The Electromagnetic Spectrum

 

The range of frequencies that make up the electromagnetic spectrum is extremely large, extending from direct current (0 Hz) through visible light (≈10⁶ GHz) and beyond. Because of this enormous span, it is convenient to divide the spectrum into bands that exhibit similar physical and propagation characteristics.

In communications engineering, we are mainly interested in those portions of the spectrum that are associated with the source or can provide carrier frequencies for transmitting information across real-world channels. The most useful regions for this purpose are the audio-frequency (AF) band (and the voiceband subset) and the radio-frequency (RF) band.

Radio Frequencies (RF) and Audio Frequencies (AF)

Within the AF and RF regions, the International Telecommunication Union (ITU) Radio Regulations (Vol 1: Articles) define standard frequency bands, summarized in Table 1 and described in the following paragraphs.

Extremely low frequency (ELF). The ELF band is characterized by its extremely narrow bandwidth and very long wavelengths, which require enormous antennas that are still electrically short and thus inefficient. For instance, a 650 kW transmitter might radiate only 150 W from a 25 km-long antenna—an efficiency of about 0.02 %. Despite this inefficiency, ELF signals experience exceptionally low attenuation—roughly 1,000 times less than that of higher RF signals—and can penetrate seawater and soil effectively. As a result, ELF is used almost exclusively for one-way submarine communications, where propagation occurs primarily by surface wave which exhibits minimal diurnal or seasonal variation and, importantly, good penetration though sea water.

Voice frequency (VF). In voice-communication systems, VF corresponds to the acoustic input (and therefore the output range of a VF communications system). Bandwidth remains limited, and efficient radiation at these frequencies would require impractically large antennas. VF therefore serves as a baseband source that must be modulated onto a higher-frequency carrier for transmission.

Very low frequency (VLF) and low frequency (LF). Both bands offer reliable long-distance propagation with low attenuation, though antennas are still physically large and bandwidths narrow. Like ELF, they are used for submarine communication, maritime radio navigation, and certain time-signal transmissions (e.g., LORAN, Omega). Propagation is primarily by surface wave, enabling beyond-line-of-sight coverage.

Medium frequency (MF). The MF band supports communication over moderate ranges. Antenna dimensions are more practical, though significant power is still required for dependable long-range service. The band is widely used for AM broadcasting, maritime and aeronautical navigation, fixed services, and amateur radio. Propagation occurs mainly by surface wave during the day and by sky wave at night when ionospheric reflection becomes effective.

High frequency (HF). HF provides long-range sky-wave propagation, allowing global communication with modest transmitter powers and antenna sizes. Channel bandwidths are typically limited to about 3 kHz, making HF suitable for voice, data, and telegraphy (but not useful for data rates above 2.4 kbps). Before the advent of satellites, HF provided the only means of reliable over-the-horizon communication. It remains in use for maritime, aeronautical, military, and amateur services.

Very high frequency (VHF) and ultra high frequency (UHF). The VHF and UHF bands are among the most heavily used for modern terrestrial communication systems. These frequencies support large bandwidths, and antennas are compact and easily directional, enabling efficient point-to-point and mobile applications. However, propagation is limited to line-of-sight and is subject to shadowing by terrain and structures. VHF and UHF are used for television broadcasting, FM radio, mobile and fixed services, ground-to-air communications, and public-safety and emergency networks.

Super high frequency (SHF) and extremely high frequency (EHF). These microwave and millimeter-wave bands provide very wide bandwidths and short wavelengths, allowing the use of small, high-gain antennas. Propagation remains line-of-sight and is subject to attenuation from rain, clouds, and atmospheric absorption. Nevertheless, through radio-relay and satellite links, these bands enable high-capacity data transmission, microwave television distribution, and satellite communication systems. SHF and EHF thus form the backbone of today’s global broadband and space-communications infrastructure.

Each region of the electromagnetic spectrum offers unique advantages and limitations in bandwidth, antenna size, and propagation behavior. As the frequency increases, available bandwidth expands and antennas become smaller, but propagation shifts from global (ELF–HF) to line-of-sight (VHF–EHF). Modern communication systems—from terrestrial broadcasting to deep-space telemetry—use specific portions of these bands to balance range, capacity, and reliability.


Frequency

Band

Wavelength

30–300 Hz

ELF

10−1 Mm

300–3000 Hz

VF

1−0.1 Mm

3–30 kHz

VLF

100−10 km

30–300 kHz

LF

10−1 km

300–3,000 kHz

MF

1−0.1 km

3–30 MHz

HF

100−10 m

30–300 MHz

VHF

10−1 m

300–3,000 MHz

UHF

1−0.1 m

3–30 GHz

SHF

100−10 mm

30–300 GHz

EHF

10−1 mm

Table 1. The AF and RF portions of the electromagnetic spectrum.

Communications Frequencies other than RF and AF

While the RF portion of the electromagnetic spectrum contains the majority of frequencies used for traditional communications, modern systems increasingly employ optical and infrared frequencies for very-high-capacity data transmission.

The development of optical fiber communication has extended practical use of the spectrum into the near-infrared and visible-light regions. In optical fibers, information is transmitted as modulated light waves rather than as radio waves. Carrier wavelengths typically range between 850 nm, 1,310 nm, and 1,550 nm, corresponding to frequencies of approximately 200–375 THz. At these frequencies, extremely high data rates (terabits per second) can be achieved with very low attenuation over long distances, making optical fiber the preferred medium for terrestrial and submarine trunk communications

Beyond guided transmission, free-space optical (FSO) and laser communication systems are increasingly used for short- and medium-range wireless links, as well as for satellite crosslinks and deep-space communication. These systems exploit narrow laser beams to achieve high directivity, large bandwidths, and strong resistance to electromagnetic interference, though they remain sensitive to atmospheric absorption, scattering, and turbulence.

At slightly lower frequencies, infrared (IR) radiation—lying between roughly 300 GHz and 430 THz—is widely used for wireless local area networks (WLANs), remote controls, and short-range point-to-point links. Infrared transmission provides immunity to radio interference and a degree of security due to its confined propagation, though it requires unobstructed line-of-sight paths.

At the upper end of the RF spectrum, terahertz frequencies (0.1–10 THz) are also emerging as candidates for ultra-high-capacity short-range communications. Research into THz technology aims to bridge the gap between millimeter-wave and optical systems, promising data rates exceeding 100 Gbps for indoor and inter-satellite links.

Collectively, these higher-frequency technologies expand the usable communications spectrum far beyond traditional RF and AF bands, enabling the next generation of broadband, optical, and space-based networks.

edVirtus Communications Courses

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