What is the principle of radio? How are radio waves divided into different bands?

(The picture shows the shortwave cross-distance phenomenon)

17. 19-meter broadcast band (15100KHz-15600KHz, HF) is an international broadcast band. Although it does not have the propagation characteristics of the 22-meter band, it is also a band that can achieve long-distance propagation. Basically, there are radio stations all day long. The propagation attenuation of this band is slightly larger at night, and there are fewer signals from radio stations late at night. After 21:00 in the evening to 06:00 in the morning, the attenuation of the radio waves is relatively large. Before that, the signal quality is relatively good and there are more radio stations.

18. 16-meter broadcast band (17550KHz -17900 KHz, HF) This is an international broadcast frequency. Some regional radio stations are active here. The signal quality is better during the day than at night. When the transmission is opened, broadcasts from more distant areas can also be heard.

19. 17m amateur band (18068KHz-18168KHz, HF) This is a shortwave band that is usually not noticed by enthusiasts. Because it is sandwiched between the 15m band and the 20m band, it has some of the propagation characteristics of the 15m band. In years with low sunspot activity, the propagation of this band is more frequent and the window of propagation is longer. Because the frequency band is narrow, this band is mostly CW and DATA signals.

20.15 meter broadcast band (18900KHz-19020KHz, HF) This is an international broadcast band, but the number of radio stations in this band is relatively small, and it is also beyond the coverage of some old radios. It is easier to receive long-distance signals in the summer, but the performance is slightly worse in the winter.

21. The 15-meter amateur band (21000KHz-21450KHz) is also a band suitable for short-wave long-distance communication. During the peak period of sunspot activity, relatively reliable long-distance communication can be achieved. The 20m band also has the phenomenon of over-range, and it is more obvious. The over-range distance is usually longer than the 20m band. In summer and winter, communication in the province or neighboring areas will be relatively difficult. At other times, when the 20m band propagation is relatively attenuated, the 15m wave still has a good long-distance communication effect. At the lower end of this band, there are many long-distance CW signals. This band is also one of the key bands for scoring in various radio competitions.

22.13 meter broadcast band (21450 KHz -21850 KHz, HF) This is an international broadcast band that can achieve long-distance communication when ionospheric propagation is open. The signal quality of this band is better during the day than at night, because the signal of this band decays greatly at night, and a few radio stations will use this band. Under normal circumstances, the background noise of this band is less than that of lower frequency bands.

23. 12-meter amateur band (24890 KHz -24990 KHz, HF) This is an amateur frequency band, but there are relatively few amateur radio stations active here. Because usually, the propagation distance of this band is not too far, it is not suitable for DX communication and is only suitable for regional communication. The bandwidth of this band is relatively narrow, and CW and DATA modes are generally the most common. This band receives less attention.

24. 11-meter broadcast band (25600 KHz -26100 KHz, HF) This is a broadcast band within the shortwave band, but almost no radio stations use this band. The transmission distance of this band is limited under normal circumstances and is not suitable for long-distance broadcasting. Generally speaking, the signal quality is better during the day than at night. During the window period of transmission opening, communication coverage at a slightly longer distance can be achieved.

25. 10-meter amateur band (28000KHz-29700KHz, HF) This band belongs to the high end of the shortwave band. Under normal circumstances, regardless of whether the sunspots are active or not, medium and short-distance communication (distance <300km) can be achieved, and the communication quality of this band is relatively high when the transmission is opened. In China, 29.60MHz is a hot frequency for many enthusiasts to conduct NFM communication, and many enthusiasts have been active here. When the transmission is opened, this band can achieve low-power long-distance transmission. The 28.490MHz USB upper sideband is often used internationally for long-distance cross-regional voice communication, and the low end of this band is also often used for long-distance CW communication. On the 10m band, due to the decrease in ionospheric density at night, it can no longer be effectively reflected, and long-distance transmission is generally opened during the day. This band also has the smallest antenna size in shortwave, and its diffraction ability is stronger than the common V/U band. It is more suitable for communication in vehicle-mounted equipment. Normally, 25W transmission power can cover an area with a radius of 20km-30km.

26. 6m amateur band (50.000MHz-54.000MHz, VHF) This band also belongs to the VHF band. The propagation mode is close to direct wave, which can ensure reliable communication within the line of sight. In open areas, it can achieve a coverage range of 10km-15km in diameter. Most shortwave radio stations and a few handheld devices, or car radio equipment have this band. This band is a commonly used medium and short-range communication band. The antenna size can be made much smaller than shortwave, and the vertical GP of 1/4 wavelength does not exceed 2 meters. In some special cases, the 6m band can achieve long-distance communication, such as the atmospheric waveguide phenomenon. The reason is that there are many unevenly cold and hot air masses and circulations in the troposphere, while the higher stratosphere is not affected. Such atmospheric physical characteristics cause the VHF radio waves to change the direction of propagation, so that the electromagnetic waves are transmitted to the distance along the interlayer between the troposphere and the stratosphere. This phenomenon is called the "atmospheric waveguide" phenomenon. Some studies have also shown that in the E layer of the ionosphere, there are areas of uneven electron density, which leads to scattering of electromagnetic waves at the low end of the very high frequency, and can achieve communication as far as 2000km. There have been records of enthusiasts achieving communication with Australia in this band (actual distance > 8200km).

27. FM broadcast band (64.000 MHz -108.000MHz, VHF) This is the most common broadcast band, and a large number of radios cover this band. This band uses different frequencies according to the division of each country and ITU. Russia and some Eastern European countries use 64.000MHz-87.000MHz, Japan uses 76.000MHz-90.000MHz, and my country and most countries use 87.000-108.000MHz. Most broadcasts have a step of 0.100MHz. In addition, in my country, there are also campuses and factories and mines that use the 64.000 MHz -87.000MHz frequency band for broadcasting. In this band, the main radio stations all use FM frequency modulation mode, and some test stations also use digital mode. The main propagation feature of this band is the space wave that is close to a straight line, which can achieve reliable communication and broadcasting within the line of sight range. In open areas without obstacles, with a good antenna, you can also receive radio stations within a range of 200km, which belongs to the FMDX range. In addition, FMDX is affected by the curvature of the earth and it is difficult to exceed a distance of 400km. Many cities use this frequency band for broadcasts of various natures, and it is relatively easy to receive within a radius of 30km-100km. This band has relatively low requirements for equipment and antennas, and it is also a relatively easy band for civilian use. Several kilowatts of power and a not too large antenna are enough to achieve a wider coverage range. Because the working mode of this band is FM, it is easy to achieve high sound quality, and the loss of sound quality during the transmission process is relatively small. It is also the band with the most stereo broadcasts. When propagating in the city, it is also a band that is not easily interfered by various stray scattered waves.

28. Aviation channel (118.000MHz-137.000MHz, VHF), this is a professional frequency band for aviation, generally used for communication between aircraft and the ground. An airport will use multiple frequencies for communication at the same time, such as tower, aircraft, approach, weather, etc. According to the law, civil use is prohibited from transmitting in this frequency band because it is easy to cause serious accidents or false interference. Aviation channels can generally be received within a radius of 60km from the airport. The main operating mode is AM amplitude modulation mode, and many communication equipment and radios have the receiving function of this band.

29. 2-meter band (144.000MHz-148.000MHz, VHF) This is a very high frequency band. The propagation mainly relies on direct waves. It can achieve reliable communication within the line of sight and has a certain diffraction ability. Enthusiasts mainly use this band for local communication. Usually, an automatic differential communication system, that is, a relay station, can be set up to achieve a longer-distance communication coverage area with a larger area. In the case of effective use of relay stations, the effective communication distance can cover an area with a diameter of 25km, so that enthusiasts with handheld or fixed devices in the area can communicate effectively. In many cases, the use of relay equipment can also enable enthusiasts in the area to better coordinate their actions, greatly reducing the communication blind spots of personnel and vehicles. This band can occasionally pass through the E layer of the ionosphere to achieve long-distance transmission, similar to a special channel, with the longest communication record of 6000km-7000km. This band is also an active satellite communication band. Some countries have launched amateur radio communication satellites. By using directional Yagi antennas or satellite antenna systems that automatically servo to capture signals, long-distance communications can be achieved using the equipment and antennas in this band.