What are Radio Waves?
Radio waves are electromagnetic waves in the radio frequency band that travel in free space (including air and vacuum). The shorter the wavelength and the higher the frequency of radio waves, the more information is transmitted in the same time.
- A type of electromagnetic wave. Electromagnetic waves with a frequency of about 10 KHz to 30,000 KHz, or a wavelength of 30,000 m to 10 m, are called radio waves because they are generated by the alternating current of an oscillating circuit and can be transmitted and absorbed through an antenna. [1]
- How radio waves travel
- For free space, since there is no obstruction in free space, the radio wave propagates only directly, and there are no other phenomena.
- For the actual propagation environment in daily life, due to the presence of various objects on the ground, the propagation of radio waves has direct, reflected, diffracted (diffraction), etc. In addition, for users indoors or in trains, there are some signal sources Due to the penetration of radio waves into the building. All these have caused the diversity and complexity of radio wave propagation, and made it difficult to study radio wave propagation.
- Direct shot
- Direct radiation in the line of sight can be seen as radio waves traveling in free space. The formula for direct wave propagation loss is the same as the path loss formula in free space: PL = 32.44 + 20lgf + 20lgd. Among them, PL is the path loss in free space, and the unit is dB. F is the frequency of the carrier, and the unit is MHz. d is the distance between the transmitting source and the receiving point, and the unit is km.
- Reflection, refraction and penetration
- When an obstacle is encountered during the propagation of electromagnetic waves, when the size of this obstacle is much larger than the wavelength of the electromagnetic wave, the electromagnetic wave will emit and refract at the boundary of different media. In addition, the medium properties of the obstacle will also affect the reflection. For good conductors, reflection does not cause attenuation; for insulators, he only reflects a part of the incident energy, and the rest is refracted into the new medium to continue to propagate; for non-ideal mediums, electromagnetic waves penetrate the medium, that is, when the medium penetrates It absorbs the energy of electromagnetic waves and produces through-fading. The penetration loss is not only related to the frequency of the electromagnetic wave, but also to the material and size of the penetrating object.
- Generally, the radio wave signal in a room is a superposition of a penetration component and a diffraction component, and the diffraction component accounts for most of it. Therefore, in general, the difference between indoor and outdoor levels of high-frequency signals (for example, 1800MHz) is greater than the difference between indoor and outdoor levels of low-frequency signals (800MHz). In addition, after the low-frequency signal enters the room, the field strength distribution is more uniform after various reflections in the room due to the poor penetration ability. However, after the high-frequency signal enters the room, part of the penetration penetrates again and the indoor signal distribution does not Too uniform, it makes users feel the signal fluctuates greatly.
- Diffraction (diffraction)
- When an obstacle is encountered during the propagation of the electromagnetic wave, when the size of the obstacle is close to the wavelength of the electromagnetic wave, the electromagnetic wave can diffract from the edge of the object. Diffraction can help cover shadow areas.
- scattering
- When an obstacle is encountered during the propagation of electromagnetic waves, the size of this obstacle is smaller than the wavelength of the electromagnetic wave, and when the number of such obstacles per unit volume is very large, scattering occurs. Scattering occurs on the surface of rough objects, small objects, or other irregular objects, such as leaves, street signs, and lampposts.
- Radio wave propagation at different distances
- Line-of-sight
- The general form of radio wave line-of-sight propagation is mainly the superposition of direct waves and ground reflected waves. As a result, the signal may be strengthened or weakened.
- Because the earth is spherical and affected by the radius of curvature of the earth, there is a limit distance Rmax for line-of-sight propagation, which is affected by the height of the transmitting antenna, the height of the receiving antenna and the radius of the earth.
- Non-line-of-sight
- The general forms of radio wave non-line-of-sight propagation are: diffracted waves, tropospheric reflected waves and ionospheric reflected waves.
- diffraction wave
- Diffraction waves are the main source of signals inside buildings or in shadowed areas. The intensity of the diffraction wave is greatly affected by the propagation environment, and the higher the frequency, the weaker the diffraction signal.
- Tropospheric reflected wave
- Tropospheric reflections are generated in the troposphere. The troposphere is a heterogeneous medium that changes over time due to weather conditions. Its reflection coefficient decreases with height. This slowly changing reflection coefficient bends the radio wave. Tropospheric reflection is used in wireless communications with a wavelength of less than 10 meters (that is, a frequency greater than 30 MHz). Tropospheric reflections are extremely random.
- Ionospheric reflected wave
- When the wave length is greater than 1 meter (ie, the frequency is less than 300 MHz), the ionosphere is a reflector. Radio waves reflected from the ionosphere may have one or more hops, so this propagation is used for long-distance communication. Like the troposphere, the ionosphere also has the characteristics of continuous fluctuations.
- There are mainly three propagation modes of radio waves from the transmitting site to the receiving site: sky wave, ground wave, and space linear wave. The characteristics of each wave are as follows:
- Ground wave: An electric wave that travels along the surface of the earth, called a ground wave. During the propagation process, because the radio waves are absorbed by the ground, its propagation distance is not far. The higher the frequency, the greater the ground absorption. Therefore, when short and ultrashort waves propagate along the ground, the distance is relatively short, generally not more than 100 kilometers, while the medium wave propagation distance is relatively long. The advantages are less affected by climate, stable signal and high communication reliability. [1]
- Radio waves are shear waves. That is, the direction of the electric field and the magnetic field are perpendicular to the direction of wave propagation. When radio waves travel in space, they must be affected by the atmosphere, especially the ionosphere, which has the most significant effect, making radio waves refract and attenuate. Among them, the larger the wavelength , The greater the refraction and attenuation.
- According to the different propagation characteristics of radio wave wavelengths, different communication services use different bands. For example, long waves are used for navigation and fixed services; short and medium waves are used for mobile services; microwave is used for radio astronomy and space communications. [2]
- Radio wave fading characteristics
- The fading of radio waves in the propagation process is a very important characteristic, which can be described on three scales: large, medium and small.
- Large scale is used to describe the median signal (regional mean). It has the power law propagation characteristic, that is, the power of the median signal is inversely proportional to a certain power that increases the distance length.
- Mesoscale is used to describe slow fading. It is the average power variation overlaid on the median level of large-scale propagation characteristics. When expressed in decibels, this change tends to be normally distributed.
- Small scale is used to describe fast fading. It usually obeys the Rayleigh probability density function, also known as Rayleigh fading.
- Doppler shift
- According to the Doppler effect, due to the relative movement between the transmitting end and the receiving end of the radio wave, there will be a difference between the frequency of the signal received by the receiving end and the frequency of the signal sent by the transmitting end. shift.
- The Doppler shift conforms to the following formula:
- v is the maximum speed of the mobile terminal
- Time dispersion and equalization
- Temporal dispersion originates from reflections, whose reflections come from objects that are several kilometers away from the receiving antenna. For example, a sequence of "1" and "0" is continuously transmitted by the base station. If the time when the distant reflected signal arrives at the mobile terminal is exactly one bit behind the direct signal, the receiving terminal will detect "0" from the direct signal and also A "1" is detected in the reflected signal, which causes inter-symbol interference, which is called time dispersion. The use of adaptive equalization technology can reduce the effect of time dispersion.
- Radio was first used in navigation, using Morse telegrams to transfer information between ships and land. Nowadays, radio has many application forms, including wireless data networks, various mobile communications, and radio broadcasts.
- Here are some of the main applications of radio technology:
Radio wave nautical
- The earliest form of sound broadcasting was a marine radio. It uses a switch to control the transmission of continuous waves or not, thereby generating a discontinuous sound signal at the receiver, that is, a Morse code.
- * AM broadcasting can spread 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 such as lightning or other sources of interference.
- Radio wave propagation characteristics
- * The sideband of FM broadcast can be used to spread digital signals such as station identification, program name profile, website address, stock market information, etc. In some countries, when moved to a new area, the FM radio can automatically find the original channel based on the sideband information.
- * VHF AM technology is used for voice stations used in marine and aviation. This allows lightweight antennas to be used on aircraft and ships.
- * Government, fire, police, and commercial radio stations typically use narrowband FM technology in dedicated frequency bands. These applications typically use 5KHz bandwidth. Compared to the 16KHz bandwidth of FM radio or TV sound, fidelity has to be sacrificed.
- * Civil or military high-frequency voice services use shortwaves for communications between ships, aircraft or isolated locations. In most cases, single-sideband technology is used, which can save half of the frequency band and use transmit power more efficiently than AM technology.
- * Terrestrial Trunked Radio (TETRA) is a digital trunked telephone system designed for military, police, emergency and other special departments.
- * Amateur radio is a radio station communication involving radio enthusiasts. Amateur radio stations can use many open frequency bands across the entire spectrum. Hobbyists use different forms of coding and techniques. Some later commercially available technologies, such as FM, single-sideband AM, digital packet radio, and satellite signal transponders, were first applied by amateurs.
Radio wave call
- * Cell phone or mobile phone is currently the most commonly used wireless communication method. Cellular phone coverage is usually divided into multiple cells. Each cell is covered by a base station transmitter. In theory, the shape of the cell is a honeycomb hexagon, which is also the source of the name of the cell phone. Currently widely used mobile phone system standards include: GSM, CDMA and TDMA. A few operators have started to provide next-generation 3G mobile communication services, and their leading standards are UMTS and CDMA2000.
- * There are two types of satellite phones: INMARSAT and Iridium system. Both systems provide global coverage services. INMARSAT uses geostationary satellites and requires directional high-gain antennas. Iridium is a low-orbit satellite system that uses a mobile phone antenna directly
Radio wave video
- * Normal analog TV signals use image amplitude modulation, sound frequency modulation and synthesis to spread in the same signal.
- * Digital TV uses MPEG-2 image compression technology, so only about half the bandwidth of analog TV signals is required.
Radio wave emergency services
- Radio wave propagation
Radio wave data transmission
- * Digital microwave transmission equipment, satellites, etc. usually use Quadrature Amplitude Modulation (QAM). QAM modulation uses both the amplitude and phase of the signal to load information. In this way, a larger amount of data can be transferred on the same bandwidth.
- * IEEE 802.11 is the current standard for wireless local area networks. It uses the 2GHz or 5GHz band and the data transmission rate is 11 Mbps or 54 Mbps.
Radio wave navigation
- * Uses active and passive radios to identify and identify objects.
- * All satellite navigation systems use satellites equipped with accurate clocks. Navigation satellites broadcast their position and timing information. The receiver simultaneously receives signals from multiple navigation satellites. The receiver calculates its distance to each satellite by measuring the propagation time of the radio wave, and then calculates its precise position.
- * The Loran system also uses the travel time of radio waves for positioning, but its transmitters are located on land.
- * VOR system is usually used for flight positioning. It uses two transmitters, a directional transmitter that always transmits and rotates at a fixed rate like a lighthouse's spotlight. When a directional transmitter faces north, another omnidirectional transmitter transmits a pulse. The aircraft can receive the signals from the two VOR stations and determine its position by extrapolating the intersection of the two beams.
- * Radio orientation is the earliest form of radio navigation. Radio orientation uses a movable loop antenna to find the direction of the station.
- radar
- * Radar estimates the distance of a target by measuring the delay of reflected radio waves. The surface type of the target is sensed by the polarization and frequency of the reflected wave.
- * The navigation radar uses ultrashort waves to scan the target area. 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 merchant ships and long-range commercial aircraft.
- Multipath transmission effect of radio waves
- * The search radar uses short wave pulses to scan the target area, usually 2 to 4 times per minute. Some search radars use the Doppler effect to distinguish moving objects from the background
- * The search radar uses a similar principle to the search radar, but scans a small area quickly and repeatedly, usually up to several times per second.
- * Weather radars are similar to search radars, but use 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 oven uses high power microwave to heat food. (Note: A common misconception is that the frequency used by microwave ovens is the resonance frequency of water molecules. In fact, the frequency used is about one-tenth the resonance frequency of water molecules.)
- power
- * Radio waves can generate weak electrostatic and magnetic forces. Under microgravity conditions, this can be used to fix the position of the object.
- * Aerospace Dynamics: There are plans to use the pressure generated by high-intensity microwave radiation as the power of the interstellar probe.
- astronomy
- * It is a radio wave signal emitted by a cosmic celestial body received through a radio astronomical telescope, which can study the physical and chemical properties of the celestial body. This discipline is called radio astronomy.