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Electromagnetic Spectrum

The Electromagnetic (EM) spectrum is just a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Scientists believe that all objects either reflect or emit light in the EM spectrum, and those reflections or emissions are simply called radiation. Radiation is energy that travels and spreads out as it goes. Radiation can occur in the visible light spectrum, and they can be seen by the human eye as color or light such as visible light that comes from a lamp. But more often, radiation occurs in the non-visible light spectrum, or the frequencies that are invisible to the human eye such as radio waves that come from a radio station.

These are just two types of electromagnetic radiation which only represents a tiny portion of the whole EM. Other examples of the EM radiation that cannot be picked up by the human eye are microwaves, infrared, ultraviolet light, X-rays, and gamma-rays. The EM spectrum can be expressed in terms of energy, wavelength, or frequency (shown in the Figure 1 on page 2). Frequency is measured in cycles per second (Hertz), wavelength is measured in meters, and energy is measured in electron volts. And, they are related to each other mathematically. The EM spectrum extends from frequencies used in the electric power grid (at the long-wavelength end) to gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometers down to fractions of the size of an atom, though in principle the spectrum is actually infinite.

Electromagnetic radiation can be described as a stream of photons, which are massless particles that are traveling in a wave-like pattern at the speed of light. Each of these photons contains a certain amount of energy, and all electromagnetic radiation consists of these photons. The difference between various types of radiation is the total amount of energy found in the photons. Radio waves have photons with very low energies and gamma-rays are radiation waves that contain very high amounts of energies (see piture on the left). While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, some low-energy gamma rays may have a longer wavelength than some high-energy X-rays. This is possible because "gamma ray" is the name given to the photons generated from nuclear decay or other nuclear and sub-nuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma ray and X-ray is related to the radiation source rather than the radiation wavelength (shown below).

Radiation

Generally, nuclear transitions are much more energetic than electronic transitions, and usually, gamma-rays are more energetic than X-rays. However, there are a few low-energy nuclear transitions that produce gamma rays that are less energetic than some of the high-energy X-rays.

Radio Frequency (RF):

The Radio frequency (RF) refers to the portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. The use of the radio spectrum is regulated by governments, and it is called the frequency allocations. In radio spectrum, the waves used for radio communication (and other purposes) are neatly divided up into bands whose wavelengths and frequencies vary over a factor of 10. In wavelength, the bands begin and end on meters times a power of ten. In frequencies, the bands begin and end on 3 times a power of 10 Hertz (Hz) because the speed of light is close to 3x108 m/s. The bands of the radio spectrum are the following: ELF, SLF, ULF, VLF, LF, MF, HF, VHF, UHF, SHF, and EHF. The ELF, SLF, ULF, and VLF bands overlap the audio frequency (AF) spectrum that is approximately 20–20,000 Hz. However, sounds are transmitted by atmospheric compression and expansion, and not by electromagnetic energy. The SHF and EHF bands are often considered to be not part of the radio spectrum and form their own microwave spectrum.

Extremely Low Frequency (ELF):

Extremely low frequency (ELF) is the band of radio frequencies from 3 Hz to 30 Hz. Elf has the ability to penetrate ocean depths to several hundred feet with little signal loss. ELF was used by the US Navy to communicate with submerged submarines. The purpose of the ELF communications system is to send short "phonetic letter spelled out" (PLSO) messages from operating authorities in the continental United States to submarines operating at normal mission speeds and depths. ELF allows submarines to be operated well below the immediate surface and enhances submarine survivability by making detection more difficult. One of the great difficulties, associated with the use of ELF for communication purposes, is the problem of generating a useful signal. The physical size of an antenna that can produce a useable signal with reasonable efficiency is quite large. And, the large size of ELF transmitters and antennas makes ELF transmission from submarines impractical. ELF was also used in the pipeline industry: maintenance of pipelines.

Super Low Frequency (SLF):

Super Low Frequency (SLF) is the frequency range from 30 HZ to 300 Hz. This frequency range includes the frequencies of AC power grids (50 Hz and 60 Hz). The radio services Saguine (USA) on 76 Hz and ZEVS (Russia) on 82 Hz also operate in this range, which is often incorrectly called Extremely Low Frequency (ELF). They both provide communication services for submarines at depth: SLF also has the ability to penetrate ocean depths to several hundred feet with little signal loss. Another used in SLF can be found on PCs, PCs with integrated sound cards are increasingly being used instead of radio receivers for this frequency range, because of their much smaller size and lower cost. Signals received by the sound card with a coil or a wire antenna are analyzed by a software Fast Fourier Transform algorithm and converted into audible sound. Typically, the voiced speech of an adult is in the frequency range from 85 to 155 Hz, and an adult female will have the frequency range from 165 to 255 Hz. However, the fundamental frequency of most speech falls below the bottom of the "voice frequency" band on the ULF.

Ultra Low Frequency (ULF):

Ultra Low Frequency (ULF) is the frequency range from 300 Hz to 3000 Hz. This band is used for communications in mines, as it can penetrate the earth: some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ULF activity. ULF propagates through conductive media (water, rock) through a phenomenon called "skin effect" as deep as 10-20 miles below the surface. Electromagnetic waves in this range are not strongly absorbed by water or the earth. One disadvantage is that, with such low frequency, one can only modulate their amplitude or frequency very slowly: they cannot carry much information. However, this is not a disadvantage if only the phase is required, as is the case for navigation systems. The wavelengths of ULF are so long that antennae may be huge. A voice frequency (VF) or voice band is one of the ULF frequencies that are used for the transmission of speech. In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz.

Very Low Frequency (VLF):

Very low frequency (VLF) refers to radio frequencies (RF) in the range of 3 kHz to 30 kHz. There is not much bandwidth in this band of the radio spectrum. Only the very simplest signals are used in this band. VLF has similar advantages to ELF. Therefore, VLF is used for the transmission of instructions to submerged submarines. Since radio waves can penetrate to a depth of roughly 10 to 40 meters (30 to 130 feet), depending on the frequency and the salinity of the water, in sea water in this frequency band. The band has more use for communications than ELF since it has a bandwidth that is large enough to provide reliable communications. VLF is not possible to transmit audio signals: all messaging is done with alphanumeric data at very low bit rates. The frequency range less than 30 kHz is also used for time signals and radio-navigation. This frequency range is subject to no control on the part of the international communications organization (International Telecommunication Union) and may be used in some states license-free (see Figure 3 at the end of the document for complete VLF transmitter).

Low Frequency (LF):

Low Frequency (LF) (sometimes called longwave) refers to Radio Frequencies (RF) in the range of 30 kHz to 300 kHz. Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. They have the property of following the curvature of the earth that making them ideal for continuous continental communications. Unlike shortwave radio, longwave signals do not reflect or refract using the ionosphere, so there are fewer phase-caused fadeouts. Instead, the D-layer of the ionosphere and the surface of the earth serve as a waveguide directing the signal. In Europe, part of the LF spectrum is used for AM broadcast service. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. Time signal stations MSF, DCF77, JJY and WWVB are found in this band. In the USA, the portion between 160 and 190 kHz can be used for experimental purposes and is sometimes called the "Lost Band". Amateur Radio operators experiment in this band.

Medium Frequency (MF):

Medium frequency (MF) (sometimes called Mediumwave radio transmissions) is the frequencies in range of 300 kHz to 3000 kHz. In most of the world, MF serves as the most common band for broadcasting. The standard AM broadcast band is 525 kHz to 1715 kHz in North America, but remains only up to 1615 kHz elsewhere. Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to ground-wave propagation. The effectiveness of ground-wave signals largely depends on ground conductivity: higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably. Many stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range. In the US, Mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz.; where as the rest of the world, the separation is 9 kHz with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts. There is no need to increase the length of an antenna, although the strength of the signal will increase as you increase the antenna length. For most of the 20th century, the radio frequency 500 kHz was reserved world wide as the Morse code international calling and distress frequency for ships on the high seas. Other services that operate in medium wave include Navtex, the Amateur Radio 160-meter band, and the obsolete LORAN-A system.

High Frequency (HF):

High frequency (HF) radio frequency ranges are between 3 MHz to 30 MHz. This range is often called shortwave. Shortwave frequencies are capable of reaching the other side of the planet since the ionosphere often reflects HF radio waves quite well. This range is extensively used for medium and long range terrestrial radio communication. However, suitability of this portion of the spectrum depends on several factors:

The distance from the transmitter to the target receiver
Time of day. During the day, higher shortwave frequencies (> 10 MHz) can travel longer distances than lower; at night, this property is reversed.
Season of the year.
Solar conditions, including the number of sunspots, solar flares, and overall solar activity. Solar flares can prevent the ionosphere from reflecting or refracting radio waves.
Type of modulation. Independent from the frequency, the receiver must be capable to receive the same modulation type of the transmitter.

Types of modulation frequently used in the shortwave frequency range are:

AM: amplitude modulation. Usually used for shortwave broadcasting, and some aeronautical communications.
NFM: Narrow-band Frequency Modulation. Normally used for VHF communication, but some NFM transmissions occur in the higher HF frequencies.
SSB: Single sideband (USB/LSB). This is used for long-range communications by ships and aircraft, for voice transmissions by amateur radio operators, and for broadcasting.
CW: Continuous/Carrier wave. It is used for Morse code communications.
DRM: Digital Radio Mondale. Digital modulation for use on bands below 30 MHz.
Various radio teletype, fax, or other systems, which require special equipment to decode.

The HF band is very popular with amateur radio operators, who can take advantage of direct, long-distance (often inter-continental) communications and the "thrill factor" resulting from making contacts in variable conditions. International shortwave broadcasting utilizes this set of frequencies, as well as a seemingly declining number of "utility" users (marine, aviation, military, and diplomatic interests), who have, in recent years, been swayed over to less volatile means of communication (for example, via satellites), but may maintain HF stations after switch-over for back-up purposes. The Citizens Band (CB) radios operate in the higher portion of the range (around 27 MHz), as do some studio-to-transmitter (STL) radio links. Some modes of communication, such as continuous wave Morse code transmissions (especially by amateur radio operators) and single sideband voice transmissions are more common in the HF range than on other frequencies, because of their bandwidth-conserving nature, but broadband modes, such as TV transmissions, are generally prohibited by HF's relatively small chunk of electromagnetic spectrum space.

Very Figh Frequency (VHF):

Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength of 10 m) to 300 MHz (wavelength of 1 m). Common uses for VHF are FM radio broadcast at 88 - 108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems (VOR in particular) and aircraft communications. VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter. Unlike HF band, the ionosphere does not usually reflect VHF radio. Therefore, the transmissions are restricted to the local area and don't interfere with transmissions thousands of kilometers away. VHF is also less affected by atmospheric noise and interference from electrical equipment than low frequencies. Two unusual propagation conditions can allow much farther range than normal:

Tropospheric ducting can occur in front of and parallel to an advancing cold weather front especially if there is a marked difference in humidity between the cold and warm air masses. A duct can form approximately 150 miles (240 km.) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of miles. For example, a 50-watt Amateur FM transmitter at 146 MHz can talk from Chicago, Illinois, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater.
Sporadic-E refers to the E-layer of the ionosphere. A sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies the same way HF band is usually reflected (skywave). For example, TV channel 2 (54 - 60 MHz) from Midland, Texas was seen in Chicago, pushing out Chicago's own TV channel 2. These patches may last for seconds, or extend into hours. FM stations from Miami, Florida; New Orleans, Louisiana; Houston, Texas and even Mexico were heard for hours in central Illinois during one such event.

VHF was also easier to construct efficient transmitters, receivers, and antennas comparing to UHF. Antennae are often made to be about one quarter or one half of the wavelengths long. Television uses several different bands between 54 to 220 MHz. Television carries more information than radio: does pictures and sounds. So, it needs broader bands for each channel. Therefore in most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as those operated by taxis and police), marine two-way audio communications, and aircraft radios.

Ultra High Frequency (UHF):

The Ultra high frequency (UHF) is a range (band) of electromagnetic waves whose frequency is between 300 MHz (Wavelength of 1 m) to 3.0 GHz (Wavelength of 10 cm). UHF and VHF are the most common frequency bands for television. Modern mobile phones (the frequency between 824 - 849 MHz) also transmit and receive within the UHF spectrum, and UHF is widely used for two-way radio communication (usually using narrowband frequency modulation) by both public service agencies and the general public. Though television broadcasting is common on UHF, there has traditionally been very little radio broadcasting in this band until fairly recently. The transmission of radio waves from one point to another is affected by many variables such as atmospheric moisture, the stream of particles from the sun called solar wind, and time of day. All radio waves are somewhat absorbed by atmospheric moisture. This reduces, or attenuates, the strength of radio signals over long distances. UHF signals are generally more degraded by moisture than lower bands such as VHF. UHF benefits less from the ionosphere that can reflect radio waves bouncing from the sky to the ground over and over, covering long distances. However, as the atmosphere warms and cools throughout the day, UHF transmissions may be enhanced by tropospheric ducting. The main advantage of UHF transmission is that the size of transmission and reception equipment (particularly antennas) is smaller. UHF signals essentially travel over line-of-sight distances. The distant transmissions cannot travel far enough to interfere with local transmissions. A great number of public safety and business communications are handled on UHF: civilian applications such as GMRS, PMR446, and UHF CB are extremely popular. Where communications greater than line-of-sight are required, a repeater is used to propagate signals that otherwise would not reach their destinations.

The next radio spectrums are super high frequency (SHF) and extremely high frequency (EHF). The SHF and EHF bands are often considered to be not part of the radio spectrum and form their own microwave spectrum.

Super High Frequency (SHF):

Standards Super High Frequency (SHF) is a radio-frequency (RF) band in the range of 3 - 30 GHz. As mentioned before, SHF and EHF are known as the microwave bands and their wavelengths are short enough to be propagated by highly directional antennas and waveguides. Propagation ranges are limited to line-of-sight, but long-distance communication can be achieved by employing a series of radio relay stations. These portions of the spectrum are used for television and high-speed data services requiring large bandwidths. Microwaves are electromagnetic waves with wavelengths longer than those of infrared light, but shorter than those of radio waves. The microwave range include ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz) signals. Microwaves have wavelengths approximately in the range of 30 cm to 1 mm. The boundaries between far infrared light, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The existence of electromagnetic waves, of which microwaves are part of the higher frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his famous Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building apparatus to produce radio waves. The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands are defined in picture below:

SHF

This frequency range used for communication with satellites, and roughly corresponds to microwave band. The microwave band is used for radar and long distance trunk telephone communications. Domestically, it is also used in microwave ovens.

Extremely High Frequency (EHF):

Extremely high frequency (EHF) is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 GHz, above which electromagnetic radiation is considered to be low (or far) infrared light. This band has a wavelength of one to ten millimeters, giving it the name millimeter band. Radio signals in this band are extremely prone to atmospheric attenuation: making them of very little use over long distances. Even over relatively short distances, rain fade is a serious problem. The rain fade caused the absorption by rain reduces signal strength. This band is commonly used in radio astronomy. In the USA, the band 38.6 - 40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit/s (gigabits per second).

Furthermore today, radio waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, T-rays, T-light, T-lux and THz, are in the region of the light spectrum between far infrared and microwaves. Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They can also penetrate fog and clouds but cannot penetrate metal or water. The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness. The proposed WiMAX standard for wireless networking, a long-range enhancement of Wi-Fi, lies within this region. Scientists are also looking to apply Terahertz technology in the armed forces, where high frequency waves will be sent at enemy troops to incapacitate them.

Here ends the radio bands. Hereafter, wavelengths are used almost exclusively, partly for traditional reasons, and partly because frequencies in the THz range (THz = 1012 Hz) are difficult to measure directly. They can be measured by heterodyning: observing the difference frequencies they make with reference signals. The most common electromagnetic spectrum, which have higher frequency than radio frequency, are Infrared radiation, Visible light radiation, Ultraviolet radiation, X-ray radiation, Gamma Ray radiation, and cosmic rays radiation.

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