How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 1051

How does a radio receive transmissions from one station and not another, and how does it turn them into audible waves? — T, Chester, VT
A radio wave contains an electric field that pushes on any electric charge it encounters. That's why, when a radio wave passes the antenna of your radio, it causes electric charges in that antenna to accelerate up and down. There is also a resonant circuit connected to the antenna—a circuit that oscillates strongly only when charge is pushed up and down the antenna at exactly the circuit's resonant frequency. If the circuit's resonant frequency is the same as that of the radio wave, the small pushes exerted on charges in the antenna add up so that charge moves more and more vigorously through the resonant circuit. But if your radio isn't tuned to the frequency of the radio wave, the overall motion of charge on the antenna and this resonant circuit is small. That's why your radio only responds to the radio transmission of one station and not others. To understand this effect, imagine pushing a child on a swing. If you push rhythmically at just the right frequency, the child will swing higher and higher. But if you push rhythmically at the wrong frequency, the child will just jitter about a bit.

Once charge is moving strongly through the resonant circuit in your radio, the radio can monitor various features of that moving charge. If the station is using the AM or amplitude modulation technique to represent sound, your radio studies the amount of charge moving back and forth through the resonant circuit. When that flow of charge—that current—is strong, it moves the speaker cone toward you and produces a compression of the air. When that current is weak, it moves the speaker cone away from you and produces a rarefaction of the air. These changes in air density and pressure reproduce the sound that the station is transmitting.

If the station is using the FM or frequency modulation technique to represent sound, your radio studies the frequency at which charge moves back and forth in the resonant circuit. Very small changes in this frequency, caused by frequency changes in the radio wave itself, are used to control the speaker cone in your radio. When the frequency is raised slightly above normal, your radio moves the speaker cone toward you and produces a compression of the air. When the frequency is lowered slightly below normal, your radio moves the speaker cone away from you and produces a rarefaction of the air. Again, these changes in air density and pressure produce sound.

         

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