installation to a rotating antenna. To do this, the
energy must be transferred from a rectangular
waveguide to a circular waveguide, and back to a
rectangular waveguide that is terminated with an
antenna. The energy put into the rectangular
waveguide is in one of the TE modes. A probe
terminates this section of rectangular waveguide.
The probe extends through the rectangular
waveguide into the circular waveguide. Energy is
put into the circular waveguide by the probe in one
of the TM modes. This exchange of energy and
modes in the two waveguides is done with very little
loss. The energy in the circular waveguide is stable
and does not rotate with the waveguide, so there is
no change in polarization. The energy from the
rotating section of the circular waveguide is
transferred through another probe to the rectangular
section of the waveguide that feeds the antenna. The
energy transmitted to the antenna is now in the
original TE mode.
2-16.
DIRECTIONAL COUPLER
Figure 85. Rectangular waveguide with circular
rotary joints.
a. The directional coupler, as the name
implies, couples (or samples) energy only from a
wave traveling in one particular direction in a
waveguide. Figure 86 shows a common type of
directional coupler, which consists of a short section
of waveguide coupled to the main-line waveguide by
means of two small holes. It contains a matched
load in one end and a probe in the other end. The
degree of coupling between the mainline waveguide
and the auxiliary is determined by the size of the two
holes.
b. The action of this waveguide is
explained through the diagrams in figures 87 and 88.
In figure 87 power is shown flowing from left to
right, and two small samples are coupled out at
points C and D. Since the two paths (C-D-F and C-
Figure 86. Directional coupler.
E-F) to the coaxial probe are the same length, the
two samples arrive at point F in phase and are picked up by the coaxial probe. With regard to the paths to the
matched load, however, path C-D-F-E is one-half wavelength longer than path C-E, because the two holes are
one-quarter wavelength apart. Therefore, the two samples arriving at point E are 180 out of phase with each
other. The
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