Electromagnetic wave propagation in wireless communication

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Electromagnetic Wave Propagation in Wireless CommunicationElectromagnetic waves in wireless communication have different propagation characteristics depending on their wavelength. The following describes the propagation characteristics by wavelength:Extremely Low Frequency (ELF) PropagationELF refers to electromagnetic waves with wavelengths of 10,000 to 100,000 kilometers (frequencies of 3 to 30 Hz). Theoretical studies have shown that the attenuation of electromagnetic waves in this band is minimal when propagating along land surfaces and seawater.Super Low Frequency (SLF) PropagationSLF refers to electromagnetic waves with wavelengths of 1,000 kilometers to 10,000 kilometers (frequencies of 30 to 300 Hz). Electromagnetic waves in this band are very stable and have very little attenuation in seawater (attenuation coefficient of 0.3dB/m at a frequency of 75Hz). They have strong penetration ability to seawater and can reach depths of over 100m.Very Low Frequency (VLF) PropagationVLF refers to electromagnetic waves with wavelengths of 10 kilometers to 100 kilometers (frequencies of 3 to 30 kHz). The frequency of VLF used in wireless communication is 10 to 30 kHz. Electromagnetic waves in this band can propagate in the waveguide formed between the earth and the lower ionosphere, and the distance can reach thousands of kilometers or even cover the globe.Low Frequency (LF) PropagationLF refers to electromagnetic waves with wavelengths of 1 kilometer to 10 kilometers (frequencies of 30 to 300 kHz). It can propagate along the earth's surface (ground wave) and by ionospheric reflection (sky wave). Although sky waves can be reflected by the ionosphere to propagate far away, the ionosphere absorbs them strongly, and the parameters of the lower ionosphere change greatly, so they are extremely unstable and the fading is serious. Ground wave propagation is mainly related to the conductivity of the earth, and the propagation is relatively stable. The propagation distance on land is generally tens to hundreds of kilometers. When sky waves and ground waves coexist, they will interfere with each other and cause fading. When ground waves propagate on the sea surface, the attenuation of electromagnetic waves is small due to the high conductivity of seawater, and the propagation distance is much longer than that on land, reaching hundreds or even thousands of kilometers. The low end (30~60kHz) of this band can penetrate seawater to a certain depth and can be used for submarine communication.Medium Frequency (MF) PropagationMF refers to electromagnetic waves with wavelengths of 100 meters to 1000 meters (frequencies of 300 to 3000 kHz). Medium waves can propagate along the earth's surface (ground wave) and by ionospheric reflection (sky wave). When medium waves propagate along the earth's surface, they are absorbed more severely by the earth's surface than longer waves. The sky wave propagation of medium waves is related to day and night changes. During the day, the D layer of the ionosphere absorbs medium waves seriously, and medium waves are difficult to propagate in the form of sky waves, and can only propagate by ground waves. At night, the D layer of the ionosphere disappears, the electron density of the E layer decreases, and the height increases, which reduces the absorption of medium waves, so medium waves can be reflected by the E layer of the ionosphere. At night, medium waves rely on ground wave propagation and sky wave propagation.High Frequency (HF) PropagationHF refers to electromagnetic waves with wavelengths of 10 meters to 100 meters (frequencies of 3 to 30 MHz). Short waves can propagate along the earth's surface (ground wave), propagate directly or diffractively in space (space wave) and by ionospheric reflection (sky wave). Due to the strong absorption of short waves by the earth's surface (compared with medium waves), and it increases significantly with the increase of frequency. Therefore, the ground wave propagation of short waves generally does not exceed tens of kilometers (at the low end of the short waves). The space wave propagation of short waves is mainly based on the direct and diffractive propagation of radio waves. Space wave propagation is not the main propagation mode of short waves. Sky wave propagation of short waves is the main propagation mode of short waves. The sky wave propagation of short waves is closely related to the changes of the ionosphere (day and night changes, seasonal and annual changes, as well as magnetic storms, sunspot changes, nuclear explosions, etc.), and is also closely related to the elevation angle and working frequency of the antenna. Due to the gradient distribution of the refractive index of the ionosphere and its relationship with frequency, there is a multipath effect in the propagation of short waves, resulting in serious fading.Very High Frequency (VHF) PropagationVHF refers to electromagnetic waves with wavelengths of 1 meter to 10 meters (frequencies of 30 to 300 MHz). Due to the severe absorption of VHF by the earth's surface, and also because VHF has strong penetration ability to the ionosphere, its refractive index in the ionosphere tends to 1, so the energy reflected by the ionosphere (sky wave) is very small. Therefore, VHF is difficult to propagate by ground wave and sky wave, but mainly by direct propagation (the so-called "line of sight" method). In addition, due to the scattering effect of atmospheric turbulence in the troposphere on VHF, VHF can realize the "over-the-horizon" propagation of VHF by using this scattering effect. VHF has a certain diffraction propagation ability. The higher the frequency, the worse the diffraction ability.Microwave PropagationMicrowave refers to electromagnetic waves with a wavelength of less than 1 meter (frequency higher than 300MHz). Currently, it is further divided into decimeter wave (Ultra High Frequency UHF), centimeter wave (Super High Frequency SHF), millimeter wave (Extremely High Frequency EHF) and sub-millimeter wave (Tremendously High Frequency THF) according to its wavelength.Microwave propagation is similar to light wave propagation and is a line-of-sight propagation. It mainly takes place within the troposphere. Generally speaking, this propagation method is relatively stable, but its propagation is also affected by atmospheric refraction and ground reflection. In addition, atmospheric turbulence in the troposphere has a scattering effect on microwaves. This scattering effect can be used to achieve over-the-horizon propagation of microwaves.Laser PropagationSince light is also an electromagnetic wave and laser is monochromatic light with good coherence. Therefore, laser propagation is also a type of electromagnetic wave propagation. Laser propagation in the atmosphere is mainly line-of-sight propagation and is easily affected by climate. Oxygen, nitrogen, carbon dioxide, and water vapor in the atmosphere absorb laser signals; the unevenness of atmospheric molecular density and dust, smoke, crystals, salt particles, microorganisms, and tiny water droplets suspended in the atmosphere all have a scattering effect on light signals. Atmospheric flow caused by air convection on the earth's surface can cause beam deflection, beam diffusion, and beam scintillation in laser propagation. Certain wavelengths of laser (such as 2-0.5μm blue-green laser) can propagate in water.