Radio wave

Radio waves are a fundamental form of electromagnetic radiation. They occupy the portion of the electromagnetic spectrum with the lowest frequencies and longest wavelengths. Typically, these waves have frequencies below 300 gigahertz (GHz). Their wavelengths are often quite large, sometimes greater than 1 meter. When the frequency rises above 1 GHz, the waves are classified as microwaves.

Because they can pass through many materials, they are essential for modern communication. They allow us to send information across the planet and into deep space.

Radio waves are generated when charged particles undergo acceleration. In artificial systems, this happens through time-varying electric currents. A device called a transmitter applies an oscillating electric current to an antenna. This antenna is a specially shaped metal conductor. As the electrons flow back and forth in the antenna, they radiate energy into space as radio waves.

To catch these waves, a second antenna is connected to a radio receiver. When the waves strike the receiving antenna, they push the electrons in the metal. This creates tiny oscillating currents that the receiver can process into signals.

Radio waves can be categorized by how they travel through the atmosphere. Long waves can undergo diffraction to bend around obstacles like mountains. These are known as ground waves because they follow the contour of the Earth. Shorter waves can reflect off the ionosphere, which is a layer of the atmosphere. These reflected waves are called skywaves and can reach beyond the visual horizon. Much shorter wavelengths travel in a straight line. This is called line-of-sight propagation, and it is limited by the visual horizon.

This method is used for cell phones, FM radio, and television.

Scientific understanding of these waves grew through several major discoveries. In 1867, the Scottish physicist James Clerk Maxwell proposed a mathematical theory of electromagnetism. His equations predicted that coupled electric and magnetic fields could travel as electromagnetic waves. In 1887, German physicist Heinrich Hertz experimentally proved Maxwell was correct. Hertz showed that these waves shared properties with light, such as refraction and diffraction. Around 1894, Guglielmo Marconi developed the first practical transmitters and receivers. Marconi later won the 1909 Nobel Prize in Physics for his contributions. The term "radio wave" replaced the older name "Hertzian wave" around 1912.

Radio waves possess specific physical properties, including polarization. Polarization refers to the direction of the wave's oscillating electric field. A wave can be plane-polarized, meaning the field oscillates in a specific vertical or horizontal plane. It can also be circularly polarized, where the field rotates as it travels.

The polarization of an antenna determines the polarization of the waves it emits. For example, a vertical dipole antenna radiates vertically polarized waves. For efficient communication, a receiving antenna must have the same polarization as the transmitter. Many natural sources, like the sun, emit unpolarized waves instead.

In the natural world, radio waves are everywhere. They are produced by lightning during atmospheric storms. Astronomical objects, such as galaxies and nebulas, also emit radio waves. All warm objects emit radio waves as part of their blackbody radiation. From a quantum perspective, radio waves can be seen as streams of particles called photons. These radio photons carry extremely small amounts of energy. For a single radio photon, the energy ranges from $10^{-22}$ to $10^{-30}$ joules. Because each photon has so little energy, antennas emit an enormous number of them every second.

Today, the use of radio waves is strictly regulated by international law. The International Telecommunication Union (ITU) coordinates these rules to prevent interference. The radio spectrum is divided into different bands based on frequency. Each band is allocated for specific uses, such as broadcasting or satellite communication. This organization allows for wireless computer networks, radar, and radio navigation. Without this coordination, our various electronic systems would constantly clash. Radio waves remain one of the most important tools in our technological landscape.