Published in Australian Longwall Magazine

Since the 1990 introduction of leaky-feeder-based systems specifically for mining, MineCom has progressed from offering a single-channel VHF FD4 system to offering a 32-channel MCA1000 VHF system, along with two UHF versions offering 16 or 64 channels over a single 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,” said managing director Brian Wilson.

“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 had a finite signal loss it was necessary to provide amplification (signal 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 base station repeaters.

“An amplifier is in fact two or three separate amplifiers in one enclosure,” Wilson said.

“That is, a voice/data UP amplifier, a voice/data down amplifier and the third, 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.”

He said while in a conventional leaky feeder system the cable length between amplifiers was well controlled in the installation phase, 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 guesswork out of the equation and assist staff to design system expansions, rather than guess or estimate the way it should be,” he said.

“(But) all of this requires human input. To take the human element out of the equation 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.

“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 had come up with a “smart solution to the problem” in the form of the SMARTControl head-end incorporating an AGC circuit. This produced 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,” Wilson said.

“The system operates in similar fashion 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.”

Wilson said where the MineCom LLC was a manually adjustable pre-set attenuator inside the amplifier, the SMARTTune in-line-amplifier incorporated a SMART attenuator.

“You 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,” he said.

“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.”

Wilson said competitors in the main used “pilot tone generators” installed at the furthest end of cable runs, and transmitted 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,” he said.