The resonant frequency of a cavity can be varied by changing any of three parameters: cavity
volume, cavity capacitance, or cavity inductance. Changing the frequencies of a cavity is known as
TUNING. The mechanical methods of tuning a cavity may vary with the application, but all methods use
the same electrical principles.
A mechanical method of tuning a cavity by changing the volume (VOLUME TUNING) is illustrated
in figure 1-62. Varying the distance d will result in a new resonant frequency because the inductance and
the capacitance of the cavity are changed by different amounts. If the volume is decreased, the resonant
frequency will be higher. The resonant frequency will be lower if the volume of the cavity is made larger.
Figure 1-62.Cavity tuning by volume.
CAPACITIVE TUNING of a cavity is shown in figure 1-63A. An adjustable slug or screw is placed
in the area of maximum E lines. The distance d represents the distance between two capacitor plates. As
the slug is moved in, the distance between the two plates becomes smaller and the capacitance increases.
The increase in capacitance causes a decrease in the resonant frequency. As the slug is moved out, the
resonant frequency of the cavity increases.
Figure 1-63A.Methods of changing the resonant frequency of a cavity. CHANGING THE CAPACITANCE.