Microwave
Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter. Different sources define different frequency ranges as microwaves. A more common definition in radio engineering is the range between 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations; the prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. Rather, it indicates that microwaves are "small", compared to the radio waves used prior to microwave technology; the boundaries between far infrared, terahertz radiation and ultra-high-frequency radio waves are arbitrary and are used variously between different fields of study. Microwaves travel by line-of-sight. At the high end of the band they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer.
Microwaves are used in modern technology, for example in point-to-point communication links, wireless networks, microwave radio relay networks, radar and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, for cooking food in microwave ovens. Microwaves occupy a place in the electromagnetic spectrum with frequency above ordinary radio waves, below infrared light: In descriptions of the electromagnetic spectrum, some sources classify microwaves as radio waves, a subset of the radio wave band; this is an arbitrary distinction. Microwaves travel by line-of-sight paths. Although at the low end of the band they can pass through building walls enough for useful reception rights of way cleared to the first Fresnel zone are required. Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about 30–40 miles.
Microwaves are absorbed by moisture in the atmosphere, the attenuation increases with frequency, becoming a significant factor at the high end of the band. Beginning at about 40 GHz, atmospheric gases begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies. Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. In a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through the troposphere. A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal; this technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter communication systems to communicate beyond the horizon, at distances up to 300 km.
The short wavelengths of microwaves allow omnidirectional antennas for portable devices to be made small, from 1 to 20 centimeters long, so microwave frequencies are used for wireless devices such as cell phones, cordless phones, wireless LANs access for laptops, Bluetooth earphones. Antennas used include short whip antennas, rubber ducky antennas, sleeve dipoles, patch antennas, the printed circuit inverted F antenna used in cell phones, their short wavelength allows narrow beams of microwaves to be produced by conveniently small high gain antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, for radar. An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing frequency reuse by nearby transmitters. Parabolic antennas are the most used directive antennas at microwave frequencies, but horn antennas, slot antennas and dielectric lens antennas are used. Flat microstrip antennas are being used in consumer devices.
Another directive antenna practical at microwave frequencies is the phased array, a computer-controlled array of antennas which produces a beam which can be electronically steered in different directions. At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as coaxial cable and parallel wire lines, have excessive power losses, so when low attenuation is required microwaves are carried by metal pipes called waveguides. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the transmitter or the RF front end of the receiver is located at the antenna; the term microwave has a more technical meaning in electromagnetics and circuit theory. Apparatus and techniques may
Radiation pattern
In the field of antenna design the term radiation pattern refers to the directional dependence of the strength of the radio waves from the antenna or other source. In the fields of fiber optics and integrated optics, the term radiation pattern may be used as a synonym for the near-field pattern or Fresnel pattern; this refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it; the far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, the radiation pattern deduced from it by computation. The far-field radiation pattern can be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can compute the near field; the far field radiation pattern may be represented graphically as a plot of one of a number of related variables, including.
Only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a linear scale, or in dB; the plot is represented as a three-dimensional graph, or as separate graphs in the vertical plane and horizontal plane. This is known as a polar diagram, it is a fundamental property of antennas that the receiving pattern of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is proved below. Therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. Note however that this applies only to the passive antenna elements. Active antennas that include amplifiers or other components are no longer reciprocal devices. Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.
The simplest antennas and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, called omnidirectional patterns; this illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry. In most antennas, the radiation from the different parts of the antenna interferes at some angles; this results in zero radiation at certain angles where the radio waves from the different parts arrive out of phase, local maxima of radiation at other angles where the radio waves arrive in phase. Therefore, the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero; the larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the radio waves in one particular direction, the lobe in that direction is larger than the others.
The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe; the other lobes, representing unwanted radiation in other directions, are called "side lobes". The side lobe in the opposite direction from the main lobe is called the "back lobe". Minor lobes represent radiation in undesired directions, so in directional antennas a design goal is to reduce the minor lobes. Side lobes are the largest of the minor lobes; the level of minor lobes is expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is termed the side lobe ratio or side lobe level. Side lobe levels of −20 dB or greater are not desirable in many applications. Attainment of a side lobe level smaller than −30 dB requires careful design and construction. In most radar systems, for example, low side lobe ratios are important to minimize false target indications through the side lobes.
For a complete proof, see the reciprocity article. Here, we present a common simple proof limited to the approximation of two antennas separated by a large distance compared to the size of the antenna, in a homogeneous medium; the first antenna is the test antenna. The second antenna is a reference antenna; each antenna is alternately connected to a transmitter having a particular source impedance, a receiver having the same input impedance. It is assumed that the two antennas are sufficiently far apart that the properties of the transmitting antenna are not affected by the load placed upon it by the receiving antenna; the amount of power transferred from the transmitter to the receiver c
Dipole antenna
In radio and telecommunications a dipole antenna or doublet is the simplest and most used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each end. A dipole antenna consists of two identical conductive elements such as metal wires or rods; the driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors; this contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, the other side connected to some type of ground. A common example of a dipole is the "rabbit ears" television antenna found on broadcast television sets; the dipole is the simplest type of antenna from a theoretical point of view.
Most it consists of two conductors of equal length oriented end-to-end with the feedline connected between them. Dipoles are used as resonant antennas. If the feedpoint of such an antenna is shorted it will be able to resonate at a particular frequency, just like a guitar string, plucked. Using the antenna at around that frequency is advantageous in terms of feedpoint impedance, so its length is determined by the intended wavelength of operation; the most used is the center-fed half-wave dipole, just under a half-wavelength long. The radiation pattern of half-wave dipoles is maximum perpendicular to the conductor, falling to zero in the axial direction, thus implementing an omnidirectional antenna if installed vertically, or a weakly directional antenna if horizontal. Most antennas in use can be seen as based on the dipole. Although they may be used as standalone low-gain antennas, they are employed as driven elements in more complex antenna designs such as the Yagi antenna and driven arrays.
Dipole antennas are used to feed more elaborate directional antennas such as a horn antenna, parabolic reflector, or corner reflector. Engineers analyze vertical antennas on the basis of dipole antennas. German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna. On the other hand, Guglielmo Marconi empirically found that he could just ground the transmitter dispensing with one half of the antenna, thus realizing the vertical or monopole antenna. For the low frequencies Marconi employed to achieve long-distance communications, this form was more practical. In the early days of radio, the thus-named Marconi antenna and the doublet were seen as distinct inventions. Now, the "monopole" antenna is understood as a special case of a dipole which has a virtual element "underground". A short dipole is a dipole formed by two conductors with a total length L less than a half wavelength. Short dipoles are sometimes used in applications.
They can be analyzed using the results obtained below for the Hertzian dipole, a fictitious entity. Being shorter than a resonant antenna its feedpoint impedance includes a large capacitive reactance requiring a loading coil or other matching network in order to be practical as a transmitting antenna. To find the far-field electric and magnetic fields generated by a short dipole we use the result shown below for the Hertzian dipole at a distance r from the current and at an angle θ to the direction of the current, as being: H ϕ = i I h L k 4 π r e i sin E θ = ζ 0 H ϕ = i ζ 0 I h L k 4 π r e i sin where the radiator consists of a current of I h e
Transmission line
In radio-frequency engineering, a transmission line is a specialized cable or other structure designed to conduct alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses; this article covers two-conductor transmission line such as parallel line, coaxial cable and microstrip. Some sources refer to waveguide, dielectric waveguide, optical fibre as transmission line, however these lines require different analytical techniques and so are not covered by this article. Ordinary electrical cables suffice to carry low frequency alternating current, such as mains power, which reverses direction 100 to 120 times per second, audio signals. However, they cannot be used to carry currents in the radio frequency range, above about 30 kHz, because the energy tends to radiate off the cable as radio waves, causing power losses.
Radio frequency currents tend to reflect from discontinuities in the cable such as connectors and joints, travel back down the cable toward the source. These reflections act as bottlenecks. Transmission lines use specialized construction, impedance matching, to carry electromagnetic signals with minimal reflections and power losses; the distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. Types of transmission line include parallel line, coaxial cable, planar transmission lines such as stripline and microstrip; the higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength. At microwave frequencies and above, power losses in transmission lines become excessive, waveguides are used instead, which function as "pipes" to confine and guide the electromagnetic waves.
Some sources define waveguides as a type of transmission line. At higher frequencies, in the terahertz and visible ranges, waveguides in turn become lossy, optical methods, are used to guide electromagnetic waves; the theory of sound wave propagation is similar mathematically to that of electromagnetic waves, so techniques from transmission line theory are used to build structures to conduct acoustic waves. Mathematical analysis of the behaviour of electrical transmission lines grew out of the work of James Clerk Maxwell, Lord Kelvin and Oliver Heaviside. In 1855 Lord Kelvin formulated a diffusion model of the current in a submarine cable; the model predicted the poor performance of the 1858 trans-Atlantic submarine telegraph cable. In 1885 Heaviside published the first papers that described his analysis of propagation in cables and the modern form of the telegrapher's equations. In many electric circuits, the length of the wires connecting the components can for the most part be ignored; that is, the voltage on the wire at a given time can be assumed to be the same at all points.
However, when the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire. A common rule of thumb is that the cable or wire should be treated as a transmission line if the length is greater than 1/10 of the wavelength. At this length the phase delay and the interference of any reflections on the line become important and can lead to unpredictable behaviour in systems which have not been designed using transmission line theory. For the purposes of analysis, an electrical transmission line can be modelled as a two-port network, as follows: In the simplest case, the network is assumed to be linear, the two ports are assumed to be interchangeable. If the transmission line is uniform along its length its behaviour is described by a single parameter called the characteristic impedance, symbol Z0.
This is the ratio of the complex voltage of a given wave to the complex current of the same wave at any point on the line. Typical values of Z0 are 50 or 75 ohms for a coaxial cable, about 100 ohms for a twisted pair of wires, about 300 ohms for a common type of untwisted pair used in radio transmission; when sending power down a transmission line, it is desirable that as much power as possible will be absorbed by the load and as little as possible will be reflected back to the source. This can be ensured by making the load impedance equal to Z0, in which case the transmission line is said to be matched; some of the power, fed into a transmission line is lost because of its resistance. This effect is called resistive loss. At high frequencies, another effect cal
Antenna array
An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship; the radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, cancelling to reduce the power radiated in other directions. When used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements. An antenna array can achieve higher gain, a narrower beam of radio waves, than could be achieved by a single element.
In general, the larger the number of individual antenna elements used, the higher the gain and the narrower the beam. Some antenna arrays are composed of thousands of individual antennas. Arrays can be used to achieve higher gain, to give path diversity which increases communication reliability, to cancel interference from specific directions, to steer the radio beam electronically to point in different directions, for radio direction finding; the term antenna array most means a driven array consisting of multiple identical driven elements all connected to the receiver or transmitter. A parasitic array consists of a single driven element connected to the feedline, other elements which are not, called parasitic elements, it is another name for a Yagi-Uda antenna. A phased array means an electronically scanned array; the beam of radio waves can be steered electronically to point in any direction over a wide angle, without moving the antennas. However the term "phased array" is sometimes used to mean an ordinary array antenna.
Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles, don't have much directivity. To create a directional antenna, which radiates radio waves in a narrow beam, two general techniques can be used. One technique is to use reflection by large metal surfaces such as parabolic reflectors or horns, or refraction by dielectric lenses to change the direction of the radio waves, to focus the radio waves from a single low gain antenna into a beam; this type is called an aperture antenna. A parabolic dish is an example of this type of antenna. A second technique is to use multiple antennas which are fed from receiver. If the currents are fed to the antennas with the proper phase, due to the phenomenon of interference the spherical waves from the individual antennas combine in front of the array to create plane waves, a beam of radio waves traveling in a specific direction. In directions in which the waves from the individual antennas arrive in phase, the waves add together to enhance the power radiated.
In directions in which the individual waves arrive out of phase, with the peak of one wave coinciding with the valley of another, the waves cancel reducing the power radiated in that direction. When receiving, the oscillating currents received by the separate antennas from radio waves received from desired directions are in phase and when combined in the receiver reinforce each other, while currents from radio waves received from other directions are out of phase and when combined in the receiver cancel each other; the radiation pattern of such an antenna consists of a strong beam in one direction, the main lobe, plus a series of weaker beams at different angles called sidelobes representing residual radiation in unwanted directions. The larger the width of the antenna and the greater the number of component antenna elements, the narrower the main lobe, the higher the gain which can be achieved, the smaller the sidelobes will be. Arrays in which the antenna elements are fed in phase are broadside arrays.
The largest array antennas are radio interferometers used in the field of radio astronomy, in which multiple radio telescopes consisting of large parabolic antennas are linked together into an antenna array, to achieve higher resolution. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a diameter equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry dishes on separate continents have been linked, creating "array antennas" thousands of miles in size. Most array antennas can be divided into two classes based on how the component antennas' axis is related to the direction of radiation. A broadside array is a one or two dimensional array in which the direction of radiation of the radio waves is perpendicular to the plane of the antennas. To radiate perpendicularly, the antennas must be fed in phase. An endfire array is a linear array in which the direction of radiation is along the line of the antennas.
The antennas must be fed with a phase difference equal to the separ
Radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: electromagnetic radiation, such as radio waves, infrared, visible light, ultraviolet, x-rays, gamma radiation particle radiation, such as alpha radiation, beta radiation, neutron radiation acoustic radiation, such as ultrasound and seismic waves gravitational radiation, radiation that takes the form of gravitational waves, or ripples in the curvature of spacetime. Radiation is categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation carries more than 10 eV, enough to ionize atoms and molecules, break chemical bonds; this is an important distinction due to the large difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, photons, respectively.
Other sources include X-rays from medical radiography examinations and muons, positrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth's atmosphere. Gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum; the word "ionize" refers to the breaking of one or more electrons away from an atom, an action that requires the high energies that these electromagnetic waves supply. Further down the spectrum, the non-ionizing lower energies of the lower ultraviolet spectrum cannot ionize atoms, but can disrupt the inter-atomic bonds which form molecules, thereby breaking down molecules rather than atoms; the waves of longer wavelength than UV in visible light and microwave frequencies cannot break bonds but can cause vibrations in the bonds which are sensed as heat. Radio wavelengths and below are not regarded as harmful to biological systems; these are not sharp delineations of the energies.
The word radiation arises from the phenomenon of waves radiating from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation; because such radiation expands as it passes through space, as its energy is conserved, the intensity of all types of radiation from a point source follows an inverse-square law in relation to the distance from its source. Like any ideal law, the inverse-square law approximates a measured radiation intensity to the extent that the source approximates a geometric point. Radiation with sufficiently high energy can ionize atoms. Ionization occurs when an electron is stripped from an electron shell of the atom, which leaves the atom with a net positive charge; because living cells and, more the DNA in those cells can be damaged by this ionization, exposure to ionizing radiation is considered to increase the risk of cancer. Thus "ionizing radiation" is somewhat artificially separated from particle radiation and electromagnetic radiation due to its great potential for biological damage.
While an individual cell is made of trillions of atoms, only a small fraction of those will be ionized at low to moderate radiation powers. The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, is a function of the damaging tendency of the type of radiation and the sensitivity of the irradiated organism or tissue. If the source of the ionizing radiation is a radioactive material or a nuclear process such as fission or fusion, there is particle radiation to consider. Particle radiation is subatomic particle accelerated to relativistic speeds by nuclear reactions; because of their momenta they are quite capable of knocking out electrons and ionizing materials, but since most have an electrical charge, they don't have the penetrating power of ionizing radiation. The exception is neutron particles. There are several different kinds of these particles, but the majority are alpha particles, beta particles and protons. Speaking and particles with energies above about 10 electron volts are ionizing.
Particle radiation from radioactive material or cosmic rays invariably carries enough energy to be ionizing. Most ionizing radiation originates from radioactive materials and space, as such is present in the environment, since most rocks and soil have small concentrations of radioactive materials. Since this radiation is invisible and not directly detectable by human senses, instruments such as Geiger counters are required to detect its presence. In some cases, it may lead to secondary emission of visible light upon its interaction with matter, as in the case of Cherenkov radiation and radio-luminescence. Ionizing radiation has many practical uses in medicine and construction, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue.
In-phase and quadrature components
In electrical engineering, a sinusoid with angle modulation can be decomposed into, or synthesized from, two amplitude-modulated sinusoids that are offset in phase by one-quarter cycle. All three functions have the same frequency; the amplitude modulated sinusoids are known as quadrature components. In some contexts it is more convenient to refer to only the amplitude modulation itself by those terms. In vector analysis, a vector with polar coordinates A,φ and Cartesian coordinates x = A cos, y = A sin, can be represented as the sum of orthogonal "components": +. In trigonometry, the expression sin can be represented by sin cos + sin sin, and in functional analysis, when x is a linear function of some variable, such as time, these components are sinusoids, they are orthogonal functions. When φ = 0, sin reduces to just the in-phase component, sin cos, the quadrature component, sin sin, is zero. A phase-shift of x → x + π/2 changes the identity to cos = cos cos + cos sin, in which case cos cos is the in-phase component.
In both conventions cos is the in-phase amplitude modulation, which explains why some authors refer to it as the actual in-phase component. We can observe that in both conventions the quadrature component leads the in-phase component by one-quarter cycle; the term alternating current applies to a voltage vs. time function, sinusoidal with a frequency f. When it is applied to a typical circuit or device, it causes a current, sinusoidal. In general there is φ, between any two sinusoids; the input sinusoidal voltage is defined to have zero phase, meaning that it is arbitrarily chosen as a convenient time reference. So the phase difference is attributed to the current function, e.g. sin, whose orthogonal components are sin cos and sin sin, as we have seen. When φ happens to be such that the in-phase component is zero, the current and voltage sinusoids are said to be in quadrature, which means they are orthogonal to each other. In that case, no electrical power is consumed. Rather it is temporarily given back, once every 1 ⁄ f seconds.
Note that the term in quadrature only implies that two sinusoids are orthogonal, not that they are components of another sinusoid. In an angle modulation application, with carrier frequency f, φ is a time-variant function, giving: sin = sin ⋅ cos ⏟ in-phase + sin ⏞ cos ⋅ sin ⏟ quadrature; when all three terms above are multiplied by an optional amplitude function, A > 0, the left-hand side of the equality is known as the amplitude/phase form, the right-hand side is the quadrature-carrier or IQ form. Because of the modulation, the components are no longer orthogonal functions, but when A and φ are varying functions compared to 2πft, the assumption of orthogonality is a common one. Authors call it a narrowband assumption, or a narrowband signal model. Orthogonality is important in many applications, including demodulation, direction-finding, bandpass sampling. IQ imbalance Constellation diagram Phasor Polar modulation Quadrature amplitude modulation Single-sideband modulation Steinmetz, Charles Proteus.
Lectures on Electrical Engineering. 3. Mineola,NY: Dover Publications. ISBN 0486495388. Steinmetz, Charles Proteus. Theory and Calculations of Electrical Apparatus 6. New York: McGraw-Hill Book Company. B004G3ZGTM. I/Q Data for Dummies
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
