Guglielmo Marchese Marconi (1874-1937) first successfully transmitted over the English Channel in 1899 and across the Atlantic in 1901. Only pulse transmisssions were possible at the time and it was remarkably good fortune that the familiar code of Samuel Morse was available. In 1920, commercial, spoken-voice broadcasting was under way and the Second World War stimulated a surge of innovation in radio communication that continues to this day.
The mining industry was the poor relation during this period and only the telephone was successfully established underground. Dust, excessive moisture, corrosive blasting fumes, a rigorously confined space and the proximity of other electrical equipment throwing off stray radio frequencies, these all conspired to hobble the innovator.
In broad terms, there are three underground radio systems. These are the “leaky feeder,” the “medium frequency inductive” and “very low frequency” systems. Perhaps there should also be added a fourth — a new advance originating in Germany using super-high frequency (shf) and to be introduced on to the North American market in 1991. The market for radio communication systems is extremely competitive and manufacturers are reluctant to disclose very much beyond generalities. But one inference gleaned from contacts with manufacturers, distributors and users is that there is still some way to go before the ultimate system is developed and those manufacturers who do develop the ideal, or close to it, will find a lively market.
Without delving too deeply into the esoterica of radio theory, it may be useful to review some of the basics. Radio frequency waves (rf) are electromagnetic waves very similar to light and possessing the same speed as light. They have been divided more or less arbitarily into the “radio spectrum,” a tabulation of which is shown. The very low frequencies (vlf) are not used for broadcasting at all but are primarily navigational aids and are also used for continuous wave telegraphy.
The geophysist has a practical interest in the global distribution of vlf stations, originally established for the nuclear submarine force, for certain of his electromagnetic exploration techniques. The frequencies can be transmitted over great distances but they have, so far, resisted attempts to use them as voice carriers. Low and medium frequencies (lf and mf) cover the ordinary, regional broadcasting range and the high frequencies (hf) are the shortwave broadcasting bands. Very high frequencies (vhf) and beyond are used for progressively greater distances represented by aeronautical, maritime and satellite communications. As is to be expected, the functions overlap considerably.
Figure 1 shows how information (for example, the spoken voice) is attached to the rf carrier. The varying pressure impulses of the voice are translated into electrical impulses of correspondingly varying strength and “fitted” on to the rf by a modulating process. If the modulation is carried out on the amplitude of the wave, the result is am (amplitude modulation), and if on the frequency, fm (frequency modulation). The fm signal is more effective than the am in combatting noise and interferece and it is therefore the preferred mode. In addition to transmitting a spoken message, the coal miner and the hard rock miner are looking for a great deal more: 2-way communication with multiple channels; the capability of monitoring: starting and stopping equipment such as fans, pumps, generators; and, finally, the use of the system for carrying tv, all with a reliability factor of as close to 100% as possible. Clearly, this is asking a lot. On the other hand, the drive for greater automation is now positively established: automatic/semi-automatic hoisting and automated underground haulage are no longer uncommon. And with robotics now on the horizon, the pressure for fresh developments is strong. In fact, by the time this article appears, Inco may have disclosed such a multi-use system, and other corporations are spending large sums on research and development in this area.
Leaky feeder systems were brought to the commercial stage in the coal mines of the United Kingdom and several hundred systems had been installed by the National Coal Board by the mid-1970s. Germany and the U.S. concentrated on the inductive, medium-frequency systems and very low frequencies have seen their most recent developments in Canada and Australia. From this list, it is evident that anyone who expects a walkie-talkie to work underground is going to be disappointed. It won’t; it will cease working as soon as the person carrying the receiver turns a corner, for radio frequencies are like light waves, they travel in straight lines. Early underground experimental work used metal reflectors to direct radio waves where they were wanted, but obviously this was not a practical measure. Another characteristic is that radio waves have a variable, but generally restricted, capability of transmitting through rock. vlf achieves the best penetration but the same frequencies cannot cary a voice transmission. The general rule is that the capability of transmitting through rock deteriorates as the frequency increases. Underground radio requires the antenna (that is, the device that propagates and directs the radio frequencies) to be close to the receiver — specifically fewer than 100 metres away.
The antenna in the leaky feeder system (sometimes called the “radiating cable system”) is a coaxial cable carrying twin concentric copper conductors arranged as shown in Figure 2. The outer copper conductor is intermittently slotted, allowing the radio signal to “leak out” wherever the cable is laid. Because the radio signal weakens as the distance from the transmitter increases, amplifiers (often referred to as “repeaters”) are situated at 300-metre to 500-metre intervals to boost the signal, both incoming and outgoing. The system is completed with the miner equipped with a walkie-talkie type of transmitter/receiver and, so long as he is within range of the leaky feeder cable, high quality, 2-way radio communication will be possible. The specialized cable is the main expense of the system, although the same cable can be used in a multi-channel mode and also for video. These advantages are denied the orthodox antenna. The radio frequencies most commonly used in the leaky feeder system are vhf but range all the way between mf and uhf.
Loop antennas of variable design are used in medium frequency inductive systems. The radio signal is directed, or randomly picked up, by a suitable metallic conductor in the mine tunnel or shaft (for example, a power cable, pipework or conveyor supports) and transmitted by these means through the underground workings. At intervals, amplifiers (or “repeaters”) are needed to recharge the signal and this is done by fitting two antennas to the appropriate conductor (e.g. electric cable). Incoming signals are thus received, amplified and then re-transmitted; the amplifier itself will generally be battery-powered. Similar radio systems substitute a dedicated wire antenna for the power cable, or the conveyor supports, with obvious benefits in clarity of reception and transmission. The dedicated antenna also provides a much higher degree of confidence in the reliability of the system. The antenna wire is only a fraction of the size of leaky feeder cable, about one quarter of an inch in diameter (as opposed to one inch) to one and three quarter inches, with a corresponding reduction in cost. However, more than ordinary care is needed in stringing the wire. It must usually be at least 12 inches from the back or walls of the drift to reduce distortion of the magnetic field surrounding the wire, a factor that has a major effect on the quality of the transmission.
Very little is known about the fourth system mentioned earlier in this article, super-high frequency, and the little information that has been forthcoming suggests it may have some elements in common with the previously described inductive systems. On the other hand, there is no dedicated wire antenna, shf is used rat
her than mf, and the signal is transmitted over a wide area by means of repeated loop antennas. Detailed information will only become available when the system comes to the North American market, sometime this year.
The capability of vlf for penetrating rock formations has been known for many years and has been used by geophysical prospectors for more than a generation. Nevertheless vlf has not appealed to radio investigators because of its inability to carry a voice transmission. This reluctance has been overcome in recent times by the need to install faster means than the injection of foul-smelling compounds into the compressed air line, to warn underground crews of electrical fires, flooding, fuel spills, and so on. Both Canada and Australia, quite independently, have reached advanced stages in producing commercial systems and it appears that developments are only a little shy of being in lock-step. Transmitters are set up on surface/underground and the signal propagated through an antenna designed for the particular location.
When it is required to send a warning signal underground, the miner’s cap lamp will flash for a predetermined interval; and in one of the systems, a one-way message is relayed to the miner via a liquid crystal display on his battery pack. Of all the systems, vlf requires the least hardware, but it is also the least versatile.
With the exception of Motorola, the following list of manufacturers/distributors is in alphabetical order:
* Motorola Canada (Toronto, Ont.)
* Andrew Antenna (Whitby, Ont.)
* BLM Mincon (Lively, Ont.)
* El-Equip Inc. (Sudbury, Ont.)
* Mine Radio Systems (Sunderland, Ont.)
* Mine Site Technologies Pty. (North Sydney, N.S.W., Australia)
* O.B. Systems & Mining (Huntingdon, W.V.)
* Rovern Mining Equipment Co. (Halifax, N.S.)
* VLF Magnetic Systems (Scarborough, Ont.)
The list is not claimed to be complete, but all the companies listed are involved in underground radio systems. The number of companies involved with other forms of underground communications, such as telephone, hardwire paging systems, telemetry, remote control, and so on, is much higher. Motorola is a major supplier of components and radios to many of the companies listed; hence its precedence.
Be the first to comment on "Underground Radio"