In this paper, a miniaturized planar rat-race coupler is proposed to achieve arbitrary power division ratio and harmonic suppression performance simultaneously. It consists of six enhanced T-shaped line sections. The T-shaped lines can be equivalent to arbitrary electrical length lines rather than the conventional λ/4 lines. The explicit design formulas are derived and the characteristic impedances variations with the freedom variable are discussed. Simulated and experimental results show the harmonic suppression to be over -35 dB, while maintaining the conventional performance at the fundamental frequency. The circuit area of the prototype is only 30.8% of the conventional coupler.
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