How Radio Works

Radio can be incredibly simple, and around the turn of the 20th century this simplicity made early experimentation possible for just about anyone. How simple can it get? Here's an example:

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.

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 radio waves were not heavily regulated.

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. Very early radio transmitters emitted a large band of frequencies at once. All they could reproduce were simple noises which could be used to communicate with Morse code. A sine wave transmitter narrows this band down to more specific frequencies which can effectively reproduce complex information like audio streams, video and internet data. The narrow frequency band also allows many transmitters to operate in an area without interfering with one another.

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. 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, etc.), 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.

You can get an idea for how a radio transmitter works by starting with a battery and a piece of wire. 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 2 inches (5 centimeters) 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:

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, as shown at left:

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:

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 and simple transmitter schematic. By sending that signal to an antenna, you can transmit the sine wave into space.

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 four common ways to modulate a sine wave:

Pulse Width Modulation (PWM)

In PWM, you simply turn the sine wave on and off. This is an easy way to send Morse code. PWM 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 PWM transmitter is able to cover the entire United States!

Amplitude Modulation (AM)

AM radio stations 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)

FM radio stations and hundreds of other wireless technologies 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. FM uses higher frequency signals than AM, which have higher fidelity but a decrease in range.

Digital Modulation

Digital modulation encodes digital information onto an analog carrier signal and provides higher fidelity without any of the typical static. In the case of things like wireless routers, digital modulation also allows the signal to be encrypted. This way, the transmitter will only send data to particular devices.

However, a digital signal that is too weak will quickly become unusable. Audio data will sound scrambled, and videos will be highly pixelated. In the U.S. over-the-air television has moved entirely over to digital transmission, and many terrestrial radio stations operate on digital antennas in addition to their analog signals.

Once you modulate a sine wave with information, you can transmit the information.

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's how it works./\r\n/

What you hear coming out of 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.

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:

To make the process easier, some kits are also available online containing most of the necessary parts in the box. The Home Science Tools crystal radio kit also comes with an electric amplifier, making the earphone unnecessary.

You now need to find and be near an AM radio station's transmitting tower (within 1 mile/1.6 kilometer or so) for this to work. Here's what you do:

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 amplifier. However, adding an amplifier like that on the educational kit will boost the signal and give it greater volume.

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 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 230 feet (70 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). So, how do you know what size antenna you need?

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) or 0.00000147 seconds. One quarter of that is 0.0000003675 seconds.

At the speed of light, electrons can travel 0.0684 miles (0.11 kilometers) 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 3 inches or 8.3 centimeters. This is why cell phones can have such short antennas.

You might have noticed that the AM radio antenna in your car is not 300 feet (91 meters) 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.

Although analog radio sources are still pervasive, digital signals like Wi-Fi and Bluetooth have taken over. In 2009, the U.S. mandated that the majority of over-the-air analog TV stations would have to switch to digital transmitters. For many radio stations, a digital format known as HD Radio is also available. However, FM signals remain the current standard, likely because a large number of older vehicles on the road still rely on AM/FM tuners.

The advantages provided by a digital radio transmission are fidelity and security. Digital signals carry the much higher data rates needed to provide things like high-definition video or wireless internet. The receiver also gets none of the noise and static that is ever-present in analog transmissions. However, the actual method of transmission can get complicated.

Since radio waves are a traditional analog signal, the transmitter needs to convert its data using a digital to analog converter. Once the receiving antenna picks up the signal, it then has to "unscramble" the data back to its original form using an analog to digital converter. This method sounds overly complex, but it does allow for things like data encryption.

Essentially, the receiving antenna must have the correct instructions to unscramble the digital data that was converted by the transmitter. Without those instructions, the data cannot be accessed. This is why people generally can't access your Wi-Fi or bluetooth without pairing with the correct devices. This process also helps cut down on radio interference in the air. Analog radio signals, on the other hand, can be accessed by anyone in the area with a functioning antenna.