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

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.