MadSci Network: Engineering

Re: How are EM waves used to transmit information?

Date: Thu Nov 27 15:14:25 2003
Posted By: Yaxun Liu, Grad student, Electrical and Computer Engineering, University of Waterloo
Area of science: Engineering
ID: 1069801887.Eg

To use EM wave to transmit information, you have to modulate the EM wave 
with the information.

The information can be anolog signal like speech or digital signal like a 
text file in your computer.

For example, you want to transmit a text file to another person with EM 
wave of 1MHz.

First you need an oscillator which can generate a continuous 1MHz EM wave, 
and you need an antenna to emit this EM wave to your receipient. It is 
somewhat like shooting a beam of light with a flashlight. Now supposing 
you read the text file bit by bit, and each bit takes 1 milli-second, you 
will get a bit stream, like


Now you use a switch to control the oscillator on and off. If for this 
millisecond the bit is 0, then the oscillator is off; if the bit is 1, the 
oscillator is on. Then the receipient will receive EM wave at some time 
and not receive EM wave at some other time. He can record the strength of 
the received EM wave and get a curve. From this curve he can recover the 
sent bit stream.

If you want to transmit speech, you can use a microphone to turn the sound 
into a voltage signal. When the sound is loud, the voltage is high; when 
the sound is low, the voltage is low. This voltage signal is then used to 
control the strength of your EM wave. This is called amplitude modulation. 
The receiver can recover the voltage signal from the received EM wave.

The knowlege of how to transmit information by EM wave is covered 
formmally in the 'Wireless Communication', 'Satellite Commuincation' 
or 'Microwave Communication' courses.

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