Loading coil
Encyclopedia
In electronics
, a loading coil or load coil is a coil
(inductor
) that does not provide coupling
to any other circuit
, but is inserted in a circuit to increase its inductance
. The need was discovered by Oliver Heaviside
in studying the disappointing slow speed of the Transatlantic telegraph cable
. He concluded additional inductance was required to prevent amplitude and time delay distortion of the transmitted signal. The mathematical condition for distortionless transmission is known as the Heaviside condition
. Previous telegraph lines were overland or shorter, hence had less delay and the need for extra inductance was not so great. Submarine communications cable
s are particularly subject to the problem, but early 20th century ones using balanced pairs were often continuously loaded by iron tape rather than discretely by load coils.
Loading coils are archaically known as Pupin coils after Mihajlo Pupin (especially when used for the Heaviside condition), and the process of inserting them is sometimes called pupinization.
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. Above the cutoff frequency, attenuation increases rapidly. The shorter the interval between the coils, the higher the cut-off frequency.
It should be emphasised that the cutoff effect is an artifact of using lumped
inductors. With loading methods using continuous distributed
inductance there is no cutoff.
Without loading coils, the line response is dominated by the resistance and capacitance of the line with the attenuation gently increasing with frequency. With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response is flat, waveform
s are undistorted and the characteristic impedance
is resistive up to the cutoff frequency. The coincidental formation of an audio frequency
filter is also beneficial in that noise is reduced.
of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. Thus, if the pair is subsequently reused to support applications that require higher frequencies (such as analog or digital carrier system
s or DSL
), any loading coils that were present on the line must be removed or replaced with one which is transparent to DSL. 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.
If they are not removed, as when the subscriber is
an extended distance (e.g. over 4 miles) from the
Central Office, DSL can not be supported. This sometimes happens in dense, growing areas (subject to frequent national numbering scheme repartitioning) such as Southern California in the late 1990s and early 21st century.
systems, which could carry 1.5 Mbit/s across that distance. Due to narrower streets and higher cost of copper, European cables had thinner wires and needed closer intervals. Intervals of a kilometer allowed European systems to carry 2 Mbit/s.
for predicting the propagation constant
of a loaded line. It is stated as;
is the propagation constant of the unloaded line
is the propagation constant of the loaded line
is the interval between coils on the loaded line
is the impedance of a loading coil and
is the characteristic impedance of the unloaded 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. This approximation can be arrived at by treating the loaded line as a constant k filter
and applying image filter theory
to it. From basic image filter theory the angular cutoff frequency and the characteristic impedance of a low-pass
constant k filter are given by;
and, 
From these basic equations the necessary loading coil inductance and coil spacing can be found;
and, 
Expressing this in terms of number of coils per cutoff wavelength yields;
The origin of the loading coil can be found in the work of Oliver Heaviside
on the theory of transmission line
s. Heaviside (1881) represented the line as a network of infinitesimally small circuit elements. By applying his operational calculus
to the analysis of this network he discovered (1887) what has become known as the Heaviside condition
. This is the condition that must be fulfilled in order for a transmission down a line to be free from distortion
. The Heaviside condition is that the line series impedance
, Z, must be proportional to the line shunt admittance
, Y, at all frequencies. In terms of the primary line coefficients this is the condition;
is the series resistance of the line per unit length
is the series self-inductance of the line per unit length
is the shunt leakage conductance of the line insulator per unit length
is the shunt capacitance between the line conductors per unit length
Heaviside was aware that this condition was not met in the practical telegraph cables in use in his day. In general, a real cable would have,
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. In order to meet the condition, the choices are therefore to try and increase G or L or to decrease R or C. Decreasing R requires larger conductors. Copper was already in use in telegraph cables and this is the very best conductor available short of using silver. Decreasing R means using more copper and a more expensive cable. Decreasing C would also mean a larger cable (although not necessarily more copper). Increasing G is highly undesirable, while it would reduce distortion, it would at the same time increase the signal loss. Heaviside considered, but rejected, this possibility which left him with the strategy of increasing L as the way to reduce distortion.
Heaviside immediately (1887) proposed several methods of increasing the inductance, including spacing the conductors further apart and loading the insulator with iron dust. Finally, Heaviside made the proposal (1893) to use discrete inductors at intervals along the line. However, he never succeeded in persuading the British GPO to take up the idea. Brittain attributes this to Heaviside's failure to provide engineering details on the size and spacing of the coils for particular cable parameters. Heaviside's eccentric character and setting himself apart from the establishment may also have played a part in their ignoring of him.
worked for the American Telephone & Telegraph Company (AT&T) and was the first to attempt to apply Heaviside's ideas to real telecommunications. Stone's idea (1896) was to use a bimetallic iron-copper cable which he had patented. This cable of Stone's would increase the line inductance due to the iron content and had the potential to meet the Heaviside condition. However, Stone left the company in 1899 and the idea was never implemented.
was another AT&T engineer working for them in their Boston facility. Campbell was tasked with continuing the investigation into Stone's bimetallic cable, but soon abandoned this in favour of the loading coils idea. This was an independent discovery, Campbell being aware of Heaviside's work in discovering the Heaviside condition, but apparently not aware of Heaviside's suggestion of using loading coils to force a line to meet it. The motivation for the change of direction was Campbell's limited budget.
Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. 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. When his calculations showed that the manholes on telephone routes were sufficiently close together to be able to insert the loading coils without the expense of either having to dig up the route or lay in new cables he changed to this new plan. The very first demonstration of loading coils on a telephone cable was on a 46-mile length of the so-called Pittsburgh cable (the test was actually in Boston, the cable had previously been used for testing in Pittsburgh) on September 6, 1899 carried out by Campbell himself and his assistant. The first telephone cable using loaded lines put into public service was between Jamaica Plain and West Newton in Boston on May 18, 1900.
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
. The cut-off phenomena of loading coils, an undesirable side-effect, can be exploited to produce a desirable filter frequency response.
Michael Pupin, inventor and Serbia
n immigrant to the USA, also played a part in the story of loading coils. Pupin filed a rival patent to the one of Campbell's. This patent of Pupin's dates from 1899. There is an earlier patent (1894, filed December 1893) which is sometimes cited as Pupin's loading coil patent but is, in fact, something different. The confusion is easy to understand, Pupin himself claims that he first thought of the idea of loading coils while climbing a mountain in 1894, although there is nothing from him published at that time.
Pupin's 1894 patent "loads" the line with capacitors rather than inductors, a scheme that has been criticised as being theoretically flawed and never put into practice. To add to the confusion, one variant of the capacitor scheme proposed by Pupin does indeed have coils. However, these are not intended to compensate the line in any way. They are there merely to restore DC continuity to the line so that it may be tested with regular equipment. Pupin states that the inductance is to be so large that it will block all AC signals above 50 Hz. Consequently, only the capacitor is adding any significant impedance to the line and "the coils will not exercise any material influence on the results before noted".
AT&T fought a legal battle with Pupin over his claim. Pupin was first to patent but Campbell had already conducted practical demonstrations before Pupin had even filed his patent (December 1899). Campbell's delay in filing was due to the slow internal machinations of AT&T.
However, AT&T foolishly deleted from Campbell's proposed patent application all the tables and graphs detailing the exact value of inductance that would be required before the patent was submitted. Since Pupin's patent contained a (less accurate) formula, AT&T was open to claims of incomplete disclosure. 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. By January 1901 Pupin had been paid $200,000 and by 1917, when the AT&T monopoly ended and payments ceased, he had received a total of $455,000.
A Danish engineer, Carl Emil Krarup
, invented a form of continuously loaded cable which solved these problems and the cable is named for him. Krarup cable has iron wires continuously wound around the central copper conductor with adjacent turns in contact with each other. This cable was the first use of continuous loading on any telecommunication cable. In 1902 Krarup both wrote his paper on this subject and saw the installation of the first cable between Helsingør (Denmark) and Helsingborg
(Sweden).
Even though Krarup cable added inductance to the line, it did not add enough to meet the Heaviside condition. AT&T searched for a better material with higher magnetic permeability
. In 1914 Gustav Elmen discovered permalloy
, a magnetic nickel-iron annealed alloy. Oliver E. Buckley
, along with his colleagues at Bell Labs
, H. D. Arnold and Elmen, c.1915 proposed a method of constructing submarine cable
using permalloy tape wrapped around the copper conductors. This construction greatly improved the performance of the cable.
The cable was tested in a trial in Bermuda in 1923. The first permalloy cable to be put into service was between New York
and Horta (Azores)
in September 1924. Permalloy cable enabled signalling speed on submarine telegraph cables to be increased to 2,000 words/min at a time when 40 words/min was considered good.
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 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. 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.
Mu-metal was invented (1923) by The Telegraph Construction and Maintenance Company Ltd., London, who made the cable, initially, for the Western Union Telegraph Co
. Western Union were in competition with AT&T and the Western Electric Company who were using permalloy (the patent for permalloy was held by Western Electric).
using electrically powered in-line repeater
s and then by fibre-optic cable
. Manufacture of loaded cable declined in the 1930s and was then superseded by other technologies post-war. Loading coils can still be found in some telephone landlines today but new installations would use more modern technology.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, a loading coil or load coil is a coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
(inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
) that does not provide coupling
Coupling (electronics)
In electronics and telecommunication, coupling is the desirable or undesirable transfer of energy from one medium, such as a metallic wire or an optical fiber, to another medium, including fortuitous transfer....
to any other circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
, but is inserted in a circuit to increase its inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
. The need was discovered by Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
in studying the disappointing slow speed of the Transatlantic telegraph cable
Transatlantic telegraph cable
The transatlantic telegraph cable was the first cable used for telegraph communications laid across the floor of the Atlantic Ocean. It crossed from , Foilhommerum Bay, Valentia Island, in western Ireland to Heart's Content in eastern Newfoundland. The transatlantic cable connected North America...
. He concluded additional inductance was required to prevent amplitude and time delay distortion of the transmitted signal. The mathematical condition for distortionless transmission is known as the Heaviside condition
Heaviside condition
The Heaviside condition, due to Oliver Heaviside , is the condition an electrical transmission line must meet in order for there to be no distortion of a transmitted signal...
. Previous telegraph lines were overland or shorter, hence had less delay and the need for extra inductance was not so great. Submarine communications cable
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean....
s are particularly subject to the problem, but early 20th century ones using balanced pairs were often continuously loaded by iron tape rather than discretely by load coils.
Loading coils are archaically known as Pupin coils after Mihajlo Pupin (especially when used for the Heaviside condition), and the process of inserting them is sometimes called pupinization.
Applications
Voice circuits
A common application of loading coils is to improve the voice-frequencyVoice frequency
A voice frequency or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech.In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz...
amplitude response characteristics of the twisted balanced pairs
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
in a telephone cable.
Loading coils inserted periodically in series with a pair of wires reduce the attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
at the higher voice frequencies up to the cutoff frequency
Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
of the low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
formed by the inductance of the coils (plus the distributed inductance of the wires) and the distributed capacitance between the wires. Above the cutoff frequency, attenuation increases rapidly. The shorter the interval between the coils, the higher the cut-off frequency.
It should be emphasised that the cutoff effect is an artifact of using lumped
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
inductors. With loading methods using continuous distributed
Distributed element model
In electrical engineering, the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit are distributed continuously throughout the material of the circuit...
inductance there is no cutoff.
Without loading coils, the line response is dominated by the resistance and capacitance of the line with the attenuation gently increasing with frequency. With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response is flat, waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s are undistorted and the characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
is resistive up to the cutoff frequency. The coincidental formation of an audio frequency
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
filter is also beneficial in that noise is reduced.
DSL
When loading coils are in place, signal attenuation remains low for signals within the passbandPassband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. Thus, if the pair is subsequently reused to support applications that require higher frequencies (such as analog or digital carrier system
Carrier system
In telecommunication, a carrier system is a multichannel telecommunications system in which a number of individual channels are multiplexed for transmission...
s or DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
), any loading coils that were present on the line must be removed or replaced with one which is transparent to DSL. Using coils with parallel capacitors will form a filter with the topology of an m-derived filter
M-derived filter
m-derived filters or m-type filters are a type of electronic filter designed using the image method. They were invented by Otto Zobel in the early 1920s. This filter type was originally intended for use with telephone multiplexing and was an improvement on the existing constant k type filter...
and a band of frequencies above the cut-off will also be passed.
If they are not removed, as when the subscriber is
an extended distance (e.g. over 4 miles) from the
Central Office, DSL can not be supported. This sometimes happens in dense, growing areas (subject to frequent national numbering scheme repartitioning) such as Southern California in the late 1990s and early 21st century.
Carrier systems
American early and middle 20th Century telephone cables had load coils at intervals of a mile (1.61 km), usually in coil cases holding many. The coils must be removed to pass high frequencies, but the coil cases provided convenient places for repeaters for digital T-carrierT-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
systems, which could carry 1.5 Mbit/s across that distance. Due to narrower streets and higher cost of copper, European cables had thinner wires and needed closer intervals. Intervals of a kilometer allowed European systems to carry 2 Mbit/s.
Radio antennae
A (mobile) radio antenna, shorter than a quarter wavelength for practical reasons, presents capacitive reactance to a transmission line. This can be canceled by inserting an equal and opposite (inductive) reactance in series, by means of a loading coil typically at the base or center of the antenna. Consequently the antenna presents a resistance (desirable) to the transmission line.Campbell equation
The Campbell equation is a relationship due to George Ashley CampbellGeorge Ashley Campbell
George Ashley Campbell was a pioneer in developing and applying quantitative mathematical methods to the problems of long-distance telegraphy and telephony. His most important contributions were to the theory and implementation of the use of loading coils and the first wave filters designed to...
for predicting the propagation constant
Propagation constant
The propagation constant of an electromagnetic wave is a measure of the change undergone by the amplitude of the wave as it propagates in a given direction. The quantity being measured can be the voltage or current in a circuit or a field vector such as electric field strength or flux density...
of a loaded line. It is stated as;
- where,
is the propagation constant of the unloaded line is the propagation constant of the loaded line is the interval between coils on the loaded line is the impedance of a loading coil and is the characteristic impedance of the unloaded 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. This approximation can be arrived at by treating the loaded line as a constant k filter
Constant k filter
Constant k filters, also k-type filters, are a type of electronic filter designed using the image method. They are the original and simplest filters produced by this methodology and consist of a ladder network of identical sections of passive components...
and applying image filter theory
Image impedance
Image impedance is a concept used in electronic network design and analysis and most especially in filter design. The term image impedance applies to the impedance seen looking in to the ports of a network. Usually a two-port network is implied but the concept is capable of being extended to...
to it. From basic image filter theory the angular cutoff frequency and the characteristic impedance of a low-pass
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
constant k filter are given by;
and, - where
and 
are the half section element values.
From these basic equations the necessary loading coil inductance and coil spacing can be found;
and, - where C is the capacitance per unit length of the line.
Expressing this in terms of number of coils per cutoff wavelength yields;
- where v is the velocity of propagation of the cable in question.
Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
on the theory of transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s. Heaviside (1881) represented the line as a network of infinitesimally small circuit elements. By applying his operational calculus
Operational calculus
Operational calculus, also known as operational analysis, is a technique by which problems in analysis, in particular differential equations, are transformed into algebraic problems, usually the problem of solving a polynomial equation.-History:...
to the analysis of this network he discovered (1887) what has become known as the Heaviside condition
Heaviside condition
The Heaviside condition, due to Oliver Heaviside , is the condition an electrical transmission line must meet in order for there to be no distortion of a transmitted signal...
. This is the condition that must be fulfilled in order for a transmission down a line to be free from distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
. The Heaviside condition is that the line series impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, Z, must be proportional to the line shunt admittance
Admittance
In electrical engineering, the admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the inverse of the impedance . The SI unit of admittance is the siemens...
, Y, at all frequencies. In terms of the primary line coefficients this is the condition;
- where;
is the series resistance of the line per unit length is the series self-inductance of the line per unit length is the shunt leakage conductance of the line insulator per unit length is the shunt capacitance between the line conductors per unit lengthHeaviside was aware that this condition was not met in the practical telegraph cables in use in his day. In general, a real cable would have,
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. In order to meet the condition, the choices are therefore to try and increase G or L or to decrease R or C. Decreasing R requires larger conductors. Copper was already in use in telegraph cables and this is the very best conductor available short of using silver. Decreasing R means using more copper and a more expensive cable. Decreasing C would also mean a larger cable (although not necessarily more copper). Increasing G is highly undesirable, while it would reduce distortion, it would at the same time increase the signal loss. Heaviside considered, but rejected, this possibility which left him with the strategy of increasing L as the way to reduce distortion.
Heaviside immediately (1887) proposed several methods of increasing the inductance, including spacing the conductors further apart and loading the insulator with iron dust. Finally, Heaviside made the proposal (1893) to use discrete inductors at intervals along the line. However, he never succeeded in persuading the British GPO to take up the idea. Brittain attributes this to Heaviside's failure to provide engineering details on the size and spacing of the coils for particular cable parameters. Heaviside's eccentric character and setting himself apart from the establishment may also have played a part in their ignoring of him.
John Stone
John S. StoneJohn Stone Stone
John Stone Stone was an American mathematician, physicist and inventor. He labored as an early telephone engineer, was influential in developing wireless communication technology, and holds dozens of key patents in the field of "space telegraphy".-Early years:Stone was born in Dover, now Manakin...
worked for the American Telephone & Telegraph Company (AT&T) and was the first to attempt to apply Heaviside's ideas to real telecommunications. Stone's idea (1896) was to use a bimetallic iron-copper cable which he had patented. This cable of Stone's would increase the line inductance due to the iron content and had the potential to meet the Heaviside condition. However, Stone left the company in 1899 and the idea was never implemented.
George Campbell
George CampbellGeorge Ashley Campbell
George Ashley Campbell was a pioneer in developing and applying quantitative mathematical methods to the problems of long-distance telegraphy and telephony. His most important contributions were to the theory and implementation of the use of loading coils and the first wave filters designed to...
was another AT&T engineer working for them in their Boston facility. Campbell was tasked with continuing the investigation into Stone's bimetallic cable, but soon abandoned this in favour of the loading coils idea. This was an independent discovery, Campbell being aware of Heaviside's work in discovering the Heaviside condition, but apparently not aware of Heaviside's suggestion of using loading coils to force a line to meet it. The motivation for the change of direction was Campbell's limited budget.
Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. After considering that his artificial line simulators used lumped
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
components rather than the distributed
Distributed element model
In electrical engineering, the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit are distributed continuously throughout the material of the circuit...
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. When his calculations showed that the manholes on telephone routes were sufficiently close together to be able to insert the loading coils without the expense of either having to dig up the route or lay in new cables he changed to this new plan. The very first demonstration of loading coils on a telephone cable was on a 46-mile length of the so-called Pittsburgh cable (the test was actually in Boston, the cable had previously been used for testing in Pittsburgh) on September 6, 1899 carried out by Campbell himself and his assistant. The first telephone cable using loaded lines put into public service was between Jamaica Plain and West Newton in Boston on May 18, 1900.
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
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
. The cut-off phenomena of loading coils, an undesirable side-effect, can be exploited to produce a desirable filter frequency response.
Michael Pupin
Serbia
Serbia , officially the Republic of Serbia , is a landlocked country located at the crossroads of Central and Southeast Europe, covering the southern part of the Carpathian basin and the central part of the Balkans...
n immigrant to the USA, also played a part in the story of loading coils. Pupin filed a rival patent to the one of Campbell's. This patent of Pupin's dates from 1899. There is an earlier patent (1894, filed December 1893) which is sometimes cited as Pupin's loading coil patent but is, in fact, something different. The confusion is easy to understand, Pupin himself claims that he first thought of the idea of loading coils while climbing a mountain in 1894, although there is nothing from him published at that time.
Pupin's 1894 patent "loads" the line with capacitors rather than inductors, a scheme that has been criticised as being theoretically flawed and never put into practice. To add to the confusion, one variant of the capacitor scheme proposed by Pupin does indeed have coils. However, these are not intended to compensate the line in any way. They are there merely to restore DC continuity to the line so that it may be tested with regular equipment. Pupin states that the inductance is to be so large that it will block all AC signals above 50 Hz. Consequently, only the capacitor is adding any significant impedance to the line and "the coils will not exercise any material influence on the results before noted".
Legal battle
Heaviside never patented his idea; indeed, he took no commercial advantage of any of his work. Despite the legal disputes surrounding this invention, it is unquestionable that Campbell was the first to actually construct a telephone circuit using loading coils. There also can be little doubt that Heaviside was the first to publish and many would dispute Pupin's priority.AT&T fought a legal battle with Pupin over his claim. Pupin was first to patent but Campbell had already conducted practical demonstrations before Pupin had even filed his patent (December 1899). Campbell's delay in filing was due to the slow internal machinations of AT&T.
However, AT&T foolishly deleted from Campbell's proposed patent application all the tables and graphs detailing the exact value of inductance that would be required before the patent was submitted. Since Pupin's patent contained a (less accurate) formula, AT&T was open to claims of incomplete disclosure. 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. By January 1901 Pupin had been paid $200,000 and by 1917, when the AT&T monopoly ended and payments ceased, he had received a total of $455,000.
Benefit to AT&T
The invention was of enormous value to AT&T. 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. When considering whether to allow Campbell to go ahead with the demonstration, their engineers had estimated that they stood to save $700,000 in new installation costs in New York and New Jersey alone. It has been estimated that AT&T saved $100 million in the first quarter of the 20th century. Heaviside, who began it all, came away with nothing. He was offered a token payment but would not accept, wanting the credit for his work. He remarked ironically that if his prior publication had been admitted it would "interfere . . . with the flow of dollars in the proper direction . . .".Krarup cable
Loading coils were not without their problems. For submarine cables where they were of most benefit, they were difficult to lay. The cable needed to be heavier and both this and the discontinuities in the profile where the coils occurred caused stresses in the cable during laying. Without great care, the cable might part and would be enormously expensive, possibly impossible, to fix. A second problem was that the material science of the time had difficulties sealing the joint between coil and cable against ingress of seawater. When this occurred, of course, the cable was ruined.A Danish engineer, Carl Emil Krarup
Carl Emil Krarup
Carl Emil Krarup was a Danish telegraph engineer who is chiefly known for the invention of a kind of loaded cable, eponymously called Krarup cable, which made improvements in the transmission of telephone signals, especially on submarine cables....
, invented a form of continuously loaded cable which solved these problems and the cable is named for him. Krarup cable has iron wires continuously wound around the central copper conductor with adjacent turns in contact with each other. This cable was the first use of continuous loading on any telecommunication cable. In 1902 Krarup both wrote his paper on this subject and saw the installation of the first cable between Helsingør (Denmark) and Helsingborg
Helsingborg
Helsingborg is a city and the seat of Helsingborg Municipality, Skåne County, Sweden with 97,122 inhabitants in 2010. Helsingborg is the centre of an area in the Øresund region of about 320,000 inhabitants in north-west Scania, and is Sweden's closest point to Denmark, with the Danish city...
(Sweden).
Permalloy cable
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
. In 1914 Gustav Elmen discovered permalloy
Permalloy
Permalloy is a nickel-iron magnetic alloy, with about 20% iron and 80% nickel content. It is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields...
, a magnetic nickel-iron annealed alloy. Oliver E. Buckley
Oliver E. Buckley
Oliver Ellsworth Buckley was an American electrical engineer known for his contributions to the field of submarine telephony.-Biography:...
, along with his colleagues at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, H. D. Arnold and Elmen, c.1915 proposed a method of constructing submarine cable
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean....
using permalloy tape wrapped around the copper conductors. This construction greatly improved the performance of the cable.
The cable was tested in a trial in Bermuda in 1923. The first permalloy cable to be put into service was between New York
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
and Horta (Azores)
Horta (Azores)
Horta is a single municipality and city in the western part of the Archipealgo of the Azores, encompassing the island of Faial. Horta has a population of about approximately 15,038 people and an area of 173.1 square kilometers. The population density is about 88 persons per square kilometer...
in September 1924. Permalloy cable enabled signalling speed on submarine telegraph cables to be increased to 2,000 words/min at a time when 40 words/min was considered good.
Mu-metal cable
Mu-metal
Mu-metal is a nickel-iron alloy that is notable for its high magnetic permeability. The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods. The name came from the Greek letter mu which represents...
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 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. 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.
Mu-metal was invented (1923) by The Telegraph Construction and Maintenance Company Ltd., London, who made the cable, initially, for the Western Union Telegraph Co
Western Union
The Western Union Company is a financial services and communications company based in the United States. Its North American headquarters is in Englewood, Colorado. Up until 2006, Western Union was the best-known U.S...
. Western Union were in competition with AT&T and the Western Electric Company who were using permalloy (the patent for permalloy was held by Western Electric).
Current practice
Loaded cable is no longer a useful technology for submarine communication cables, having first been superseded by co-axial cableCoaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
using electrically powered in-line repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s and then by fibre-optic cable
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
. Manufacture of loaded cable declined in the 1930s and was then superseded by other technologies post-war. Loading coils can still be found in some telephone landlines today but new installations would use more modern technology.
See also
- Electrical lengtheningElectrical lengtheningElectrical lengthening is the modification of an aerial which is shorter than a whole-number multiple of a quarter of the radiated wavelength, by means of a suitable electronic device, without changing the physical length of the aerial, in such a way that it corresponds electrically to the next...
- Antenna tunerAntenna tunerAn antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...
- Constant k filterConstant k filterConstant k filters, also k-type filters, are a type of electronic filter designed using the image method. They are the original and simplest filters produced by this methodology and consist of a ladder network of identical sections of passive components...
- Unloaded phantom