ASYMMETRIC FOUR-PORT HYBRIDS 3
A scattering matrix exists for every linear, passive, and time-invariant network,
and it is possible to deduce important general properties of junctions containing
a number of ports by invoking such junction properties as reciprocity and power
conservation. Since the entries of the scattering matrix S , impedance matrix Y ,
and admittance matrix Z of a symmetric network are linear combinations of the
circuit eigenvalues, their direct evaluation or measurement provides an alternative
formulation of network parameters. Therefore, the relation between the scattering
matrix and other circuit matrices is important and is described in Chapter 2.
1.3 ASYMMETRIC FOUR-PORT HYBRIDS
Many different types of power dividers, with and without isolation between out-
put ports, are used for various applications. They perform a variety of functions,
such as splitting and combining power in mixers (hybrids), sampling power from
sources for level control, separating incident and reflected signals in network
analyzers, and dividing power among a number of loads. Certain power dividers,
which provide isolation between their output ports, are branch-line hybrids, ring
hybrids, and parallel-coupled directional couplers, and their two outputs are
in phase or out of phase by 90
◦
or 180
◦
. These power dividers are shown in
Fig. 1.1(a), where the direction of power flow is indicated when power is fed
into port
1
. As shown, the direction of the ring hybrid is the same as that of
the parallel-coupled directional coupler, but the two output signals of the ring
hybrid are in phase or 180
◦
out of phase, whereas those of the parallel-coupled
directional coupler are 90
◦
out of phase. The branch-line hybrid in Fig. 1.1(b)is
same as that in Fig. 1.1(c), in that the two output signals are out of phase by 90
◦
,
but the power division directions are different from each other. Thus, the branch-
line coupler (hybrid) is called a forward coupler, whereas the parallel-coupled
directional coupler is called a backward c oupler.
1.3.1 Asymmetric Ring Hybrids
The first conventional ring hybrid to be treated in Chapter 3 was investigated
by Tyrrel in 1947 [24]. Tyrrel tried to explain ring hybrids using the concept of
waveguide T-junctions, and described two types of hybrid circuits, one involving
a ring or loop transmission line and the other relying on the symmetry properties
of certain four-arm junctions. After he described the fundamental characteris-
tics of distributed circuit hybrids, a number of workers discussed the perfor-
mance of practical wideband realizations constructed in coaxial line [25–27]
and stripline [28]. One of them was that two coupled-line filters were used for a
wideband ring hybrid in the 1950s. In 1961, Pon [29] derived design equations
for ring hybrids with arbitrary power divisions. In 1968, March [28] developed a
wideband ring hybrid, adapting one coupled-line filter instead of a three-quarter-
wavelength transmission line, which causes narrowband responses.
In the 1980s, as uniplanar techniques emerged for MMIC applications, there
were several researchers developed small broadband ring hybrids which employed