2-11.
BENDS
To have energy move from one end of a
waveguide to the other without reflections or
standing waves, the size, shape, and dielectric
material of the waveguide must be constant
throughout its entire length. Any abrupt change in
the size or shape of the waveguide will cause
Figure 76. Waveguide transformer.
reflections; therefore, any change must be very
gradual unless special devices are used. When it is
necessary to change the shape or direction of a
waveguide, then bends, twists, or terminations are
used.
These are sometimes called waveguide
plumbing.
a. Twisted Bends. In some installations it
is necessary to change the direction of the
waveguide or to rotate the electromagnetic field.
When a waveguide is terminated with an antenna,
Figure 77. Twisted section of waveguide.
the electromagnetic field may have to be rotated so
that the antenna can be properly polarized. This can
be done by twisting the waveguide, as shown in
figure 77. The twist is gradual and is extended over
two wavelengths or more to prevent reflections.
b. Gradual Bends. When the direction of a
waveguide is changed, a gradual bend is used, as
shown in figure 78. Some bends may be 90 and
others may be more or less than 90. The radius of
the bend must be greater than two wavelengths to
minimize reflections.
Figure 78. Gradual bends.
c. Sharp Bends. Some installations may
require a sharp bend, as shown in figure 79. These
bends are bent twice at 45,, one-quarter wavelength
apart. Reflections do occur in these bends, but the
combination of the direct reflection at one bend and
the inverted reflection at the other bend will cancel.
The fields then appear as though no reflection had
occurred.
Figure 79. Sharp bends.
344 L2
77
Previous Page
