"Radio waves" transmit music, conversations, pictures and
data invisibly through the air, often over millions of miles
-- it happens every day in thousands of different ways! Even
though radio waves are invisible and completely undetectable
to humans, they have totally changed society. Whether we are
talking about a cell
phone, a baby
monitor, a cordless
phone or any one of the thousands of other wireless
technologies, all of them use radio waves to communicate.
Here are just a few of the everyday technologies that
depend on radio waves:
The funny thing is that, at its core, radio is an
incredibly simple technology. With just a couple of electronic
components that cost at most a dollar or two, you can build
simple radio transmitters and receivers. The story of how
something so simple has become a bedrock technology of the
modern world is fascinating!
In this edition of HowStuffWorks,
we will explore the technology of radio so that you can
completely understand how invisible radio waves make so many
How Simple is Radio? Radio can be incredibly
simple, and around the turn of the century this simplicity
made early experimentation possible for just about anyone. How
simple can it get? Here's an example:
Find an AM radio and tune it to an area of the dial
where you hear static.
Now hold the battery near the antenna and quickly tap
the two terminals of the battery with the coin (so that you
connect them together for an instant).
You will hear a crackle in the radio that is caused by
the connection and disconnection of the coin.
By tapping the terminals of a 9-volt battery
with a coin, you can create radio waves that an AM radio
Your battery/coin combination is a radio transmitter! It's
not transmitting anything useful (just static), and it will
not transmit very far (just a few inches, because it's not
optimized for distance). But if you use the static to tap out
code, you can actually communicate over several inches
with this crude device!
If you want to get a little more elaborate, use a metal
file and two pieces of wire. Connect the handle of the file to
one terminal of your 9-volt battery. Connect the other piece
of wire to the other terminal, and run the free end of the
wire up and down the file. If you do this in the dark, you
will be able to see very small 9-volt sparks running along the
file as the tip of the wire connects and disconnects with the
file's ridges. Hold the file near an AM radio and you will
hear a lot of static.
In the early days of radio, the transmitters were called spark
coils, and they created a continuous stream of sparks at
much higher voltages (e.g. 20,000 volts). The high voltage
created big fat sparks like you see in a spark
plug, and they could transmit farther. Today, a
transmitter like that is illegal because it spams the entire
spectrum, but in the early days it worked fine and was
very common because there were not many people using radio
Understanding the Basics As seen in the
previous section, it is incredibly easy to transmit with
static. All radios today, however, use continuous sine
waves to transmit information (audio, video, data). The
reason that we use continuous sine waves today is because
there are so many different people and devices that want to
use radio waves at the same time. If you had some way to see
them, you would find that there are literally thousands of
different radio waves (in the form of sine waves) around you
right now -- TV broadcasts, AM and FM radio broadcasts, police
and fire radios, satellite
TV transmissions, cell phone conversations, GPS signals,
and so on. It is amazing how many uses there are for radio
waves today (see How the
Radio Spectrum Works to get an idea). Each different radio
signal uses a different sine wave frequency, and that
is how they are all separated.
Any radio setup has two parts:
The transmitter takes some
sort of message (it could be the sound of someone's voice,
pictures for a TV set, data
for a radio modem or whatever), encodes it onto a sine wave
and transmits it with radio waves. The receiver receives the
radio waves and decodes the message from the sine wave it
receives. Both the transmitter and receiver use
antennas to radiate and capture the radio signal.
monitor is about as simple as radio technology gets. There
is a transmitter that sits in the baby's room and a receiver
that the parents use to listen to the baby. Here are some of
the important characteristics of a typical baby monitor:
Modulation: Amplitude Modulation (AM)
Frequency range: 49 MHz
Number of frequencies: 1 or 2
Transmitter power: 0.25 watts
(Don't worry if
terms like "modulation" and "frequency" don't make sense right
now -- we will get to them in a moment.)
A typical baby monitor, with the receiver on
the left and the transmitter on the right: The
transmitter sits in the baby's room and is essentially a
mini "radio station." The parents carry the receiver
around the house to listen to the baby. Typical
transmission distance is limited to about 200 feet (61
A cell phone is also a radio and is a much more
sophisticated device (see How Cell
Phones Work for details). A cell phone contains both a
transmitter and a receiver, can use both of them
simultaneously, can understand hundreds of different
frequencies, and can automatically switch between frequencies.
Here are some of the important characteristics of a typical
analog cell phone:
Modulation: Frequency Modulation (FM)
Frequency range: 800 MHz
Number of frequencies: 1,664 (832 per provider, two
providers per area)
Transmitter power: 3 watts
A typical cell phone contains both a
transmitter and a receiver, and both operate
simultaneously on different frequencies. A cell phone
communicates with a cell
phone tower and can transmit 2 or 3 miles (3-5
Simple Transmitters You can get an idea for
how a radio transmitter works by starting with a battery and a
piece of wire. In How
Electromagnets Work, you can see that a battery sends
electricity (a stream of electrons) through a wire if you
connect the wire between the two terminals of the battery. The
moving electrons create a magnetic field surrounding the wire,
and that field is strong enough to affect a compass.
Let's say that you take another wire and place it parallel
to the battery's wire but several inches (5 cm) away from it.
If you connect a very sensitive voltmeter to the wire, then
the following will happen: Every time you connect or
disconnect the first wire from the battery, you will sense a
very small voltage and current in the second wire; any
changing magnetic field can induce an electric field in a
conductor -- this is the basic principle behind any electrical
The battery creates electron flow in the first wire.
The moving electrons create a magnetic field around the
The magnetic field stretches out to the second wire.
Electrons begin to flow in the second wire whenever the
magnetic field in the first wire changes.
One important thing to notice is that electrons flow in the
second wire only when you connect or disconnect the battery. A
magnetic field does not cause electrons to flow in a wire
unless the magnetic field is changing. Connecting and
disconnecting the battery changes the magnetic field
(connecting the battery to the wire creates the magnetic
field, while disconnecting collapses the field), so electrons
flow in the second wire at those two moments.
To create a simple radio transmitter, what you want to do
is create a rapidly changing electric current in a
wire. You can do that by rapidly connecting and disconnecting
a battery, like this:
When you connect the battery, the voltage in
the wire is 1.5 volts, and when you disconnect it, the
voltage is zero volts. By connecting and disconnecting a
battery quickly, you create a square wave that
fluctuates between 0 and 1.5
A better way is to create a continuously varying electric
current in a wire. The simplest (and smoothest) form of a
continuously varying wave is a sine wave like the one shown
A sine wave fluctuates smoothly between, for
example, 10 volts and -10
By creating a sine wave and running it through a wire, you
create a simple radio transmitter. It is extremely easy to
create a sine wave with just a few electronic components -- a
and an inductor
can create the sine wave, and a couple of transistors
can amplify the wave into a powerful signal (see How
Oscillators Work for details, and here
is a simple transmitter schematic). By sending that signal to
an antenna, you can transmit the sine wave into space.
One characteristic of a sine wave is its
frequency. The frequency of a sine wave is the
number of times it oscillates up and down per second.
When you listen to an AM radio broadcast, your radio is
tuning in to a sine wave with a frequency of around
1,000,000 cycles per second (cycles per second is also
known as hertz). For example, 680 on the AM dial
is 680,000 cycles per second. FM radio signals are
operating in the range of 100,000,000 hertz, so 101.5 on
the FM dial is a transmitter generating a sine wave at
101,500,000 cycles per second. See How
the Radio Spectrum Works for details.
Transmitting Information If you have a sine
wave and a transmitter that is transmitting the sine wave into
space with an antenna, you have a radio station. The only
problem is that the sine wave doesn't contain any information.
You need to modulate the wave in some way to encode
information on it. There are three common ways to modulate a
Pulse Modulation - In PM, you simply turn the
sine wave on and off. This is an easy way to send Morse
code. PM is not that common, but one good example of it is
the radio system that sends signals to radio-controlled
clocks in the United States. One PM transmitter is able
to cover the entire United States!
Amplitude Modulation - Both AM radio stations and
the picture part of a TV signal use
amplitude modulation to encode information. In amplitude
modulation, the amplitude of the sine wave (its peak-to-peak
voltage) changes. So, for example, the sine wave produced by
a person's voice is overlaid onto the transmitter's sine
wave to vary its amplitude.
Frequency Modulation - FM radio stations and
hundreds of other wireless technologies (including the sound
portion of a TV signal,
cordless phones, cell phones, etc.) use frequency
modulation. The advantage to FM is that it is largely immune
to static. In FM, the transmitter's sine wave frequency
changes very slightly based on the information signal.
Once you modulate a sine wave with information, you can
transmit the information!
Receiving an AM Signal Here's a real world
example. When you tune your car's AM radio to a station -- for
example, 680 on the AM dial -- the transmitter's sine wave is
transmitting at 680,000 hertz (the sine wave repeats 680,000
times per second). The DJ's voice is modulated onto that
carrier wave by varying the amplitude of the transmitter's
sine wave. An amplifier amplifies the signal to something like
50,000 watts for a large AM station. Then the antenna sends
the radio waves out into space.
So how does your car's AM radio -- a receiver -- receive
the 680,000-hertz signal that the transmitter sent and extract
the information (the DJ's voice) from it? Here are the steps:
Unless you are sitting right beside the transmitter,
your radio receiver needs an antenna to help it pick
the transmitter's radio waves out of the air. An AM antenna
is simply a wire or a metal stick that increases the amount
of metal the transmitter's waves can interact with.
Your radio receiver needs a tuner. The antenna
will receive thousands of sine waves. The job of a tuner is
to separate one sine wave from the thousands of radio
signals that the antenna receives. In this case, the tuner
is tuned to receive the 680,000-hertz signal.
Tuners work using a principle called resonance.
That is, tuners resonate at, and amplify, one
particular frequency and ignore all the other frequencies in
the air. It is easy to create a resonator
with a capacitor
and an inductor
(check out How
Oscillators Work to see how inductors and capacitors
work together to create a tuner).
The tuner causes the radio to receive just one sine wave
frequency (in this case, 680,000 hertz). Now the radio has
to extract the DJ's voice out of that sine wave. This is
done with a part of the radio called a detector or
demodulator. In the case of an AM radio, the detector
is made with an electronic component called a diode.
allows current to flow through in one direction but not the
other, so it clips off one side of the wave, like this:
The radio next amplifies the clipped signal and
sends it to the speakers
(or a headphone). The amplifier is made of one or more
transistors (more transistors means more amplification and
therefore more power to the speakers).
What you hear
coming out the speakers is the DJ's voice!
In an FM radio, the detector is different, but everything
else is the same. In FM, the detector turns the changes in
frequency into sound, but the antenna, tuner and amplifier are
largely the same.
The Simplest AM Receiver In the case of a
strong AM signal, it turns out that you can create a simple
radio receiver with just two parts and some wire! The process
is extremely simple -- here's what you need:
A diode - You can get a diode for
about $1 at Radio Shack. Part number 276-1123 will do.
Two pieces of wire - You'll need about 20 to 30
feet (15 to 20 meters) of wire. Radio Shack part number
278-1224 is great, but any wire will do.
A small metal stake that you can drive into the
ground (or, if the transmitter has a guard rail or metal
fence nearby, you can use that)
A crystal earphone - Unfortunately, Radio Shack
does not sell one. However, Radio Shack does sell a Crystal
Radio Kit (part number 28-178) that contains the
earphone, diode, wire and a tuner (which means that you
don't need to stand right next to the transmitter for this
to work), all for $10.
You now need to find and be near an AM radio station's
transmitting tower (within a mile/1.6 km or so) for this to
work. Here's what you do:
Drive the stake into the ground, or find a convenient
metal fence post. Strip the insulation off the end of a
10-foot (3-meter) piece of wire and wrap it around the
stake/post five or 10 times to get a good solid connection.
This is the ground wire.
Attach the diode to the other end of the ground wire.
Take another piece of wire, 10 to 20 feet long (3 to 6
meters), and connect one end of it to the other end of the
diode. This wire is your antenna. Lay it out on the ground,
or hang it in a tree, but make sure the bare end does not
touch the ground.
Connect the two leads from the earplug to either end of
the diode, like this:
Now if you put the earplug in your ear, you will hear the
radio station -- that is the simplest possible radio receiver!
This super-simple project will not work if you are very far
from the station, but it does demonstrate how simple a radio
receiver can be.
Here's how it works. Your wire antenna is receiving all
sorts of radio signals, but because you are so close to a
particular transmitter it doesn't really matter. The nearby
signal overwhelms everything else by a factor of millions.
Because you are so close to the transmitter, the antenna is
also receiving lots of energy --
enough to drive an earphone! Therefore, you don't need a tuner
or batteries or anything else. The diode acts as a detector
for the AM signal as described in the previous section. So you
can hear the station despite the lack of a tuner and an
Antennas You have probably noticed that
almost every radio you see (like your cell phone, the radio in
your car, etc.) has an antenna. Antennas come in all
shapes and sizes, depending on the frequency the antenna is
trying to receive. The antenna can be anything from a long,
stiff wire (as in the AM/FM radio antennas on most cars) to
something as bizarre as a satellite
dish. Radio transmitters also use extremely tall antenna
towers to transmit their signals.
The idea behind an antenna in a radio transmitter is to
launch the radio waves into space. In a receiver, the idea is
to pick up as much of the transmitter's power as possible and
supply it to the tuner. For satellites
that are millions of miles away, NASA uses huge dish antennas
up to 200 feet (60 meters ) in diameter!
The size of an optimum radio antenna is related to the
frequency of the signal that the antenna is trying to transmit
or receive. The reason for this relationship has to do with
the speed of light, and the distance electrons can
travel as a result. The speed of light is 186,000 miles per
second (300,000 kilometers per second).
Let's say that you are trying to build a radio tower for
radio station 680 AM. It is transmitting a sine wave with a
frequency of 680,000 hertz. In one cycle of the sine wave, the
transmitter is going to move electrons in the antenna in one
direction, switch and pull them back, switch and push them out
and switch and move them back again. In other words, the
electrons will change direction four times during one cycle of
the sine wave. If the transmitter is running at 680,000 hertz,
that means that every cycle completes in (1/680,000)
0.00000147 seconds. One quarter of that is 0.0000003675
seconds. At the speed of light, electrons can travel 0.0684
miles (0.11 km) in 0.0000003675 seconds. That means the
optimal antenna size for the transmitter at 680,000 hertz is
about 361 feet (110 meters). So AM radio stations need very
tall towers. For a cell phone working at 900,000,000 (900
MHz), on the other hand, the optimum antenna size is about 8.3
cm or 3 inches. This is why cell phones can have such short
You might have noticed that the AM radio antenna in your
car is not 300 feet long -- it is only a couple of feet long.
If you made the antenna longer it would receive better, but AM
stations are so strong in cities that it doesn't really matter
if your antenna is the optimal length.
You might wonder why, when a radio transmitter transmits
something, radio waves want to propagate through space away
from the antenna at the speed of light. Why can radio waves
travel millions of miles? Why doesn't the antenna just have a
magnetic field around it, close to the antenna, as you see
with a wire attached to a battery? One simple way to think
about it is this: When current enters the antenna, it does
create a magnetic field around the antenna. We have also seen
that the magnetic field will create an electric field (voltage
and current) in another wire placed close to the transmitter.
It turns out that, in space, the magnetic field created by the
antenna induces an electric field in space. This electric
field in turn induces another magnetic field in space, which
induces another electric field, which induces another magnetic
field, and so on. These electric and magnetic fields
(electromagnetic fields) induce each other in space at the
speed of light, traveling outward away from the antenna.
For more information on radio and related topics, check out
the links on the next page.