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Home » Physics Homework Help » Quantum Physics » Radiowaves, Transmission
Radiowaves, Transmission
The radiowaves are the electromagnetic waves of frequency ranging from 500 MHz to about 1000 MHz. These waves are used in the field of radio communication. With reference to the frequency range and wavelength range, the radiowaves have been divided into various categories shown in table under:

Frequency range and wavelength range of radio-waves:

Frequency band Frequency range Wavelength range Main use
Very-Low-Frequency (VLF) 3 kHz to 30kHz 10 km to 100 km Long distance point to point communication
Low-Frequency (LF) 30 kHz to 300 kHz 1 km to 10 km Marine and navigational purposes
Medium Frequency (MF) 300 kHz to 3 MHz 100 m to 1 km Marine and broadcasting purposes
High Frequency (HF) 3 MHz to 30 MHz 10 m to 100 m Communication of all types
Very-High Frequency(VHF) 30MHz to 300 MHz 1 m to 10 m T.V., Radar and air navigation
Ultra-High Frequency (UHF) 300 MHz to 3000 MHz 10 cm to 1 m Radar and microwave communication
Super-High Frequency (SHF) 3 GHz to 30 GHz 1 cm to 10 cm Radar, radio relays and navigation purposes
Extremely-High Frequency (EHF) 30 GHz to 300 GHz 1 mm to 1 cm Optical fibre communication

Transmission medium or communication channel

The transmission medium or communication channel is a link through which information/message signal may propagate from the source to the destination, without any noise or distortion. It is a sort of electronic roadways along which signals travel.

Broadly, transmission media have been divided into two types:

1. Guided Transmission Medium.

2. Unguided Transmission Medium.

Guided Transmission Medium: it is that communication medium or channel which is used in signal communication, for point to point contact between the transmitter and receiver. For example, parallel wire lines, twisted pair and co-axial cable are guided transmission media. Optical fibres are other examples of guided transmission medium. Guided transmission medium is used in line communication.

Unguided transmission medium: it is that communication medium which is used, where there is no point to point contact between the transmitter and receiver. Free space is an example of unguided transmission medium. It is used in space communication and satellite communication.

The characteristics and quality of transmission medium depend upon

(i) Nature of transmission medium,

(ii) Nature of signal.

Thus, basically there are two types of communications (1) space communication (2) line communication.

Band width of transmission medium

There are different band widths for different message signals. In fact, the different transmission media for communication of signals offer different band widths. The commonly used transmission media are wire, free space and optical fibre cable. The wire cables (i.e. parallel wire lines, twisted pair wire lines and co-axial wire lines) is a transmission medium for the message signal which offers a band width of about 750 MHz. those cables are generally operated below 18 GHz.

The free space is a transmission medium for the propagation of radiowaves having range of frequencies from a few hundred kHz to a few GHz. The different ranges of frequencies are used for different purposes as indicated in the table below:

The optical fibre is a communication medium for a signal wave in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fibre can offer a transmission band width in excess of 100 GHz.

Some important wireless communication frequency bands

Name of service Frequency band Remarks
Standard amplitude modulated broadcast 540-1600 kHz  
F.M. Broadcast 88 to 108 MHz  
Television 54-72 MHz
76-88 MHz
174-216 MHz
420-890 MHz
Very high frequency (VHF) T.V.
Ultra violet frequency (UHF) T.V.
Cellular mobile radio 896-901 MHz Mobile to base station
Base station to mobile
Satellite communication 5.925-6.425 GHz
3.7-4.2 GHz
Uplink
Downlink

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