Symbol 7 in Figure N6-1 represents an antenna. An antenna is used to radiate radio energy.
Symbol 8 in Figure N6-1 represents an earth ground.
Symbol 4 in Figure N6-3 represents an NPN transistor.
A PNP transistor can amplify a small signal using low voltages.
Symbol 1 in Figure T6-2 represents a fixed-value capacitor. Symbol 3 in Figure T6-2 represents a variable capacitor. A a capacitor stores energy electrostatically and opposes a change in voltage.
The capacitance of a capacitor is determined by the he material between the plates, the area of one side of one plate, the number of plates and the spacing between the plates.
An inductor stores energy electromagnetically and opposes a change in current. As an iron core is inserted in a coil, the coil's inductance increases.