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When designing a system, you need to first consider the regulatory and RF environment where the system will operate. This item will be sent through the Global Shipping Programme and includes international tracking. Items delivered internationally may be subject to customs processing depending on the item's declared value.
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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. Estimated delivery dates - opens in a new window or tab include seller's dispatch time, and will depend on postal service selected. If the item comes direct from a manufacturer, it may be delivered in non-retail packaging, such as a plain or unprinted box or plastic bag. Find out more about your rights as a buyer - opens in a new window or tab and exceptions - opens in a new window or tab.
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Import charges previously quoted are subject to change if you increase you maximum bid amount. 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.
During the more than 40 years that I have worked with EMC, I have come across overhead cranes that move while unattended, nuclear power plants that accidently shut down and even ATM machines that dispense money because of the RF environment. If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable.
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.
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.
Finding EMC (electromagnetic compatibility) problems at this late stage can often result in wasted time and money using hit-and-miss fixes. 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. Increasing G is highly undesirable, while it would reduce distortion, it would at the same time increase the signal loss. Often, over design as well as under design is done because no thought was given to EMC at the beginning of the project.In response to the many EMC problems I have seen in products that are either failing while in the field or that fail their compliance testing while in their final stages of design, I created a three-day EMC by Your Design Seminar (Ref. My goal was to help digital and analog design engineers become aware of EMC pitfalls, and more importantly, techniques they can incorporate to minimize EMC problems.
At the same time, engineers must not add to a producta€™s cost by overdesigning their product.
These specifications will define the maximum emissions allowed from a system and the maximum interference the system must withstand while still functioning properly. Therefore, a producta€™s enclosure must reduce both the emissions leaving the product and the interference trying to enter the product. The amount of reduction of the radio waves passing through the surface is generally referred to as enclosurea€™s shielding effectiveness (SE).Shielding is one of many ways you can control a systema€™s emissions and minimize immunity issues.

If you plan to use an existing enclosure, then you must test a prototype to confirm it will comply as part of the new design.
If you design a new enclosure, it must meet EMC emissions and immunity requirements in addition to meeting aesthetic, mechanical, and cost constraints. Designing for both emissions and immunity up front will avoid the possibly of having to redesign the enclosure should the product later be sold in the European Union, which requires immunity testing. Determine shielding effectivenessWhen designing a new enclosure, you need to know the amount of attenuation (reduction of radio waves passing into or out of the enclosure) that is required. To do this, you need to know which regulatory requirements apply to your product: FCC, European Union, Military, RTCA, etc.
Before doing so, I would recommend reading the example of a site survey found in an old medical standard, MDS 201-0004 that defines the ambient conditions found in several medical environments. Knowing the regulation and the emissions, you can again determine the amount of shielding needed. You can estimate the amount of shielding needed by calculating the emissions from a design making a few assumptions. Shielding is accomplished in primarily two ways: reflected loss (R) and absorption loss (A).
There is a third loss related to multiple reflections but it is generally not much of a factor. Reflection loss is the loss when a wave strikes a surface (see Ein in Figure 1) due to the impedance mismatch.
The current density in a material created by an incoming signal decreases exponentially through the material.The other major contributor to shield attenuation is from absorption loss, which is the dissipation of the energy in the form of heat as the wave passes through the material (Figure 1). As can be seen in the equation above, if the impedance of the shield increases, the shielding effectiveness decreases.
The good news is that absorption loss increases with frequency and the two complement each other such that across the entire frequency spectrum, any conductive material used as a shield and strong enough to stand by itself will provide enough shielding for almost all applications, including most military enclosures. Practical shields require penetrations and are generally not made with continuous bonding between various parts of the cabinet.Reflective loss can also be from conductive coatings. The coating can be made using paint, loaded with conductive material such as silver, nickel, copper and even silver-plated carriers.
The size of the conductive film does not matter as long as it is square (equal length sides). To use the formal concept of Ohm per square would require the film to be cut into a square rectangle, damaging the material. Using this modified 2 Ohms per square, even though it does not meet the strictest definition, will assure about 33 dB of attenuation. The two pennies prevent breaking through any surface film, which would prevent contact with another surface when joining two sections of an enclosure. When using a conductive coating, make sure the surfaces have a smooth transition because tight corners can develop turbulence during spraying, causing a high resistance surface to develop. When checking for quality on this type of enclosure, be sure to place pennies on opposite sides of a rib, a corner, or a transition. The conductivity of the plastic is associated with the aspect ratio of the conductive fill, the volume fraction and the conductivity of the material. The larger the aspect ratio, the better the conductivity but the harder the material is to use.
One problem with fills is it changes the shrinkage of the plastic which can require new tooling. Magnetic shield is typically accomplished by shunting the fields away from the problem area.
I was once called to an office building where a video monitor was being exposed to about 1000 mG of 60 Hz flux. We ended up putting Mu metal on the wall and floor as well as adding a special enclosure for the monitor. The walls and floor reduced the field to about 50 mG and the special enclosure reduced it to about 6 MG. Mu Metal on a wall shields a monitor from a source of magnetic flux.Making the enclosureFigure 6 shows that even a practical box is far from ideal because ita€™s built from multiple pieces that may need continuous bonding.
Besides these discontinuities, the box has yet to have penetrations for cables entering or leaving the enclosure, an air vent for cooling, and possibly viewing apertures. In Figure 7B and 7C, an aperture distorts the current flow and thus creates a voltage drop across the slot, which can cause energy to pass through it.
The distortion is virtually the same in both parts B and C because the maximum dimension, which is the diagonal distance from one corner of the aperture to the other, determines the leakage. Figure 7D shows that multiple openings, because of the more uniform current, create less voltage drop than across a single, larger gap. Figure 7E stresses the point that ita€™s the maximum dimension of the opening that matters most.(E)Figure 7. The maximum dimension of an opening in a shield determines how much energy can pass through.It is the maximum dimension of the slot, not the area, that determines the field that passes through. One of the ways to reduce the slot size when bringing two pieces of material together is to use gasket material. Figure 8 shows how gasketing comes in many different styles and some even require special consideration be made when constructing the enclosure.
Gaskets can reduce slot size, thus reducing the amount of energy that passes through the enclosure.Conductive tape is often used to cover a seam, especially while testing.
It is generally best not to use conductive tape in production, except in special cases, because of its cost and its short lifetime.
Conductive window used for shielding against unwanted signals.Cables are another major source of energy leaving or entering an enclosure.
Figure 10 shows the use of a commercial power line filter which can be an expensive addition. Its effect can be completely nullified by improper lead dress, as is seen in the top photo, where the noisy wires are coupled to the quiet wires by routing the wires in the same harness. In the bottom photo, by keeping the input and output of the filter separate from each other, the filter can function as it was designed.Figure 10.
Running wires from a noisy side along with those from the filtered side can cause energy to couple into the filtered wires, thus nullifying the filter.After the best techniques have been implemented on the internal circuit boards and wiring, the equipment housing becomes the final barrier. As previously mentioned, low frequency magnetic fields are harder to shield against than electric fields or plane waves.
Use a good conducting shield for electric fields, plane waves, and high-frequency magnetic fields. For almost all applications, any solid shield thick enough to be made into an enclosure will supply adequate absorption loss, giving a total attenuation sufficient for most applications.ConclusionIn general, shielding effectiveness is determined by seams and joints, not the material.
Until the last hole is treated, the best metal box might appear useless.When designing a metal housing a€?alla€? metal parts need to be well bonded. Often finger stock or gaskets are needed to bond the metal section of an enclosure together.
After the enclosure is designed, the shield integrity may need to be restored at cooling holes, viewing apertures and component holes. Remember, use two pennies and an Ohm meter to check its quality.Be sure to filter or treat all cables entering or leaving an enclosure.
Seminar and Workshop, a€?EMC by your Design.a€?a€? Henry Ott, author of a€?Electromagnetic Compatibility Engineering,a€? Wiley 2009 - Used by permission.
He holds a BSEE from University of Illinois at Urbana and has over 44 years experience in the EMC and electrical engineering fields.
Don has served as a special consultant to the Lawrence Livermore National Laboratory and the Nuclear Regulatory Commission and also consults for companies worldwide.
Council of EMC Laboratories (USCEL), has served on the board of directors of the IEEE EMC Society, is a member of the IEEE EMC Standards Development Committees, serves as "angel" for several standards and is an iNARTE certified EMC Engineer. He has taught at the University of Wisconsin, Oakton Community College, and currently teaches EMC By Your Design Seminars.

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