Impedance-Transforming Transdirectional Coupled Line Coupler Based on a Suspended Substrate

A new design of a quadrature 3-dB impedance-transforming transdirectional (TRD) coupled line coupler is proposed. The structure contains a base dielectric substrate with a grounded base and a suspended substrate with coupled strips, elevated with a small air gap, above a metal pad, which is galvanically connected to the grounded base of the base substrate by vias. The simulation was performed using the numerical conformal transformation technique with the center frequency calculation error of no more than 11.6%. The measured characteristics of the manufactured TRD coupler turned out to be close to the calculated ones and amounted to the following values in terms of the return loss of the upper line and isolation between the ports of the reference line of 10 dB: operating frequency band of 2.2\u20132.62 GHz (17%); amplitude and phase balances between the output ports of 4.4\u00b10.5 dB and 84\u00b13 deg., respectively; return loss of the reference line and isolation between the output ports of the upper line of more than 17 dB. A unique advantage of TRD coupler, which is absent in other types of the coupled line couplers (contra- and co-directional), is DC isolation between the input port and a pair of output ports, which is combined with a double impedance transformation (50:25). This will allow finding application of the new TRD coupler in development of balanced amplifiers, mixers, reflective phase shifters with increased controllable phase shift, phased antenna arrays, etc.