Fundamentally, there are three methods of coupling waveguide energy into or out of a waveguide: probe, loop, and aperture. Probe, or capacitive, coupling waveguide energy is illustrated in Figure 1. Its action is the same as that of a quarter-wave Marconi antenna. When the probe is excited by an RF signal, an electric field is set up [Figure 1(a)]. The probe should be located in the center of the dimension and a quarter wavelength, or odd multiple of a quarter Wavelength, from the short-circuited end, as illustrated in Figure 1(b).
This is a point of maximum E field and, therefore, is a point of maximum coupling between the probe and the field. Usually, the probe is fed with a short length of coaxial cable. The outer conductor is connected to the waveguide wall, and the probe extends into the guide but is insulated from it, as shown in Figure 1(c). The degree of coupling may be varied by varying the length of the probe, removing it from the center of the E field, or shielding it.
In a pulse-modulated radar system, there are wide sidebands on either side of the carrier frequency. In order that a probe does not discriminate too sharply against frequencies that differ from the carrier frequency, a wideband probe may be used. This type of probe is illustrated in Figure 1(d) for both low- and high-power usage.
Figure 2 illustrates loop, or inductive, coupling. The loop is placed at a point of maximum H field in the guide. As shown in Figure 2(a), the outer conductor is connected to the guide, and the inner conductor forms a loop inside the guide. The current flow in the loop sets up a magnetic field in the guide. This action is illustrated in Figure 2(b). As shown in Figure 2(c), the loop may be placed in a number of locations. The degree of loop coupling may be varied by the rotation of the loop.
The third method of coupling is aperture, or slot, coupling. This type of coupling is shown in Figure 3. Slot A is at an area of maximum E field and is a form of electric field coupling. Slot B is at an area of maximum H field and is a form of magnetic field coupling. Slot C is at an area of maximum E and H field and is a form of electromagnetic coupling.
