6.
Some characteristics of resonant cavities.
a. You have seen that a resonant cavity is a space bounded by walls.
When you excite the cavity by applying electrical energy to it, all the
energy is contained inside the walls.
b. When we apply energy to ordinary tuned circuits and transmission-
line tuned circuits, we usually speak in terms of voltage and current. But
when we excite a resonant cavity, we cannot speak only in terms of voltage
and current because most of the energy inside the walls is contained in the
electric and magnetic fields. There is some electron flow, however, but it
is limited to a thin layer of metal on the inside surface of the walls.
Sometimes, the inside surfaces of the walls are silver-plated or gold-plated
to reduce heat losses due to resistance. Plating the walls makes the Q of
the cavity even higher, and cavities with a Q of 30,000 are not uncommon.
There is no current flow on the outside of the cavity, so there is no
radiation loss. Since current flows only on the inside walls, some cavities
are made from nonconducting materials.
The inside walls are then sprayed
with a thin layer of metal, or covered with metal foil.
7.
Comparing resonant cavities with transmission lines.
a. Resonant cavities are similar to transmission lines in two ways:
(1) The fundamental resonant frequency
of
transmission
lines
and
resonant cavities is determined by their size.
(2) Transmission lines and resonant cavities can have different forms
or shapes.
b. Let's examine both of these similarities.
8.
First, size determines resonant frequency.
You know that the size of a transmission line determines its
resonant frequency.
For example, a one-quarter wavelength section of
transmission line acts as a parallel resonant circuit, and it is the
shortest length that oscillates. Similarly, the area within the walls of a
resonant cavity determine the resonant frequency. A resonant cavity must be
at least one-half wavelength in dimension to oscillate.
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