Hybrid Directional Couplers

Directional couplers are used to combine or split power from or to transmitters and
their loads. These may also be referred to as hybrid combiners, splitters,
or diplexers. Applications range from printed circuit functions realized in micro
strip, to combining 0.5-MWatt transmitters.

In a hybrid network, power fed into any port is split equally between the two adjacent
ports, and provided that the loads are perfectly matched, no power reaches the
opposite port. Consideration must also be given to the phase difference between the
input and the two output voltages.
This gives two classes of hybrid:
Quadrature types. These have two output voltages, differing in phase by 90° and
two planes of symmetry.
Sum-and-difference types. These have two output voltages either in phase (0°)
or 180° out of phase with each other, depending on which port is used as the
input. They have a symmetry of only one plane.
If the paths between ports are labeled according to phase change in proceeding from
one port to the next, the properties may be seen. Some types have ports that are balanced,
or not directly connected to ground, so voltage phases are ambiguous, and
alternative (or relative) phases are given.
If power from a transmitter is split between two loads (for example, antennas) by a
simple transformer and T junction, there is a high probability that failure of one load
will result in a large change in the amount of power arriving at the remaining load.
A hybrid with a balancing load used to split the power overcomes this problem. Equal amplitude forward waves, with phases appropriate to
the hybrid, always reach each termination, and any reflected power is split equally
between the transmitter and the load on port D. Any change in voltage applied to one
load, brought about by a change in the other, results from a mismatch of the load D or
the transmitter. The worst VSWR that can be presented to a transmitter by a single load
fault is 50 percent (VSWR 3:1).

Equal loads, arranged to be fed with equal quadrature currents, will receive their
correct relative currents regardless of mismatch. Amatched load is always presented to
the transmitter, and all reflected power is transferred to load D. By substituting sources
for load ports, the power of multiple transmitters may be combined, as shown in Figure
13.16, to one load (for example, antenna). If a transmitter fails, the remaining transmitters
will still be correctly terminated and deliver their power to the load.