2.7.5 What Frequencies Are Used for Communications?
- What Is the Electromagnetic Spectrum?
- What Are Audio Frequencies and Radio Frequencies?
- What Are the Main Radio Frequency Bands?
- What Are VHF, UHF, and Microwave Frequencies?
- Why Do Different Services Use Different Frequencies?
- Why Do Mobile Phones Use UHF and Microwave Frequencies?
- Why Do Satellite Systems Use Microwave Frequencies?
- What Frequencies Are Used by Fiber-Optic Systems?
- What Is Free-Space Optical Communication?
- What Are Terahertz Communications?
- How Does Frequency Affect Antenna Size?
- Will Communications Systems Continue to Move to Higher Frequencies?
Every communications system—from a simple telephone call to a satellite television broadcast—relies on the transmission of signals using a particular range of frequencies. Different communications services use different portions of the electromagnetic spectrum because each frequency range possesses unique characteristics that influence coverage, bandwidth, antenna size, propagation behavior, and system capacity.
The choice of frequency is therefore one of the most important decisions in the design of any communications system. Low frequencies can travel enormous distances but support relatively little information. High frequencies can support vast amounts of information but often require line-of-sight paths and are more susceptible to environmental effects.
Understanding how communications systems use different frequencies helps explain why submarine communications use one part of the spectrum, AM radio another, cellular networks another, and fiber-optic systems yet another.
What Is the Electromagnetic Spectrum?
The electromagnetic spectrum is the complete range of electromagnetic frequencies, extending from direct current (0 Hz) through radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
Communications systems make use of only a portion of this spectrum, although that portion is still enormous. Frequencies used for communications range from a few hertz in specialized submarine systems to hundreds of terahertz in optical fiber networks.
All electromagnetic waves travel through free space at approximately the speed of light (c = 3 x 108 m/s). The principal difference between the various regions of the spectrum is frequency and its associated wavelength.
As frequency increases:
- Wavelength decreases.
- Available bandwidth generally increases.
- Antenna size decreases.
- Propagation characteristics change.
These relationships strongly influence which frequencies are used for particular applications.
What Are Audio Frequencies and Radio Frequencies?
Communications engineers are particularly interested in two broad regions of the spectrum:
Audio frequencies (AF). Audio frequencies (AF) correspond to sounds that can be heard by the human ear. The audio frequency range extends approximately from 20 Hz to 20 kHz. Within this range lies the voice frequency (VF) band used for speech communications. Human speech contains most of its useful energy between approximately 300 Hz and 3.4 kHz. This range became the basis of traditional telephone systems. Audio frequencies are important because they represent many of the original information sources that communications systems must convey.
Radio frequencies (RF). Radio frequencies occupy the portion of the electromagnetic spectrum extending approximately from 3 kHz to 300 GHz. Most traditional communications systems operate somewhere within this range. Radio frequencies are particularly useful because they can be transmitted efficiently through space using antennas. The RF spectrum is divided into several standard bands, each possessing distinct characteristics and applications.
What Are the Main Radio Frequency Bands?
To simplify spectrum management, the International Telecommunication Union (ITU) divides the radio spectrum into standard frequency bands. These bands are identified using internationally recognized abbreviations.
Extremely low frequency (ELF). 30-300 Hz. ELF signals have extremely long wavelengths and can penetrate seawater effectively. Because of these properties, ELF has been used primarily for one-way communications with submerged submarines. The principal disadvantage is that available bandwidth is extremely limited.
Very low frequency (VLF). 300-3,000 Hz. VLF signals can travel great distances by surface-wave propagation and also penetrate seawater. Applications include:
- Submarine communications,
- Navigation systems,
- Time-signal transmissions.
Low frequency (LF). 30-300 kHz. LF supports reliable long-distance communications and navigation services. Historically it was widely used for maritime and aeronautical navigation systems.
Medium frequency (MF). 300-3,000 kHz. MF is best known as the traditional AM broadcasting band. Signals propagate primarily via surface waves during the day and may travel much farther at night due to ionospheric reflection.
High frequency (HF). 3-30 MHz. HF is often called the shortwave band. One of its most important characteristics is its ability to support long-distance sky-wave propagation via the ionosphere. HF communications have historically been used for:
- International broadcasting.
- Maritime communications.
- Aviation.
- Military communications.
- Amateur radio.
Before satellites, HF provided the principal means of global radio communication.
What Are VHF, UHF, and Microwave Frequencies?
Most modern terrestrial wireless systems operate above HF.
Very High Frequency (VHF). 30-300 MHz. VHF supports relatively wide bandwidths and practical antenna sizes. Applications include:
- FM radio broadcasting.
- Television broadcasting.
- Marine communications.
- Aircraft communications.
- Land-mobile radio systems.
Propagation is primarily line-of-sight.
Ultra high frequency (UHF). 300-3,000 MHz. UHF supports even larger bandwidths and smaller antennas. Applications include:
- Cellular networks.
- Television broadcasting.
- Public-safety radio.
- Wi-Fi.
- GPS.
Because of its favorable balance between coverage and capacity, UHF is one of the most heavily used portions of the spectrum.
Super high frequency (SHF). 3-30 GHz. SHF corresponds to microwave frequencies. Applications include:
- satellite communications,
- radar,
- microwave radio-relay systems, and
- broadband wireless networks.
The shorter wavelengths permit highly directional antennas and very large bandwidths.
Extremely high frequency (EHF). 30-300 GHz. EHF encompasses millimeter-wave frequencies. Applications include:
- advanced radar systems,
- 5G and future wireless systems,
- high-capacity satellite links, and
- scientific applications.
Although enormous bandwidths are available, atmospheric absorption and rain attenuation become increasingly important.
Why Do Different Services Use Different Frequencies?
Different frequency bands provide different trade-offs between:
- range,
- bandwidth,
- antenna size,
- equipment complexity, and
- propagation reliability.
For example:
- Low frequencies can travel around the curvature of the Earth and penetrate obstacles effectively, but they support relatively little bandwidth.
- High frequencies support enormous bandwidths and high data rates but often require line-of-sight paths and are more sensitive to environmental effects.
- No single frequency range is ideal for all applications.
Instead, engineers select frequencies that best match the operational requirements of the service.
Why Do Mobile Phones Use UHF and Microwave Frequencies?
Cellular systems must simultaneously support:
- millions of users,
- high data rates,
- portable antennas, and
- frequency reuse.
These requirements are best satisfied at UHF and microwave frequencies. At these frequencies:
- Antennas can be small enough to fit inside mobile devices.
- Large bandwidths are available.
- Signals can be reused efficiently in neighboring cells.
Modern cellular systems therefore operate primarily between several hundred megahertz and several gigahertz.
Why Do Satellite Systems Use Microwave Frequencies?
Satellite communications require frequencies that can pass through the atmosphere with relatively low attenuation while also supporting high-capacity links.
Most satellite systems therefore operate in microwave bands such as L-band, S-band, C-band, X-band, Ku-band, and Ka-band.
These frequencies provide a good balance between:
- atmospheric penetration,
- antenna size,
- available bandwidth, and
- system capacity.
As demand for capacity grows, newer systems are increasingly using higher-frequency Ka-, Q-, and V-band allocations.
What Frequencies Are Used by Fiber-Optic Systems?
Modern communications are not limited to radio frequencies. Most long-distance communications traffic now travels through optical fibers using infrared light. Typical optical carrier wavelengths include: 850 nm, 1,310 nm, and 1,550 nm.
These correspond to frequencies of approximately 200–375 THz, thousands of times higher than microwave radio frequencies.
The enormous bandwidth available at optical frequencies allows fiber-optic systems to support data rates measured in terabits per second. Consequently, optical fiber forms the backbone of the modern Internet and global telecommunications infrastructure.
What Is Free-Space Optical Communication?
Free-space optical (FSO) communication uses laser beams to transmit information through the atmosphere or space. Applications include:
- building-to-building links,
- satellite crosslinks, and
- deep-space communications.
Laser systems offer:
- very high bandwidth,
- excellent directivity, and
- resistance to radio interference.
However, they are sensitive to cloud, fog, dust, and atmospheric turbulence.
What Are Terahertz Communications?
Terahertz (THz) communications occupy the region between microwaves and infrared light. Typically: 0.1 THz to 10 THz. Research into terahertz communications seeks to exploit the vast bandwidth available in this region for future ultra-high-speed networks. Potential applications include:
- wireless links exceeding 100 Gbps,
- data-center interconnections,
- satellite communications, and
- future 6G systems.
Although still largely experimental, terahertz technology represents a promising area of communications research.
How Does Frequency Affect Antenna Size?
Frequency and wavelength are directly related by λ = c f. As frequency increases, wavelength decreases. Since many antennas have dimensions related to wavelength, higher frequencies generally permit smaller antennas.
For example:
- An HF antenna may be tens of meters long.
- A cellular antenna may be only a few centimeters long.
- A millimeter-wave antenna may be measured in millimeters.
This relationship strongly influences the frequencies chosen for portable and mobile systems.
Will Communications Systems Continue to Move to Higher Frequencies?
In general, yes.
As demand for bandwidth increases, communications systems continue to expand into higher-frequency regions of the spectrum. Historically, communications evolved from:
- telegraphy,
- voice telephony,
- radio broadcasting,
- microwave links,
- satellite systems, and
- optical fiber systems.
Each step generally involved the use of higher frequencies and wider bandwidths. Future systems are expected to make increasing use of:
- millimeter waves,
- Terahertz frequencies, and
- optical communications.
However, lower frequencies will remain important for applications requiring long range, obstacle penetration, and wide-area coverage.
Summary
Communications systems use a wide range of frequencies, from a few hertz for specialized submarine communications to hundreds of terahertz in optical fiber networks. Different portions of the electromagnetic spectrum provide different combinations of range, bandwidth, antenna size, and propagation characteristics.
Low frequencies support long-range communications but offer limited bandwidth, while high frequencies support enormous data rates but often require line-of-sight paths. Understanding how these frequency bands differ is essential to understanding why modern communications systems—from AM radio and cellular networks to satellites and optical fibers—operate where they do within the electromagnetic spectrum.
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