Single-wire transmission line

Last updated

A single-wire transmission line (or single wire method) is a method of transmitting electrical power or signals using only a single electrical conductor. This is in contrast to the usual use of a pair of wires providing a complete circuit, or an electrical cable likewise containing (at least) two conductors for that purpose.

Contents

The single-wire transmission line is not the same as the single-wire earth return system, which is not covered in this article. The latter system relies on a return current through the ground, using the earth as a second conductor between ground terminal electrodes. In a single-wire transmission line there is no second conductor of any form.

History

As early as the 1780s Luigi Galvani first observed the effect of static electricity in causing the legs of a frog to twitch, and observed the same effect produced just due to certain metallic contacts with the frog involving a complete circuit. The latter effect was correctly understood by Alessandro Volta as an electric current inadvertently produced by what would become known as a voltaic cell (battery). He understood that such a current required a complete circuit to conduct the electricity, even though the actual nature of electric currents was not at all understood (only a century later would the electron be discovered). All subsequent development of electrical motors, lights, etc. relied on the principle of a complete circuit, generally involving a pair of wires, but sometimes using the ground as the second conductor (as with commercial telegraphy).

At the end of the 19th century, Nikola Tesla demonstrated that by using an electrical network tuned to resonance it was possible to transmit electric power using only a single conductor, with no need for a return wire. This was spoken of as the "transmission of electrical energy through one wire without return". [1] [2]

In 1891, 1892, and 1893 demonstration lectures with electrical oscillators before the AIEE at Columbia College, N.Y.C., the IEE, London, the Franklin Institute, Philadelphia, and National Electric Light Association, St. Louis, it was shown that electric motors and single-terminal incandescent lamps can be operated through a single conductor without a return wire. Although apparently lacking a complete circuit, such a topology effectively obtains a return circuit by virtue of the load's self-capacitance and parasitic capacitance. [3] [4]

Thus coils of the proper dimensions might be connected each with only one of its ends to the mains from a machine of low E. M. F., and though the circuit of the machine would not be closed in the ordinary acceptance of the term, yet the machine might be burned out if a proper resonance effect would be obtained. [5]

The final reference to "burning out" a machine was to emphasize the ability of such a system to transmit a large power given a proper impedance match, as can be obtained through electrical resonance.

Theory

This observation has been rediscovered several times, and described, for instance, in a 1993 patent. [6] Single-wire transmission in this sense is not possible using direct current and totally impractical for low frequency alternating currents such as the standard 50–60 Hz power line frequencies. At much higher frequencies, however, it is possible for the return circuit (which would normally be connected through a second wire) to utilize the self- and parasitic capacitance of a large conductive object, perhaps the housing of the load itself. Although the self-capacitance of even large objects is rather small in ordinary terms, as Tesla himself appreciated it is possible to resonate that capacitance using a sufficiently large inductor (depending on the frequency used), in which case the large reactance of that capacitance is cancelled out. This allows a large current to flow (and a large power to be supplied to the load) without requiring an extremely high voltage source. Although this method of power transmission has long been understood, it is not clear whether there has been any commercial application of this principle for power transmission.

Single conductor waveguides

As early as 1899, Arnold Sommerfeld published a paper [7] predicting the use of a single cylindrical conductor (wire) to propagate radio frequency energy as a surface wave. Sommerfeld's "wire wave" was of theoretical interest as a propagating mode, but this was decades before technology existed for the generation of sufficiently high radio frequencies for any such experimentation, let alone practical applications. What's more, the solution described an infinite transmission line without consideration of coupling energy into (or out of) it.

Of particular practical interest, though, was the prediction of a substantially lower signal attenuation compared to using the same wire as the center conductor of a coaxial cable. Contrary to the previous explanation of the full transmitted power being due to a classical current through a wire, in this case the currents in the conductor itself are much smaller, with the energy transmitted in the form of an electromagnetic wave (radio wave). But in this case, the presence of the wire acts to guide that wave toward the load, rather than radiating away.

The reduction of ohmic losses compared to using coax (or other two-wire transmission lines) is especially an advantage at higher frequencies where these losses become very large. Practically speaking, use of this transmission mode below microwave frequencies is very problematic due to the very extended field patterns around the wire. The fields associated with the surface wave along the conductor are significant out to many conductor diameters, therefore metallic or even dielectric materials inadvertently present in these regions will distort the propagation of the mode and typically will increase propagation loss. Although there is no wavelength dependence to this dimension in the transverse direction, in the direction of propagation it is necessary to have a minimum of one half wave of conductor length to fully support the propagating mode. For these reasons, and at frequencies available prior to about 1950, the practical disadvantages of such transmission completely outweighed the reduced loss due to the wire's finite conductivity.

Goubau line

In 1950 Georg Goubau revisited Sommerfeld's discovery of a surface wave mode along a wire, but with the intent of increasing its practicality. [8] One major goal was to reduce the extent of the fields surrounding the conductor so that such a wire would not require an unreasonably large clearance. Another problem was that Sommerfeld's wave propagated exactly at the speed of light (or the slightly lower speed of light in air, for a wire surrounded by air). That meant that there would be radiation losses. The straight wire acts as a long wire antenna, robbing the radiated power from the guided mode. If the propagation velocity can be reduced below the speed of light then the surrounding fields become evanescent, and are thus unable to propagate energy away from the area surrounding the wire.

Goubau investigated the beneficial effect of a wire whose surface is structured (rather than an exact cylinder) such as would be obtained using a threaded wire. More significantly, Goubau proposed the application of a dielectric layer surrounding the wire. Even a rather thin layer (relative to the wavelength) of a dielectric will reduce the propagation velocity sufficiently below the speed of light, eliminating radiation loss from a surface wave along the surface of a long straight wire. This modification also had the effect of greatly reducing the footprint of the electromagnetic fields surrounding the wire, addressing the other practical concern. [9]

Finally, Goubau invented a method for launching (and receiving) electrical energy from such a transmission line. The patented [10] Goubau line (or "G-line") consists of a single conductor coated with dielectric material. At each end is a wide disk with a hole in the center through which the transmission line passes. The disk may be the base of a cone, with its narrow end connected typically to the shield of coaxial feed line, and the transmission line itself connecting to the center conductor of the coax.

Even with the reduced extent of the surrounding fields in Goubau's design, such a device only becomes practical at UHF frequencies and above. With technological development at terahertz frequencies, where metallic losses are yet greater, the use of transmission using surface waves and Goubau lines appears promising. [11]

E-Line

From 2003 through 2008 patents were filed for a system using Sommerfeld's original bare (uncoated) wire, but employing a launcher similar to that developed by Goubau. [12] [13] It was promoted under the name "E-Line" through 2009. [14] This line is claimed to be completely non-radiating, propagating energy by a previously ignored transverse-magnetic (TM) wave. The intended application is high information rate channels using existing power lines for communications purposes. [15]

See also

Related Research Articles

<span class="mw-page-title-main">Tesla coil</span> Electrical resonant transformer circuit invented by Nikola Tesla

A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits.

In electrical engineering, electrical length is a dimensionless parameter equal to the physical length of an electrical conductor such as a cable or wire, divided by the wavelength of alternating current at a given frequency traveling through the conductor. In other words, it is the length of the conductor measured in wavelengths. It can alternately be expressed as an angle, in radians or degrees, equal to the phase shift the alternating current experiences traveling through the conductor.

<span class="mw-page-title-main">Surface wave</span> Physical phenomenon

In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at the interface between two fluids with different densities. Elastic surface waves can travel along the surface of solids, such as Rayleigh or Love waves. Electromagnetic waves can also propagate as "surface waves" in that they can be guided along with a refractive index gradient or along an interface between two media having different dielectric constants. In radio transmission, a ground wave is a guided wave that propagates close to the surface of the Earth.

<span class="mw-page-title-main">Transmission line</span> Cable or other structure for carrying radio waves

In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmission must be taken into account. This applies especially to radio-frequency engineering because the short wavelengths mean that wave phenomena arise over very short distances. However, the theory of transmission lines was historically developed to explain phenomena on very long telegraph lines, especially submarine telegraph cables.

<span class="mw-page-title-main">Transmission medium</span> Conduit for signal propagation

A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate sound, and a transmission medium for sounds may be air, but solids and liquids may also act as the transmission medium. Vacuum or air constitutes a good transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by absorption or reflection or refraction at the interfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable is used as transmission media.

<span class="mw-page-title-main">Waveguide</span> Structure that guides waves efficiently

A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities decrease according to the inverse square law as they expand into three-dimensional space.

<span class="mw-page-title-main">Alternating current</span> Electric current that periodically reverses direction

Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

<span class="mw-page-title-main">Coaxial cable</span> Electrical cable type with concentric inner conductor, insulator, and conducting shield

Coaxial cable, or coax is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ; many coaxial cables also have a protective outer sheath or jacket. The term coaxial refers to the inner conductor and the outer shield sharing a geometric axis.

<span class="mw-page-title-main">Antenna (radio)</span> Electrical device

In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

<span class="mw-page-title-main">Spark-gap transmitter</span> Type of radio transmitter

A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark. Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the wireless telegraphy or "spark" era, the first three decades of radio, from 1887 to the end of World War I. German physicist Heinrich Hertz built the first experimental spark-gap transmitters in 1887, with which he proved the existence of radio waves and studied their properties.

<span class="mw-page-title-main">Goubau line</span> Single wire transmission line used to conduct radio waves at UHF and microwave frequencies

A Goubau line or Sommerfeld–Goubau line, or G-line for short, is a single-wire transmission line used to conduct radio waves at UHF and microwave frequencies. The dielectric coated transmission line was invented by F. Harms in 1907 and George J. E. Goubau in 1950, based on work on surface waves on wires from 1899 by Arnold Sommerfeld. It is used as a feedline at UHF to link high frequency transmitters and receivers to their antennas, and in scientific research.

<span class="mw-page-title-main">Waveguide (radio frequency)</span> Hollow metal pipe used to carry radio waves

In radio-frequency engineering and communications engineering, waveguide is a hollow metal pipe used to carry radio waves. This type of waveguide is used as a transmission line mostly at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, and microwave radio links.

James R. Wait was a Canadian electrical engineer and engineering physicist. In 1977, he was elected as a member of National Academy of Engineering in Electronics, Communication & Information Systems Engineering for his contributions to electromagnetic propagation engineering as it affects communication and geophysical exploration.

<span class="mw-page-title-main">Microwave cavity</span>

A microwave cavity or radio frequency (RF) cavity is a special type of resonator, consisting of a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. The structure is either hollow or filled with dielectric material. The microwaves bounce back and forth between the walls of the cavity. At the cavity's resonant frequencies they reinforce to form standing waves in the cavity. Therefore, the cavity functions similarly to an organ pipe or sound box in a musical instrument, oscillating preferentially at a series of frequencies, its resonant frequencies. Thus it can act as a bandpass filter, allowing microwaves of a particular frequency to pass while blocking microwaves at nearby frequencies.

<span class="mw-page-title-main">Waveguide filter</span> Electronic filter that is constructed with waveguide technology

A waveguide filter is an electronic filter constructed with waveguide technology. Waveguides are hollow metal conduits inside which an electromagnetic wave may be transmitted. Filters are devices used to allow signals at some frequencies to pass, while others are rejected. Filters are a basic component of electronic engineering designs and have numerous applications. These include selection of signals and limitation of noise. Waveguide filters are most useful in the microwave band of frequencies, where they are a convenient size and have low loss. Examples of microwave filter use are found in satellite communications, telephone networks, and television broadcasting.

<span class="mw-page-title-main">World Wireless System</span> Proposed telecommunications and electrical power delivery system by Nikola Tesla

The World Wireless System was a turn of the 20th century proposed telecommunications and electrical power delivery system designed by inventor Nikola Tesla based on his theories of using Earth and its atmosphere as electrical conductors. He claimed this system would allow for "the transmission of electric energy without wires" on a global scale as well as point-to-point wireless telecommunications and broadcasting. He made public statements citing two related methods to accomplish this from the mid-1890s on. By the end of 1900 he had convinced banker J. P. Morgan to finance construction of a wireless station based on his ideas intended to transmit messages across the Atlantic to England and to ships at sea. His decision to change the design to include wireless power transmission to better compete with Guglielmo Marconi's new radio based telegraph system was met with Morgan's refusal to fund the changes. The project was abandoned in 1906, never to become operational.

<span class="mw-page-title-main">Planar transmission line</span> Transmission lines with flat ribbon-like conducting or dielectric lines

Planar transmission lines are transmission lines with conductors, or in some cases dielectric (insulating) strips, that are flat, ribbon-shaped lines. They are used to interconnect components on printed circuits and integrated circuits working at microwave frequencies because the planar type fits in well with the manufacturing methods for these components. Transmission lines are more than simply interconnections. With simple interconnections, the propagation of the electromagnetic wave along the wire is fast enough to be considered instantaneous, and the voltages at each end of the wire can be considered identical. If the wire is longer than a large fraction of a wavelength, these assumptions are no longer true and transmission line theory must be used instead. With transmission lines, the geometry of the line is precisely controlled so that its electrical behaviour is highly predictable. At lower frequencies, these considerations are only necessary for the cables connecting different pieces of equipment, but at microwave frequencies the distance at which transmission line theory becomes necessary is measured in millimetres. Hence, transmission lines are needed within circuits.

<span class="mw-page-title-main">Frequency selective surface</span>

A frequency-selective surface (FSS) is any thin, repetitive surface designed to reflect, transmit or absorb electromagnetic fields based on the frequency of the field. In this sense, an FSS is a type of optical filter or metal-mesh optical filters in which the filtering is accomplished by virtue of the regular, periodic pattern on the surface of the FSS. Though not explicitly mentioned in the name, FSS's also have properties which vary with incidence angle and polarization as well - these are unavoidable consequences of the way in which FSS's are constructed. Frequency-selective surfaces have been most commonly used in the radio frequency region of the electromagnetic spectrum and find use in applications as diverse as the aforementioned microwave oven, antenna radomes and modern metamaterials. Sometimes frequency selective surfaces are referred to simply as periodic surfaces and are a 2-dimensional analog of the new periodic volumes known as photonic crystals.

<span class="mw-page-title-main">Performance and modelling of AC transmission</span>

Performance modelling is the abstraction of a real system into a simplified representation to enable the prediction of performance. The creation of a model can provide insight into how a proposed or actual system will or does work. This can, however, point towards different things to people belonging to different fields of work.

References

  1. ↑ "Why did Tesla make his coil in the first place?", 21st Century Books.
  2. ↑ Nikola Tesla, Talking with the Planets". Collier's Weekly, February 19, 1901, pp. 4–5.
    "Some ten years ago, I recognized the fact that to convey electric currents to a distance it was not at all necessary to employ a return wire, but that any amount of energy might be transmitted by using a single wire. I illustrated this principle by numerous experiments, which, at that time, excited considerable attention among scientific men."
  3. ↑ Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination, American Institute of Electrical Engineers, Columbia College, N.Y., May 20, 1891.
  4. ↑ Experiments with Alternate Currents of High Potential and High Frequency, Institution of Electrical Engineers Address, London, February 1892.
  5. ↑ On Light and Other High Frequency Phenomena, Franklin Institute, Philadelphia, February 1893, and National Electric Light Association, St. Louis, March 1893.
  6. ↑ U.S. Patent 6,104,107 , "Method and apparatus for single line electrical transmission".  Avramenko, et al.
  7. ↑ A. Sommerfeld, Ann. Phys. u. Chemie (Neue Folge) 67-1, 233 (1899)
  8. ↑ Georg Goubau, "Surface waves and their Application to Transmission Lines," Journal of Applied Physics, Volume 21, Nov. (1950)
  9. ↑ U.S. Patent 2,685,068 , "Surface wave transmission line". George J. E. Goubau
  10. ↑ U.S. Patent 2,921,277 , "Launching and receiving of surface waves". George J. E. Goubau
  11. ↑ Tahsin Akalin, "Single-wire transmission lines at terahertz frequencies", IEEE Transactions on Microwave Theory and Techniques (IEEE-MTT), Volume 54, Issue 6, June 2006 Page(s): 2762 - 2767
  12. ↑ U.S. Patent 7,009,471 , "Method and apparatus for launching a surfacewave onto a single conductor transmission line using a slotted flared cone". Glenn E. Elmore
  13. ↑ U.S. Patent 7,567,154 , " Surface wave transmission system over a single conductor having E-fields terminating along the conductor " Glenn E. Elmore
  14. ↑ "E-Line". Corridor Systems Inc. 2010. Retrieved November 6, 2013.
  15. ↑ Glenn Elmore (July 27, 2009). "Introduction to the Propagating TM Wave on a Single Conductor" (PDF). Corridor Systems. Retrieved November 6, 2013.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.