Boundary Conditions in a Waveguide

Figure 3-24.—Simple electric fields. Figure 3-25.—Magnetic field on a single wire. the H lines take opposite directions between adjacent turns, the field between the turns is canceled. Inside and outside the coil, where the direction of each H field is the same, the fields join and form continuous H lines around the entire coil. A similar action takes place in a waveguide. Figure 3-26.—Magnetic field on a coil. BOUNDARY CONDITIONS IN A WAVEGUIDE The travel of energy down a waveguide is similar, but not identical, to the travel of electromagnetic waves in free space. The difference is that the energy in a waveguide is confined to the physical limits of the guide. Two conditions, known as BOUNDARY CONDITIONS, must be satisfied for energy to travel through a waveguide. The first boundary condition (illustrated in fig. 3-27, view A can be stated as follows: For an electric field to exist at the surface of a conductor, it must be perpendicular to the conductor. The in view Figure 3-27.—E field boundary condition. opposite of this boundary condition, shown B, is also true. An electric field CANNOT exist parallel to a perfect conductor. The second boundary condition, which is illustrated in figure 3-28, can be stated as follows: For a varying magnetic field to exist, it must form closed loops in parallel with the conductors and be perpendicular to the electric field. 3-13