Using Leaky Feeder
We’ve been selling Times Microwave Leaky Feeder since 2006. We’ve installed it ourselves and guided Resellers through the short learning curve necessary to know what it will do and what it won’t. It’s installed in tunnels, underground buildings, a TV Editing suite so cluttered with steel racks that no normal signal from an antenna could get through, runways and particularly hotels.
Other typical locations include mines, metal-hulled ships, nuclear power plants and tall buildings with metal supporting structures – in fact anywhere that the structure makes RF communications difficult, or where low power levels are required so that the spectrum can be used in other areas as well.
Diplexers can allow the cable to carry other frequencies at the same time, so the same cable can provide extended mobile phone coverage with 900 and 1800 MHz signals being carried simultaneously. (N.B. If you are considering deployment of 900/1800 MHz mobile phone repeaters in buildings in the UK check the Ofcom website and the requirements of the Wireless Telegraphy Act.) Patient Monitoring Systems in hospitals using RFID signals along the same cable are under development.
There are a few tricks of the trade for Leaky Feeder so here’s what you need to know.
What is Leaky Feeder?
| It’s very similar to the RF cables used to connect an Access Point to an antenna. With RF cables everything possible is done to keep the signal inside and deliver as much of it as possible at the other end. Leaky feeder has a different outer shield, in fact two half-shields, and the shields are designed to allow the signal to leak out evenly along its length. This gives better performance than cables made with a slotted outer jacket and a lower loss of signal along the length of the cable. Really it’s a cable that acts as a long cylindrical antenna. Placed in a roof void, above false ceiling tiling or between ceiling and floorboards, it will radiate its signal into the adjacent rooms above and below.
Unlike some other brands of cable, the signal radiates evenly all round, so it is not necessary to mark the ‘back’ of the cable and install with the front radiating in a particular direction. Installed between floors, Times Microwave Leaky Feeder covers the floor above and the floor below, not just one or the other.
Why use leaky Feeder rather than Access Points and antenna?
Is it legal?
On 2.4 GHz the legal constraint is a maximum of 20 dBm radiated power at the antenna, but because the signal from the Leaky Feeder drops off with distance it can be fed with higher power signals. A 1 watt amplifier is most often used. The AP feeds the amp and the amp feeds the Leaky Feeder. Yes, it’s legal from a wireless point of view. Be aware that for installation inside UK buildings cabling must comply with fire regulations and Times Microwave LF comes in two types – the cheaper PVC jacketed version must not be used inside a building, only the FPRE fire-resistant low smoke version is suitable. Other manufacturers may not manufacture their cable with the required fire resistance. If you installed it and you used the wrong type then make sure you’re public liability covers you for several million!
How far does the signal travel?
In our experiments several years ago we tested a 25 metre length of Leaky Feeder fed by a very basic budget D Link wireless router, with a power output of 18 dBm, less than 100 mW. We successfully roamed with a laptop and had a good connection 20 feet away through a building – which three solid 9” brick walls in the path. Adding a 1 watt amp gave a very solid connection at 30 metres in free air, which we can say from experience will drop to about 20 metres if there are walls in the way. This kind of distance is more than enough to serve many buildings: we’d expect to be able to place Leaky feeder in the loft space of a three-storey building with plasterboard ceilings and timber floorboards and get a good connection on the ground floor. If the building was made of steel-reinforced concrete we’d would expect to get through one floor because concrete with steel reinforcement, or thick stone walls, will impede signal at 2.4 GHz quite substantially. Nonetheless, judicious placement will still serve two floors as the majority of concrete-built buildings have tiled ceilings with a void above them. Times Microwave Leaky Feeder is very light – lighter than LMR400 cable for example – so laying it on top of ceiling tiles makes for a very easy install option.
How long can the cable be?
There have been successful UK deployments of 400 metres. Both ends of the cable are held at 50 ohm impedance by the use of a terminating resistor rated at 2 watts, enabling 4 watts power dissipation, plenty of margin when using bi-directional amplifiers rated up to 3 watts. The Connector set we supply includes the necessary T-piece and the terminating resistors.
Does it use standard N Plug and Jack connectors? How is it mounted?
Yes, standard connectors suitable for LMR600 may be used. Fitting is slightly different insofar as the plug is slid up under the outer jacket but our preferred method is not to slide under the jacket but crimp un the radiating sheath and add two layers of glue-lined heat shrink. This gives a very solid connector to cable fit. See the diagrams at the bottom of this document for the Times Micowave suggestion.
In most cases it’s a ‘lay-in,’ so mounting issues are minimal, but don’t run LF in next to a metal pipe! If it needs to be wall mounted Times Microwave say it must be 50mm clear of the wall, so a standoff is required or the capacitance effect of the wall will impair the radiation pattern. A version of the cable – Nu-Trac – is available which can be cleated to a wall or floor with metal cleats. In practice a lay of cable on top of ceiling tiles or plasterboard works fine.
Do I need an amplifier?
It depends on the length of the cable run and the choice of Access Point. Ubiquiti Bullet 2 High Power would the lowest-rated AP we would advise and these would be suitable for a small installation. In all other cases a 1 Watt bi-directional amplifier will do the job nicely and we keep them in stock. The amps accept a maximum of 100 mw at the input so devices with a higher output must be selected o a lower level output to avoid damaging the amplifier input circuits. 17 dB works well.
We have an Evaluation Kit consisting of 40 metres of Leaky Feeder, an amp, a connector set and cables to hook to Access Points presenting N jack, SMA-RP jack or TNC-RP jack and we can include cables to connect to something other than these on request. The kit is available on hire at £20 a week + VAT, plus carriage in both directions. Hiring the Evaluation Kit allows you to try out what Leaky Feeder can do and get that all-important ‘feel’ for its capabilities without having to purchase cable for yourself.
Although you can order on our website it’s much better to phone or drop us an email so we can advise on its use in your particular circumstances and determine exactly what you need to do the job.
We hope the above information helps open up some new possibilities to cover provide wifi connectivity in places that would otherwise be impossible. Feel free to call us on 01295 266277 for more information.
See below for the Times Micowave N Plug fitting procedure.