The directional property of all directional couplers is produced through the use of two sep- arate 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 wave- guides. 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 mo- ment 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, the