All previously discussed types of transmission line consist of two distinct conductors where the current flows in opposite directions. In a microstrip line the strip on top of the substrate is the signal conductor and the ground plane is the return conductor. Hence, such transmission lines can carry DC current and support TEM (or quasi-TEM) waves as a fundamental wave mode.
A rectangular waveguide consist of only one conductor. The metallic conductor encloses a rectangular volume (see Figure 4.14a). The width of the volume is a and the height is b. Usually the width is greater than the height (a ≥ b). A commonly used ratio is a = 2b. Table 4.2 shows some examples of commonly used dimensions of rectangular waveguide cross-sections. It is intuitively clear that a one-conductor line cannot support DC current. The first propagating wave type is a non-TEM wave type called TE10 (or H10). This mode has a transversal electric field but magnetic field components in the direction of propagation. Figure 4.14b shows the orientation of the electric field in a cross-section of the rectangular waveguide. In the next section we will take a closer look at the wave type and derive the cut-off frequency that is associated with that wave mode.