A magnetometer or magnetic sensor is an instrument that measures magnetism—either the magnetization of a magnetic material like a ferromagnet, or the direction, strength, or relative change of a magnetic field at a particular location. A compass is a simple type of magnetometer, one that measures the direction of an ambient magnetic field; the first magnetometer capable of measuring the absolute magnetic intensity was invented by Carl Friedrich Gauss in 1833 and notable developments in the 19th century included the Hall effect, still used. Magnetometers are used for measuring the Earth's magnetic field and in geophysical surveys to detect magnetic anomalies of various types, they are used in the military to detect submarines. Some countries, such as the United States and Australia, classify the more sensitive magnetometers as military technology, control their distribution. Magnetometers can be used as metal detectors: they can detect only magnetic metals, but can detect such metals at a much larger depth than conventional metal detectors.
In recent years, magnetometers have been miniaturized to the extent that they can be incorporated in integrated circuits at low cost and are finding increasing use as miniaturized compasses. Magnetic fields are vector quantities characterized by both direction; the strength of a magnetic field is measured in units of tesla in the SI units, in gauss in the cgs system of units. 10,000 gauss are equal to one tesla. Measurements of the Earth's magnetic field are quoted in units of nanotesla called a gamma; the Earth's magnetic field can vary from 20,000 to 80,000 nT depending on location, fluctuations in the Earth's magnetic field are on the order of 100 nT, magnetic field variations due to magnetic anomalies can be in the picotesla range. Gaussmeters and teslameters are magnetometers that measure in units of gauss or tesla, respectively. In some contexts, magnetometer is the term used for an instrument that measures fields of less than 1 millitesla and gaussmeter is used for those measuring greater than 1 mT.
There are two basic types of magnetometer measurement. Vector magnetometers measure the vector components of a magnetic field. Total field magnetometers or scalar magnetometers measure the magnitude of the vector magnetic field. Magnetometers used to study the Earth's magnetic field may express the vector components of the field in terms of declination and the inclination. Absolute magnetometers measure the absolute magnitude or vector magnetic field, using an internal calibration or known physical constants of the magnetic sensor. Relative magnetometers measure magnitude or vector magnetic field relative to a fixed but uncalibrated baseline. Called variometers, relative magnetometers are used to measure variations in magnetic field. Magnetometers may be classified by their situation or intended use. Stationary magnetometers are installed to a fixed position and measurements are taken while the magnetometer is stationary. Portable or mobile magnetometers are meant to be used while in motion and may be manually carried or transported in a moving vehicle.
Laboratory magnetometers are used to measure the magnetic field of materials placed within them and are stationary. Survey magnetometers are used to measure magnetic fields in geomagnetic surveys; the performance and capabilities of magnetometers are described through their technical specifications. Major specifications include; the inverse is the cycle time in seconds per reading. Sample rate is important in mobile magnetometers. Bandwidth or bandpass characterizes. For magnetometers with no onboard signal processing, bandwidth is determined by the Nyquist limit set by sample rate. Modern magnetometers may perform averaging over sequential samples. Achieving a lower noise in exchange for lower bandwidth. Resolution is the smallest change in a magnetic field. A magnetometer should have a resolution a good deal smaller than the smallest change one wishes to observe. Quantization error is caused by recording roundoff and truncation of digital expressions of the data. Absolute error is the difference between the readings of a magnetometer true magnetic field.
Drift is the change in absolute error over time. Thermal stability is the dependence of the measurement on temperature, it is given as a temperature coefficient in units of nT per degree Celsius. Noise is the random fluctuations generated by electronics. Noise is given in units of n T / H z. Sensitivity is the larger of the resolution. Heading error is the change in the measurement due to a change in orientation of the instrument in a constant magnetic field; the dead zone is the angular region of magnetometer orientation in which the instrument produces poor or no measurements. All optically pumped, proton-free precession, Overhauser magnetometers experience some dead zone effects. Gradient tolerance is the ability of a ma
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of a pipe. The Venturi effect is named after an Italian physicist. In fluid dynamics, an incompressible fluid's velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy, thus any gain in kinetic energy a fluid may occur due to its increased velocity through a constriction is balanced by a drop in pressure. By measuring the change in pressure, the flow rate can be determined, as in various flow measurement devices such as venturi meters, venturi nozzles and orifice plates. Referring to the adjacent diagram, using Bernoulli's equation in the special case of steady, inviscid flows along a streamline, the theoretical pressure drop at the constriction is given by: p 1 − p 2 = ρ 2 where ρ is the density of the fluid, v 1 is the fluid velocity where the pipe is wider, v 2 is the fluid velocity where the pipe is narrower.
The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. When a fluid system is in a state of choked flow, a further decrease in the downstream pressure environment will not lead to an increase in the mass flow rate. However, mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction; this is the principle of operation of a de Laval nozzle. Increasing source temperature will increase the local sonic velocity, thus allowing for increased mass flow rate but only if the nozzle area is increased to compensate for the resulting decrease in density; the Bernoulli equation is invertible, pressure should rise when a fluid slows down. If there is an expansion of the tube section, turbulence will appear and the theorem will not hold. Notice that in all experimental Venturi tubes, the pressure in the entrance is compared to the pressure in the middle section.
The output section is never compared with them. The simplest apparatus is a tubular setup known as a Venturi tube or a venturi. Fluid flows through a length of pipe of varying diameter. To avoid undue aerodynamic drag, a Venturi tube has an entry cone of 30 degrees and an exit cone of 5 degrees. Venturi tubes are used in processes where permanent pressure loss is not tolerable and where maximum accuracy is needed in case of viscous liquids. Venturi tubes are more expensive to construct than simple orifice plates, both function on the same basic principle. However, for any given differential pressure, orifice plates cause more permanent energy loss. Both venturis and orifice plates are used in industrial applications and in scientific laboratories for measuring the flow rate of liquids. A venturi can be used to measure the volumetric flow rate, Q. Since Q = v 1 A 1 = v 2 A 2 p 1 − p 2 = ρ 2 Q = A 1 2 ρ ⋅ p 1 − p 2 2 − 1 = A 2 2 ρ ⋅ p 1 − p 2 1 − 2 A venturi can be used to mix a liquid with a gas.
If a pump forces the liquid through a tube connected to a system consisting of a venturi to increase the liquid speed, a short piece of tube with a small hole in it, last a venturi that de
Paul B. MacCready Jr. was an American aeronautical engineer. He was the founder of AeroVironment and the designer of the human-powered aircraft that won the first Kremer prize, he devoted his life to developing more efficient transportation vehicles that could "Do more with less". Born in New Haven, Connecticut to a medical family, MacCready was an inventor from an early age and won a national contest building a model flying machine at the age of 15. "I was always the smallest kid in the class... by a good bit, was not coordinated, not the athlete type, who enjoyed running around outside, was kind of immature, not the comfortable leader, teenager type. And so, when I began getting into model airplanes, getting into contests and creating new things, I got more psychological benefit from that than I would have from some of the other typical school things."MacCready graduated from Hopkins School in 1943 and trained as a US Navy pilot before the end of World War II. He received a BS in physics from Yale University in 1947, an MS in physics from Caltech in 1948, a PhD in aeronautics from Caltech in 1952.
In 1951, MacCready founded Meteorology Research Inc, to do atmospheric research. Some of MacCready's work as a graduate student involved cloud seeding, he was an early pioneer of the use of aircraft to study meteorological phenomena, he started gliding after World War II and was a three-time winner of the Richard C. du Pont Memorial Trophy, awarded annually to the U. S. National Open Class Soaring Champion. In 1956, he became the first American pilot to become the World Soaring Champion, he invented a device that told pilots the best speed to fly a glider, depending on conditions and based on the glider's rate of sink at different air-speeds. Glider pilots still use the "MacCready speed ring". In the 1970s, he guaranteed a business loan for a friend, which subsequently failed, leaving him with a $100,000 debt; this was the motivation he needed to compete for the £50,000 Kremer prize for human-powered flight, on offer for 18 years. With Dr. Peter B. S. Lissaman, he created the Gossamer Condor; the Condor stayed aloft for seven minutes while it completed the required figure eight course, thereby winning the first Kremer prize in 1977.
The award-winning plane was constructed of aluminium tubing, plastic foam, piano wire, bicycle parts, mylar foil for covering. Kremer offered another £100,000 for the first human-powered crossing of the English Channel. MacCready took up the challenge and in 1979, he built the Condor's successor, the Gossamer Albatross, won the second Kremer prize flying from England to France, he received the Collier Trophy, awarded annually for the greatest achievement in aeronautics or astronautics, for his design and construction of the Albatross. He created solar-powered aircraft such as the Gossamer Penguin and the Solar Challenger, he was involved in the development of NASA's solar-powered flying wings such as the Helios, which surpassed the SR-71's altitude records and could theoretically fly on Mars. MacCready collaborated with General Motors on the design of the Sunraycer, a solar-powered car, on the EV-1 electric car. In 1985, he was commissioned to build a halfscale working replica of the pterosaur Quetzalcoatlus for the Smithsonian Institution, following a workshop in 1984, which concluded that such a replica was feasible.
The completed remote-controlled flying reptile, with a wingspan of 18 feet, was filmed over Death Valley, California in 1986 for the Smithsonian's IMAX film On the Wing. It flew several times before being damaged in a crash at an airshow at Andrews AFB in Maryland; the launch of the pterosaur model came off well but the radio transmitter link failed due to the interference from some of the many base communications devices. The model crashed at the runway side, breaking at the neck from the force of impact. MacCready helped to sponsor the Nissan Dempsey/MacCready Prize which has helped to motivate developments in racing-bicycle technology, applying aerodynamics and new materials to allow for faster human-powered vehicles, he was the founder and Chairman of AeroVironment Inc. a public company that develops unmanned surveillance aircraft and advance power systems. AV flew a prototype of the first airplane to be powered by hydrogen fuel cells, the Global Observer. MacCready died on August 2007 from metastatic melanoma.
He was a skeptic. He was survived by his wife Judy, his three sons Parker and Marshall and two grandchildren. Induction to the U. S. Soaring Hall of Fame, 1954 Otto Lilienthal Medal of the Fédération Aéronautique Internationale, 1956 California Institute of Technology, Distinguished Alumni Award, 1978, Collier Trophy, 1979, by the National Aeronautics Association Reed Aeronautical Award, 1979, by the American Institute of Aeronautics and Astronautics Edward Longstreth Medal, 1979, by the Franklin Institute Engineer of the Century Gold Medal, 1980, by the American Society of Mechanical Engineers Spirit of St. Louis Medal, 1980 Inventor of the Year Award, 1981, by the Association for the Advancement of Invention and Innovation Klemperer Award, 1981, Organisation Scientifique et Technique du Vol à Voile, Germany I. B. Laskowitz Award, 1981, New York Academy of Science The Lindbergh Award, 1982, by the Lindbergh Foundation ("to a person who contributes to achieving
A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, share several key functional parts; this is the most common type of transformer used in electric power transmission and appliances to convert mains voltage to low voltage to power electronic devices. They are available in power ratings ranging from mW to MW; the insulated laminations minimizes eddy current losses in the iron core. Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings; the rectangular cores are made up of stampings in E-I shape pairs, but other shapes are sometimes used. Shields between primary and secondary may be fitted to reduce EMI, or a screen winding is used. Small appliance and electronics transformers may have a thermal cut-out built into the winding, to shut-off power at high temperatures to prevent further overheating.
Doughnut-shaped toroidal transformers save space compared to E-I cores, may reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring, tape for insulation. Toroidal transformers have a lower external magnetic field compared to rectangular transformers, can be smaller for a given power rating. However, they cost more to make, as winding requires slower equipment, they can be mounted by a bolt through the center, using washers and rubber pads or by potting in resin. An autotransformer has one winding, tapped at some point along the winding. Voltage is applied across a portion of the winding, a higher voltage is produced across another portion of the same winding; the equivalent power rating of the autotransfomer is lower than the actual load power rating. It is calculated by: load VA × /Vin. For example, an auto transformer that adapts a 1000 VA load rated at 120 volts to a 240 volt supply has an equivalent rating of at least: 1,000 VA / 240 V = 500 VA.
However, the actual rating must be at least 1000 VA. For voltage ratios that don't exceed about 3:1, an autotransformer is cheaper, lighter and more efficient than an isolating transformer of the same rating. Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks. By exposing part of the winding coils of an autotransformer, making the secondary connection through a sliding carbon brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for wide voltage adjustment in small increments; the induction regulator is similar in design to a wound-rotor induction motor but it is a transformer whose output voltage is varied by rotating its secondary relative to the primary—i.e. Rotating the angular position of the rotor, it can be seen as a power transformer exploiting rotating magnetic fields. The major advantage of the induction regulator is that unlike variacs, they are practical for transformers over 5 kVA.
Hence, such regulators find widespread use in high-voltage laboratories. For polyphase systems, multiple single-phase transformers can be used, or all phases can be connected to a single polyphase transformer. For a three phase transformer, the three primary windings are connected together and the three secondary windings are connected together. Examples of connections are wye-delta, delta-wye, delta-delta, wye-wye. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth, the earth connection point is the center point of a wye winding. If the secondary is a delta winding, the ground may be connected to a center tap on one winding or one phase may be grounded. A special purpose polyphase transformer is the zigzag transformer. There are many possible configurations that may involve more or fewer than six windings and various tap connections. Grounding or earthing transformers let three wire polyphase system supplies accommodate phase to neutral loads by providing a return path for current to a neutral.
Grounding transformers most incorporate a single winding transformer with a zigzag winding configuration but may be created with a wye-delta isolated winding transformer connection. This is a specialized type of transformer which can be configured to adjust the phase relationship between input and output; this allows power flow in an electric grid to be controlled, e.g. to steer power flows away from a shorter link to a longer path with excess capacity. A variable-frequency transformer is a specialized three-phase power transformer which allows the phase relationship between the input and output windings to be continuously adjusted by rotating one half, they are used to interconnect electrical grids with the same nominal frequency but without synchronous phase coordination. A leakage transformer called a stray-field transformer, has a higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, sometimes adjustable with a set screw.
This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. In this case, it is short-circuit inductance, acting as a current limiting parameter; the output and input currents are low enough to prevent thermal overload under all load conditions—even if the secondary is shorted. Le
Polar curve (aerodynamics)
A polar curve is a graph which contrasts the sink rate of an aircraft with its horizontal speed. Knowing the best speed to fly is important in exploiting the performance of a glider. Two of the key measures of a glider’s performance are its minimum sink rate and its best glide ratio known as the best'glide angle'; these occur at different speeds. Knowing these speeds is important for efficient cross-country flying. In still air the polar curve shows that flying at the minimum sink speed enables the pilot to stay airborne for as long as possible and to climb as as possible, but at this speed the glider will not travel as far as if it flew at the speed for the best glide; when in sinking air, the polar curve shows that best speed to fly depends on the rate that the air is descending. Using Paul MacCready's theory, the optimal speed to fly for best cross country speed may be in excess of the speed for the best glide angle to get out of the sinking air as as possible; the glide ratio is expressed as the ratio of the distance travelled to height lost in the same time.
The ratio of the horizontal speed versus the vertical speed gives the same answer. (If the glider flies at 40 knots for an hour and experiences a 2-knot sink rate, it will travel 40 nautical miles and descend 2 nautical miles. The glide ratio is 20 using both methods; the effect of wind and sink on best glide speed is to move the curve within the plot by the amount of each component. That is, if flying into a headwind, with no vertical air movement, the curve would move left toward the origin by an amount equal to the velocity of the wind; the effect is the tangent line for best glide speed moves further down the graph for an increasing best glide speed but a lower best glide ratio. Thus, when flying into a head wind, the best glide speed is higher but the best glide ratio is lower. Conversely, for a tail wind, the polar curve moves away from the origin so that best glide speed is lower and the effective glide ratio is improved. In lift, move the curve up for a lower best glide speed and better glide ratio.
In sink, move the curve down for a higher best glide speed and a worse glide ratio. The effect is that when flying between thermals, you would slow down in rising air and speed up when encountering sink. Wind with lift/sink would move the plot the according amount for each component. By measuring the rate of sink at various air-speeds a set of data can be accumulated and plotted on a graph; the points can be connected by a line known as the ‘polar curve’. Each type of glider has a unique polar curve; the curve can be degraded with debris such as bugs and rain on the wing. Published polar curves will be shown for a clean wing in addition to a dirty wing with bug splats represented by small pieces of tape applied to the leading edge of the wing; the origin for a polar curve is where the sink rate is zero. In the first diagram a line has been drawn from the origin to the point with minimum sink; the slope of the line from the origin gives the glide angle, because it is the ratio of the distance along the airspeed axis to the distance along the sink rate axis.
A whole series of lines could be drawn from the origin to each of the data points, each line showing the glide angle for that speed. However the best glide angle is the line with the least slope. In the second diagram, the line has been drawn from the origin to the point representing the best glide ratio; the air-speed and sink rate at the best glide ratio can be read off the graph. Note that the best glide ratio is shallower than the glide ratio for minimum sink. All the other lines from the origin to the various data points would be steeper than the line of the best glide angle; the line for the best glide angle will only just graze the polar curve, i.e. it is a tangent. A polar curve is in a strict sense. Although this is never the case for any "polar curve" in aerodynamics, tradition continues to call lots of curves expressing flight or profile characteristics "polar curves", despite all of them being expressed in Cartesian Orthogonal coordinates; the origin of this tradition has faded from knowledge.
Drag coefficient Lift coefficient Angle of attack Lift Glider Performance Airspeeds An animated explanation of the basic polar curve, with modifications for sinking or rising air and for head- or tailwinds. Reichmann, Helmut. Streckensegelflug. Motorbuch Verlag. ISBN 3-613-02479-9
Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as altitude and direction. They improve safety by allowing the pilot to fly the aircraft in level flight, make turns, without a reference outside the aircraft such as the horizon. Visual flight rules require an airspeed indicator, an altimeter, a compass or other suitable magnetic direction indicator. Instrument flight rules additionally require a gyroscopic pitch-bank and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, a clock. Flight into Instrument meteorological conditions require radio navigation instruments for precise takeoffs and landings; the term is sometimes used loosely as a synonym for cockpit instruments as a whole, in which context it can include engine instruments and communication equipment. Many modern aircraft have electronic flight instrument systems. Most regulated aircraft have these flight instruments as dictated by the US Code of Federal Regulations, Title 14, Part 91.
They are grouped according to pitot-static system, compass systems, gyroscopic instruments. The altimeter shows the aircraft's altitude above sea-level by measuring the difference between the pressure in a stack of aneroid capsules inside the altimeter and the atmospheric pressure obtained through the static system, it is adjustable for local barometric pressure which must be set to obtain accurate altitude readings. As the aircraft ascends, the capsules expand and the static pressure drops, causing the altimeter to indicate a higher altitude; the opposite effect occurs. With the advancement in aviation and increased altitude ceiling, the altimeter dial had to be altered for use both at higher and lower altitudes. Hence when the needles were indicating lower altitudes i.e. the first 360-degree operation of the pointers was delineated by the appearance of a small window with oblique lines warning the pilot that he or she is nearer to the ground. This modification was introduced in the early sixties after the recurrence of air accidents caused by the confusion in the pilot's mind.
At higher altitudes, the window will disappear. The airspeed indicator shows the aircraft's speed relative to the surrounding air, it works by measuring the ram-air pressure in the aircraft's Pitot tube relative to the ambient static pressure. The indicated airspeed must be corrected for nonstandard pressure and temperature in order to obtain the true airspeed; the instrument is color coded to indicate important airspeeds such as the stall speed, never-exceed airspeed, or safe flap operation speeds. The VSI senses changing air pressure, displays that information to the pilot as a rate of climb or descent in feet per minute, meters per second or knots; the compass shows. Errors include Variation, or the difference between magnetic and true direction, Deviation, caused by the electrical wiring in the aircraft, which requires a Compass Correction Card. Additionally, the compass is subject to Dip Errors. While reliable in steady level flight it can give confusing indications when turning, descending, or accelerating due to the inclination of the Earth's magnetic field.
For this reason, the heading indicator is used for aircraft operation, but periodically calibrated against the compass. The attitude indicator shows the aircraft's relation to the horizon. From this the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon; this is a primary instrument for instrument flight and is useful in conditions of poor visibility. Pilots are trained to use other instruments in combination should its power fail; the heading indicator displays the aircraft's heading with respect to magnetic north when set with a compass. Bearing friction causes drift errors from precession, which must be periodically corrected by calibrating the instrument to the magnetic compass. In many advanced aircraft, the heading indicator is replaced by a horizontal situation indicator which provides the same heading information, but assists with navigation; these include the Turn-and-Slip Indicator and the Turn Coordinator, which indicate rotation about the longitudinal axis.
They include an inclinometer to indicate if the aircraft is in Coordinated flight, or in a Slip or Skid. Additional marks indicate a Standard rate turn; these include Attitude Director Indicator. The HSI combines the magnetic compass with a Glide slope; the navigation information comes from a VOR/Localizer, or GPS. The ADI is an Attitude Indicator with computer-driven steering bars, a task reliever during instrument flight; the VOR indicator instrument includes a Course deviation indicator, Omnibearing Selector, TO/FROM indicator, Flags. The CDI shows an aircraft's lateral position in relation to a selected radial track, it is used for orientation, tracking to or from a station, course interception. On the instrument, the vertical needle indicates the lateral position of the selected track. An horizontal needle allows the pilot to follow a glide slope when the instrument is used with an ILS; the Automatic direction finder indicator instrument can be a fixed-card, movable card, or a Radio magnetic indicator.
An RMI is remotely coupled to a gyrocompass so that it automatically rotates
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are