Electromagnetic Spectrum Categories

Radio Frequencies (3kHz to 300 GHz)

Radio wave frequencies consist of a large portion of the electromagnetic spectrum from 3kHz up to 300 GHz (which correspond to wavelengths of 100km down to 1 mm). This part of the spectrum is widely used for communications and different parts of the radio frequency band are used for different applications. For example, long wavelength communication is good for communicating long distances because the waves are so long that they bend around large obstacles and even around mountains. Short wavelength communication is good for high data rates because the higher the frequency, the more data that can be transmitted. Communication at frequencies about 300 GHz is not effective for wireless communications because these signals do not travel through the atmosphere very well.

The radio frequencies span a large section of the electromagnetic spectrum and is often split into subsections such as:

  • VLF (Very Low Frequency): 3 to 30 kHz. Communication over VLF is low bandwidth (see Nyquist), but the large wavelengths (up to 100km) allow VLF signals to travel around mountain ranges. VLF radio waves can also travel through sea water more than other frequencies
  • LF (Low Frequency): 30 to 300 kHz. LF waves also have long wavelengths (up to 10km) and can travel around mountains. They also effectively travel as ground waves by following the curve of the earth.
  • MF (Medium Frequency): 300 to 3000 kHz. AM radio stations broadcast within the MF band. MF signals travel by ground waves as well as by skywaves (i.e., reflecting or refracting ooff the ionosphere).
  • HF (High Frequency): 3 to 30 MHZ.  Shortwave radio uses the HF band. International broadcasting stations can use this band to cover large ranges because HF frequencies can also travel by skywave propagation.
  • VHF (Very High Frequency): 30 to 300 MHz. The VHF band is used for FM radio and over-the-air TV broadcast. VHF signals do not travel by skywave and have limited range as ground waves because of their shorter wavelength. Most transmission is based on line-of-site
  • UHF (Ultra High Frequency): 300 to 3000 MHz. UHF signals typically do not travel far and are nearly exclusively line of sight transmissions. Moisture in the air also has a larger attenuating effect on UHF signals compared to lower frequency ones. Using a broad definitions of microwave transmissions, UHF falls in to the broad definition of microwave (300 MHz to 300 GHz). The high data rates as well as the availability of the spectrum, including several bands that can be used license free make UHF a popular communication band. Common applications using UHF include BlueTooth, WiFi, cellular, and cordless telephones.
  • SHF (Super High Frequency): 3 to 30 GHz. SHF frequencies also fall in the microwave band. This band is used for satellite communications, WiFi (5GHz band) and point-to-point microwave communication. Propagation in this band is solely via line of sight.
  • EHF (Extremely High Frequency): 30 to 300 GHz. This band is also called millimeter wave because the wavelengths or 1-10 mm in length. This band is mostly unused for communications, because the small wavelength leads to high atmospheric losses. There are both licensed and unlicensed bands of EHF reserved for high-speed data links but these bands are mostly used experimentally at this point. (In)famously, millimeter waves are used in scanners to screen passengers in some airports.
  • THF (Tremendously High Frequency): 300 to 3000 GHz. This frequency range is no longer in the RF band, but I wanted to add it anyway because it fits in to the <Superlative> High Frequency naming convention

You’ll notice that all of these frequency ranges begin with a ‘3’. This might seem unusual at first, but the reason is that when you convert the frequencies to wavelengths, you’ll find that the wavelength ranges all start with ‘1’.

\lambda = \frac{c}{f}            (\lambda is the wavelength, c is the speed of light and c is the frequency)

It has also struck me as strange that the different frequency descriptors are kind of set in and arbitrary order. Who was it that decided that Very High < Ultra High < Super High < Extremely High < Tremendously High?

Microwave Frequencies (300 MHz to 300 GHz)

Microwaves fall within the RF band at the upper end of that band. They have many uses including:

  • broadcasting transmissions because microwaves pass easily through the earth’s atmosphere and have a higher bandwidth than the rest of the radio spectrum. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van.
  • Radar to detect the range, speed, and other characteristics of remote objects
  • Many communications protocols including Bluetooth (2.4GHz), IEEE802.11g, n, ac (2.4GHz), WiMAX (Worldwide Interoperability for Microwave Access at 2-11GHz), IEEE802.11a(5GHz), Wide Area Mobile Broadband Wireless Access (1.6-2.3GHz), GSM.
  • Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies
  • Generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
  • Transmit power over long distances
  • MASERs which are devices similar to LASERs but work at microwe frequencies

Infrared Frequencies (300 GHz to 400 THz)

The infrared spectrum is often divided into three parts, but the three parts vary depending on which organization is doing the division.

  • Far-infrared. This is the part of infrared that is farthest from visible light and the lower end borders on the microwave spectrum. Far infrared is not used for communications but it transfers its energy in the form of heat when it is absorbed by human bodies.
    The International Commission on Illumination calls this band IR-C and assigns it the frequency range 300 GHz to 100 THz (1 mm to 10 μm wavelength). ISO 20473 sets it as 300GHz to 6 THz, and astronomers set it as 850 GHz to 12THz
  • Mid-infrared. The International Commission on Illumination calls this band IR-B and assigns it the frequency range 1oo THz to 215 THz (3 μm to 1.4 μm wavelength). ISO 20473 sets it as 12 THz to 100 THz, and astronomers set it as 12THz to 120 THz
  • Near-infrared are closest to visible light and can actually be captured by digital cameras.  The IrDA standards for infrared communications use the 850-900nm wavelength which falls in the near-infrared band. The International Commission on Illumination calls this band IR-A and assigns it the frequency range 215 THz to 430 THz (3 μm to 1.4 μm wavelength). ISO 20473 sets it as 100 THz to 385 THz, and astronomers set it as 120 THz to 430 THz

Visible Radiation (380 THz to 750 THz)

Visible light is a very narrow range on the EM scale, but it is obviously very important because it is the range that the human eye is sensitive to. The colours in order from lowest frequency to highest frequency are red, orange, yellow, green blue, violet. Visible light is used for communication over fiber optic cables which carry modulated visible light signals and over free space optics which transmits modulated light signals through free space. Free space optics require line of sight and anything that would block visible light would of course block communication.

Ultraviolet Light (750 THz – 30 PHz)

Ultraviolet or UV light is at frequencies just beyond the visible spectrum. It is highly energetic and can break chemical bonds making molecules reactive. Sunburn is caused by the highly energetic effect of UV light.

X-Rays (30 PHz to 30000 PHz)

X-rays do not have any communications applications; they are mostly used for medical imaging and crystallography. X-rays are even more highly energetic than UV light and are therefore potentially more dangerous. Fortunately exposure to X-Rays is rare except during medical imaging.

Gamma Rays (2.4 EHz and up)

Gamma rays are at the upper end of the EM spectrum. Astronomers monitor gamma rays to study regions of space and physicists use them to study radioisotopes. The Earth’s atmosphere blocks gamma rays so exposure to them on Earth is nearly 0.