A new method to design an unequal Wilkinson power divider (UWPD) with a high power-dividing ratio is presented. By employing dual transmission lines, the ratio of highest to lowest characteristic impedance in the UWPD is effectively reduced while maintaining the main frequency response, thus the conventional narrow width in the microstrip can be avoided in practical implementation. For verification, a 5:1 microstrip UWPD operating at 1 GHz has been designed, fabricated and tested. Results show that there is good agreement between calculation and measurement.Introduction: Unequal Wilkinson power dividers (UWPDs) have asymmetric structures, which are more generalised and complicated than equal ones [1]. In general, the main difficulty in designing microstrip extremely UWPDs operating at single band [2] or dual-band [3-7] is the realisation of high characteristic impedance transmission lines. To overcome such practical difficulty for UWPDs, many novel technologies, such as a meander-shaped DGS [8], offset double-sided parallel-strip lines (ODSPSL) [9], grooved substrates [10], and a T-shaped equivalent structure [11], have been developed. However, design and implementation using the DGS, ODSPSL and grooved substrates are not very easy for many engineers, while the transmission line stubs [11] decrease the final operating bandwidth.In this Letter, we propose another novel design method of extremely UWPDs. In this proposed UWPD, equivalent impedance transformers based on dual transmission lines (DTLs), which have been applied to miniaturise the branch-line coupler [12] and improve the isolation of the parallel coupler [13], are used to replace the quarter-wavelength impedance transformers with too low characteristic impedances. Therefore, equivalent structures with DTLs make the ratio of highest to lowest characteristic impedances decrease compared with the conventional one. In particular, the proposed UWPD does not include any reactive components, transmission line stubs and via holes, and can be easily realised on the planar PCB procedure or monolithic microwave integrated circuit (MMIC). In the final microstrip experiment, good agreement between the calculated and measured results can be observed.