20. Summary of tuning methods.
a. There are four principal methods
of
tuning
a
cavity:
using
a
plunger, a diaphragm, a screw, or a paddle.
b. The plunger and diaphragm methods of tuning vary the size of the
cavity to change its resonant frequency.
c. The screw and paddle methods of tuning change the resonant frequency
of the cavity without varying the size.
d. Now that you know the various methods used to tune resonant
cavities, let's get back to the main purpose of resonant cavities, that is,
to generate frequencies higher than 1000 megahertz. First of all, you know
that a resonant cavity must be excited to make it oscillate, that is, energy
must be fed to it from an external source. Once the cavity is oscillating,
we must couple the energy out of the cavity. There are three principle ways
that energy is put into or removed from a cavity. You use the same methods
for either operation. These three methods are described in the paragraphs
that follow.
21. Loop coupling.
a. A conductor in the shape of a loop is inserted into the area of
maximum magnetic lines of force as shown in Part A of Figure 79. In effect,
this method of coupling is comparable to coupling between the primary and
secondary of a transformer as shown in Part B of Figure 79. When the loop
is perpendicular to the lines of force, maximum energy is transferred. This
fact is also true of a transformer.
The coils of the transformer are
parallel to each other, and the magnetic lines of force are perpendicular to
the coils.
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