Figure 69. Probe coupling.
(1) For maximum coupling between the probe and the field, the probe is one-quarter wavelength
away from the shorted (closed) end of the waveguide. The probe will work equally well if it is
placed at a three-quarter-wavelength distance from the shorted end.
(2) Usually the probe is fed with a coaxial line, as shown in figure 69. This coaxial line is limited to
extremely short lengths to avoid loss of energy. Varying the distance between the probe and the
shorted end of the waveguide matches the impedance between the coaxial line and the
waveguide. The end of the waveguide is fitted with a movable plunger which moves the shorted
end of the waveguide closer to or farther from the probe. Usually the position of the probe and
the-shorted end of the waveguide is predetermined by the factory and is fixed permanently.
(3) The degree of excitation, or the amount of energy put into the waveguide, is controlled by varying
the depth of the probe into the waveguide. To increase excitation the probe is moved deeper into
the waveguide, and to decrease excitation the probe depth is decreased. The desired depth of the
probe is usually one-quarter wavelength so that the action of the probe will be similar to that of a
quarter-wave antenna. The depth of the probe also assists in matching impedance between the
coaxial line and the waveguide.
(4) When a probe is used to couple energy out of a waveguide, it is placed in a similar position on the
opposite end of the waveguide. When the probe at the output end of the waveguide matches the
probe at the input end of the waveguide, the energy is coupled out of the waveguide with no
change in efficiency.
b. Loop Coupling. To put energy into the waveguide with maximum efficiency, a small loop is placed
in the waveguide at a point of maximum magnetic field intensity. You will recall, the probe was inserted at a
point of maximum electric field intensity. For comparison, notice that the loop is placed in a waveguide at a point
of maximum magnetic field intensity.
(1) The loop is a one-half wavelength in circumference and is similar to a parallel resonant circuit.
The current through the loop