16. First, plunger tuning.
You see in Part A of Figure 78 that one wall of the cavity is a
movable plunger, resembling a piston in a cylinder. You adjust the plunger
to change the size of the cavity.
The effect is similar to what happens
when you change the length of a coil. Inserting the plunger deeper reduces
the cavity and inductance, causing the frequency to go up. If you withdraw
the plunger, the inductance goes up and frequency goes down.
17. Next, diaphragm tuning.
An example of diaphragm tuning is shown in Part B of Figure 78. You
see that one wall of the cavity is pleated. You adjust this wall, called a
diaphragm, to reduce or increase the size of the cavity.
The effect is
similar to what happens when you change the distance between the plates of a
capacitor. If you make the distance between the diaphragm and the bottom of
the cavity smaller, capacitance increases, causing the frequency to go down.
If you make the distance larger, the capacitance goes down and frequency
goes up.
18. Now, screw tuning.
An example of screw tuning, sometimes called slug tuning, is shown
in Part C of Figure 78. An adjustable screw is inserted through one wall of
the cavity. Turning the screw changes the inductance of the cavity. As you
turn the screw so that it penetrates further into the cavity, you confine
the magnetic field to a smaller space, and reduce the inductance.
This
increases the resonant frequency.
As you turn the screw in the opposite
direction, you shorten its extent into the cavity.
This increases its
inductance and the resonant frequency goes down.
19. Paddle tuning.
Another method similar to screw tuning is paddle tuning. Part D of
Figure 78 shows that one wall of the cavity has a paddle that you rotate.
Turning the paddle to a position more nearly perpendicular to the magnetic
field lowers the cavity inductance and raises the cavity resonant frequency.
Turning the paddle to a position more nearly parallel to the magnetic field
raises the cavity inductance, and lowers the cavity resonant frequency.
Both the screw and paddle methods of tuning compare to inserting a brass
slug into an IF coil to adjust the inductance.
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