Re: How do cellphones work?

Hello, Kaitlin. 

Cellular phones work by using the sound waves from your voice to control 
("modulate") a radio wave.  When you talk, you transmit sound waves - 
kind of like  the waves  you can make travel down a rope when you 
someone holds the other side and you move your side up and down 
rapidly.  Sound waves are like rapid movements of the air - you can't see 
the sound wave, but the movement of the air goes from someone else 
through the air to your ear.

Here is what the sound wave from you saying "Hello" would look like if you 
could see it:

(just look at the green squiggle, and ignore the white arrow).

Radio waves are like light waves, but invisible. A very long time ago, a very 
smart man named James Maxwell came up with a set of four 
mathematical equations (now called the Maxwell equations) that 
predicted that we could generate electromagnetic waves - waves that 
change electrically and magnetically. Depending on how fast the electrical 
and magnetic waves change, you can have different types of 
electromagnetic waves, including light as I mentioned before, but also 
X-rays and microwaves (like in your microwave oven), and ...RADIO 
WAVES!


Here is a radio wave modulated by a sound wave (the radio wave is called 
the "carrier wave" and the sound wave is called  the "audio wave" in the 
picture below)


You can hear an FM wave here:

http://mmd.foxtail.com/Tech/FM.wav

This is called Frequency Modulation - the change in the sound or audio 
wave causes a change in the  how quickly the radio or carrier wave 
changes (the frequency is how quickly the radio wave changes). 
Frequency Modulation is abbreviated "FM". The FM radio that you might 
listen to in your car uses Frequency Modulation to send you music by 
radio waves.   Some cellular phones use FM to send your voice on radio 
waves. Other cellular phones use an approach called digital modulation 
that we can talk about if you want to ask a follow-up question.

Cellular phones transmit the sound wave-modulated Radio Wave from 
the cellular phone to a Base Station that includes a large antenna that 
looks something like this:

 
The antenna and the base station receive the radio signal, pull out the 
sound wave information and send it through the phone line to the person 
you are talking to on the cellular phone.

Then, the person you are talking to responds to you, telling you how things 
are going or the latest gossip or whatever. Their voice goes along the 
telephone line to the base station near your cellular phone, and then the 
base station modulates the other person's voice onto a radio wave and 
sends  that radio wave to your cellular phone.