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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
Radio Frequency(RF) Fundamentals

An energy wave transmitted by a transmitter antenna is called radio wave. It is a complex form of energy containing both electric and magnetic fields. As such radio waves are also called as EM waves or radiation. Like the light and heat waves, these can be reflected, refracted and diffracted in the atmosphere.

The basic shape of the wave generated by a transmitter is a sine wave. However, once radiated into the space, it may not retain the characteristics of the sine wave in total. There is bound to be distortion.

Radiowaves are often referred to by their wavelength(in meters) rather than by frequency(in Hertz). A wavelength is the space occupied by one full cycle of a radio wave at a given time. The frequency and wavelength are related by,
Frequency(f) =
In the EM spectrum, the frequencies in the range 3KHz to 300GHz are called Radio Frequencies(RF) as they are commonly uses in radio communications.

This part of the RF spectrum(3KHz - 300GHz) is divided into bands. Each band is 10 times higher in frequency than the one immediately below it. The RF bands are listed in the table.

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