10. A brief review.
a. Combinations of coils and capacitors are impractical as resonant
circuits above 500 megahertz.
b. Transmission lines are impractical as resonant circuits above 1000
megahertz.
c. Therefore, at microwave frequencies, a resonant cavity is used as a
tuned circuit instead of LC circuits or transmission lines.
d. A resonant cavity is a hollow chamber with conducting walls.
e. All energy generated in a resonant cavity is contained within its
walls.
f. Resonant cavities have very little resistance and a very high Q.
g. Now that you know the characteristics of a resonant cavity, let's
see how they work.
11. We can best describe the operation of a resonant cavity in terms of
fields.
a. Since resonant cavity electron flow is limited to the inside walls,
we will explain the operation of a cavity in terms of fields. We will also
use the voltage and current curves associated with transmission lines to
represent the field intensities.
b. Current flow in an antenna creates a magnetic field, called the H
field, that encircles the antenna. The magnetic field produces an electric
field, called the E field, at right angles to itself. This combination of H
and E fields, shown in Figure 75, is called the electromagnetic field.
107
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