When a radio wave is radiated from the transmitting antenna, it spreads in all directions with decrease in amplitude with increasing distance because of spreading of the electromagnetic energy through larger surface areas and it follows inverse square law. This law states that the intensity of the radiating waves is inversely proportional to the square of the distance from the radiator.

Electromagnetic wave in the frequency spectrum of 0.001 to 10¹⁶ Hertz are termed as radio waves, but broadly speaking by radio waves, in this chapter, mean that band of electromagnetic energy which covers the frequency range from few kHz.