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Thread: Leaky Feeder Systems

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    Dr. Reinhard H. Wöhlbier

    Dr. Reinhard H. Wöhlbier, Dr.-Ing.


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    Leaky Feeder Systems

    SMARTControl® and SMARTTune®

    MineCom has released additional products under its SMART product line up, SMARTControl® a new version it’s MCA1000 VHF Head End with ALC & SMARTTune® an in-line-amplifier featuring an Automatic Cable Length and Gain Compensation circuit.

    MineCom was the first company in Australia to introduce a new generation of leaky feeder based systems specifically for mining back in 1990. Since those early days, MineCom has progressed from offering a single channel VHF FD4 system to offering a 32 channel MCA1000 VHF system AND two UHF versions offering 16 or 64 channels over a single leaky feeder cable. Both the VHF and UHF systems utilise MineCom’s new SMARTCabel leaky feeder cable.

    Leaky feeder cable based communication systems utilise a leaky feeder cable especially designed for mines by an English university some 25 years ago. The leaky feeder cable is used to carry the signals from the base station repeater out to the extents of the mine, with the RF signals radiating out from the cable evenly along the route. The same cable also acts as the receiving antenna and carries the signals from handheld portables, vehicle radios and data radio modems back to the base station repeater.

    As the cable has a finite signal loss it is necessary to provide amplification (boost) at routine intervals along the cable length. Therefore small in-line-amplifiers are installed in order to boost the RF signals as they travel TO and FROM the base station repeaters. The gain of the in-line-amplifiers has to match the signal losses as they pass along the cable. In this way it is possible to cover an entire mine which may have an active tunnel network of around 100 or even 200 kilometres (or more). An ‘amplifier’ is in fact 2 or 3 separate amplifiers in one enclosure, i.e. a Voice/Data UP amplifier, a Voice/Data DOWN amplifier and third being a VIDEO amplifier (which can be configured to transmit video images back UP to the surface OR configured to transmit FM radio station programs from the surface DOWN, depending on the system requirement)

    In a conventional leaky feeder system, the cable length between amplifiers is well controlled in the installation phase. The maximum gain of the amplifiers may be fixed or it may have a number of stepped gain settings, as in the case of MineCom’s LLC (Line-Length-Compensation) feature where there is a selectable attenuator fitted on the Base side of the amplifier in the common path i.e. it attenuates both the up (RX) as well as the down (TX) path signals.

    In the case of the Base Transmit amplifier circuit, should the level of the received signal be such that the amplifier will over amplify, then the Automatic Gain Control (AGC) circuit will operate and reduce the gain of the amplifier by adding attenuation in the Base Transmit Amplifier circuit. In this case, the gain of the signal in the Base Receive path is not affected.
    The Base Receive Amplifier has a similar circuit to the Base Transmit Amplifier, if there is too much gain the AGC will reduce the excess gain.
    For instance it would operate if two amplifiers were placed too close together i.e. 150 meters apart and if someone stood underneath an amplifier and transmitted the AGC would operate.

    In an earlier statement we mentioned that “In a conventional leaky feeder system, the cable length between amplifiers is well controlled in the installation phase”. However from that point on anything goes, mainly due to a lack of system training, high turnover in staff, and lack of design tools to assist mine staff. MineCom provides its customers with a range of software ‘tools’, designed to take the guess work out of the equation and assist staff to design system expansions, rather than guess or estimate the way it should be.
    The MineCom “CHANNEL CALCULATOR” software will tell you which radio channels will be free from crosstalk and work best on a leaky feeder cable which is different to transmitting via an antenna on the surface.
    The “SYSTEM DESIGN CALCULATOR” software will tell you how far after a branch unit that you MUST install another amplifier. These are just 2 of the range of tools provided. The full range of design software packages are provided free of charge to MineCom customers.

    All of this requires human input, BUT what if we took the human element out of the equation. To do this accurately we would need a micro PC that can sense/measure the cable losses in the path between the amplifiers and then adjust the gain of BOTH amplifiers accordingly. But to work efficiently, this circuit should be automatic and not require human intervention to be activated, nor should it require someone to go up and down the cable adjusting every amplifier either, it would have to be an automatic self-adjusting function, if it is to work correctly.

    MineCom have come up with a SMART solution to the problem, a new SMARTControl® Head End incorporating an AGC circuit producing a ‘consistent, set output level’ into all four leaky feeder cables. It is normal for the RF power output of a leaky feeder Head End to vary, dependant on the number of carriers (channels) being transmitted at any one time. In the SMARTControl head-end this output power level is held at a constant level at all times.

    The system operates in a similar method to a Trunking system or a Cellular system where one of the channels acts as the control channel. In the case of the SMARTControl head-end, the Diagnostic modem is transmitting continuously. Although any voice channel transmitter could be used for the same purpose and would be if the Diagnostic modem failed.

    Where the MineCom LLC® is a manually adjustable preset attenuator inside the amplifier, the new MineCom SMARTTune® in-line-amplifier incorporates a SMART attenuator.
    Literally install the SMARTTune® amplifiers in the leaky feeder cable and the amplifier will set its gain according to the level of input signal received from the previous amplifier. As all amplifiers include AGC in the common path, the level of output signal in the Base TX direction will be constant. This constant output power is used by the next amplifier in the system as a measure of the gain required to set its output level to the required amount. The shorter the cable, the less gain is required, if the cable is longer, then more gain will be applied etc.

    Competitors in the main, use ‘pilot tone generators’, which are installed at the far end of the cable runs, and transmit a continuous carrier back to the Head End, providing the amplifiers with a reference level. Unfortunately ‘pilot tone generators’ create more problems than they fix, and play havoc with Video and Data signals.
    The last time MineCom used ‘pilot tone generators’ was back in the days of the original design, the FD-4 system 14 years ago.

    While some mines will continue to layout a system design, and follow the system design, which is the most economical method as far as leaky feeder systems go. Mines who wish to go the most expedient route can utilise MineCom’s SMARTControl and SMARTTune devices WITHOUT the problems they currently experience.

    MineCom is dedicated to providing the best leaky feeder systems in the world.

    Note:- The names MineCom, “the leaky feeder people”, SMARTControl, SMARTTune and MineCom-LLC are registered trade marks of MineCom. The words as used in conjunction with leaky feeder systems “CHANNEL CALCULATOR”, “SYSTEM DESIGN CALCULATOR”, ‘consistent, set output level’ are the copyright of MineCom.

    For more information, please visit:
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