Slotted cables , also known as leaky cables , leakage cables or radiator cables , are electrical leaky waveguides that are used as elongated antennas , for example for radio, mobile radio or WLAN signals. They are coaxial cables with small slots or openings in the outer conductor (shield) through which the RF signal can be emitted or received over the entire length of the cable.
Long interior areas in particular can be supplied with radio services evenly and cost-effectively using slotted cables. The terms leak cable and radiator cable are based on the idea that part of the high-frequency power leaks or radiates radially out of the cable.
The openings in the outer conductor form inhomogeneities in the waveguide at which HF energy can escape from the cable. The electromagnetic field between the inner and outer conductors, here initially the magnetic field component, generates a surface current in the shield. The redirection of the surface current around the opening in the screen creates a dipole moment at the location of the slot. As a result, each slot acts as an aperture radiator, and the magnetic field component of the electromagnetic field between the outer and inner conductor is radiated proportionally. The electric field component of the wave also penetrates the opening, but perpendicular to the magnetic field component. The radiation behavior of a slot cable can be influenced and adjusted by the size, shape and spacing of the openings.
While normal coaxial cables aim to minimize losses (the signal power arriving at the end of the cable should be as close as possible to the power fed in at the start of the line), here part of the power fed in leaves the cable in a controlled manner at each slot. In contrast to the signal attenuation by loss of resistance in a cable, it is not converted into heat, but is available as a radio signal in the irradiated room.
The line impedance is usually 50 ohms or 75 ohms, as is common in high-frequency technology . Slotted cables usually have a high frequency bandwidth , i. H. a wide range of frequencies can be transmitted. If the slots lie over only one radial angular segment along the cable, a directional effect can only be seen in the immediate vicinity of the slot cable . In the far range the radiation characteristic is uniformly radial. The signal attenuation increases over the cable length and the relatively low antenna gain decreases over the cable length. This effect limits the usable length of a slotted cable. He can in principle z. B. compensate by increasing the number of slots towards the end of the cable.
With the help of slotted cables, long narrow areas can preferably be supplied with radio services. Areas of application are e.g. B .:
- Automation: For robots that are e.g. B. can move on rail lines, the slotted cable is laid along a guide rail and covers the entire route section with radio signals, eg. B. WLAN, so that an interruption-free and interference-free communication with a robot can take place.
- Buildings : For new buildings with public traffic, such as shopping centers, office complexes, hospitals, airports and industrial buildings, secure radio coverage with BOS radio is usually prescribed. In order to ensure this, the slotted cable is designed as a loop and fed from both ends in order to continue to ensure a supply in the event of an interruption. Another example is the radio coverage of the Allianz Arena with TETRA , for which approx. 2.5 kilometers of slotted cable was laid throughout the stadium.
- Mining : Split cables are also used in modern mines. As in tunnels, the advantage over conventional antennas is that "illumination" of the mine is possible despite curves and branches.
- U-Bahn : Passengers can use the GSM and UMTS via a slotted cable laid throughout the tunnel (e.g. Hamburg U-Bahn since 2006, Hamburg S-Bahn , Hanover light rail and Berlin U-Bahn ). Other uses are the supply with BOS radio (e.g. Stuttgart city railway ) or company radio . In the Hamburg subway, all public mobile phone networks in GSM and UMTS, the digital BOS radio and the digital company radio are fed into the broadband radiator cable.
- Road tunnel : In addition to the supply with GSM, as well as UMTS, LTE and partly already with 5G , radio signals of the BOS radio and radio (to inform the motorists) are often transported here. Example: In the Elbe tunnel in Hamburg , a. the radio station NDR 2 is fed in via a slotted cable. In an emergency, it is possible to replace the radio signal in the tunnel with a broadcast signal for announcements to inform drivers.
- Railway tunnel : On the ICE high-speed line Cologne – Rhein / Main , all tunnels are supplied with GSM 900 and DCS 1800 via slotted cables. The slotted cable used can also be used for the transmission of UMTS at a later point in time . In Switzerland, the tunnel supply from GSM-R is ensured via slotted cables. In addition, GSM (only until the end of 2020), UMTS and LTE services are fed in via the slotted cables in all rail tunnels. The feed-in of 5G services is currently being prepared.
- Railway : Slotted cables are used on the railway track for the Euroloop transmission system of the European train control system ETCS Level 1 .
- On the train : Passengers are supplied with GSM, UMTS, LTE and 5G services (currently only GSM in Germany) via a slotted cable in the ceiling of the wagon. This ensures the connection to the cellular network via an in-train repeater and an external antenna. The advantage of using a slotted cable compared to a conventional antenna is the lower point radiation exposure of the passengers due to the more even distribution of the transmission power over the area covered by mobile radio.
- Aircraft : In modern aircraft cabins, slotted cables are used to supply the cabin with GSM and WLAN. Here, the transmission power is too low by orders of magnitude to cause undesired influences on the control electronics of the aircraft.
- Aviation : Instrument landing system (ILS): end-fire glideslope antenna (signal propagation is independent of ground reflections) and landing course transmitter antennas.
- Alois Krischke: Rothammels Antennenbuch. 11th edition, Franckh-Kosmos-Verlags-GmbH, Stuttgart 1995, ISBN 3-440-07018-2
- Tunnel radio coverage - explanation of tunnel radio with slotted cables
- "Repeaters do not protect train passengers" - comparison of repeaters with slotted cables and conventional antennas