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 Figure 3-28.—H field boundary condition.

Since an E ﬁeld causes a current ﬂow that in turn produces an H field, both fields always exist at the same time in a waveguide. If a system satisfies one of these boundary conditions, it must also satisfy the other since neither field can exist alone.

WAVEFRONTS WITHIN A WAVE GUIDE

Electromagnetic energy transmitted into space consists of electric and magnetic ﬁelds that are at right angles (90 degrees) to each other and at right angles to the direction of propagation. A simple analogy to establish this relationship is by use of the right-hand rule for electromagnetic energy, based on the POYNTING VECTOR. It indicates that a screw (right-hand thread) with its axis perpendicular to the electric and magnetic fields will advance in the direction of propagation if the E field is rotated to the right (toward the H field). This rule is illustrated in figure 3-29.

Figure 3-29.—The Poynting vector.

The combined electric and magnetic fields form a wavefront that can be represented by alternate negative and positive peaks at half-wavelength intervals, as illustrated in figure 3-30. Angle Q is the direction of travel of the wave with respect to some reference axis.

Figure 3-30.—Wavefronts in space.

The reflection of a single wavefront off the “b” wall of a waveguide is shown in figure 3-31. The wavefront is shown in view A as small particles, In views B and C particle 1 strikes the wall and is bounced back from the wall without losing velocity. If the wall is perfectly flat, the angle at which it the wall, known as the angle of incidence (θ), is the same as the angle of reflection (Φ) An instant after particle 1 strikes the wall, particle 2 strikes the wall, as shown

Figure 3-31.—Reflection of a single wavefront. 3-14