Sound and pictures become a signal
A microphone turns sound into a tiny electric signal. A TV camera does the same for light, so a picture becomes an electric signal too. This signal is a copy of the sound or picture, written in the language of electricity.
This signal is too weak to travel far. So we need a helper.
The carrier wave and the tower
A carrier wave is a strong radio wave of one fixed frequency. The signal is put onto it. This is called modulation.
- AM (amplitude modulation): the signal changes the height (amplitude) of the carrier.
- FM (frequency modulation): the signal changes the frequency of the carrier a little.
The tower's antenna sends the wave out. Radio waves travel at the speed of light, c = 3 × 108 m/s. Wavelength comes from c = f × λ.
The receiver: tuning
Many stations send at the same time. Each has its own carrier frequency. A radio or TV has a tuner. Turning the dial picks one frequency, and the others are ignored. Then the receiver removes the carrier and keeps the signal. A speaker turns it into sound; a screen turns it into light.
Mobile networks and cells
A phone call is two-way. So a phone has a small transmitter and a small receiver. A network divides an area into cells. Each cell has a base station (a cell tower). Your phone talks to the nearest tower.
Why small cells? Each tower can serve only so many calls. Towers that are far apart can re-use the same frequencies, so more people can talk. When you move, the network passes your call to the next tower. This is called a handover. The towers are joined to each other by cables, so a call can reach any phone.
Key formulas and definitions
- c = f × λ (c = 3 × 10^8 m/s)
- λ = c / f, f = c / λ
- 1 kHz = 10^3 Hz, 1 MHz = 10^6 Hz, 1 GHz = 10^9 Hz
- Modulation: AM changes amplitude, FM changes frequency
Worked examples
1. An FM station broadcasts at 100 MHz. Find the wavelength of its carrier.
λ = c / f = (3 × 10^8) / (100 × 10^6) = 3 m.
2. An AM station has wavelength 300 m. Find its frequency.
f = c / λ = (3 × 10^8) / 300 = 10^6 Hz = 1 MHz (1000 kHz).
3. A mobile network works at 900 MHz. How long is its wave?
λ = (3 × 10^8) / (900 × 10^6) = 0.33 m, about 33 cm.
4. Why can two far-apart cell towers use the same frequency?
Their waves are weak by the time they reach each other, so they do not disturb each other. This is frequency re-use.
Common mistakes
- Thinking the sound itself travels through the air as a radio wave. Only the electric signal rides on the radio wave.
- Mixing up AM and FM. AM changes the height (amplitude); FM changes the frequency.
- Forgetting to change MHz to Hz in c = f × λ. 100 MHz = 100 × 10^6 Hz.
- Thinking a phone talks straight to another phone. It talks to a tower first.