Radio waves
Radio waves refer to electromagnetic waves in the radio frequency band that propagate in free space (including air and vacuum). Radio technology is the technology of transmitting sound or other signals through radio waves.
The principle of radio technology is that changes in the strength of the current in a conductor produce radio waves. This phenomenon is used to load information onto radio waves through modulation. When the radio wave propagates through space and reaches the receiving end, the change in the electromagnetic field caused by the radio wave will generate a current in the conductor. By extracting information from current changes through demodulation, the purpose of information transmission is achieved.
Discovery
Maxwell first clarified the theoretical basis of electromagnetic wave propagation in his paper "The Dynamic Theory of Electromagnetic Fields" submitted to the Royal Society. This work was completed between 1861 and 1865.
Heinrich Rudolf Hertz first verified Maxwell's theory through experiments between 1886 and 1888. He demonstrated that radio radiation has all the properties of a wave and discovered that the equations of the electromagnetic field can be expressed in terms of partial differential equations, often called wave equations.
On Christmas Eve in 1906, Reginald Fessenden used the heterodyne method to achieve the first radio broadcast in history in Massachusetts, USA. Fessenden broadcast himself playing "Silent Night" on violin and reciting excerpts from the Bible. The Marconi Research Center in Chelmsford, England, launched the world's first regularly broadcast radio entertainment program in 1922!
Invention
There is some controversy as to who invented radio.
In 1893, Nikola Tesla demonstrated the first public demonstration of wireless communications in St. Louis, Missouri, USA. In lectures to the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated the basic principles of radio communications. The instrument he built contained all the basic elements of radio systems before the invention of the vacuum tube.
Guglielmo Marconi holds what is generally regarded as the world's first patent for radio technology, British Patent No. 12039, "Improvements in Electrical Pulse and Signal Transmission Technology and Required Equipment."
Nikola Tesla patented radio technology in the United States in 1897. However, the U.S. Patent Office revoked his patent in 1904 and instead granted Marconi a patent for his invention of radio. This move may have been the result of the influence of Marconi's financial backers in the United States, including Edison and Andrew Carnegie. In 1909, Marconi and Karl Ferdinand Braun received the Nobel Prize in Physics "for their contribution to the invention of wireless telegraphy".
In 1943, shortly after Tesla’s death, the U.S. Supreme Court reaffirmed that Tesla’s patents were valid. This decision recognized that his invention preceded Marconi's patent. Some believe the decision was made for obvious financial reasons. This way the U.S. government during World War II could avoid paying patent royalties to Marconi.
In 1898, Marconi opened the world's first radio factory in Hall Street, Chelmsford, England, employing about 50 people.
Purpose of Radio
The earliest application of radio was in navigation, using Morse telegraph to transmit information between ships and land. Today, radio has many applications, including wireless data networks, various mobile communications, and radio broadcasts.
Here are some of the major applications of radio technology:
Communications
Sound
* The earliest form of sound broadcasting was marine radiotelegraphy. It uses a switch to control whether to transmit continuous waves, thereby producing intermittent sound signals at the receiver, that is, Morse code.
* AM radio broadcasts music and sound. AM broadcasting uses amplitude modulation technology, that is, the greater the volume received at the microphone, the greater the energy emitted by the station. Such signals are susceptible to interference from sources such as lightning or other interference sources.
* FM radio can transmit music and sounds with higher fidelity than AM radio. For frequency modulation, the greater the volume received at the microphone, the higher the frequency of the transmitted signal. FM radio operates in the very high frequency band (Very High Frequency, VHF). The higher the frequency band, the larger the frequency bandwidth it has, so it can accommodate more radio stations. At the same time, the propagation of radio waves with shorter wavelengths is closer to the linear propagation characteristics of light waves.
* The sidebands of FM broadcasts can be used to spread digital signals such as radio station logos, program name introductions, website addresses, stock market information, etc. In some countries, when moved to a new area, FM radios can automatically search for the original channel based on sideband information.
* Voice radio stations used in navigation and aviation apply VHF amplitude modulation technology. This enables the use of lightweight antennas on aircraft and ships.
* Radio stations used by government, fire, police and commercial companies often use narrowband FM technology on dedicated frequency bands. These applications typically use 5KHz bandwidth. Compared with the 16KHz bandwidth of FM radio or TV sound, fidelity has to be sacrificed.
* Civilian or military high frequency voice services use short waves for communication between ships, aircraft or isolated locations. In most cases, single sideband technology is used, which saves half the frequency band compared to amplitude modulation technology and uses transmit power more efficiently.
* Terrestial Trunked Radio (TETRA) is a digital trunked telephone system designed for special departments such as the military, police, and emergency services.
Telephone
* Cellular or mobile phones are currently the most common form of wireless communication. Cellular phone coverage areas are typically divided into multiple cells. Each cell is covered by a base station transmitter. Theoretically, the cells are shaped like a hexagonal honeycomb, which is where the cell phone gets its name. Currently widely used mobile phone system standards include: GSM, CDMA and TDMA. A few operators have begun to provide next-generation 3G mobile communication services, and their leading standards are UMTS and CDMA2000.
* Satellite phones exist in two forms: INMARSAT and Iridium systems. Both systems provide global coverage. INMARSAT uses geostationary satellites and requires directional, high-gain antennas. Iridium is a low-orbit satellite system that directly uses mobile phone antennas
TV
* Usually analog TV signals use image amplitude modulation, accompanying audio frequency and synthesis to spread in the same signal.
* Digital TV uses MPEG-2 image compression technology, which requires only about half the bandwidth of analog TV signals.
Emergency services
* Emergency position indicating radio beacons (EPIRBs), emergency positioning transmitters or personal positioning beacons are used to locate personnel in emergencies. Or small radio transmitters that measure positioning via satellites. Its function is to provide rescuers with the precise location of the target in order to provide timely rescue.
Data transmission
* Digital microwave transmission equipment, satellites, etc. usually use Quadrature Amplitude Modulation (QAM). The QAM modulation method uses both the amplitude and phase of the signal to load information. In this way, a larger amount of data can be transmitted on the same bandwidth.
* IEEE 802.11 is the current standard for wireless LAN. It uses the 2GHz or 5GHz frequency band and has a data transfer rate of 11 Mbps or 54 Mbps.
Identification
* Active and passive radio devices can be used to identify and indicate the identity of objects.
Others
* Amateur radio is a radio station communication involving radio amateurs. Amateur radio stations can use many open frequency bands throughout the spectrum. Hobbyists use different forms of coding and techniques. Some of the technologies that became commercially available, such as frequency modulation, single sideband amplitude modulation, digital packet radio and satellite signal transponders, were first implemented by amateurs.
Navigation
* All satellite navigation systems use satellites equipped with precise clocks. Navigation satellites broadcast their position and timing information. The receiver receives signals from multiple navigation satellites at the same time. The receiver measures the propagation time of the radio wave to determine its distance to each satellite, and then calculates its precise position.
* The Loran system also uses the propagation time of radio waves for positioning, but its transmitting stations are all located on land.
* VOR systems are commonly used for flight positioning. It uses two transmitters, one directional transmitter that always emits and rotates at a fixed rate like a lighthouse spotlight. When the directional transmitter faces north, another omnidirectional transmitter emits a pulse. The aircraft can receive signals from two VOR stations and determine its position by calculating the intersection point of the two beams.
* Radio directionality is the earliest form of radio navigation. Directional radio uses a movable loop antenna to find the direction of a station.
Radar
* Radar estimates the distance of a target by measuring the delay of reflected radio waves. and senses the surface type of the target through the polarization and frequency of the reflected wave.
* Navigation radar uses ultra-short waves to scan target areas. The general scanning frequency is two to four times per minute, and the terrain is determined by reflected waves. This technology is commonly used on commercial ships and long-distance commercial aircraft.
* Multi-purpose radars usually use the frequency bands of navigation radars. However, the pulses it emits are modulated and polarized to determine the surface type of the reflector. Excellent multi-purpose radar can identify heavy rain, land, vehicles and more.
* Search radar uses short-wave pulses to scan the target area, usually 2-4 times per minute. Some search radars use the Doppler effect to distinguish moving objects from the background
* Homing radars use a similar principle to search radars, but by quickly and repeatedly scanning a smaller area, they can usually Up to several times per second.
* Weather radar is similar to search radar, but uses circularly polarized waves and wavelengths that are easily reflected by water droplets. Some weather radars also use the Doppler effect to measure wind speed.
Heating
* Microwave ovens use high-power microwaves to heat food. (Note: A common misunderstanding is that the frequency used by a microwave oven is the maximum vibration frequency of water molecules. In fact, the frequency used is about one-tenth of the maximum vibration frequency of water molecules.)
Power
* Radio waves can produce weak electrostatic and magnetic forces. This can be used to fix the position of objects in microgravity conditions.
* Aerospace power: There are plans to use the pressure generated by high-intensity microwave radiation as the power for interstellar probes.
Astronomy
* The radio wave signals emitted by cosmic objects received through radio astronomical telescopes can be used to study the physical and chemical properties of celestial bodies. This subject is called radio astronomy.