will encounter an impedance mismatch between the
waveguide and space. Part of the energy will be
radiated into space, and part will be reflected back
into the waveguide because of the impedance
mismatch. The reflected energy will cause standing
waves in the waveguide.
b. Despite this loss of efficiency, the
waveguide radiator is sometimes used to radiate
energy into space. The waveguide opening is an
aperture, and the size and shape of this aperture
determines the polar distribution and gain of the
radiator. Because the waveguide radiator is the open
Figure 95. Waveguide radiator.
end of the waveguide, the aperture dimensions are
the waveguide dimensions.
c. Strange as it may seem, the gain of the waveguide radiator is somewhat greater than the gain of a
dipole. The polar distribution in the electric plane is similar to the figure-8 pattern of the dipole. The waveguide
radiator also has a greater tuning range than the dipole. The tuning range limits are the same as the waveguide
limits. As the frequency of the energy in the waveguide is increased to where the waveguide dimension is more
than one wavelength, the energy is attenuated. Also, if the frequency is decreased sufficiently, it will reach the
cutoff frequency of the waveguide, and propagation ceases.
The fact that a waveguide radiator has greater gain than a dipole might lead you to think that it is a simple
way to radiate energy into space. Actually it is simple, but there is still the problem of the impedance mismatch
between the waveguide and space. An impedance mismatch prevents maximum transfer of energy, so energy is
lost in the waveguide because of standing waves.
a. If a waveguide is terminated with a
dipole, as shown in figure 96, a good impedance
match can be obtained. It is much simpler to excite
a dipole from a waveguide than from a coaxial line.
To excite a dipole from a waveguide, the dipole is
mounted on a web that fits into the open end of the
waveguide. The web is mounted in the waveguide
so that it is parallel with the wide side of the
waveguide, and this places the dipole so that it is
parallel to the E lines in the waveguide.
Figure 96. Dipole termination of a waveguide.