The Functioning of Radio Technology

Electromagnetic radio waves are among the most groundbreaking innovations in technology for the 20th and 21st centuries. Though invisible to the naked eye, countless high-frequency waves travel through the air surrounding you each day. These waves enable wireless communication for devices such as car radios, smartphones, and Wi-Fi internet. Thanks to radio waves, exchanging information has become faster and more convenient than ever before.

Below are just a few examples of everyday technologies that rely on radio waves:

Even technologies like radar and microwave ovens rely on radio waves. Communication and navigation satellites wouldn’t function without them, nor could modern aviation—airplanes rely on numerous radio systems. The Wi-Fi networks we use daily for work, home, and school are also entirely dependent on radio waves for data transfer.

Ironically, the underlying technology of radio is quite simple. With just a few inexpensive electronic parts, you can build basic radio transmitters and receivers for only a dollar or two. In this article, we'll delve into the fundamentals of radio technology so that you can fully grasp how these invisible radio waves make so much possible.

The Most Basic Radio

Radio technology can be surprisingly simple, and at the dawn of the 20th century, this simplicity allowed nearly anyone to experiment with it. Just how simple can it be? Here's a demonstration:

Your battery/coin setup is essentially a radio transmitter! Although it won’t send anything meaningful (just static noise), and its range is very short (a few inches, since it’s not designed for long-distance transmission), you can still use the static to communicate over short distances with Morse code using this basic device.

A (Slightly) More Advanced Radio

For a slightly more advanced version, take a metal file and two pieces of wire. Attach one wire to the handle of the 9-volt battery, and connect the other wire to the second terminal. Run the free end of the wire up and down the ridged surface of the file. If you do this in the dark, you’ll see tiny sparks from the 9-volt battery as the wire tip touches and separates from the file’s ridges. Hold the file near an AM radio, and you’ll hear plenty of static noise.

In the early days of radio, transmitters were known as spark coils, generating a steady stream of sparks at much higher voltages (around 20,000 volts). These high voltages produced large, visible sparks like those in a spark plug, and allowed signals to travel farther. Though such transmitters are now illegal due to their interference across the radio spectrum, they were commonly used in the early days when radio waves were not yet heavily regulated.

Fundamentals of Radio: The Components

As discussed earlier, transmitting with static is remarkably simple. However, modern radios utilize continuous sine waves to transmit information. Early radio transmitters emitted a wide frequency band all at once, and could only generate basic sounds for Morse code communication. Sine wave transmitters, on the other hand, focus on specific frequencies, enabling the transmission of more complex data such as audio, video, and internet content. The narrow range of frequencies also allows multiple transmitters to function simultaneously without causing interference.

The reason we use continuous sine waves today is due to the large number of people and devices vying for radio waves at the same time. If you could somehow see them, you'd notice thousands of different radio waves (in the form of sine waves) all around you right now — TV broadcasts, AM/FM radio stations, police and fire radios, satellite TV signals, cell phone calls, GPS transmissions, and more. It's remarkable how many uses there are for radio waves today. Each unique radio signal occupies a specific sine wave frequency, which is how they are kept separate.

Every radio system consists of two main components:

The transmitter captures a message, which could be anything from someone's voice, images for a TV set, or data for a radio modem, encodes it onto a sine wave, and sends it out through radio waves. The receiver then picks up these radio waves and decodes the information from the sine wave. Both the transmitter and the receiver use antennas to send and receive the radio signals.

Basic Transmitters

To understand how a radio transmitter works, begin with a battery and a piece of wire. When you connect the wire between the two terminals of the battery, it allows electricity (a flow of electrons) to travel through the wire. As the electrons move, they generate a magnetic field around the wire, which is strong enough to influence a compass.

Now, imagine placing another wire parallel to the first one, about 2 inches (5 centimeters) away from it. If you connect a sensitive voltmeter to the second wire, you'll notice that every time the first wire is connected or disconnected from the battery, a small voltage and current will be detected in the second wire. This occurs because any changing magnetic field can induce an electric field in a conductor — this is the fundamental principle of an electrical generator. So here's what happens:

One crucial point to observe is that electrons in the second wire only flow when the battery is either connected or disconnected. A static magnetic field does not cause electron flow unless it is changing. Connecting the battery generates the magnetic field, and disconnecting the battery collapses it. Hence, the electrons only flow in the second wire at the two moments when the magnetic field changes.

Building Your Own Basic Transmitter

To make a simple radio transmitter, the goal is to create a rapidly fluctuating electric current in a wire. This can be achieved by quickly connecting and disconnecting a battery, as demonstrated on the left:

A more effective approach is to generate a continuously varying electric current in the wire. The simplest and most fluid form of such a current is a sine wave, as illustrated below:

By generating a sine wave and passing it through a wire, you can create a basic radio transmitter. Creating a sine wave is remarkably simple with just a few electronic parts — a capacitor and an inductor can generate the sine wave, while a couple of transistors can amplify it into a powerful and simple transmitter schematic. Sending the signal to an antenna will broadcast the sine wave into space.

Sending Signals

With a sine wave and a transmitter broadcasting the sine wave through an antenna, you essentially have a radio station. However, the sine wave itself lacks any useful content. To carry information, you need to modulate the wave in some manner. There are four primary techniques for modulating a sine wave:

Pulse Width Modulation (PWM)

With PWM, you essentially switch the sine wave on and off. This simple approach is perfect for transmitting Morse code. Although PWM isn't very common, one notable application is in the radio system that transmits signals to radio-controlled clocks across the United States. In fact, a single PWM transmitter can cover the entire country!

Amplitude Modulation (AM)

AM radio stations use amplitude modulation to encode information. In this method, the amplitude of the sine wave (its peak-to-peak voltage) is altered. For instance, the sine wave of a person’s voice is superimposed onto the transmitter’s sine wave to modify its amplitude.

Frequency Modulation (FM)

FM radio stations, along with many other wireless technologies, utilize frequency modulation. The key benefit of FM is its resistance to static. In FM, the frequency of the transmitter’s sine wave shifts ever so slightly in response to the information signal. FM signals operate at higher frequencies than AM, which results in better sound quality, though with a shorter range.

Digital Modulation

Digital modulation encodes digital data onto an analog signal, offering superior fidelity and minimal static interference. Technologies like wireless routers employ digital modulation, which also has the advantage of enabling encryption. This ensures the transmitter communicates only with designated devices.

However, if a digital signal is too weak, it becomes unusable very quickly. Audio will become garbled, and video will appear pixelated. In the U.S., over-the-air television has fully transitioned to digital transmission, and many radio stations now use digital antennas alongside their traditional analog signals.

Once you encode information onto a sine wave, you can transmit that information.

Receiving an AM Signal

Let’s consider a practical example. When you tune your car’s AM radio to a station — say, 680 on the AM dial — the transmitter is emitting a sine wave at 680,000 hertz (the wave completes 680,000 cycles per second). The DJ’s voice is encoded onto this carrier wave by varying the amplitude of the sine wave. The signal is then amplified, often up to 50,000 watts for large AM stations, and transmitted via the antenna, sending the radio waves into the air.

How does your car's AM radio, a receiver, capture the 680,000-hertz signal transmitted and extract the information, such as the DJ's voice, from it? Here's how the process works.

What comes out of the speakers is the DJ's voice!

FM radios use a different detector, but the rest of the system is nearly identical. In FM, the detector translates frequency changes into sound, while the antenna, tuner, and amplifier operate much the same way as in AM radios.

The Simplest AM Receiver

For a strong AM signal, you can actually build a basic radio receiver with just two components and some wire. The procedure is very straightforward — here’s what you’ll need to get started:

To simplify things, there are kits available online that contain most of the required components. The Home Science Tools crystal radio kit also includes an electric amplifier, eliminating the need for an earphone.

Now, you’ll need to be close to an AM radio station's transmitting tower (roughly within 1 mile or 1.6 kilometers) for it to function. Here’s what you need to do next:

Once you place the earplug in your ear, you'll hear the radio station — that’s the most basic form of a radio receiver! Although this simple setup won’t work if you’re far from the station, it effectively illustrates how a radio receiver can be incredibly simple.

Here’s how it works: Your wire antenna picks up a variety of radio signals, but because you’re so close to the transmitter, the nearby signal dominates everything else by millions of times. With your proximity to the transmitter, the antenna also gathers substantial energy — enough to power an earphone! This eliminates the need for a tuner or batteries. The diode serves as the detector for the AM signal, as explained earlier. So, you can still hear the station without a tuner or amplifier. However, adding an amplifier, like the one in the educational kit, enhances the signal and increases its volume.

Understanding Antennas

You've probably noticed that nearly every radio device you encounter (like your phone, car radio, etc.) is equipped with an antenna. Antennas vary in form and size, depending on the frequency they are designed to capture. They can range from a long, rigid wire (such as the AM/FM antennas on cars) to something as unusual as a satellite dish. Radio transmitters also rely on towering antenna structures to send out their signals.

The function of an antenna in a radio transmitter is to send radio waves into the atmosphere. In a receiver, the goal is to capture as much of the transmitter’s energy as possible and direct it into the tuner. For satellites situated millions of miles away, NASA employs massive dish antennas that can be as large as 230 feet (70 meters) in diameter.

The size of a perfect radio antenna is connected to the frequency of the signal it’s meant to transmit or receive. This connection stems from the speed of light and the distance that electrons can travel. The speed of light is 186,000 miles per second (300,000 kilometers per second). So, how do you determine the ideal antenna size?

Antenna: Real-life Examples

Imagine you're tasked with constructing a radio tower for station 680 AM. This station broadcasts a sine wave at a frequency of 680,000 hertz. During each cycle of this sine wave, the transmitter will push electrons in one direction, reverse the flow, push them outward again, and reverse the flow once more. Essentially, the electrons will change direction four times per cycle. At 680,000 hertz, one complete cycle occurs in 0.00000147 seconds. A 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. Therefore, the ideal antenna size for a 680,000 hertz transmitter is roughly 361 feet (110 meters). This explains why AM radio stations require very tall towers. In contrast, a cell phone working at 900,000,000 (900 MHz) would need an antenna just 3 inches (8.3 centimeters) long, which is why cell phone antennas are so short.

You may have noticed that the AM radio antenna in your car isn’t 300 feet (91 meters) long but only a few feet. A longer antenna would improve reception, but since AM stations are strong in cities, the antenna length doesn’t significantly impact performance.

You might wonder why, when a radio transmitter sends a signal, radio waves travel outward through space at the speed of light. Why don’t they just stay near the antenna, like the magnetic field around a wire connected to a battery? A simple way to visualize this is: when current flows into the antenna, it does generate a magnetic field around it.

We've also observed that the magnetic field creates an electric field (voltage and current) in any wire placed near the transmitter. In space, the magnetic field produced by the antenna induces an electric field, which in turn creates another magnetic field, which induces yet another electric field, and this process continues. These electric and magnetic fields (electromagnetic fields) continuously induce each other at the speed of light as they spread outward from the antenna.

Analog vs. Digital Radio

While analog radio is still common, digital signals like Wi-Fi and Bluetooth have largely taken over. In 2009, the U.S. mandated that most over-the-air analog TV stations must transition to digital transmitters. Many radio stations have adopted HD Radio, a digital format. However, FM signals continue to be the most widely used, largely because many older vehicles still rely on AM/FM radios.

The key benefits of digital radio transmission are higher fidelity and security. Digital signals can carry much larger amounts of data, enabling services such as high-definition video or wireless internet. Additionally, digital radio does not suffer from the static and interference that often affects analog signals. However, the transmission process itself can be more intricate.

Since radio waves are traditionally analog, the transmitter must use a digital to analog converter to encode the data. When the signal reaches the receiving antenna, an analog to digital converter is used to 'decode' the data back to its original format. This process may seem complicated, but it enables advanced features like data encryption.

In essence, the receiving antenna must have the proper decryption instructions to access the digital data transmitted by the sender. Without these instructions, the data remains inaccessible. This is why devices typically need to pair with one another to access Wi-Fi or Bluetooth networks. This encryption also minimizes radio interference. On the other hand, anyone with a working antenna can easily access analog radio signals.