Wideband Microstrip Coupled-Line Ring Hybrids for High Power-Division Ratios

Wideband coupled-line ring hybrids are presented for high power-division ratios. For this, an equivalent circuit to make the characteristic impedance of a transmission-line section lower is firstly investigated, based on a Π-type lumped-element equivalent circuits. Design formulas of four types of equivalent circuits (L_Π-, LT-, LS2-, and LS1-types) are then derived, and N is introduced for more design flexibility and wideband performance. The LS2-type with N=1 is composed of one transmission-line section and two identical series inductances at both ends, which cannot be used for the ring hybrids with the high power-division ratios. To avoid the problem, modified LS2-type (MLS2-type) with N is additionally suggested, and that with N=1 is intensively discussed. Using two of those, LS1-type with N=2 and MLS2-type with N=1, the coupled-line ring hybrids with 13- and 11-dB power-division ratios are fabricated. The measured results show good agreement with those predicted.