Figure 75.
c. The electromagnetic field in a resonant cavity may be in different
positions depending upon the size of the cavity, the method of coupling, and
the frequency of the signal fed into the cavity. The different positions,
or patterns of the fields are called modes.
12. What are modes?
a. So far, you know that a resonant cavity can be one of several
different shapes and, when excited, the cavity contains an electromagnetic
field.
The electric field is always perpendicular to the magnetic field,
but the pattern (mode) of the combined fields inside the cavity may vary.
b. For example, the electric field may be parallel to the width,
height, or length of the cavity. These patterns or forms of operation are
only three among many possibilities that we term "modes of operation." As a
repairman, it is not necessary for you to be skilled in mode operation.
But, it will help in your work to know there are different ways (or modes)
to excite a cavity.
Therefore, an explanation of the PRIMARY mode of
operation follows.
13. The PRIMARY mode of operation.
a. The PRIMARY mode of exciting a resonant cavity is shown in Figure
76. The arrows in Part A of Figure 76 represent the electric (E) field and
indicate the field direction and intensity. Consider the electric field as
voltage, and the areas of maximum voltage are where the lines are the
densest.
You can see that there is one complete electric field pattern
along the length and another across the width of the cavity.
These
dimensions are one-half wavelength.
108
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