As seen from Example 7.3, a single-hole coupler has a relatively narrow bandwidth, at
least in terms of its directivity. However, if the coupler is designed with a series of coupling
holes, the extra degrees of freedom can be used to increase this bandwidth. The principle
of operation and design of such a multihole waveguide coupler is very similar to that of the
multisection matching transformer.
First let us consider the operation of the two-hole coupler shown in Figure 7.18. Two
parallel waveguides sharing a common broad wall are shown, although the same type of
structure could be made in microstrip line or stripline form. Two small apertures are spaced
λg/4 apart and couple the two guides. A wave entering at port 1 is mostly transmitted
through to port 2, but some power is coupled through the two apertures. If a phase reference
is taken at the first aperture, then the phase of the wave incident at the second aperture will
be −90◦. Each aperture will radiate a forward wave component and a backward wave
component into the upper guide; in general, the forward and backward amplitudes are
different. In the direction of port 3, both wave components are in phase because both have
traveled λg/4 to the second aperture. However, we obtain a cancellation in the direction of
port 4 because the wave coming through the second aperture travels λg/2 further than the
wave component coming through the first aperture. Clearly, this cancellation is frequency
sensitive, making the directivity a sensitive function of frequency. The coupling is less
frequency dependent since the path lengths from port 1 to port 3 are always the same.