Childrens Illustrated Encyclopedia

426

pioneers of radio

In 1864, Scottish physicist James Clerk

Maxwell developed the theory of

electromagnetic waves, which are the

basis of radio. In 1888, Heinrich Hertz,

a German physicist, discovered radio

waves. Italian Guglielmo Marconi

(1874-1937, right) created the

first radio system in 1895, and

in 1901, he transmitted radio

signals across the Atlantic.

RAdIo StudIo

A microphone converts sound

waves from the announcer’s

voice into electrical

signals, which are

then transmitted

as radio waves.

RAdIo fRequenCIeS

Radio waves consist of rapidly

oscillating (varying) electric

and magnetic fields. the rate

of oscillation is called the

frequency of the wave, measured

in hertz (Hz). one hertz equals

one oscillation per second; one

kilohertz (kHz) equals 1,000

hertz. Bands of certain

frequencies are used

to transmit different

kinds of information.

Communications satellites

pick up and rebroadcast radio

programs using super-high-

frequency waves with frequencies

of more than 3 million kHz.

Long waves

(30-300 kHz) can

travel about 600

miles (1,000 km).

They are used for

national broadcasts

and to send

information

to ships.

RAdIo ReCeIveR

When radio waves reach the antenna

of a radio, they produce tiny varying

electric currents in the antenna. As the

tuner knob is turned, an electronic

circuit selects a single frequency from

these currents corresponding to a radio

channel. the signal is then converted

into sound waves. Modern digital radios,

as above, receive signals coded using a

computer code. this gives better sound

quality and access to more stations.

A transmitter receives

radio programmes by

cable from the studio. The

transmitter antenna beams

radio waves that spread

out like ripples in water.

Television

shows are 

carried on UHF

(ultra-high-

frequency) radio

waves (300,000-

3,000,000 kHz).

Dish sends

and receives

radio waves

VHF (very-high-

frequency) radio

waves (30,000-

300,000 kHz) move

in straight lines so they

cannot travel over the

horizon. Police, fire

brigade, and citizens’

band radios use VHF

waves for short-range

communications.

Many radio stations transmit programs

on the medium-wave band. These

medium-frequency (300-3,000 kHz)

channels are restricted to within a

couple of hundred miles.

International radio stations and

amateur radio enthusiasts use

short-wave radio signals. Short

waves (3,000-30,000 kHz) can

travel great distances. They

bounce around the world,

reflected off Earth’s surface 

and a layer of the atmosphere

called the ionosphere.

eARly RAdIo WAS often called “the wireless” because radio uses invisible

waves instead of wires to carry messages from one place to another. today,

radio waves are an important means of communicating sounds, pictures,

and data all over the world. Within the circuits of a radio transmitter,

rapidly varying electric currents generate radio waves of different lengths

that travel to a radio receiver. Radio waves are a type of electromagnetic

(e M) wave, similar to light and x-rays. like these waves, radio waves travel

at the speed of light, 186,000 miles (300,000 km) per second, nearly one

million times the speed of sound waves. Radio waves can travel through

the air, solid materials, or even empty space, but are

sent most efficiently by putting the transmitting

antenna on high ground like a hill.

MoRSe Code

early radio signals

consisted of beeps,

made by tapping a

key. operators

tapped out a

message using

a series of

short and long

beeps called

Morse code,

invented by

Samuel Morse

(1791-1872) in 1837.

Radio

Find out more

Astronomy

navigation

Sound

telephones

television

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