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Radio is a technology that uses radio waves for signaling and communication. Radio waves are electromagnetic waves with frequencies between 30 Hz and 300 GHz. They are generated by an electronic device called a transmitter that is connected to the antenna of the radiated wave and received by a radio receiver connected to another antenna. Radio is widely used in modern technology in radio communication, radar, radio navigation, remote control, remote sensing and other applications. In radio communication, for radio and television broadcasting, mobile phones, two-way radio, wireless networks and satellite communications and many other purposes, by modulating radio signals, radio waves are used to carry information throughout the space from the transmitter to the receiver. (information signals are applied to the radio waves by changing certain aspects of the radio waves). In the radar used to locate and track aircraft, ships, spaceships and missiles, a beam of radio wave emitted by the radar transmitter reflects away from the target object, and the reflected wave reveals the location of the object. In a radio navigation system such as GPS and VOR, a mobile receiver receives a radio signal from a navigation radio beacon whose position is known, and by accurately measuring the arrival time of the radio wave, the receiver can calculate its position on the earth. In the wireless radio remote control equipment such as UAV, garage door opener and keyless entry system, the radio signal sent from the controller equipment controls the action of the remote equipment.

Applications of radio waves that do not involve long-distance transmission, such as RF heating used in industrial processes and microwave ovens, as well as medical uses such as diathermy and MRI machines, are generally not called radios. The noun radio is also used to indicate a radio receiver for broadcasting.

Radio waves were first discovered and studied by German physicist Heinrich Hertz in 1886. The first practical radio transmitters and receivers were developed by Italian Guglielmo Marconi in 1895-6, and radio began commercial use around 1900. The transmission of radio waves is strictly regulated by law and coordinated by an international body called the International Telecommunication Union (ITU), which distributes the frequency bands in the radio spectrum to different uses.

Accelerated charges radiate radio waves. They are artificially generated by time-varying current, which consists of electrons flowing back and forth in a metal conductor called an antenna, thereby accelerating the current. During transmission, the transmitter generates RF AC applied to the antenna. The antenna radiates the power in the current in the form of radio waves. When waves hit the antenna of a radio receiver, they push electrons back and forth in the metal, creating tiny alternating currents. The radio receiver connected to the receiving antenna detects the oscillation current and amplifies it.

When they travel far away from the transmitting antenna, the radio waves will spread, so their signal strength (watt strength per square meter) will be reduced, so the radio transmission can only be received within the limited range of the transmitter, and the distance depends on the transmitter power, antenna radiation pattern, receiver sensitivity, noise level and whether there are obstacles between the transmitter and the receiver. The omnidirectional antenna transmits or receives radio waves in all directions, while the directional antenna or high gain antenna transmits radio waves in the form of beams in a specific direction, or receives waves from only one direction.

Radio waves travel in vacuum at the speed of light, and in air at a speed very close to the speed of light. Therefore, the wavelength of radio waves (that is, the distance between adjacent wave peaks, in meters) is inversely proportional to the frequency.

Other types of electromagnetic waves besides radio waves; infrared, visible, ultraviolet, X-ray and gamma ray can also transmit information and be used for communication. The wide use of radio waves in the field of telecommunications is mainly due to the ideal propagation characteristics brought about by its large wavelength. Radio waves have the ability to travel through the atmosphere, leaves and most building materials, and can bend around obstacles by diffraction, and unlike other electromagnetic waves, they tend to be scattered rather than absorbed by objects larger than their wavelength.

The modulated radio wave carrying the information signal occupies a certain frequency range. See Fig. The information (modulation) in the radio signal is usually concentrated in the narrow band (called sideband (sb)) above and below the carrier frequency. The highest frequency (in Hertz) of the frequency range occupied by a radio signal, that is, the highest frequency minus the lowest frequency, is called its bandwidth (BW). For any given signal-to-noise ratio, no matter where the bandwidth is located in the RF spectrum, a certain amount of bandwidth can carry the same amount of information (data rate in bits per second), so the bandwidth is the measurement capacity of information carrying. The bandwidth required for radio transmission depends on the data rate of the transmitted information (modulated signal) and the spectral efficiency of the modulation method used. How much data can be transmitted per kilohertz bandwidth. Different types of information signals transmitted by radio have different data rates. For example, a television (video) signal has a higher data rate than an audio signal.

Radio spectrum is the total range of radio frequencies that can be used for a given area communication, and it is a limited resource. Each radio transmission occupies a portion of the total available bandwidth. Radio bandwidth is considered to be an economic commodity, which has money cost and increasing demand. In some parts of the radio spectrum, the right to use the frequency band or even a single radio channel is bought and sold for millions of dollars. Therefore, there is an incentive to adopt technology to minimize the bandwidth used by radio services.

In recent years, it has been transited from analog to digital radio transmission technology. Part of the reason for this is that by using the data compression algorithm, digital modulation can usually transmit more information (greater data rate) than analog modulation in a given bandwidth, which can reduce the redundancy of the data to be sent and improve the modulation efficiency. The other reason for the transition is that digital modulation has higher anti-interference than analog, digital signal processing chip has higher power and flexibility than analog circuit, and can use the same digital modulation to transmit various types of information.

Because it is a kind of fixed resource, more and more users need it, so the radio spectrum becomes more and more crowded in recent decades, and the need to use it more effectively is promoting many other radio innovations, such as cluster radio system, spread spectrum (ultra wideband) transmission, frequency multiplexing, dynamic spectrum management, frequency pool and cognitive radio.

Broadcasting is a one-way information transmission from a radio transmitter to a receiver belonging to the public audience. As the radio wave weakens with distance, it can only receive the radio station within a limited distance of its transmitter. A system broadcast from a satellite can usually be received across the country or continent. Older terrestrial radio and television are paid for by commercials or the government. In subscription systems such as satellite TV and satellite radio, customers pay monthly fees. In these systems, the radio signal is encrypted and can only be decrypted by the receiver, which is controlled by the company. If the customer does not pay the bill, it can be deactivated.

Broadcast uses several parts of the radio spectrum, depending on the type of signal being transmitted and the desired target audience. Long wave and medium wave signals can reliably cover hundreds of kilometers, but the information carrying capacity is more limited, so they are most suitable for use with audio signals (voice and music), and natural and artificial radio noise will reduce the sound quality of data sources. The short band has a larger potential range, but is more susceptible to interference from distant radio stations and atmospheric conditions that affect changes in reception.

In the very high frequency band greater than 30 MHz, the influence of the earth's atmosphere on the signal range is small, and the line of sight propagation becomes the main mode. These higher frequencies allow television broadcasting to require a lot of bandwidth. Because there are few natural and artificial noise sources at these frequencies, high quality audio transmission can be achieved by using frequency modulation.