Generalised capacitive‐loading method for miniaturisation of forward‐wave directional coupled‐line couplers with arbitrary strip impedances and coupling levels

Abstract

This study demonstrates a capacitive reactive loading method for forward‐wave couplers based on coupled lines. With the method, a miniaturised forward‐wave coupler can be designed by loading either a symmetric or an asymmetric coupled line with shunt capacitors symmetrically or asymmetrically. The method is a generalised version of all previous shunt capacitive loading methods for miniaturisation of forward‐wave coupled‐line couplers since it provides complete control of not only the maximum coupling level, but also the impedances of the two strips. For the first time, a forward‐wave coupled‐line coupler is demonstrated that provides an arbitrary maximum coupling level based on 50‐Ω lines. Therefore, lengthy and/or complicated impedance matching networks can be eliminated, a more effective miniaturisation is enabled, and performance degradation associated with the additional circuitry is minimised. Experimental results of various forward‐wave couplers based on quasi‐symmetric and asymmetric coupled lines with various coupling levels and strip impedances are provided to demonstrate the versatility of the proposed method.

Development of broadband forward-wave directional couplers loaded by periodic patterned ground structure

“…The couplers proposed in Refs. and are fabricated according to asymmetric structures, and have large bandwidth. However, their asymmetric configurations makes the analysis and design process complicated.…”

Forward‐wave directional coupler based on slow‐wave coupled microstrip lines

“…Besides, an impedance-matching network could be needed, for example in [4], which complicates the overall design and eventually limits the bandwidth. Basic concept to reduce the FWDC length is explained in [6]. It consists in increasing the self-capacitance of the coupled lines which implies in return an increase in the difference between the even-and odd-mode phase velocities.…”