Different bands of electromagnetic waves vary in frequency, wavelength, energy, and propagation methods, which gives them unique advantages in applications. Below, we will provide a detailed analysis of the characteristics of major electromagnetic wave bands outside of medium and short waves, as well as their applications in communication and other fields:
1. Extremely Low Frequency and Super Low Frequency Bands (ELF, SLF, ULF, VLF)#
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Frequency and Wavelength Characteristics
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Extremely Low Frequency (ELF): Typically between 3–30 Hz, with wavelengths reaching tens of thousands of kilometers.
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Super Low Frequency (SLF) and Ultra Low Frequency (ULF): Frequencies are approximately between 30–300 Hz and 300–3000 Hz, respectively, with still very long wavelengths.
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Very Low Frequency (VLF): Generally between 3 kHz and 30 kHz, with wavelengths ranging from tens to hundreds of kilometers.
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Propagation Characteristics
Due to their extremely long wavelengths, waves in these bands can bypass terrain and seawater, demonstrating good penetration capabilities through media such as underground and seawater. However, their data bandwidth is very limited, resulting in low transmission rates. -
Main Applications
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Submarine Communication: ELF and VLF can penetrate seawater, suitable for long-range low-rate command communication with submarines.
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Navigation and Time Signals: For example, LF (Low Frequency) signals are used to transmit standard time signals and navigation information.
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2. Low Frequency and Medium Wave (LF, MF, AM Broadcasting)#
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Frequency and Wavelength Characteristics
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Low Frequency (LF): Generally between 30 kHz and 300 kHz, with wavelengths over 1 kilometer.
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Medium Frequency (MF): Approximately 300 kHz to 3 MHz, encompassing the traditional amplitude modulation (AM) broadcasting band, which falls under this category.
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Propagation Characteristics
Low and medium frequency electromagnetic waves can propagate via ground waves, and during nighttime when ionospheric conditions are better, they can propagate long distances via sky waves. However, due to their longer wavelengths, their bandwidth and data transmission capabilities are relatively limited. -
Main Applications
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Broadcast Communication: AM broadcasting and maritime, aviation navigation often use low and medium frequency signals.
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Time and Positioning Signals: For example, LF signals are commonly used in some long-distance positioning and time calibration systems.
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3. High Frequency and Very High Frequency Bands (HF, VHF, UHF)#
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Frequency and Wavelength Characteristics
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High Frequency (HF): 3–30 MHz, with wavelengths around 10 to 100 meters, suitable for long-distance shortwave communication.
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Very High Frequency (VHF): 30–300 MHz, corresponding to wavelengths of about 1 to 10 meters, primarily used for television, FM broadcasting, and aviation communication.
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Ultra High Frequency (UHF): 300 MHz–3 GHz, with wavelengths of about 10 centimeters to 1 meter, commonly found in mobile communications, satellite broadcasting, and some radar systems.
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Propagation Characteristics
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HF: Utilizes ionospheric reflection (sky wave propagation), capable of covering extremely long distances, but signals are easily affected by atmospheric and ionospheric conditions.
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VHF and UHF: Mainly rely on line-of-sight propagation, providing stable transmission with good anti-interference performance, but transmission distances are limited by terrain obstructions, typically used for local or satellite communications.
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Main Applications
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Long-Distance Communication and Amateur Radio: HF bands are frequently used for international communications and cross-ocean contacts among amateur radio enthusiasts.
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Broadcasting, Television, and Mobile Communication: VHF/UHF are widely used in civilian broadcasting, mobile phone communication, wireless local area networks, and satellite television.
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4. Microwaves and Higher Frequency Bands (SHF, EHF, THz)#
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Frequency and Wavelength Characteristics
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Microwaves (SHF): 3–30 GHz, with wavelengths from about 10 millimeters to 1 centimeter.
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Extremely High Frequency (EHF): 30–300 GHz, with wavelengths between 1 millimeter and 10 millimeters.
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Terahertz Waves (THz): Between microwaves and infrared, with frequencies approximately ranging from 0.1 to 10 THz, corresponding to wavelengths between 0.03 millimeters and 3 millimeters.
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Propagation Characteristics
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Microwaves and EHF signals, due to their short wavelengths, primarily use line-of-sight propagation and are susceptible to atmospheric absorption, rain fade, and obstruction by obstacles; however, they support wide bandwidths for high-speed data transmission.
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Although THz waves have the potential for extremely high data transmission rates, their propagation distances are short and they are particularly sensitive to water vapor in the atmosphere, thus currently being researched and applied mainly in short-distance, high-speed communication and special imaging and detection.
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Main Applications
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Satellite Communication, Radar, and Wireless Broadband: The microwave band is used in satellite links, radar systems, and point-to-point wireless communication.
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Data Centers and Next-Generation Wireless Communication: EHF, millimeter waves, and some THz technologies are being explored for 5G/6G communication, providing new avenues for ultra-high-speed, low-latency data transmission.
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Imaging and Detection: THz waves also have unique advantages in medical imaging, non-destructive testing, food safety, and concealment detection.
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5. Infrared, Visible Light, Ultraviolet, and High-Energy Rays (X-rays, γ-rays)#
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Infrared
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Characteristics: Wavelengths range from 0.75μm to 1mm, with energy lower than visible light but higher than microwaves.
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Applications: Widely used in fiber optic communication, infrared thermal imaging, night vision devices, and some short-distance high-speed wireless optical communication (e.g., free-space optical communication).
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Visible Light
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Characteristics: Wavelengths are approximately 390–750nm, which is the portion visible to the human eye.
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Applications: Used in optical communication (such as laser communication), fiber optic transmission, display technology, and general lighting.
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Ultraviolet
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Characteristics: Wavelength range is approximately 10–400nm, with relatively high energy, and some ultraviolet can cause chemical reactions.
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Applications: Mainly applied in scientific detection, disinfection, sterilization, and material analysis, with fewer applications in communication.
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X-rays and γ-rays
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Characteristics: Extremely short wavelengths, very high frequencies, and very high energy.
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Applications: Due to their high energy, they have strong penetration capabilities, primarily used in medical imaging (such as X-ray examinations), industrial non-destructive testing, and nuclear physics research, typically not used for conventional communication.
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Summary#
Different electromagnetic wave bands each have their unique transmission and physical characteristics, thus excelling in various communication systems:
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Low and extremely low frequency bands can penetrate seawater and underground, suitable for long-distance low-speed transmission and communication in special environments;
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Medium to high frequency (HF, VHF, UHF) bands are suitable for broadcasting, shortwave international communication, and line-of-sight communication;
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Microwaves and higher frequency bands (SHF, EHF, and THz) are becoming important candidates for satellite communication, radar, and next-generation wireless networks due to their wide bandwidth and high data rate advantages;
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Infrared, visible light, etc., leverage the properties of light in fiber optic and free-space optical communication, while ultraviolet, X-rays, and γ-rays, due to their high energy, are mainly used in imaging and detection fields rather than conventional data communication.
This multi-band, multi-mode application forms the cornerstone of modern electromagnetic wave systems, while also demonstrating the complementarity of different technologies in meeting various transmission needs. When choosing the appropriate band, it is necessary to weigh multiple factors such as propagation distance, data rate, anti-interference capability, and environmental adaptability.