3 Transmission Media Part 2

8/11/2019 3 Transmission Media Part 2

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ASET

Transmission

Media

Part 2

Module 1


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ASETUnguided Media

Unguided media, or wireless communication, transport

electromagnetic waves without using a physical conductor.

Instead the signals are broadcast though air and are available

to anyone who has a device capable of receiving them.

The section of the electromagnetic spectrum defined as radio

communication is divided into eight ranges, called bands,

each regulated by government authorities.


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ASETWireless transmission waves


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ASET

Electromagnetic spectrum for wirelesscommunication


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ASETPropagation of Radio Waves

Radio technology considers the earth as surrounded by twolayers of atmosphere: the troposphere and theionosphere.

The troposphere is the portion of the atmosphere

extending outward approximately 30 miles from the earth'ssurface.

The troposphere contains what we generally think of asair. Clouds, wind, temperature variations, and weather in

general occur in the troposphere. The ionosphere is the layer of the atmosphere above the

troposphere but below space.


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Propagation methods


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Ground propagation. In ground propagation, radio wavestravel through the lowest portion of the atmosphere, huggingthe earth. These low-frequency signals emanate in alldirections from the transmitting antenna and follow thecurvature of the planet. The distance depends on the power

in the signal. In Sky propagation, higher-frequency radio waves radiate

upward into the ionosphere where they are reflected back toearth. This type of transmission allows for greaterdistances with lower power output.

In Line-of-Sight Propagation, very high frequency signalsare transmitted in straight lines directly from antenna toantenna.


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ASETBands

Band Range Propagation Application

VLF 330 KHz Ground Long-range radio navigation

LF 30300 KHz GroundRadio beacons and

navigational locators

MF 300 KHz3 MHz Sky AM radio

HF 330 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF 30300 MHzSky and

line-of-sight

VHF TV,

FM radio

UHF 300 MHz

3 GHz Line-of-sight UHF TV, cellular phones,paging, satellite

SHF 330 GHzLine-of-

sightSatellite communication

EHF 30300 GHzLine-of-

sightLong-range radio navigation


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ASET

transmission and reception are achieved by meansof an antenna

directional

transmitting antenna puts out focused beam transmitter and receiver must be aligned

omnidirectional

signal spreads out in all directions

can be received by many antennas


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ASETOmni directional Antennas


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Unidirectional antennas


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ASETRadio

radio is omnidirectional and microwave is directional

Radio is a general term often used to encompass

frequencies in the range 3 kHz to 300 GHz.

Mobile telephony occupies several frequency bandsjust under 1 GHz.


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ASETInfrared

Uses transmitters/receivers (transceivers) thatmodulate noncoherent infrared light.

Transceivers must be within line of sight of each

other (directly or via reflection ). Unlike microwaves, infrared does not penetrate walls.

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3 Transmission Media Part 2