Leaky feeder

Slot cable, also known as leaky cable, leakage line or antenna cables are electrical leaky waveguide used as an elongated antennas, for example for broadcasting, mobile or Wi-Fi signals. It is coaxial with the small slits or openings in the outer conductor (screen) through which the RF signal can be radiated or received over the whole length of the cable.

By slot cable long indoor areas can be supplied evenly and cheap (eg tunnel ) with radio services in particular. The names leaky cable and lamp cable is based on the idea that a part of the RF power leaking out radially from the cable or irradiated.

Function

The openings in the outer conductor forming inhomogeneities of the waveguide, at which RF energy can escape from the cable. The electromagnetic field between the inner and outer conductor, this first portion of the magnetic field, in the screen creates a surface current. The diversion of surface flow around the opening in the screen produces a dipole moment at the location of the slot. As a result, each slit acts as an aperture antenna, and the magnetic field component of the electromagnetic field between the outer and inner conductor is emitted proportionately. The electric field component of the wave penetrates the opening as well, but perpendicular to the magnetic field component. By size, shape and spacing of the openings can be the radiation of a slot cable influence and adjust.

While aiming at normal coaxial cables as small as possible losses ( the incoming signal power at the cable end should be as the fed at the beginning of the line performance same ), here leaves a portion of the supplied power cable checked at each slot. Unlike the signal attenuation due to loss of a resistance cable, is thereby not converted into heat, but is as a radio signal in the exposed space.

Properties

The line impedance is usually 50 ohms or 75 ohms, as is common in high-frequency technology. Leaky cables usually have a high frequency bandwidth, i.e., a wide range of radio standards, to be transmitted. If the slots with only a radial angular segment along the cable, the cable slot can be seen a directional effect only in the immediate vicinity. In the far field the radiation is uniform radially. Signal attenuation is growing over the cable length and the relatively low antenna gain decreases over the cable length. This effect limits the usable length of a slot cable. It can in principle be compensated by an increase in the number of slots toward the cable end for example.

Applications

With the help of slot cables long narrow areas can preferably be supplied with wireless services. Areas of application are eg:

• Automation: With robots that can move, for example on railway lines, the slot cable is laid along a guide rail and covers the entire section using radio signals, such as WLAN, from so that a constant, error -free communication with a robot can take place.

• Building: most secure radio coverage with bad radio is required for the construction of new buildings with public public, such as shopping centers, office complexes, hospitals, airports, and industrial buildings. To ensure this, the cable is designed as a slot loop and fed from both ends, in the case of an interruption to continue to ensure a supply. As another example, the wireless coverage of the Allianz Arena is mentioned with TETRA, for about 2.5 km slot cable was laid in the entire stadium.

• Mining: Even in modern mines slot cables are used. As in tunnels, the saving over conventional antennas that an " illumination " of the mine despite curves and junctions is possible.

• Subway: About a misplaced throughout the tunnel slot cable, is the passengers allows the use of the GSM and UMTS ( eg U -Bahn Hamburg since 2006, S -Bahn Hamburg, Hanover Stadtbahn and U -Bahn Berlin ). Other uses are in the supply of bad radio ( eg Stadtbahn Stuttgart) or mobile radio. In the U -Bahn Hamburg all public mobile telephone networks in GSM and UMTS, the BOS digital radio and digital radio operation are fed into the broadband lamp cable.

• Road Tunnel: In addition to the supply of GSM and radio signals of bad radio and broadcasting services are often transported here ( to inform motorists ). Example: The Elbe Tunnel in Hamburg, inter alia, the radio station NDR 2 is fed through a slot cable. In an emergency it is even possible here to replace the broadcast signal with a transmission signal for announcements to inform motorists in the tunnel.

• Railway tunnel: On the ICE high-speed line Cologne-Rhine/Main all tunnels are supplied GSM 900 and DCS 1800 via cable slot. The slit cable is used can be used at a later time for the transmission of UMTS. In Switzerland, the tunnel supply of GSM-R is ensured via cable slot.

• Railway: On the railroad track slot cable for the transmission system EUROLOOP the European ETCS Level 1 are used.

• By train: About a leaky cables in the ceiling of the car passengers are supplied with GSM or UMTS mobile communications ( GSM only in Germany at the moment ). This is a in -train repeaters and an external antenna connection to the wireless network secure. Advantage of using a slit cable compared to a conventional antenna here is the less selective exposure of the passengers through uniform distribution of the transmission power on the powered mobile surface.

• Air: In modern aircraft cabins leaky cables come to provide passengers with GSM and WLAN are used. Here, the transmit powers are orders of magnitude too low to cause undesirable influences on the electronic control system of the aircraft.

• Aviation: Instrument Landing System (ILS ): end-fire glide slope antenna ( the signal propagation is from bottom reflections independent) and localizer antennas.