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A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. Loading coils are historically also known as Pupin coils after Mihajlo Pupin, especially when used for the Heaviside condition and the process of inserting them is sometimes called pupinization. The concept of loading coils was discovered by Oliver Heaviside in studying the problem of slow signalling speed of the first transatlantic telegraph cable in the 1860s. A common application of loading coils is to improve the voice-frequency amplitude response characteristics of the twisted balanced pairs in a telephone cable.
Loading coils inserted periodically in series with a pair of wires reduce the attenuation at the higher voice frequencies up to the cutoff frequency of the low-pass filter formed by the inductance of the coils (plus the distributed inductance of the wires) and the distributed capacitance between the wires.
Without loading coils, the line response is dominated by the resistance and capacitance of the line with the attenuation gently increasing with frequency.
When loading coils are in place, signal attenuation remains low for signals within the passband of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. Enormous antenna loading coil used in a powerful longwave radiotelegraphy transmitter in New Jersey in 1912. In many cases for practical reasons it is necessary to make the antenna shorter than the resonant length. So to make an electrically short antenna resonant, an inductor called a loading coil is inserted in series with the antenna. The loading coil is usually inserted at the base of the antenna, between it and the transmission line (base loading), but sometimes it is inserted in the center of the antenna element itself (center loading). The Campbell equation is a relationship due to George Ashley Campbell for predicting the propagation constant of a loaded line. A more engineer friendly rule of thumb is that the approximate requirement for spacing loading coils is ten coils per wavelength of the maximum frequency being transmitted.[2] This approximation can be arrived at by treating the loaded line as a constant k filter and applying image filter theory to it.
The phenomenon of cutoff whereby frequencies above the cutoff frequency are not transmitted is an undesirable side effect of loading coils (although it proved highly useful in the development of filters). The origin of the loading coil can be found in the work of Oliver Heaviside on the theory of transmission lines.
Heaviside was aware that this condition was not met in the practical telegraph cables in use in his day. This is mainly due to the low value of leakage through the cable insulator, which is even more pronounced in modern cables which have better insulators than in Heaviside's day. Heaviside immediately (1887) proposed several methods of increasing the inductance, including spacing the conductors further apart and loading the insulator with iron dust. Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. Campbell's work on loading coils provided the theoretical basis for his subsequent work on filters which proved to be so important for frequency-division multiplexing. Michael Pupin, inventor and Serbian immigrant to the USA, also played a part in the story of loading coils. Pupin's 1894 patent "loads" the line with capacitors rather than inductors, a scheme that has been criticised as being theoretically flawed[20] and never put into practice. A Danish engineer, Carl Emil Krarup, invented a form of continuously loaded cable which solved these problems and the cable is named for him. Even though the Krarup cable added inductance to the line, this was insufficient to meet the Heaviside condition. Mu-metal has similar magnetic properties to permalloy but the addition of copper to the alloy increases the ductility and allows the metal to be drawn into wire.
Mu-metal was invented in 1923 by The Telegraph Construction and Maintenance Company Ltd., London,[41] who made the cable, initially, for the Western Union Telegraph Co. Continuous loading of cables is expensive and hence is only done when absolutely necessary. Loaded cable is no longer a useful technology for submarine communication cables, having first been superseded by co-axial cable using electrically powered in-line repeaters and then by fibre-optic cable. Bray, J, Innovation and the Communications Revolution, Institute of Electrical Engineers, 2002. Heaviside, O, Electrical Papers, American Mathematical Society Bookstore, 1970 (reprint from 1892).
Huurdeman, AA, The worldwide history of telecommunications, Wiley-IEEE, 2003 ISBN 0471205052. Kragh, H, "The Krarup Cable: Invention and Early Development", Technology and Culture, Vol. May, Earl Chapin, "Four millions on 'permalloy'—to win!", Popular Mechanics, pages 947-952, volume 44, number 6, December 1925 ISSN 0032-4558. Neki dan mi frend prica kako se iznenadio dobrim zvukom kad je poslije ohoho godina iz znatizelje stavio kazetu u player!
Mesh Fiber Tape, View Fibre Mesh Tape, RUIFIBER-Mesh Fiber Tape Product Details from Shanghai Ruifiber Industry Co., Ltd. He concluded additional inductance was required to prevent amplitude and time delay distortion of the transmitted signal. The capacitors are not discrete components but represent the distributed capacitance between the closely spaced wire conductors of the line, this is indicated by the dotted lines.
Because twisted pair is a balanced format, half the loading coil must be inserted in each leg of the pair to maintain the balance. With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response is flat, waveforms are undistorted and the characteristic impedance is resistive up to the cutoff frequency.
Thus, if the pair is subsequently reused to support applications that require higher frequencies (such as analog or digital carrier systems or DSL), any loading coils that were present on the line must be removed or replaced with ones which are transparent to DSL.


Monopole and dipole radio antennas are designed to act as resonators for radio waves; the power from the transmitter, applied to the antenna through the antenna's transmission line, excites standing waves of voltage and current in the antenna element. An antenna shorter than a quarter wavelength presents capacitive reactance to the transmission line. The inductive reactance of the coil is equal and opposite to, and cancels, the capacitive reactance of the antenna, so the loaded antenna presents a pure resistance to the transmission line, preventing energy from being reflected.
Heaviside (1881) represented the line as a network of infinitesimally small circuit elements.
In order to meet the condition, the choices are therefore to try to increase G or L or to decrease R or C. Finally, Heaviside made the proposal (1893) to use discrete inductors at intervals along the line.[7] However, he never succeeded in persuading the British GPO to take up the idea. Campbell was tasked with continuing the investigation into Stone's bimetallic cable, but soon abandoned it in favour of the loading coil. After considering that his artificial line simulators used lumped components rather than the distributed quantities found in a real line, he wondered if he could not insert the inductance with lumped components instead of using Stone's distributed line.
Pupin filed a rival patent to the one of Campbell's.[16] This patent of Pupin's dates from 1899.
To add to the confusion, one variant of the capacitor scheme proposed by Pupin does indeed have coils. Fearing that there was a risk that the battle would end with the invention being declared unpatentable due to Heaviside's prior publication, they decided to desist from the challenge and buy an option on Pupin's patent for a yearly fee so that AT&T would control both patents. Telephone cables could now be used to twice the distance previously possible, or alternatively, a cable of half the previous quality (and cost) could be used over the same distance. Krarup cable has iron wires continuously wound around the central copper conductor with adjacent turns in contact with each other. Mu-metal cable is easier to construct than permalloy cable, the mu-metal being wound around the core copper conductor in much the same way as the iron wire in Krarup cable. Western Union were in competition with AT&T and the Western Electric Company who were using permalloy. Lumped loading with coils is cheaper but has the disadvantages of difficult seals and a definite cutoff frequency. Manufacture of loaded cable declined in the 1930s and was then superseded by other technologies post-war.
You may redistribute it, verbatim or modified, providing that you comply with the terms of the GFDL. It worked surprisingly well."The guys from the commercial product management were very happy with the proposal, but were in favour of 30 minutes – and they were right.
A cheap, portable proposal which, hopefully, would find a willing market."As far as I remember, with this little machine, we were happy with 80Hz~5kHz [frequency response], which was in balance with the loudspeaker, the simple electronics and final stage and the existing tape performance.
It seems people didn’t remember to use Dolby B or even have it and so they were listening to bright, compressed audio which could be noisy too. The biggest surprise was the world wide revolution it caused in the individual availability of music. If you require further details regarding the transaction data, please contact the supplier directly. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission cables. The mathematical condition for distortion-free transmission is known as the Heaviside condition. The loading coils prevent the audio (voice) signal from being distorted by the line capacitance. The coincidental formation of an audio frequency filter is also beneficial in that noise is reduced.
Using coils with parallel capacitors will form a filter with the topology of an m-derived filter and a band of frequencies above the cut-off will also be passed. This sometimes happens in dense, growing areas such as Southern California in the late 1990s and early 21st century. Due to narrower streets and higher cost of copper, European cables had thinner wires and used closer spacing.
To be resonant, the antenna must have a physical length of one quarter of the wavelength of the radio waves used (or a multiple of that length). Some of the applied power is reflected back into the transmission line and travels back toward the transmitter.
By applying his operational calculus to the analysis of this network he discovered (1887) what has become known as the Heaviside condition.[4][5] This is the condition that must be fulfilled in order for a transmission line to be free from distortion. Brittain attributes this to Heaviside's failure to provide engineering details on the size and spacing of the coils for particular cable parameters. Stone's idea (1896) was to use a bimetallic iron-copper cable which he had patented.[9] This cable of Stone's would increase the line inductance due to the iron content and had the potential to meet the Heaviside condition. His was an independent discovery, Campbell was aware of Heaviside's work in discovering the Heaviside condition, but unaware of Heaviside's suggestion of using loading coils to enable a line to meet it. There is an earlier patent[17] (1894, filed December 1893) which is sometimes cited as Pupin's loading coil patent but is, in fact, something different. A further advantage with mu-metal cable is that the construction lends itself to a variable loading profile whereby the loading is tapered towards the ends.
A compromise scheme is patch loading whereby the cable is continuously loaded in repeated sections. Loading coils can still be found in some telephone landlines today but new installations would use more modern technology.


Dollar Amount, 1774 to present" (Contemporary Standard of Living measure) MeasuringWorth, April 2013.
What sort of team were you working with?"Lou Ottens: "The group of engineers that I got to manage in the late 1950s and early 1960s consisted of mostly young people with experience in the design or manufacture of record-playing and tape-recording equipment in Eindhoven-based groups. Actually, we had some initial jamming problems with the cassettes, which we at first had blamed on static charging. Were you concerned or was using Dolby (at least, in theory) a welcome addition?"Lou Ottens: "We didn’t like it at all. The term is also used for inductors in radio antennas, or between the antenna and its feedline, to make an electrically short antenna resonant at its operating frequency. Previous telegraph lines were overland or shorter, hence had less delay and the need for extra inductance was not as great. The windings of the loading coil are wound such that the magnetic flux induced in the core is in the same direction for both windings.
This increases the flux linkages, without which the number of turns on the coil would need to be increased. At resonance the antenna acts electrically as a pure resistance, absorbing all the power applied to it from the transmitter.
This causes standing waves on the transmission line (a standing wave ratio (SWR) greater than one) which waste energy, and can even overheat the transmitter.
The Heaviside condition is that the series impedance, Z, must be proportional to the shunt admittance, Y, at all frequencies. Copper was already in use in telegraph cables and this is the very best conductor available short of using silver. However, Stone left the company in 1899 and the idea was never implemented.[10] Stone's cable was an example of continuous loading, a principle that was eventually put into practice is other forms, see for instance Krarup cable later in this article. They are there merely to restore DC continuity to the line so that it may be tested with regular equipment. Flangeless hubs facilitate smaller dimensions because one reel diminishes as the other grows in diameter.
Were there any specific problems you recall with the prototypes?Lou Ottens: “The EL 3585 had a capstan of 3mm diameter, but for a very small successor there was no room for the same size of flywheel. Had you decided upon maximum dimensions to work to?"Lou Ottens: "Because our aim was to make a pocket recorder, it should fit into the side pocket of my tweed jacket. Were other options explored, such as a new tape speed to deliver even better results?"Lou Ottens: "As long as there was no technical revolution at stake, international standardisation was a holy cow with Philips, and rightly so. It complicated our products and it destroyed the international standard we had established with the competition.
A number of the key personnel that played an important role have passed away since those days. Submarine communications cables are particularly subject to the problem, but early 20th century installations using balanced pairs were often continuously loaded with iron wire or tape rather than discretely with loading coils, which avoided the sealing problem. He remarked ironically that if his prior publication had been admitted it would "interfere . A second problem was that the material science of the time had difficulties sealing the joint between coil and cable against ingress of seawater. Therefore we chose  a 2mm capstan."For the test models we used 2mm stamping punches, which were beautifully straight and polished. How was this decided?"Lou Ottens: "Making a very small hub does not help much, because it does not gain much tape space. The phosphor-bronze spring which carries the felt pad had been chosen to avoid eventual magnetism of a steel spring in the direct neighbourhood of the recording head.
I was of the opinion that the rapid improvement of the tape performance by the international tape manufacturers would make the Dolby system obsolete, apart from the professional field, where the genius Dolby was normally active."I was afraid that it would cause a lot of confusion in the market.
The drywall tape is the ideal material to giving you the best solution for wall and ceiling problems. Increasing G is highly undesirable, while it would reduce distortion, it would at the same time increase the signal loss. A piece of rigid leader tape that clicked between the outer rims of the flanges made it possible to be handled without de-reeling. In the component division of Philips there was a considerable activity going for the miniaturisation of electronic components.
It led to a rather complicated and not very reliable recorder, but it had an important innovation."Goldmark proposed a narrower tape of improved quality. We decided to minimize the tape exposure, so we didn’t want any big holes in the upper or lower surface of the cassette."As a result, the recording head, the erase head and the pressure roller had to move in. Was the specification designed to take these heavier cassette reels with thinner tape or did they come later?"Lou Ottens: "Originally, we planned only the C60 and the C90, if I remember well. The components, like electrolytic capacitors, ceramic capacitors, resistors, transistors (ICs were not yet a reality),     switches, potentiometers and connectors were made fit for direct mounting on a printed circuit board.
At the time of the original design we did not know much of the future tapes and their behaviour.



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