The Conspec radio product line is an inductive radio system designed specifically for use in underground applications.  The radios operate in the frequency range 280 to 520 Khz and utilize frequency modulation (FM) to provide clear voice and data communications between mobile and fixed-location transceivers underground and hard wired communication devices on the surface.

Most Conspec radios are certified for use in methane air environments.  

Loop antennas are used to generate and receive magnetically coupled energy to and from electrical conductors.  At the frequencies of operation used, the electrical conductors carry the induced signal current at an attenuation rate much lower than that which is present through air.  The signal carrier can often be the high voltage power distribution system, telephone wiring, metal pipes, or a combination thereof.  As an option, a dedicated signal carrier can be installed to provide coverage in areas that do not contain adequate electrical conductors.  

All radio transceivers, both fixed and mobile, are designed to receive on a single frequency, designated F1 (LOCAL). Each radio can transmit on F1 or a second frequency, designated F2 (DISTANCE).  The operator determines the transmit frequency with a selector switch located on the radio.  

Transmitting on F1 (LOCAL) allows one radio to talk directly to another.  This is possible when the radios are close enough to one another and adequate signal carriers exist between the two.  

By transmitting on F2 (DISTANCE), a radio is capable of talking to a repeater or repeater network.  The repeater network will receive the F2 transmission and retransmit the signal on F1 to other radios.  Repeater networks allow radios to talk to one another greater distances apart than what is possible with direct transmission on F1.

Propagation Characteristics

Experience has shown the best electrical conductors for use as signal carriers are insulated copper wiring not grounded to earth at any location.  Dedicated signal wires and non-shielded telephone cables match this description.

High-tension electrical distribution cables tend to perform well as signal carriers.  However, signal can often be lost to earth at load centers and switch boxes due to the electrical shield being grounded to earth at those locations.  In addition, electrical noise generated by underground machinery tends to be more pronounced on power cables.

In some instances, metallic water and air pipes have been used with success as signal carriers.  Pipes are normally suspended on chains connected to roof bolts, and tend to loose signal current to earth quite rapidly.  In most cases pipes, railroad track, and metallic structures not insulated from earth will prove inadequate as signal carriers.

Antenna Position and Range

All Conspec Radios and Repeaters use similar or identical transmit power levels and receiver sensitivities.  The antenna type and its position with respect to the signal carrier will determine, along with the signal carrier propagation characteristics, the distance two radio devices can be from one another and still communicate effectively.

A loop antenna is a magnetic dipole, that produces a directional response in both transmit and receive mode.  The amount of energy coupled onto or from a signal carrier by an antenna is affected by how it is located.  The following discussions will assume all antennas are positioned in the most favourable direction.

Several general rules apply to operating range:

1.                  The closer an antenna operates to a signal carrier, the better the energy coupling will be.

2.                  As an antenna moves away from a signal carrier, transmit coupling dies off faster than receive coupling.

3.                  The stronger the signal coupling, the further distance two radio devices can operate from each other.

4.                  The larger a loop is, the stronger its signal coupling strengths will be.  

To illustrate the above mentioned rules, consider a typical installation in which a dedicated signal carrier is used throughout as diagrammed in the following figure.



If both radios use loop antennas and touch their antenna to the signal carrier (D1 and D2 equal 0.0 feet), ranges of 15,000 feet can be expected.  When D1 and D2 are both increased to eight feet, the range will be reduced to approximately 3,000 feet.  If both radios are using small loop antennas, such as those used on Handheld radios, the range that can be expected to drop to 1,000 feet.  Rules 1, 3, and 4 are being applied.

To understand how rule 2 affects range, consider the case in which Radio 1 has its antenna touching the signal carrier and Radio 2 has its antenna positioned so D2 is eight feet.  Radio 1 will be able to transmit to Radio 2 about 8,000 feet.  On the other hand, Radio 2 will be able to transmit back to Radio 1 about 4,000 feet.

Most applications for radios underground involve mobile transceivers placed on vehicles or carried by personnel.  It is necessary for each radio to communicate two-way with all others.  It should be obvious that mobile applications will be very limited without the use of repeaters, since the usable range will be no greater than several thousand feet.

Repeater Networks

Repeaters take advantage of the increased range made possible when antennas are touching the signal carrier.  The least complicated repeater network uses a single repeater as shown in the figure below.

Simple Repeater Network


The single repeater, called a SIMPLE repeater, receives on frequency F2 and retransmits on F1.  Both its antennas are mounted so they are in contact with the signal carrier.  A mobile radio of the Handheld type can typically transmit to the repeater at a range of about 1,500 feet and receive from the repeater about 3,000 feet away.  With the inclusion of the SIMPLE repeater, the overall range has been increased from 1,000 feet to about 3,000 feet (1,500 feet on both sides of the repeater).

Simple repeater networks find applications in small mines, rescue team communication systems, and continuous miner sections in coal.

The first expansion of repeater networks beyond the Simple repeater involves the Backbone Repeater Network.  The backbone Repeater Network concept is shown in the following diagram.

Backbone Repeater Network

There are two repeater types used in a Backbone Network: Access and Distribution.  Access repeaters receive frequency F2 and re-transmit on a new frequency F3.  Distribution repeaters receive frequency F3 and re-transmit on F1.  The frequency F3 is used solely to communicate between repeaters, and since all the repeater antennas are touching the signal carrier, the distance the repeaters can operate apart from each other is quite large.

Access and Distribution repeaters are placed, as needed and in various quantities, at regular intervals.  Since the transmit range of the Distribution repeater is larger than the receive range of the Access, there are normally fewer repeaters of the Distribution type required in a given installation.  The following diagram is a pictorial representation of what a complete Backbone Network might look like.

When a mobile radio transmits on F2 to a nearby Access repeater, the Access repeater will re-transmit to all Distribution repeaters on F3.  The Distribution repeaters will re-transmit on F1, the universal receive frequency, to all mobile and fixed location radios throughout the underground complex.

The limiting factor in end-to-end coverage in a Backbone Network is encountered when the Access repeaters on the far ends can no longer reach the Distribution repeaters on the opposite side.  This limit is typically reached at a point where most moderate sized mines and underground complexes can be covered in main man ways and roadways.

Bringing an additional radio frequency and repeater type to the network can increase the end-to-end coverage further.  This system is called a Hub Repeater Network, and its principle of operation is detailed in the following diagram.

Three repeater types are used in the Hub Network: Access, Hub and Distribution.  Access repeaters receive F2 from mobile radios and re-transmit on frequency F4.  The Hub repeater receives F4 and re-transmits on F3.  Distribution repeaters receive F3 and re-transmits on F1.

The new frequency, F4, allows a second path for repeater-to-repeater communications.  Placement and numbers of Access and Distribution repeaters follow the general principles outlined in the discussion of the Backbone Repeater Network.  In a Hub Repeater Network, there is only a single Hub repeater.  The pictorial representation of a typical Hub network is shown below.

The Hub repeater location is selected to be near the center of the underground complex.  Mobile radios transmit to nearby Access repeaters on F2.  The Access repeaters re-transmit only to the Hub repeater on F4.  The Hub re-transmits to all Distribution repeaters, which in turn, re-transmit to other mobile radios throughout the complex.

The Hub Repeater Network allows an end-to-end communication range roughly twice that of the Backbone Repeater Network.  It also is easier to implement the Hub network in situations where the underground complex is complicated or three-dimensional.

Wired Repeater Networks

Each of the three repeater networks previously described has the advantage of communicating between repeaters via radio paths only.  Using existing electrical conductors as signal carriers, this makes installation and re-configuration simple and cost effective.

Underground complexes which extend beyond the capabilities of the Simple, Backbone, or Hub Repeater Networks can be equipped with a variety of wired repeater networks.  In this type system, some or all repeaters are connected to one another with 2-conductor wires.  The wired repeater networks are normally custom designed to meet the needs of a specific location.

 Copyright 2000 - Conspec Controls Ltd.