Introduction to How Radio Works

"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 list goes on and on... Even things like radar and microwave ovens depend on radio waves. Things like communication and navigation satellites would be impossible without radio waves, as would modern aviation -- an airplane depends on a dozen different radio systems. The current trend toward wireless Internet accessuses radio as well, and that means a lot more convenience in the future!

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 article, we will explore the technology of radio so that you can completely understand how invisible radio waves make so many things possible!

By tapping the terminals of a 9-volt battery with a coin, you can create radio waves that an AM radio can receive!

The Simplest 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:

  • Take a fresh 9-volt battery and a coin.
  • 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.

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 Morse code, you can actually communicate over several inches with this crude device!

A (Slightly) More Elaborate Radio

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 entireradio spectrum, but in the early days it worked fine and was very common because there were not many people using radio waves.

Radio Basics: The Parts

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
  • The receiver

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.

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 m).

Radio Basics: Real-life Examples

baby 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 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 km).


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 generator. So:

  • The battery creates electron flow in the first wire.
  • The moving electrons create a magnetic field around the wire.
  • 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.

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 volts.

Simple Transmitters: Make Your Own

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:

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 below:

A sine wave fluctuates smoothly between, for example, 10 volts and -10 volts.

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 capacitor 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.


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 sine wave: