How radio waves carry information
now that you know what radio is and how radio waves are generated and how they transmit energy it is time to have a look at how radio waves are used to transmit information. The radio waves we've discussed so far didn't carry any information, so they were pretty uninteresting to the receiver.
So, how do radio waves carry information? The answer is modulation. In modulation a radio wave (also called the "carrier signal") is changed by the signal we want to send to the receiver, for example a song, or, in a wireless network, some data from a computer. This changing of the carrier signal is called modulation.
The receiver of such a modulated signal must know the type of modulation that was used to change the carrier signal. With that knowledge the receiver will be able to demodulate the received signal. In other words, the receiver can obtain the information that was modulated onto the carrier signal.
Amplitude modulation (AM)
The most common type of modulation is amplitude modulation or AM, which is used in radio broadcasting. In amplititude modulation the amplitude, or height of the wave is changed by a second wave signal, like the sound of a song.
In this Wikipedia animation, created by Berserkerus, you see how in AM a signal changes the amplititude of the radio wave. The uppermost wave is the signal we want to send to the receiver.You can see how a top (crest) of the signal wave is reflected in the AM wave by a higher amplitude.
The amplitude of the AM wave corresponds to the voltage the wave was created with, or, the level of energy.
In frequency modulation the frequency of the wave is changed to represent the signal to be transmitted. Note how a top (crest) in the signal we want to send is represented in the FM signal by a higher frequency and a bottom (trough) in the signal is represented by a lower frequency in the FM signal.
Modulation in wireless networks
Now, AM and FM are two relatively simple types of modulation. In wireless communication the router and network cards use far more complicated forms of modulation, mostly Orthogonal Frequency Division Multiplex (ODFM). ODFM is a type of modulation that uses multiple carrier waves of different frequency to simultaneously carry information.
Explaining ODFM in detail is outside the scope of this article, because it would very soon become a complicated mathematics course.
What is important to understand from these examples is that a carrier wave can be changed by the signal you want to send and that on the receiving end, the signal that was used to change the carrier signal can be obtained again. This is how information is carried by a radio wave and this is how information is sent in your wireless network at home.