# Get the Knowledge that sets you free...Science and Math for K8 to K12 students

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## EM Waves - Communication System

In wireless communication, Electromagnetic waves(EM) are required to carry the signals containing the message from the transmitter (Tx) to a receiver(Rx). The block diagram of a general communication system is shown in figure.

The properties of EM waves :
• They propagate through free space at the velocity of light ie 3 × 108m/s(or 186,000 miles per second).
• They consist of moving fields of electric and magnetic forces.
• The lines of forces in electric and magnetic fields are at right angles to each other. Both of them are perpendicular to the direction of propagation (fig). In other words, in an EM wave, the direction of electric field, magnetic field and propagation are mutually perpendicular. Hence EM waves are termed as transverse in nature.
• These spread uniformly in all directions from a point source.
EM Spectrum
Band Range of frequency Range of wavelength
Audio frequency (AF) 20 to 20,000 Hz 15,000,000 to 15,000 m
Radio frequency (RF) 10 kHz to 300,000 MHz 30,000 m to 0.1 cm
Very low frequency (VLF) 10 to 30 kHz 30,000 to 10,000 m
Low frequency (LF) 30 to 300 kHz 10,000 to 1,000 m
Medium frequency (MF) 300 to 3,000 kHz 1,000 to 100 m
High frequency (HF) 3 to 30 MHz 100 to 10 m
Very high frequency (VHF) 30 to 300 MHz 10 to 1 m
Ultra high frequency (UHF) 300 to 3,000 MHz 100 to 10 cm
Super high frequency (SHF) 3,000 to 30,000 MHz 10 to 1 cm
Extremely high frequency (EHF) 30,000 to 300,000 MHz 1 to 0.1 cm
High and infrared 106 to 3.9 × 108 MHz 0.03 to 7.6 × 10–5 cm
Visible spectrum 3.9 × 108 to 7.9 × 108 MHz 7.6 × 10–5 to 3.8 × 10–5 cm
Ultraviolet 7.9 × 108 to 2.3 × 1010 MHz 3.8 × 10–5 to 1.3 × 10–6 cm
X–rays 2.0 × 109 to 3.0 × 1013 MHz 1.5 × 10–5 to 1.0 × 10–9 cm
Gamma rays 2.3 × 1012 to 3.0 × 1014 MHz 1.3 × 10–8 to 1.0 × 10–10 cm
Cosmic rays > 4.8 × 1015 MHz < 6.2 × 10–12 cm