Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line; the rays or waves may be diffracted, reflected, or absorbed by the atmosphere and obstructions with material and cannot travel over the horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency due to diffraction, radio waves can travel as ground waves, which follow the contour of the Earth; this enables AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between 1 and 30 MHz, can be reflected back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a global reach. However, at frequencies above 30 MHz and in lower levels of the atmosphere, neither of these effects are significant. Thus, any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense.
Therefore, since the ability to visually see a transmitting antenna corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon". In practice, the propagation characteristics of these radio waves vary depending on the exact frequency and the strength of the transmitted signal. Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests. Low-powered microwave transmitters can be foiled by tree branches, or heavy rain or snow; the presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first Fresnel zone should be free of obstructions. Reflected radiation from the surface of the surrounding ground or salt water can either cancel out or enhance the direct signal.
This effect can be reduced by raising either or both antennas further from the ground: The reduction in loss achieved is known as height gain. See Non-line-of-sight propagation for more on impairments in propagation, it is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave used 4⁄3 earth radius to compute clearances along the path. Although the frequencies used by mobile phones are in the line-of-sight range, they still function in cities; this is made possible by a combination of the following effects: 1⁄r 4 propagation over the rooftop landscape diffraction into the "street canyon" below multipath reflection along the street diffraction through windows, attenuated passage through walls, into the building reflection and attenuated passage through internal walls and ceilings within the buildingThe combination of all these effects makes the mobile phone propagation environment complex, with multipath effects and extensive Rayleigh fading.
For mobile phone services, these problems are tackled using: rooftop or hilltop positioning of base stations many base stations. A phone can see at least three, as many as six at any given time. "sectorized" antennas at the base stations. Instead of one antenna with omnidirectional coverage, the station may use as few as 3 or as many as 32 separate antennas, each covering a portion of the circular coverage; this allows the base station to use a directional antenna, pointing at the user, which improves the signal to noise ratio. If the user moves from one antenna sector to another, the base station automatically selects the proper antenna. Rapid handoff between base stations the radio link used by the phones is a digital link with extensive error correction and detection in the digital protocol sufficient operation of mobile phone in tunnels when supported by split cable antennas local repeaters inside complex vehicles or buildingsA Faraday cage is composed of a conductor that surrounds an area on all sides and bottom.
Electromagnetic radiation is blocked. For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, parts of trains and ships; the same problem can affect signals in buildings with extensive steel reinforcement. The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere without an atmosphere, the radio horizon would be a circle; the radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Radio wave propagation is affected by atmospheric conditions, ionospheric absorption, the presence of obstructions, for example mountains or trees. Simple formulas that include the effect of the atmosphere give the range as: h o r i z o n m i l e s ≈ 1.23 ⋅ h e i g h t
CommScope Inc. which opened in 1976, is a multi-national network infrastructure provider company based in Hickory, North Carolina, United States. CommScope is a 1997 spin-off of General Instrument and has over 20,000 employees worldwide, with customers in over 130 countries. CommScope manufactures SYSTIMAX and Uniprise brands of Enterprise infrastructure of copper Unshielded Twisted Pair cabling, connector panels and fiber optic cabling, connector panels and metals. CommScope manufactures environmentally secure cabinets for FTTN and DSL applications. In 1975, CommScope was a product line of Superior Continental Cable. 29-year-old Frank Drendel headed a team charged with selling the failing product line. Frank Drendel and Jearld Leonhardt founded CommScope in August 1976 after raising $5.1 million to purchase the CommScope product line. Two years CommScope and Valtech merged under the Valtech name. In 1979, Valtech donated fiber optics line and equipment to link the U. S. House of Representatives to the C-SPAN studios, enabling live broadcasting of U.
S. Congressional proceedings for the first time. Continuing in the 1980s, Valtech sold to M/A-COM, Inc. and CommScope became part of the Cable Home Group for M/A-COM. In 1983, CommScope formed the Network Cable division for the local area network, data communications, television-receive only and specialized wire markets. In 1986, M/A-COM, Inc. sells, the Cable Home Group to General Instrument Corporation. CommScope became a division of General Instrument. In 1990, CommScope opened a new manufacturing facility in Claremont, North Carolina to answer the increased demand for unshielded twisted pair, television receive-only cables and personal computer cables. In 1997, General Instrument split into three independent, publicly traded companies, one of, CommScope. In July 1997, Frank Drendel rang the bell at the New York Stock Exchange to announce CommScope going public. In 2000, CommScope opened its new global headquarters in North Carolina. In 2004, CommScope acquired Avaya's Connectivity Solutions business and inherited the SYSTIMAX brand, a company best known for their enterprise cabling systems.
Avaya’s Carrier Solutions – which offered products designed for switching and transmission applications in telephone central offices and secure environmental enclosures—also became a part of CommScope. This acquisition doubled the size of CommScope. In 2007, CommScope acquired the global wireless infrastructure provider Andrew Corporation. Through its new Andrew brand, CommScope became a global leader in radio frequency subsystem solutions for wireless networks and again doubled CommScope’s size. In 2011, the Carlyle Group, a global alternative asset manager, acquired CommScope; this acquisition makes CommScope owned by the Carlyle Group and removed from the New York Stock Exchange. This acquisition changed the executive structure at CommScope. Eddie Edwards was appointed president and chief executive officer, succeeding Frank Drendel, who served as CommScope’s CEO since the company’s founding in 1976. Drendel continued as the chairman of the board. In October 25, 2015, CommScope celebrated its initial public offering as the NASDAQ.
In 2015, CommScope acquired TE Connectivity’s Broadband Network Solutions division. In 2015, CommScope acquired Airvana, a held company that specialized in small cell solutions for wireless networks. In November 2016, the Carlyle Group announced the sale of its remaining stock. On November 8, 2018, CommScope announced that it would acquire ARRIS in a cash deal valued at $7.4 billion including the repayment of debt. This acquisition brings back together two of the former General Instrument companies from the 1997 split. Transaction was completed on April 4, 2019. On October 25, 2010, The Carlyle Group announced it would pay $31.50 a share, or about $2.98 billion, to take CommScope private. In July 2011, CommScope received the 2010 Communications Solutions Product of the Year Award from Technology Marketing Corporation for its product, Wired For Wireless. On August 2, 2013, CommScope filed with the SEC to raise up to $750 million in an initial public offering; the initial public offering and first public trading took place on October 25, 2013 at NASDAQ.
On May 16, 2017, CommScope introduced a High Speed Migration platform. In 2004 the company acquired Avaya's Connectivity Solutions business; the Avaya business acquisition included the legacy intellectual property and patents from Western Electric, AT&T, Lucent Technologies and Avaya. In June 2007, CommScope acquired Andrew Corporation for $2.6 billion. Andrew's products included antennas, amplifiers, transceivers, as well as software and training for the broadband and cellular industries. In January 2015, CommScope agreed to purchase a unit of TE Connectivity for $3 billion; the transaction closed August 28, 2015. In October 2015, CommScope acquired a manufacturer of small cells and femtocells. In November 2018 CommScope agreed to purchase Arris International, maker of a variety of networking equipment including set top boxes, WiFi routers and Ethernet switches for $7.4 billion. In February 2019, Arris shareholders approved the purchase. Marvin S. Edwards, Jr. – CommScope’s President and Chief Executive Officer Alexander W. Pease - Executive Vice President and Chief Financial Officer Robert W. Granow - Senior Vice President, Corporate Controller, Principal Accounting Officer Frank M. Drendel – CommScope’s Founder and Chairman of the Board Austin A. Adams - Audit Committee Stephen C.
Gray - Compensation Committee L. William Krause - Compensation and Nominating Committee Joanne M. Maguire - Chair of
Transit Wireless is an American telecommunication company founded in 2005, based in New York City. It specializes in building wireless communication infrastructure using distributed antenna system networks to provide Wi-Fi and cellular phone coverage in the places that are unreachable by traditional cellular phone services such as in the underground portions of the New York City Subway. In 2010, the company was injected with financial support from infrastructure company BAI Communications for its first project with the New York City Transit Authority, which consisted of adding wireless access to subway stations; the company is now a subsidiary of BAI Communications. Official website
7 July 2005 London bombings
The 7 July 2005 London bombings referred to as 7/7, were a series of coordinated terrorist suicide attacks in London, United Kingdom, which targeted commuters travelling on the city's public transport system during the morning rush hour. Four radical Islamic terrorists separately detonated three homemade bombs in quick succession aboard London Underground trains across the city and a fourth on a double-decker bus in Tavistock Square; the train bombings occurred on the Circle line near Aldgate and at Edgware Road, on the Piccadilly line near Russell Square. Fifty-two people of 18 different nationalities, all of whom were UK residents, were killed, more than 700 were injured in the attacks, making it Britain's deadliest terrorist incident since the 1988 bombing of Pan Am Flight 103 near Lockerbie and England's deadliest since the 1974 Birmingham pub bombings, as well as the country's first Islamist suicide attack; the explosions were caused by triacetone triperoxide IEDs packed into backpacks.
The bombings were followed two weeks by a series of attempted attacks that failed to cause injury or damage. The 7 July attacks occurred the day. At 8:49 am, on Thursday 7 July 2005, three bombs were detonated on board London Underground trains within 50 seconds of each other: The first bomb exploded on a six-car London Underground C69 and C77 Stock Circle line sub-surface train, number 204, travelling eastbound between Liverpool Street and Aldgate; the train had left King's Cross St Pancras about eight minutes earlier. At the time of the explosion, the train's third car was 100 yards along the tunnel from Liverpool Street; the parallel track of the Hammersmith & City line between Liverpool Street and Aldgate East was damaged in the blast. The second bomb exploded in the second car of another six-car London Underground C69 and C77 Stock Circle line sub-surface train, number 216, which had just left platform 4 at Edgware Road and was travelling westbound towards Paddington; the train had left King's Cross St Pancras about eight minutes previously.
There were several other trains nearby at the time of the explosion. Two other trains were at Edgware Road: an unidentified train on platform 2 and a southbound Hammersmith & City line service that had just arrived at platform 1. A third bomb was detonated on a 6-car London Underground 1973 Stock Piccadilly line deep-level Underground train, number 311, travelling southbound from King's Cross St Pancras to Russell Square; the device exploded one minute after the service departed King's Cross, by which time it had travelled about 500 yards. The explosion occurred at the rear of the first car of the train—number 166—causing severe damage to the rear of that car as well as the front of the second one; the surrounding tunnel sustained damage. It was thought that there had been six, rather than three, explosions on the Underground network; the bus bombing brought the reported total to seven. The erroneous reporting can be attributed to the fact that the blasts occurred on trains that were between stations, causing wounded passengers to emerge from both stations, giving the impression that there was an incident at each.
Police revised the timings of the tube blasts: initial reports had indicated that they occurred during a period of half an hour. This was due to initial confusion at London Underground, where the explosions were believed to have been caused by power surges. An early report, made in the minutes after the explosions, involved a person under a train, while another described a derailment. A code amber alert was declared by LU at 09:19, LU began to cease the network's operations, ordering trains to continue only to the next station and suspending all services; the effects of the bombs are understood to have varied due to the differing characteristics of the tunnels in which they occurred: The Circle line is a "cut and cover" sub-surface tunnel, about 7 m deep. As the tunnel contains two parallel tracks, it is wide; the two explosions on the Circle line were able to vent their force into the tunnel, reducing their destructive force. The Piccadilly line is a deep-level tunnel, up to 30 m below the surface and with narrow single-track tubes and just 15 cm clearances.
This confined space reflected the blast force. One hour after the attacks on the London Underground, a fourth bomb was detonated on the top deck of a number 30 double-decker bus, a Dennis Trident 2 operated by Stagecoach London and travelling its route from Marble Arch to Hackney Wick. Earlier, the bus had passed through the King's Cross area as it travelled from Hackney Wick to Marble Arch. At its final destination, the bus started the return route to Hackney Wick, it left Marble Arch at 9 am and arrived at Euston bus station at 9:35 am, where crowds of people had been evacuated from the tube and were boarding buses. The explosion at 9:47 am in Tavistock Square ripped off the roof and destroyed the rear portion of the bus; the blast took place near BMA House, the headquarters of the British Medical Association, on Upper Woburn Place. A number of doctors and medical staff in or near that building were able to provide immediate emergency assistance. Witnesses reported seeing "half a bus flying through the air".
BBC Radio 5 Live and Th
Ultra high frequency
Ultra high frequency is the ITU designation for radio frequencies in the range between 300 megahertz and 3 gigahertz known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency or microwave frequency range. Lower frequency signals fall into lower bands. UHF radio waves propagate by line of sight, they are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, numerous other applications. The IEEE defines the UHF radar band as frequencies between 1 GHz. Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz. Radio waves in the UHF band travel entirely by line-of-sight propagation and ground reflection. UHF radio waves are blocked by hills and cannot travel far beyond the horizon, but can penetrate foliage and buildings for indoor reception.
Since the wavelengths of UHF waves are comparable to the size of buildings, trees and other common objects and diffraction from these objects can cause fading due to multipath propagation in built-up urban areas. Atmospheric moisture reduces, or attenuates, the strength of UHF signals over long distances, the attenuation increases with frequency. UHF TV signals are more degraded by moisture than lower bands, such as VHF TV signals. Since UHF transmission is limited by the visual horizon to 30–40 miles and to shorter distances by local terrain, it allows the same frequency channels to be reused by other users in neighboring geographic areas. Public safety, business communications and personal radio services such as GMRS, PMR446, UHF CB are found on UHF frequencies as well as IEEE 802.11 wireless LANs. The adopted GSM and UMTS cellular networks use UHF cellular frequencies. Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required; when conditions are right, UHF radio waves can travel long distances by tropospheric ducting as the atmosphere warms and cools throughout the day.
The length of an antenna is related to the length of the radio waves used. Due to the short wavelengths, UHF antennas are conveniently short. UHF wavelengths are short enough that efficient transmitting antennas are small enough to mount on handheld and mobile devices, so these frequencies are used for two way land mobile radio systems, such as walkie-talkies, two way radios in vehicles, for portable wireless devices. Omnidirectional UHF antennas used on mobile devices are short whips, sleeve dipoles, rubber ducky antennas or the planar inverted F antenna used in cellphones. Higher gain omnidirectional UHF antennas can be made of collinear arrays of dipoles and are used for mobile base stations and cellular base station antennas; the short wavelengths allow high gain antennas to be conveniently small. High gain antennas for point-to-point communication links and UHF television reception are Yagi, log periodic, corner reflectors, or reflective array antennas. At the top end of the band slot antennas and parabolic dishes become practical.
For satellite communication and turnstile antennas are used since satellites employ circular polarization, not sensitive to the relative orientation of the transmitting and receiving antennas. For television broadcasting specialized vertical radiators that are modifications of the slot antenna or reflective array antenna are used: the slotted cylinder, zig-zag, panel antennas. UHF television broadcasting fulfilled the demand for additional over-the-air television channels in urban areas. Today, much of the bandwidth has been reallocated to land mobile, trunked radio and mobile telephone use. UHF channels are still used for digital television. UHF spectrum is used worldwide for land mobile radio systems for commercial, public safety, military purposes. Many personal radio services use frequencies allocated in the UHF band, although exact frequencies in use differ between countries. Major telecommunications providers have deployed voice and data cellular networks in UHF/VHF range; this allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet.
UHF radars are said to be effective at tracking stealth fighters, if not stealth bombers. UHF citizens band: 476–477 MHz Television broadcasting uses UHF channels between 503 and 694 MHz Fixed point-to-point Link 450.4875 - 451.5125 MHz Land mobile service 457.50625 - 459.9875 MHz Mobile satellite service: 406.0000 - 406.1000 MHz Segment and Service examples: Land mobile for private, Australian and Territory Government, Rail industry and Mobile-Satellite 430–450 MHz: Amateur radio 470–806 MHz: Terrestrial television 1452–1492 MHz: Digital Audio Broadcasting Many other frequency assignments for Canada and Mexico are similar to their US counterparts 380–399.9 MHz: Terrestrial Trunked Radio service for emergency use 430–440 MHz: Amateur ra
Rail transport is a means of transferring of passengers and goods on wheeled vehicles running on rails known as tracks. It is commonly referred to as train transport. In contrast to road transport, where vehicles run on a prepared flat surface, rail vehicles are directionally guided by the tracks on which they run. Tracks consist of steel rails, installed on ties and ballast, on which the rolling stock fitted with metal wheels, moves. Other variations are possible, such as slab track, where the rails are fastened to a concrete foundation resting on a prepared subsurface. Rolling stock in a rail transport system encounters lower frictional resistance than road vehicles, so passenger and freight cars can be coupled into longer trains; the operation is carried out by a railway company, providing transport between train stations or freight customer facilities. Power is provided by locomotives which either draw electric power from a railway electrification system or produce their own power by diesel engines.
Most tracks are accompanied by a signalling system. Railways are a safe land transport system. Railway transport is capable of high levels of passenger and cargo utilization and energy efficiency, but is less flexible and more capital-intensive than road transport, when lower traffic levels are considered; the oldest known, man/animal-hauled railways date back to the 6th century BC in Greece. Rail transport commenced in mid 16th century in Germany in the form of horse-powered funiculars and wagonways. Modern rail transport commenced with the British development of the steam locomotives in the early 19th century, thus the railway system in Great Britain is the oldest in the world. Built by George Stephenson and his son Robert's company Robert Stephenson and Company, the Locomotion No. 1 is the first steam locomotive to carry passengers on a public rail line, the Stockton and Darlington Railway in 1825. George Stephenson built the first public inter-city railway line in the world to use only the steam locomotives all the time, the Liverpool and Manchester Railway which opened in 1830.
With steam engines, one could construct mainline railways, which were a key component of the Industrial Revolution. Railways reduced the costs of shipping, allowed for fewer lost goods, compared with water transport, which faced occasional sinking of ships; the change from canals to railways allowed for "national markets" in which prices varied little from city to city. The spread of the railway network and the use of railway timetables, led to the standardisation of time in Britain based on Greenwich Mean Time. Prior to this, major towns and cities varied their local time relative to GMT; the invention and development of the railway in the United Kingdom was one of the most important technological inventions of the 19th century. The world's first underground railway, the Metropolitan Railway, opened in 1863. In the 1880s, electrified trains were introduced, leading to electrification of tramways and rapid transit systems. Starting during the 1940s, the non-electrified railways in most countries had their steam locomotives replaced by diesel-electric locomotives, with the process being complete by the 2000s.
During the 1960s, electrified high-speed railway systems were introduced in Japan and in some other countries. Many countries are in the process of replacing diesel locomotives with electric locomotives due to environmental concerns, a notable example being Switzerland, which has electrified its network. Other forms of guided ground transport outside the traditional railway definitions, such as monorail or maglev, have been tried but have seen limited use. Following a decline after World War II due to competition from cars, rail transport has had a revival in recent decades due to road congestion and rising fuel prices, as well as governments investing in rail as a means of reducing CO2 emissions in the context of concerns about global warming; the history of rail transport began in the 6th century BC in Ancient Greece. It can be divided up into several discrete periods defined by the principal means of track material and motive power used. Evidence indicates that there was 6 to 8.5 km long Diolkos paved trackway, which transported boats across the Isthmus of Corinth in Greece from around 600 BC.
Wheeled vehicles pulled by men and animals ran in grooves in limestone, which provided the track element, preventing the wagons from leaving the intended route. The Diolkos was in use for over 650 years, until at least the 1st century AD; the paved trackways were later built in Roman Egypt. In 1515, Cardinal Matthäus Lang wrote a description of the Reisszug, a funicular railway at the Hohensalzburg Fortress in Austria; the line used wooden rails and a hemp haulage rope and was operated by human or animal power, through a treadwheel. The line still exists and is operational, although in updated form and is the oldest operational railway. Wagonways using wooden rails, hauled by horses, started appearing in the 1550s to facilitate the transport of ore tubs to and from mines, soon became popular in Europe; such an operation was illustrated in Germany in 1556 by Georgius Agricola in his work De re metallica. This line used "Hund" carts with unflanged wheels running on wooden planks and a vertical pin on the truck fitting into the gap between the planks to keep it going the right way.
The miners called the wagons Hunde from the noise. There are many references to their use in central Europe in the 16th century; such a transport system was used by German miners at Cal
Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables have an insulating outer sheath or jacket; the term coaxial comes from the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880. Coaxial cable is a type of transmission line, used to carry high frequency electrical signals with low losses, it is used in such applications as telephone trunklines, broadband internet networking cables, high speed computer data busses, carrying cable television signals, connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, needed for it to function efficiently as a transmission line. Coaxial cable is used as a transmission line for radio frequency signals.
Its applications include feedlines connecting radio transmitters and receivers to their antennas, computer network connections, digital audio, distribution of cable television signals. One advantage of coaxial over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors; this allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable provides protection of the signal from external electromagnetic interference. Coaxial cable conducts electrical signal using an inner conductor surrounded by an insulating layer and all enclosed by a shield one to four layers of woven metallic braid and metallic tape; the cable is protected by an outer insulating jacket. The shield is kept at ground potential and a signal carrying voltage is applied to the center conductor; the advantage of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield.
Conversely and magnetic fields outside the cable are kept from interfering with signals inside the cable. Larger diameter cables and cables with multiple shields have less leakage; this property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the environment or for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, computer and instrumentation data connections; the characteristic impedance of the cable is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors. In radio frequency systems, where the cable length is comparable to the wavelength of the signals transmitted, a uniform cable characteristic impedance is important to minimize loss; the source and load impedances are chosen to match the impedance of the cable to ensure maximum power transfer and minimum standing wave ratio.
Other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, shield quality. Coaxial cable design choices affect physical size, frequency performance, power handling capabilities, flexibility and cost; the inner conductor might be stranded. To get better high-frequency performance, the inner conductor may be silver-plated. Copper-plated steel wire is used as an inner conductor for cable used in the cable TV industry; the insulator surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of the dielectric insulator determine some of the electrical properties of the cable. A common choice is a solid polyethylene insulator, used in lower-loss cables. Solid Teflon is used as an insulator; some coaxial lines have spacers to keep the inner conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield; this allows the cable to be flexible, but it means there are gaps in the shield layer, the inner dimension of the shield varies because the braid cannot be flat.
Sometimes the braid is silver-plated. For better shield performance, some cables have a double-layer shield; the shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers, such as "quad-shield", which uses four alternating layers of foil and braid. Other shield designs sacrifice flexibility for better performance; those cables cannot be bent as the shield will kink, causing losses in the cable. When a foil shield is used a small wire conductor incorporated into the foil makes soldering the shield termination easier. For high-power radio-frequency transmission up to about 1 GHz, coaxial cable with a solid copper outer conductor is available in sizes of 0.25 inch upward. The outer conductor is corrugated like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric. One brand name for such cable is Heliax. Coaxial cables require an internal structure of an insulating material to maintain the spacing between the center conductor and shield.