Radio wave

Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths. They usually have frequencies below 300 gigahertz and wavelengths longer than 1 millimeter, about the size of a grain of rice. Radio waves travel at the speed of light in a vacuum and a little slower in the Earth's atmosphere.

These waves are created when charged particles speed up or slow down, such as in time-varying electric currents. We can find natural radio waves from lightning and astronomical objects. People also make radio waves using a device called a transmitter connected to an antenna, which sends out the waves. Another antenna connected to a radio receiver can pick up these waves.

Radio waves are very important in our daily lives. They are used for many things like radio communication, broadcasting, radar, radio navigation, communications satellites, and wireless computer networks. Different frequencies of radio waves behave differently in the atmosphere, which helps them travel over long distances or around obstacles.

Discovery and exploitation

Main article: History of radio

Radio waves were first predicted in 1867 by a scientist named James Clerk Maxwell. He used math to show that electric and magnetic fields could travel as waves. Later, in 1887, another scientist named Heinrich Hertz made these waves in his lab and showed they acted like light. Around 1894–1895, an inventor named Guglielmo Marconi built the first practical radio machines to send and receive these waves. He won a Nobel Prize for this work, and radio became used for business around 1900. The name "radio wave" replaced the older name "Hertzian wave" around 1912.

Generation and reception

Radio waves are made when charged particles move fast. Natural sources of radio waves include radio noise from lightning and other actions in Earth's atmosphere. We can also get signals from space objects like the Sun, galaxies, and nebulas. Warm objects also give off high-frequency radio waves.

Radio waves can be created using electric currents—the flow of electrons—in a special metal part called an antenna. A device called a radio transmitter sends these electric currents through the antenna, which then sends out radio waves. These waves can be caught by another antenna connected to a radio receiver. When the radio waves reach the receiving antenna, they move the electrons back and forth, making small electric currents that the receiver can detect.

Properties

Radio waves travel at the speed of light in a vacuum, which is empty space. When they go through things like air, they slow down a bit. The distance from one high point of a wave to the next is called its wavelength. This distance changes based on how often the wave repeats, known as its frequency.

Radio waves also have a property called polarization. This is the direction the wave moves up and down. If it moves sideways, it is horizontal polarization. If it moves toward the sky or ground, it is vertical polarization. Some waves circle as they travel, called circular polarization. A radio needs to match the polarization of the wave to get a good signal.

Propagation characteristics

Main article: Radio propagation

Radio waves help us talk to each other because they can go through weather, leaves, and buildings. Longer radio waves can bend around things like walls or hills, which is helpful for many kinds of communication.

When radio waves move, they can bounce off things, change direction, or get weaker. Different frequencies are better for different uses. For example, cell phones and television use radio waves that go in straight lines. Other types of radio waves can bend around the Earth or bounce off the sky to reach faraway places. Higher frequency waves can get weaker over long distances because air can block them.

Radio communication

Main article: Radio

In radio communication, we use radio waves to send information through the air. A radio transmitter takes a signal—like sound from a microphone, pictures from a video camera, or data from a computer—and changes it into radio waves. These waves travel through the air to a receiver.

The receiver catches these waves and changes them back into the original signal. It uses special parts to pick out one station’s signal and makes it louder and clearer. The signal can then make sounds, pictures, or data that we can see or hear. Each station uses a different frequency so they don’t mix up.

Biological and environmental effects

Further information: Medical applications of radio frequency

Radio waves are a type of non-ionizing radiation. This means they do not have enough energy to change atoms or molecules or damage DNA. When radio waves are absorbed by materials, they mainly cause heating. This is like the warmth from a space heater or a wood fire. The electric field of the wave makes certain molecules vibrate, which raises the temperature. This is how microwave ovens cook food. It has also been used for over 100 years in medical treatments to heat deep body tissues and help them heal.

Unlike some other types of waves, radio waves can go inside materials and biological tissues. How far they go depends on their frequency and the material's properties. Looking directly at a strong source of radio waves, such as near a working radio transmitter, can sometimes harm the eye by causing too much heat buildup. Radio waves can be blocked by metal screens or enclosures, known as a Faraday cage, if the holes in the screen are very small compared to the wavelength of the waves.

Measurement

Radio waves have electric and magnetic parts. We measure them with special units. We use volts per meter (V/m) for the electric part and amperes per meter (A/m) for the magnetic part. These units help us know how strong the radio wave is.

We also use a unit called power density. This tells us how much power is in a certain area, like milliwatts per square centimeter (mW/cm²). We use this when we are far from the source.

electromagnetic field, field strength, far field