Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; the chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and building industries, such as building facades and window frames; the oxides and sulfates are the most useful compounds of aluminium. Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals; because of these salts' abundance, the potential for a biological role for them is of continuing interest, studies continue.
Of aluminium isotopes, only 27Al is stable. This is consistent with aluminium having an odd atomic number, it is the only aluminium isotope that has existed on Earth in its current form since the creation of the planet. Nearly all the element on Earth is present as this isotope, which makes aluminium a mononuclidic element and means that its standard atomic weight equates to that of the isotope; the standard atomic weight of aluminium is low in comparison with many other metals, which has consequences for the element's properties. All other isotopes of aluminium are radioactive; the most stable of these is 26Al and therefore could not have survived since the formation of the planet. However, 26Al is produced from argon in the atmosphere by spallation caused by cosmic ray protons; the ratio of 26Al to 10Be has been used for radiodating of geological processes over 105 to 106 year time scales, in particular transport, sediment storage, burial times, erosion. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
The remaining isotopes of aluminium, with mass numbers ranging from 21 to 43, all have half-lives well under an hour. Three metastable states are known, all with half-lives under a minute. An aluminium atom has 13 electrons, arranged in an electron configuration of 3s23p1, with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three ionization energies of aluminium are far lower than the fourth ionization energy alone. Aluminium can easily surrender its three outermost electrons in many chemical reactions; the electronegativity of aluminium is 1.61. A free aluminium atom has a radius of 143 pm. With the three outermost electrons removed, the radius shrinks to 39 pm for a 4-coordinated atom or 53.5 pm for a 6-coordinated atom. At standard temperature and pressure, aluminium atoms form a face-centered cubic crystal system bound by metallic bonding provided by atoms' outermost electrons; this crystal system is shared by some other metals, such as copper. Aluminium metal, when in quantity, is shiny and resembles silver because it preferentially absorbs far ultraviolet radiation while reflecting all visible light so it does not impart any color to reflected light, unlike the reflectance spectra of copper and gold.
Another important characteristic of aluminium is its low density, 2.70 g/cm3. Aluminium is a soft, lightweight and malleable with appearance ranging from silvery to dull gray, depending on the surface roughness, it is nonmagnetic and does not ignite. A fresh film of aluminium serves as a good reflector of visible light and an excellent reflector of medium and far infrared radiation; the yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has stiffness of steel, it is machined, cast and extruded. Aluminium atoms are arranged in a face-centered cubic structure. Aluminium has a stacking-fault energy of 200 mJ/m2. Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.
Aluminium is the most common material for the fabrication of superconducting qubits. Aluminium's corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the bare metal is exposed to air preventing further oxidation, in a process termed passivation; the strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is reduced by aqueous salts in the presence of dissimilar metals. In acidic solutions, aluminium reacts with water to form hydrogen, in alkaline ones to form aluminates—protective passivation under these conditions is negligible; because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium. However, because
Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables have an insulating outer sheath or jacket; the term coaxial comes from the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880. Coaxial cable is a type of transmission line, used to carry high frequency electrical signals with low losses, it is used in such applications as telephone trunklines, broadband internet networking cables, high speed computer data busses, carrying cable television signals, connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, needed for it to function efficiently as a transmission line. Coaxial cable is used as a transmission line for radio frequency signals.
Its applications include feedlines connecting radio transmitters and receivers to their antennas, computer network connections, digital audio, distribution of cable television signals. One advantage of coaxial over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors; this allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable provides protection of the signal from external electromagnetic interference. Coaxial cable conducts electrical signal using an inner conductor surrounded by an insulating layer and all enclosed by a shield one to four layers of woven metallic braid and metallic tape; the cable is protected by an outer insulating jacket. The shield is kept at ground potential and a signal carrying voltage is applied to the center conductor; the advantage of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield.
Conversely and magnetic fields outside the cable are kept from interfering with signals inside the cable. Larger diameter cables and cables with multiple shields have less leakage; this property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the environment or for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, computer and instrumentation data connections; the characteristic impedance of the cable is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors. In radio frequency systems, where the cable length is comparable to the wavelength of the signals transmitted, a uniform cable characteristic impedance is important to minimize loss; the source and load impedances are chosen to match the impedance of the cable to ensure maximum power transfer and minimum standing wave ratio.
Other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, shield quality. Coaxial cable design choices affect physical size, frequency performance, power handling capabilities, flexibility and cost; the inner conductor might be stranded. To get better high-frequency performance, the inner conductor may be silver-plated. Copper-plated steel wire is used as an inner conductor for cable used in the cable TV industry; the insulator surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of the dielectric insulator determine some of the electrical properties of the cable. A common choice is a solid polyethylene insulator, used in lower-loss cables. Solid Teflon is used as an insulator; some coaxial lines have spacers to keep the inner conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield; this allows the cable to be flexible, but it means there are gaps in the shield layer, the inner dimension of the shield varies because the braid cannot be flat.
Sometimes the braid is silver-plated. For better shield performance, some cables have a double-layer shield; the shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers, such as "quad-shield", which uses four alternating layers of foil and braid. Other shield designs sacrifice flexibility for better performance; those cables cannot be bent as the shield will kink, causing losses in the cable. When a foil shield is used a small wire conductor incorporated into the foil makes soldering the shield termination easier. For high-power radio-frequency transmission up to about 1 GHz, coaxial cable with a solid copper outer conductor is available in sizes of 0.25 inch upward. The outer conductor is corrugated like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric. One brand name for such cable is Heliax. Coaxial cables require an internal structure of an insulating material to maintain the spacing between the center conductor and shield.
A discone antenna is a version of a biconical antenna in which one of the cones is replaced by a disc. It is mounted vertically, with the disc at the top and the cone beneath. Omnidirectional, vertically polarized and with gain similar to a dipole, it is exceptionally wideband, offering a frequency range ratio of up to 10:1; the radiation pattern in the vertical plane is quite narrow, making its sensitivity highest in the direction of the horizon and rather less for signals coming from close by. On February 6, 1945, Armig G. Kandoian of New York City was awarded U. S. patent number 2,368,663 (assignor to Federal Telephone and Radio Corporation for a "broad band antenna", from an application made on May 15, 1943. Excerpt from the Kandoian patent: In keeping with progress made during the last few years in the development of ultra-high frequency radio technique, applications thereof to aircraft communication, direction finding, so forth, it has become necessary to develop special antennas and antenna systems suitable for installation on such aircraft.
Flying conditions are such that these antennas must be small and rigid in their construction and offer a minimum of wind resistance, in order that the flying efficiency of the aircraft will be unimpaired. In accordance with my invention I have provided a small rigid antenna suitable for mounting on the surface of the fuselage or other component of the airplane structure and in certain embodiments I have provided a streamlined protecting shield or housing covering or so cooperating with the construction of the antenna system as to reduce wind resistance; the discone antenna has a useful frequency range of at least 10 to 1. When employed as a transmitting antenna, it is less efficient than an antenna designed for a more limited frequency range. SWR is 1.5:1 or less over several octaves of frequency. A discone antenna consists of three main parts: the disc, the cone, the insulator; the disc: The disc should have an overall diameter of 0.7 times a quarter wavelength of the antenna's lowest frequency.
The antenna's feed point is at the center of the disc. It is fed with 50-ohm coaxial cable, with the center conductor connected to the disc, the outer conductor to the cone; the cone: The length of the cone should be a quarter wavelength of the antenna's lowest operating frequency. The cone angle is from 25 to 40 degrees; the insulator: The disc and cone must be separated by an insulator, the dimensions of which determine some of the antenna's properties on near its high frequency limit. A discone may be made from solid metal sheet, practical for small indoor UHF antennas, such as for Wi-Fi. At lower frequencies a sufficient number of metal wires or rods in a spoke configuration is used to approximate a solid surface; this reduces wind loading. The spokes may be made of stiff wire, brazing rods or coat hanger wire; the optimal number of rods comprising the disc and cone is quoted as being from 8 to 16. The discone's wideband coverage makes it attractive in commercial, amateur radio and radio scanner applications.
The discone's inherently wideband nature permits it to broadcast undesirable spurious emissions from faulty or improperly filtered transmitters. A vertical whip may be affixed to the center of the disc in order to extend the low frequency response, but this may compromise efficiency at higher frequencies. In this configuration, at lower frequencies the discone may more resemble a ground plane antenna or a coaxial dipole. Antenna Antenna types Biconical antenna Very high frequency Ultra high frequency Radio scanner Amateur radio UHF Discone Antenna The Discone Antenna Parabolic Discone Broadband radial discone antenna: Design and measurements Discone antenna basics
A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance over dipole antennas—or omnidirectional antennas in general—when greater concentration of radiation in a certain direction is desired. A high-gain antenna is a directional antenna with a narrow radiowave beam width; this narrow beam width allows more precise targeting of the radio signals. Most referred to during space missions, these antennas are in use all over Earth, most in flat, open areas where no mountains lie to disrupt radiowaves. By contrast, a low-gain antenna is an omnidirectional antenna with a broad radiowave beam width, that allows the signal to propagate reasonably well in mountainous regions and is thus more reliable regardless of terrain. Low-gain antennas are used in spacecraft as a backup to the high-gain antenna, which transmits a much narrower beam and is therefore susceptible to loss of signal.
All practical antennas are at least somewhat directional, although only the direction in the plane parallel to the earth is considered, practical antennas can be omnidirectional in one plane. The most common types are the Yagi antenna, the log-periodic antenna, the corner reflector antenna, which are combined and commercially sold as residential TV antennas. Cellular repeaters make use of external directional antennas to give a far greater signal than can be obtained on a standard cell phone. Satellite Television receivers use parabolic antennas. For long and medium wavelength frequencies, tower arrays are used in most cases as directional antennas; when transmitting, a high-gain antenna allows more of the transmitted power to be sent in the direction of the receiver, increasing the received signal strength. When receiving, a high gain antenna captures more of the signal, again increasing signal strength. Due to reciprocity, these two effects are equal—an antenna that makes a transmitted signal 100 times stronger will capture 100 times as much energy as the isotropic antenna when used as a receiving antenna.
As a consequence of their directivity, directional antennas send less signal from directions other than the main beam. This property may be used to reduce interference. There are many ways to make a high-gain antenna. Horn antennas can be constructed with high gain, but are less seen. Still other configurations are possible—the Arecibo Observatory uses a combination of a line feed with an enormous spherical reflector, to achieve high gains at specific frequencies. Antenna gain is quoted with respect to a hypothetical antenna that radiates in all directions, an isotropic radiator; this gain, when measured in decibels, is called dBi. Conservation of energy dictates. For example, if a high gain antenna makes a 1 watt transmitter look like a 100 watt transmitter the beam can cover at most 1⁄100 of the sky. In turn this implies that high-gain antennas must be physically large, since according to the diffraction limit, the narrower the beam desired, the larger the antenna must be. Antenna gain can be measured in dBd, gain in Decibels compared to the maximum intensity direction of a half wave dipole.
In the case of Yagi type aerials this more or less equates to the gain one would expect from the aerial under test minus all its directors and reflector. It is important not to confuse dBd. Gain is dependent on the number of elements and the tuning of those elements. Antennas can be tuned to be resonant over a wider spread of frequencies but, all other things being equal, this will mean the gain of the aerial is lower than one tuned for a single frequency or a group of frequencies. For example, in the case of wideband TV antennas the fall off in gain is large at the bottom of the TV transmitting band. In the UK this bottom third of the TV band is known as group A. Other factors may affect gain such as aperture, efficiency; these factors are easy to improve without adjusting other features of the antennas or coincidentally improved by the same factors that increase directivity, so are not emphasized. High gain antennas are the largest component of deep space probes, the highest gain radio antennas are physically enormous structures, such as the Arecibo Observatory.
The Deep Space Network uses 35 m dishes at about 1 cm wavelengths. This combination gives the antenna gain of about 100,000,000, making the transmitter appear about 100 million times stronger, a receiver about 100 million times more sensitive, provided the target is within the beam; this beam can cover at most one hundred millionth of the sky, so accurate pointing is required. Use of high gain and Millimeter-
A batwing or super turnstile antenna is a type of broadcasting antenna used at VHF and UHF frequencies, named for its distinctive shape which resembles a bat wing or bow tie. Stacked arrays of batwing antennas are used as television broadcasting antennas due to their omnidirectional characteristics. Batwing antennas generate a horizontally polarized signal; the advantage of the "batwing" design for television broadcasting is. It was the first used television broadcasting antenna. Batwing antennas are a specialized type of crossed dipole antenna, a variant of the turnstile antenna. Two pairs of identical vertical batwing-shaped elements are mounted at right angles around a common mast. Element “wings” on opposite sides are fed as a dipole. To generate an omnidirectional pattern, the two dipoles are fed 90° out of phase; the antenna radiates horizontally polarized radiation in the horizontal plane. Each group of four elements at a single level is referred to as a bay; the radiation pattern is close to omnidirectional but has four small lobes in the directions of the four elements.
To reduce power radiated in the unwanted axial directions, in broadcast applications multiple bays fed in phase are stacked vertically with a spacing of one wavelength, to create a collinear array. This generates an omnidirectional radiation pattern with increased horizontal gain, suitable for terrestrial broadcasting; the "batwing" shape of the elements is used because it gives the antenna a wide bandwidth of 20% of operating frequency at a VSWR of 1.1:1. This makes the antenna design suitable for broadcasters who wish to use a single antenna to transmit multiple television signals and thus made the batwing the preferred antenna for lowband TV stations in the early days of broadcast television. Turnstile antenna Y. T. Lo and S. W. Lee "Antenna Handbook" Vol III: Antenna Applications. ISBN V10 0442015941 / ISDN V13 978-0442015947 Markley, Don. "Television antenna systems."'Broadcast Engineering.' 1 Apr 2004. Milligan, Thomas A.'Modern antenna design.' Wiley-IEEE Press, 2005. ISBN 978-0-471-45776-3 Sclater, Neil.'Electronics technology handbook.'
McGraw-Hill Professional, 1999. ISBN 0-07-058048-0
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
In electronics, a remote control is a component of an electronic device used to operate the device from a distance wirelessly. For example, in consumer electronics, a remote control can be used to operate devices such as a television set, DVD player, or other home appliance, from a short distance. A remote control is a convenience feature for the user, can allow operation of devices that are out of convenient reach for direct operation of controls. In some cases, remote controls allow a person to operate a device that they otherwise would not be able to reach, as when a garage door opener is triggered from outside or when a Digital Light Processing projector, mounted on a high ceiling is controlled by a person from the floor level. Early television remote controls used ultrasonic tones. Present-day remote controls are consumer infrared devices which send digitally-coded pulses of infrared radiation to control functions such as power, channels, track change, fan speed, or other features varying from device to device.
Remote controls for these devices are small wireless handheld objects with an array of buttons for adjusting various settings such as television channel, track number, volume. For many devices, the remote control contains all the function controls while the controlled device itself has only a handful of essential primary controls; the remote control code, thus the required remote control device, is specific to a product line, but there are universal remotes, which emulate the remote control made for most major brand devices. Remote control has continually evolved and advanced in the 2000s to include Bluetooth connectivity, motion sensor-enabled capabilities and voice control. In 1894, the first example of wirelessly controlling at a distance was during a demonstration by the British physicist Oliver Lodge, in which he made use of a Branly's coherer to make a mirror galvanometer move a beam of light when an electromagnetic wave was artificially generated; this was further refined by radio innovators Guglielmo Marconi and William Preece, at a demonstration that took place on December 12, 1896, at Toynbee Hall in London, in which they made a bell ring by pushing a button in a box, not connected by any wires.
In 1898 Nikola Tesla filed his patent, U. S. Patent 613,809, named Method of an Apparatus for Controlling Mechanism of Moving Vehicle or Vehicles, which he publicly demonstrated by radio-controlling a boat during an electrical exhibition at Madison Square Garden. Tesla called his boat a "teleautomaton". In 1903, Leonardo Torres Quevedo presented the Telekino at the Paris Academy of Science, accompanied by a brief, making an experimental demonstration. At the same time, he obtained a patent in France, Great Britain, the United States; the Telekino consisted of a robot. With the Telekino, Torres-Quevedo laid down modern wireless remote-control operation principles and was a pioneer in the field of remote control. In 1906, in the presence of the king and before a great crowd, Torres demonstrated the invention in the port of Bilbao, guiding a boat from the shore, he would try to apply the Telekino to projectiles and torpedoes but had to abandon the project for lack of financing. The first remote-controlled model airplane flew in 1932, the use of remote control technology for military purposes was worked intensively during the Second World War, one result of this being the German Wasserfall missile.
By the late 1930s, several radio manufacturers offered remote controls for some of their higher-end models. Most of these were connected to the set being controlled by wires, but the Philco Mystery Control was a battery-operated low-frequency radio transmitter, thus making it the first wireless remote control for a consumer electronics device. Using pulse-count modulation, this was the first digital wireless remote control; the first remote intended to control a television was developed by Zenith Radio Corporation in 1950. The remote, called "Lazy Bones," was connected to the television by a wire. A wireless remote control, the "Flashmatic," was developed in 1955 by Eugene Polley, it worked by shining a beam of light onto one of four photoelectric cells, but the cell did not distinguish between light from the remote and light from other sources. The Flashmatic had to be pointed precisely at one of the sensors in order to work. In 1956, Robert Adler developed "Zenith Space Command," a wireless remote.
It was mechanical and used ultrasound to change the volume. When the user pushed a button on the remote control, it struck a bar and clicked, hence they were called a "clicker," but it sounded like a "clink" and the mechanics were similar to a pluck; each of the four bars emitted a different fundamental frequency with ultrasonic harmonics, circuits in the television detected these sounds and interpreted them as channel-up, channel-down, sound-on/off, power-on/off. The rapid decrease in price of transistors made possible cheaper electronic remotes that contained a piezoelectric crystal, fed by an oscillating electric current at a frequency near or above the upper threshold of human hearing, though still audible to dogs; the receiver contained a microphone attached to a circuit, tuned to the same frequency. Some problems with this method were that the receiver could be triggered accidentally by occurring noises or deliberately by metal against glass, for example, some people could hear the lower ultrasonic harmonics.
The impetus for a more complex type of television remote control came in 1973, with the development of the Ceefax teletext service by the BBC. Most commercial remote controls at that tim