Shifting signals from copper to fibre

The distribution of radio frequency signals is experiencing rapid changes

By Morand Fachot

RF or r.f. (radio frequency) signals, used to carry multimedia, voice or data content, may reach end-users through cable or over the air via electromagnetic wave propagation or satellite reception. Yet they rely primarily on cables for distribution from source to transmitters and antennas and final connection to receiving equipment. RF signals can be transmitted electrically or optically via coaxial cable or optic fibre; sometimes both approaches are used together in integrated networks.

Uplink dish for broadcasting (Photo: SES S.A.)
Uplink dish for broadcasting (Photo: SES S.A.)

Coaxial cables

A coaxial cable is an electric cable that acts as a waveguide for RF signals. It consists of an inner conductor (single solid or stranded wires), surrounded by a dielectric (non-conducting) insulator that separates it from an outer metallic conductor made up of metal tubing or braided strands. The outer conductor acts as a shield, preventing external and internal leakage of electromagnetic signals and interference. These elements are contained within an outer sheath that protects them from the environment; they all share the same axis, hence the term coaxial.

Coaxial cables are used in many areas, such as mobile and microwave communication, wireless broadcasting and radar systems in aviation and marine.

Multiple types, multiple standards

A wide variety of coaxial cables enables specific frequency bands to be catered for as well as the multitude of applications and environments in which the cables are used. The most familiar coaxial cable is the flexible sort found in the home environment for bringing analogue TV signals from antennas or cable networks to sets.

Other types of cables, including hard line, semi-rigid and rigid are used in domains such as broadcasting or two-way telecommunication and data transmission. They have to meet stringent technical criteria to prevent external interference and protect them against environmental hazards. As many cannot be bent, they also need special connectors.

Many International Standards are required to set the electromagnetic and physical characteristics of coaxial cables and associated components. Preparing these is the task of IEC SC (Subcommittee) 46A: Coaxial cables. SC 46A "is responsible for standardization activities related to coaxial cables used primarily in ICT (Information and Communications Technology), microwave and multimedia distribution networks and systems".

To leak or not to leak

The purpose of the shielding layer is to prevent leakage; that is, the passage of signals from and into the inner conductor.

Defective shielding may let unwanted signals penetrate the conductor and weaken or interfere with the RF signals it transmits. Conversely, external leakage of the signal carried by a coaxial cable results in a poorer RF signal and possible electromagnetic interference that can disrupt nearby devices or signals.

However, a certain type of coaxial cable is specifically designed to leak signals, both from and to the cable. These so-called "leaky" or "radiating" cables have slots cut into their shield that enable them to act as both antenna and feeder cables by allowing signals to be sent or received. These cables are mainly used for wireless communication systems in long, narrow and enclosed areas that cannot be covered effectively by conventional antenna signals – for example, elevator or mine shafts, or tunnels, like those of London Underground, which uses this type of cables.

SC 46A has published a specific standard for "leaky" cables. IEC 61196-4, Coaxial communication cables – Part 4: Sectional specification for radiating cables. This standard "covers the requirements for flexible and semi-flexible radiating coaxial communication cables (…) specifies preferred ratings and characteristics (…) and enables selection of the appropriate tests and performance levels (…)"

Light at the end of the cable

For many applications, metal wires such as coaxial cables have been replaced by optical fibres for the transmission of RF signals for multimedia services or ICT data.

Optical fibre-based systems present many benefits as they allow very large amounts of data to be transmitted over long distances almost instantaneously. Their widespread introduction has made possible the rapid and extensive development of the information society.

Optical fibres are made up of a transparent core surrounded by a transparent but less refractive material that keeps light in the core, allowing it to act as a waveguide.

Electrical signals, such as RF signals, are changed into optical signals using an electrical to optical fibre converter. They are then transmitted down optical fibres. If required, repeaters are used to regenerate the signal over distance.

Hundreds of fibres can be bundled in a single cable, allowing large amounts of data to be transmitted simultaneously at great speed. This makes fibre an ideal medium for multimedia, telecommunication and ICT applications.

In addition to these applications, the introduction of Smart Grids is expected to create an additional demand for fibre-based networks.

Optical fibres are highly adaptable: they can be fitted nearly anywhere indoors or outdoors. They may be laid in ducts, sewer networks, water pipes, high-pressure gas pipes, along or inside power cables. They can be buried in the ground, in the pavement, used in self-supporting cables or submerged in the sea…

The fibre optic market is expanding rapidly. IEC TC 86 and its SCs prepare International Standards for fibre optic-based systems. This TC and its SCs are very active, preparing and releasing many International Standards.

Multimedia world

One sector that relies extensively on both coaxial and fibre optics is the broadcast industry, from content production to signal distribution via satellite, terrestrial broadcast or cable networks.

The dramatic rise in the number of TV channels has driven the need for increased transmission capacity. The number of national channels in Europe had swollen from 47 in 1990 to 9  800by2010,a trend also observed elsewhere in the world. The introduction of interactive and data services, of HDTV (high-definition television) and 3D TV has further increased the search for even higher capacity.

The signal distribution infrastructure in satellite ground stations and CATV (cable TV) head-ends still relies heavily on coaxial cables.

HFC (hybrid fibre-coaxial) networks, employing a combination of optical fibre and coaxial cable, have been widely deployed for broadband networks by CATV operators since the early 1990s.

According to the German DEV Systemtechnik GmbH company, only about a quarter of all satellite communication and CATV installations currently use optical transmission and distribution of RF signals. The need for reliable and larger capacity solutions for signal transport and distribution at RF installations means that fibre-based systems are set to become ever more present in this sector.

TC 86 and its SCs are well prepared to support this expansion with their extensive work on International Standards for optic fibre-based systems..

Armoured optical fibre cable Armoured optical fibre cable
Coaxial cables: slots in shielding of leaky cable (right) allow signal to be received/sent Coaxial cables: slots in shielding of leaky cable (right) allow signal to be received/sent
Uplink dish for broadcasting (Photo: SES S.A.) Uplink dish for broadcasting (Photo: SES S.A.)