Coupled-line co-directional coupler with inherent harmonic suppression and its wideband implementation

In the study, a novel implementation of the coupled-line co-directional (CL-COD) coupler is proposed for the first time, and the wideband realizations are also demonstrated. The CL-COD coupler, which is composed of inductor loaded parallel coupled line and a pair of open-circuited stubs, has the advantages of smaller size and inherent harmonics suppression compared with the traditional lambda/4 branch line coupler. Design equations of the single-section CL-COD coupler are derived by the even-odd mode decomposition analysis and a prototype is designed for validation. The experimental results indicate that compared with the traditional single-section lambda/4 branch line coupler, the designed single-section CL-COD coupler shows a size reduction of 64.1%, a bandwidth enhancement of 13%, and a second harmonic suppression of more than 20 dB. By cascading multi-sections of the CL-COD couplers, wideband CL-COD coupler with harmonics suppression is also realized. A computer-aided program is applied for solving the circuit parameters of the multi-section cascaded CL-COD coupler. Besides, the two- and three-section cascaded 3-dB CL-COD couplers are designed for guidance. Finally, a 3-dB three-section cascaded CL-COD coupler is fabricated and measured. Compared with the traditional three-section cascaded lambda/4 branch line coupler, the prototype shows a size reduction of 61.9% with an additional second order harmonic suppression.