The directional property of all directional couplers is produced through the use of two separate
waves or wave components, which add in phase at the coupled port and are canceled
at the isolated port. One of the simplest ways of doing this is to couple one waveguide
to another through a single small hole in the common broad wall between the two waveguides.
Such a coupler is known as a Bethe hole coupler, two versions of which are shown
in Figure 7.16. From the small-aperture coupling theory of Section 4.8, we know that an
aperture can be replaced with equivalent sources consisting of electric and magnetic dipole
moments [6]. The normal electric dipole moment and the axial magnetic dipole moment
radiate with even symmetry in the coupled guide, while the transverse magnetic dipole moment
radiates with odd symmetry. Thus, by adjusting the relative amplitudes of these two
equivalent sources, we can cancel the radiation in the direction of the isolated port, while
enhancing the radiation in the direction of the coupled port. Figure 7.16 shows two ways in
which these wave amplitudes can be controlled; in the coupler shown in Figure 7.16a