Hybrid couplers can be built using a bilevel microstrip structure in w
hich two strips are positioned broadside, one above the other. This si
mple geometry provides large coupling factors without the need for rig
orous manufacturing tolerances. Remarkably, if the dielectric constant
of one of the layers can be freely chosen, ideal hybrid coupler perfo
rmance is achievable. Even when the choice of dielectric constant is c
onstrained, good results can be obtained. A technique for achieving op
timal design has been developed, and design curves are shown. Experime
nts confirm design predictions. The reentrant coupler may be analyzed
as an interconnection of two bilevel couplers of the kind described he
re. This provides a simpler analysis than methods used earlier, and in
some cases yields more accurate results.