Energy Propagation in Waveguides

As shown in figure 3-23, the widest dimension of a waveguide is called the “a” dimension and determines the range of operating frequencies. The narrowest dimension determines the power-handling capability of the waveguide and is called the “b” dimension. Figure 3-23.—Labeling waveguide dimensions, NOTE: This method of labeling waveguides is not standard in all texts, Different methods may be used in other texts on microwave principles, but this method is in accordance with Navy Military Standards (MIL-STDS). In theory, a waveguide could function at an infinite number of frequencies higher than the designed frequency; however, in practice, an upper frequency limit is caused by modes of operation, which will be discussed later. If the frequency of a signal is decreased so much that two quarter-wavelengths are longer than the wide dimension of a waveguide, energy will no longer pass through the waveguide. This is the lower frequency limit, or CUTOFF FREQUENCY of a given waveguide. In practical applications, the wide dimension of a waveguide is usually 0.7 wavelength at the operating frequency. This allows the waveguide to handle a small range of frequencies both above and below the operating frequency. The “b” dimension is governed by the breakdown potential of the dielectric, which is usually air. Dimensions ranging from 0.2 to 0.5 wavelength are common for the “b” sides of a waveguide. ENERGY PROPAGATION IN WAVEGUIDES Since energy is transferred through waveguides by electromagnetic fields, you need a basic understand- ing of field theory. Both electric (E FIELD) and magnetic fields (H FIELD) are present in waveguides, and the interaction of these fields causes energy to travel through the waveguide. This action is best understood by first looking at the properties of the two individual fields. E Field An electric field exists when a difference of potential causes a stress in the dielectric between two points. The simplest electric field is one that forms between the plates of a capacitor when one plate is made positive compared to the other, as shown in view A of figure 3-24. The stress created in the dielectric is an electric field. Electric fields are represented by arrows that point from the positive toward the negative potential. The number of arrows shows the relative strength of the field. In view B, for example, evenly spaced arrows indicate the field is evenly distributed. For ease of explanation, the electric field is abbreviated E field, and the lines of stress are called E lines. H Field The magnetic field in a waveguide is made up of magnetic lines of force that are caused by current flow through the conductive material of the waveguide. Magnetic lines of force, called H lines, are continuous closed loops, as shown in figure 3-25. All of the H lines associated with current are collectively called a magnetic field or H field. The strength of the H field, indicated by the number of H lines in a given area, varies directly with the amount of current. Although H lines encircle a single, straight wire, they behave differently when the wire is formed into a coil, as shown in figure 3-26. In a coil the individual H lines tend to form around each turn of wire. Since 3-12