A Novel Planar Impedance-Transforming Tight-Coupling Coupler and Its Applications to Microstrip Baluns

A novel impedance-transforming coupled-line coupler with strong coupling coefficient is proposed in this paper. Its analytical design methodology and applications to microstrip baluns are presented. For demonstration purposes, four numerical examples of this proposed impedance-transforming coupler with different terminated impedances are given. The calculated results show that this proposed coupler not only matches two different groups of terminated impedances but also covers the total power-dividing ratio range from 0 dB to infinite value. The measured results of the fabricated coupler verify the proposed design approach. Finally, this kind of coupler is used to construct a novel planar microstrip balun. This proposed balun has many advantages including planar structure without any via holes, strong coupled lines, and wide operation bandwidth. The experimental results of this proposed balun operating at approximately 2 GHz demonstrated these proposed ideas.