The variable capacitor

 A variable capacitor is an electrical component that is used to control the capacitance of a circuit. It is able to alter its capacitance in a range of values by changing the spacing between its two conducting plates or by varying the area of one or both of its conducting plates. This type of capacitor is also known as a varicap, varactor, or tuning capacitor, and it is commonly used in radio tuning circuits, in applications where the length of an antenna needs to be tuned, and in voltage-controlled oscillators. Additionally, variable capacitors can be used in more complex applications such as in frequency synthesizers and phase-locked loops.