When a quarter-wavelength probe is inserted into a waveguide and supplied with microwave energy, it will act as a quarter-wave vertical antenna. Positive and negative wavefronts will be radiated, as shown in figure 1-24. Any portion of the wavefront traveling in the direction of arrow C will rapidly decrease to zero because it does not fulfill either of the required boundary conditions. The parts of the wavefronts that travel in the directions of arrows A and B will reflect from the walls and form reverse-phase wavefronts. These two wavefronts, and those that follow, are illustrated in figure 1-25. Notice that the wavefronts crisscross down the center of the waveguide and produce the same resultant field pattern that was shown in figure 1-23.

Figure 1-24. - Radiation from probe placed in a waveguide.

Figure 1-25A. - Wavefronts in a waveguide.

Figure 1-25B. - Wavefronts in a waveguide.

Figure 1-25C. - Wavefronts in a waveguide.

The reflection of a single wavefront off the "b" wall of a waveguide is shown in figure 1-26. 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 strikes the wall, known as the angle of incidence, is the same as the angle of reflection and are measured perpendicular to the waveguide surface. An instant after particle 1 strikes the wall, particle 2 strikes the wall, as shown in view (C), and reflects in the same manner. Because all the particles are traveling at the same velocity, particles 1 and 2 do not change their relative position with respect to each other. Therefore, the reflected wave has the same shape as the original. The remaining particles as shown in views (D), (E) and (F) reflect in the same manner. This process results in a reflected wavefront identical in shape, but opposite in polarity, to the incident wave.

Figure 1-26. - Reflection of a single wavefront.

Figure 1-27, views (A) and (B), each illustrate the direction of propagation of two different electromagnetic wavefronts of different frequencies being radiated into a waveguide by a probe. Note that only the direction of propagation is indicated by the lines and arrowheads. The wavefronts are at right angles to the direction of propagation. The angle of incidence (q) and the angle of reflection (;) of the wavefronts vary in size with the frequency of the input energy, but the angles of reflection are equal to each other in a waveguide. The CUTOFF FREQUENCY in a waveguide is a frequency that would cause angles of incidence and reflection to be zero degrees. At any frequency below the cutoff frequency, the wavefronts will be reflected back and forth across the guide (setting up standing waves) and no energy will be conducted down the waveguide.

Figure 1-27A. - Different frequencies in a waveguide.

Figure 1-27B. - Different frequencies in a waveguide.

The velocity of propagation of a wave along a waveguide is less than its velocity through free space (speed of light). This lower velocity is caused by the zigzag path taken by the wavefront. The forward-progress velocity of the wavefront in a waveguide is called GROUP VELOCITY and is somewhat slower than the speed of light.

The group velocity of energy in a waveguide is determined by the reflection angle of the wavefronts off the "b" walls. The reflection angle is determined by the frequency of the input energy. This basic principle is illustrated in figure 1-28. As frequency is decreased, the reflection angle decreases causing the group velocity to decrease. The opposite is also true; increasing frequency increases the group velocity.

Figure 1-28A. - Reflection angle at various frequencies. LOW FREQUENCY

Figure 1-28B. - Reflection angle at various frequencies. MEDIUM FREQUENCY

Figure 1-28C. - Reflection angle at various frequencies. HIGH<?Pub Caret> FREQUENCY

Q.14 What interaction causes energy to travel down a waveguide?
Q.15 What is indicated by the number of arrows (closeness of spacing) used to represent an electric field?
Q.16 What primary condition must magnetic lines of force meet in order to exist?
Q.17 What happens to the H lines between the conductors of a coil when the conductors are close together?
Q.18 For an electric field to exist at the surface of a conductor, the field must have what angular relationship to the conductor?
Q.19 When a wavefront is radiated into a waveguide, what happens to the portions of the wavefront that do not satisfy the boundary conditions?
Q.20 Assuming the wall of a waveguide is perfectly flat, what is the angular relationship between the angle of incidence and the angle of reflection?
Q.21 What is the frequency called that produces angles of incidence and reflection that are perpendicular to the waveguide walls?
Q.22 Compared to the velocity of propagation of waves in air, what is the velocity of propagation of waves in waveguides?
Q.23 What term is used to identify the forward progress velocity of wavefronts in a waveguide?