|MadSci Network: Engineering|
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 01101000111.... 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.
Try the links in the MadSci Library for more information on Engineering.