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.
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.
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.
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.
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.
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.
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.