"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, they have totally changed society. Whether we are talking about a cell phone, a baby monitor, FM radio or any of the thousands of other wireless technologies, all of them use radio waves to communicate.
Just think about all of the things you see and use every day 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!
Now, 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!
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 (cm), 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!
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 radio spectrum, but in the early days it worked fine and was very common because there were not many people using radio waves.
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 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 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.
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.
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.
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 1.5v, 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.
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)(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 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. For example, 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.
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!
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 (see How Oscillators Work, as well as this page, 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. 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 changes in frequency into sound, but the antenna, tuner and amplifier are largely the same.
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.
When a radio transmitter transmits something, you might wonder why 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 instead just have a magnetic field around it, close to the antenna, as you see with a wire attached to the 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 also 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.