Analysis and design of coupled line impedance transformers

Impedance transformers employing coupled line sections that are less than quarter wavelengths are presented in this paper. Exact solutions relating the coupled line parameters and arbitrary impedance transformation ratios are derived. This is unlike previous coupled line transformers, which were designed based on electromagnetic simulations and provide only impedance transformation ratios of 1:4. Various design curves demonstrating a wide range of practical coupled line parameter values are presented and validated with experimental results. A 50 Ω to 170 Ω impedance transformer using λ/8 coupled line section achieved over 10 dB return loss from 0.5 to 0.9 GHz.