The information signal should be converted into electronic signals, when free space is the channel. Such signal consists of both electric and magnetic fields. The so-called electromagnetic signals are also referred to as radio frequency (RF) waves.
RF waves oscillate i.e., amplitude of the electric and magnetic fields vary at the specific rate. These oscillations may occur at a very low frequency or at an extremely high frequency. This entire range of frequency is the electromagnetic spectrum. Electromagnetic radiation has an electric field and magnetic field which oscillate in phase perpendicular to each other and to the direction of propagation. Radio frequency band is the frequency or the rate of oscillation within the range of about 3Hz to 300GHz.
The frequency spectrum extends from subsonic frequencies (a few hertz) to cosmic rays (1022Hz). The International Radio Consultative Committee (CCIR) bands are given below.
Extremely low frequency (ELF) :
ELF range varies from 30Hz to 300Hz and includes ac power signals and low frequency telemetry signals.
Voice frequencies (VFs) :
VF range varies from 300Hz to 3000Hz. Standard telephone channels has 300Hz to 3000Hz bandwidth.
Very low frequency(VLF) :
VLF range varies from 3 KHz to 30 KHz, which includes the upper end of human hearing range. They are used in sub-marine applications.
Low frequency (LF):
LF range varies from 30 KHz to 300 KHz and is used in marine and aeronautical navigation.
Medium frequency(MF):
MF range varies from 300 KHz to 3 MHz and is used for commercial AM broadcasting (535 KHz to 1605 KHz).
High frequency(HF):
HF ranges vary from 3 MHz to 30 MHz and are called short waves. Most of the two-way communications use this range. Amateur radio and citizen band radio also use signals in this range.
Very high frequency (VHF):
VHF range is from 30 MHz to 300 MHz, these are used for mobile, radio, marine, aeronautical communications, commercial FM broadcasting (88 MHz to 108 MHz ) and commercial TV broadcasting (54 MHz to 216 MHz)- channels (2-13).
Ultra high frequency(UHF):
UHF range is from 300 MHz to 3 GHz. It is used for commercial TV broadcasting of channels 14-83, land mobile communication, cellular phones military services, certain radar and navigation systems, microwave and satellite radio systems. Frequencies above 1 GHz are microwaves.
Super high frequency(SHF):
SHF range varies from 3 GHz to 30 GHz and is used in microwave and satellite communication.
Extremely high frequency (EHF):
EHF range varies from 30 GHz to 300 GHz.
