved bandwidth values. The coupler consists of three coupled-line sections of different electrical lengths and characteristic impedances. First, design equations are obtained by imposing the coupler conditions using a lossless transmission line model. Input impedance matching, isolation, phase, and amplitude imbalances, all four conditions for both the sum and the difference port excitations are considered for bandwidth calculations. Then, an algorithm is developed to solve for the coupled-line parameters. Considering the limitations of fabrication, guidelines are provided for selecting the right physical parameters according to bandwidth requirement. As an example, a rat race coupler is fabricated that occupies 10% area of a conventional coupler without compromising the bandwidth values. Measurement results shows that the coupler provides 50% of 15 dB return loss bandwidth, 41.7% of 20 dB isolation bandwidth, 15% of ±5° phase imbalance bandwidth, and 62.5% of ±0.5 dB amplitude imbalance bandwidth which are more than those of a conventional 3λ/2 rat race coupler.
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