The Electromagnetic Spectrum and Radio Waves
The range of all possible wavelengths of electromagnetic waves is described using the Electromagnetic spectrum. The diagram of the spectrum is shown below.
Figure: Electromagnetic Spectrum
As shown above, visible light is but a tiny fraction of the spectrum. The part highlighted in red is the Radio spectrum, or the range of radio waves. These are the waves with wavelengths longer than the IR or Infrared waves. The frequencies of radio waves range from 300GHz to 3KHZ and their wavelengths are typically from around 1mm to 100km range. Like all Electromagnetic waves, radio waves do not need any medium for propagation and they travel at speed of light. Radio waves are used for numerical other communication systems, our focus shall however remain on wireless and cellular communication.
Below table shows the further bifurcation of the radio spectrum.
30 – 300 Hz
30 – 300 MHz
3 – 30 kHz
300 – 3000 MHz
30 – 300 kHz
3 – 30 GHz
300 – 3000 kHz
30 – 300 GHz
3 – 30 MHz
Table: Radio spectrum division
In the above classification system, radio waves are divided in decades, into bands whose frequencies would vary over a factor of 10 and its exponents. Following is a brief explanation of each of these bands.
ELF: The Extra Low Frequency Band.
Extra Low Frequency (ELF) are generally produced by accidental or are natural. It is like white noise and electrical ‘hum’ encountered in almost all circuits. These frequencies are also produces as a result of solar wind and interaction atmospheric charges. Geophysicists generally record these frequencies and use their reading to study natural phenomenon. This band may be put in use for communication with submarines when very low amount of information is to be passed as loss due to absorption is very great.
ULF: Ultra Low Frequency Band.
Unlike the ELF, Electromagnetic waves of the Ultra Low Frequency Band (ULF) are very lightly absorbed by water or the earth. They are therefore more suitable for communication with submarines. The major difficulty with such low frequencies is that the modulation becomes very slow hence the information carrying capacity of the signal is greatly reduced.
VLF: Very Low Frequency Band.
Though used in navigation systems, the information capacity of a VLF signal is usually quite low.
LF: Low Frequency Band.
From the LF or low frequency band onwards, the radio spectrum can be effectively used for communication as path loss due to absorption is greatly reduced. The waves in this band have a great advantage because they are capable of reflecting and refracting at the ionosphere, hence can be made to travel along the surface of the earth. This band is used to communicate over oceans and around the world.
MF: Medium Frequency Band.
Unlike the LF band, the waves of the medium frequency band or the MF band are not reflected or refracted too well by the ionosphere. However, it can be used over long range without much path loss. It is usually allocated for AM radio communication systems.
HF: High Frequency Band.
At the frequencies of the High frequency or the HF band, the wave lengths become quite short as compared to the earlier bands of the radio spectrum. Therefore this band is also known as the short wave band. As the number of waves that are transmitted in every second increases, the amount of information that can fit in a signal of certain time length increases. Therefore with HF band it is possible to transmit more information over shorter time.
VHF: Very High Frequency Band.
The very high frequency band or the VHF band includes the signals for TV aand FM radio. Note that at these frequencies pathloss is very less but they operate within the line of sight range. Also the Antennaes required to receive these signals are about 1/4th or 1/2th of the wavelength of the signal.
UHF: Ultra High Frequency Band.
The ultra high frequency band or the UHF band includes TV signals and cellular phones signals.
SHF: Super High Frequency Band.
The super high frequency band or the SHF band are Used for communication with satellites.
EHF: Extra High Frequency Band.
The extra high frequency band or the EHF band are not very useful for communication (as of yet). This is because of the encoding techniques are limited. The apparatus and techniques to encode such a high frequency signal would be very complex and expensive.
This is all about the radio band.