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
-,
LS
2-, and
LS
1-types) are then derived, and
N
is introduced for more design flexibility and wideband performance. The
LS
2-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
LS
2-type (
MLS
2-type) with
N
is additionally suggested, and that with
N
=1 is intensively discussed. Using two of those,
LS
1-type with
N
=2 and
MLS
2-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.