CA1296063C

CA1296063C - Medium frequency mine communication system

Info

Publication number: CA1296063C
Application number: CA000566887A
Authority: CA
Inventors: Larry G. Stolarczyk; William E. Mondt; Gerald J. Boese; Kurt A. Smoker; Seth A. Smith; James L. Zappanti; Marvin L. Hasenack, Jr.; Edward D. Moore
Original assignee: Stolar Inc
Current assignee: Stolar Inc
Priority date: 1987-05-29
Filing date: 1988-05-16
Publication date: 1992-02-18
Anticipated expiration: 2009-02-18
Also published as: NZ224586A; AU620531B2; US4879755A; ZA883469B; JPH01125129A; EP0292950A2; CN1034835A; CA1319172C; EP0292950A3; AU6246390A; AU1667588A; CN1012928B; AU599797B2; AU6246290A; US5301082A; AU627354B2

Abstract

ABSTRACT OF THE INVENTION

A method for using an underground mine communication system to effect minewide communication and an intrinsically safe current limiter circuit for insuring that electrical equipment in the system will not cause incendiary conditions. The underground mine communication system comprises a plurality of repeaters and medium frequency radios, including mobile, portable and personal-carried radios, coupled to electrical conductors and natural waveguides existing in the earth by tuned loop antennas.
Messages transmitted by the radios are carried to the repeaters by the conductors or coal seam waves. The repeaters amplify, replicate and retransmit the message at two different frequencies for transmission of the message to a surface base station and to other radios in the system. A paging system, which has a separate set of repeaters, is also coupled to the network of electrical conductors and natural waveguides by tuned loop antennas. The paging system alerts miners to contact the surface base station.
Radios, pagers and repeaters in the system are equipped with the intrinsically safe current limiter circuit to preclude the development of incendiary conditions. The current limiter circuit comprises a series arrangement of a current trip circuit, a redundant current trip circuit and a current limiting field effect transistor controlled by a feedback control amplifier.

Description

~2~

Medium FFequency Mine Communication System BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates to a communication system ~or use in underground mines and more particularly to a mine communication system which includes a network o medium ~requency transceivers and double-unit repeaters. The transceivers are magnetically coupled to electrical and natural waveguide conductors within the mine by loop antennas and are protected from incinerary conditions by an intrinsically safe current limiter circuit.

Descri~tion of the Prior Art ~;~ It has long been known that medium ~requency (MF) electromagnetic waves propagate through natural media, such as coal and rock, as well as through electrical conductors such as track, wire rope and electrical wiring that e~ist in underground mines. Efforts have been made to exploit the propagation properties of ~F
signals to develop "wireless" communication systems.
A wireless underground mine communication system would 2~ improve both mine productivity and ~ine sa~ety. In a 1980 paper, Larry G. StoIarczyk proposed such a system.
His cellular mine communication system exploited both ~; ~ the conductor mode of transmission and the natural waveguide mode of transmission for effecting MF radio : ~ io communication within the mine. The system utilized a cellular repeater to provide a means for two mobile transceivers to communicate with each other. A
` communication link to the surface and to other , ;

~2~ 3 repeaters was provided by a two-wire transmission line (i.e., a telephone line) over which voice signals in the audio frequency range were transmitted. L.
Stolarczyk, The Design of a Cellular MF Radio ~ , Reprint from National Telecommunications Conference (Nov. 30 -Dec. 4, 1980). Thus, this system suf~ered ~rom the inability of sur~ace stations and repeaters within the system to communicate with each other using radio frequency signals. This would become a serious proble~ if the telephone line was severed during a mine disaster, ~or example. This early system, which included a transceiver and a loop antenna attached to a vest worn by a miner, was described in more detail by L. Stolarczyk and R. Chufo in SYstem Desi~n and Performance of an MF Radio Communication_System for nder~round Mining, (Sept. 1981). The loop antennas used in these early systems were second order tuned loop antennas.

More recently, the MF wireless communication technique has been expanded to include a radio com~unication system which provides for the radio control o~ a mine train-loading operation. H.
Dobroski and L. Stolarczyk, Control and Monitoring_via Medium-Freq~nc~ Techni~ues and Existin~ Mine Conductors, IEEE Transactions on Industry Applications, Vol. lA-21, No. 4 (July/August 1985).
In this system, MF radio signals are induced on existing conductors through the use of air core line couplers.

Many attempts have also been made at using ferrite couplers to induce MF radio signals on conductors. These suffer from the problem that no ~6~6~

single ~errite material functions satisfactorily as both a receiving and a transmitting line coupler.

Finally, surveys have been published which review the attempts at developing var;ous types oE loop antennas for use in wireless mine communication systems. R. Lagace, D. Curtis, JO Foulkes and J.
Rothery, Transmit Antennas t~ Pcrtable VLF to MF
Wlreless Mine Communications, ~SMB Contract Final Report (H0346045), Task C, Task Order No. 1 (Arthur D.
Little, Inc.) May 1977~

Loop antennas ~or use in mine communication systems that have been incorporated into a bandolier type garment have long been known. See, e.g. B.A.
Austin and G.P. Lambert, An Interim Report on the Radio Communication System Installed Underground at Greenside Coller~, Apex Mines Limited, Chamber of `:
Mines of South Africa Research Report No. 39/77, Project No. CSlC10 (1977). The bandolier design suffers from the ~act that as the miner's chest moves, the loop area of the antenna changes, thus changing the area and inductance of the antenna.
':
A separate direction in which mine communication methodology has developed is that of emergency commu-nications. Two types o~ emergency mine communication systems are the seismic method and the borehole method. In thc seismic method, a trapped miner 3n transmits seismic vibrations by pounding on a rail or roof bolt. These signals are detected by surface geophones. After computer analysis o~ the arrival times o~ the seismic signals, the location of the trapped miner can be determined. The seismic method has proven inadequate because the deployment of --4~

geophone arrays is time consuming and voice communication is impossible. Additionally, the technique requires that the miner not be seriously injured so that he can pound on a rail or rooE bolt in order to be detected~

In the borehole method, probes are lowered down boeeholes in order to provide two-way voice communi-cations with trapped miners. This method is not satisfactory because set-up drilling is time consuming and useless if the exact location of a trapped miner is not known.

A safety requirement for all electrical equipment used in mines i5 that the e~uipment be intrinsically safe. Intrinsically safe equipment is incapable of releasing su~ficient electrical or thermal energy, under norrnal or abnormal conditions, to cause ignition of a specific hazardous atmosphere mixture in its most ; 20 easily ignited concentration. IEEE Standard Dictionary of Electrical_and Electronics Terms, 3rd Edition, p. 463 (1984). To satisfy this requirement, the Mine Safety and ~lealth Administration (MSE~A) and the U.K. ~eaIth and Sa~ety Executive (HSE) require that batteries be protected with a fuse and series resistor circuit. The fuse is designed to blow out before th~ temperature of the resistor reaches a certain temperature. ~ disadvantage of this circuit is that it requires that larger batteries be used to compensate Eor the voltage drop across the resistor.
The use of larger batteries increases the size of mine equipment and decreases the capacity of the batteries.

.~

:

~Z~6`3 SUMMARY OF THE PRESENT INVENTION

It is there~ore an object of the present invention to provide an underground mine communication system in which the plurality of repeaters and surface base stations can communicate with each other using mediu~ ~requency (MF) radio signals, It is another object of the present invention to provide a method Eoe communicating in an underground mine communication system whereby MF radio signals can be transmitted long distances to a sur~ace base station.

; lS It is another object of the present invention to provide an improved method for inductively coupling MF
signals onto electrical conductors.
;::
It is another obiect of the present invention to provide a method ~or inductively coupling MF signals onto natural waveguides.
:
It is another object of the present invention to provide an improved current limiter circuit which prevents incendiary conditions from developing in radios within the under~round mine communication system.

It is another object o~ the present invention to provide a transmitter for use within the mine communication system that is optimized for maximum ; ~ ef~iciency in ~enerating a loop magnetic moment.

It is another object of the present invention to ~ 35 provide a portable radio for use within the ; communication system that can be carried by miners in emer~ency situations.
' ' .

~Z~

It is another object of the present invention to provide a personal carried radio ~or use within the communication system that can be used with a plurality of antennas.

It is another object of the present invention to provide a paging system for calling or warning miners.

It is another object of the present invention to provide a vertical loop antenna unit for use within the communication system that can be worn by a miner and in which the loop area will remain constant.

It is another object of the present invention to provide a mobile horizontal loop antenna for use within the communication system that can be mounted on a vehicle.

Briefly, the present invention includes a method for using an underground mine com~unication system to effect minewide communications and an intrinsically sa~e current limiter circuit for insuring that electrical equipment in the system will not cause incendiary conditions. The system includes portable radios, personal-carried radios, pagers, mobile radios and surface station radios all linked by a backbone network of repeaters. The repeaters are double transceivers tightly coupled to electrical conduc~ors existing in the mine. The pagers have a separate ;; ~ repeater network. Messages transmitted by radios in the system are inductively coupled onto the electrical conductors at one ~requency by vertical and horizontal loop antennas on the radios. ~he message is received by the rFpeaters which then amplify, replicate and :: ~
:
:

transmit the message at two diEEerent frequencies~
This allows the original radio message to be transmitted long distances to the surface stations as well as to other radios in the systems. The pagers include a signal light which is activated by a digitally encoded signal sent from a pager base station on the surface. This allows key personnel underground, to be contacted independently of the radio system. Each radio, pager and repeater in the system is equipped with an intrinsically safe (IS) current limiter circuit which insures that a fault in the system will not result in incendiary conditions.
The IS current limiter circuit comprises a series arrangement of a current trip circuit, a redundant current trip circuit and a current limiting field effect transistor ~FET). The current limiting FET is driven to a high resistance state by a Eeedback control ampli~ier whenever excessive current demand is drawn through the circuit. The current trip circuit then latches the system in an open position until the current draining fault is removed. A heat responsive thermistor attached to the current limiting FET
supplements the Eeed back control ampliEier. The translnitter unit of each radio and repeater in the system is designed Eor maximum e~ficiency in generating a loop magnetic moment. This is accomplished by optimizing the ratio of the power dissipated in the vertical loop antenna to the bandwidth of the Erequency modulated MF carrier signal. In the present invention, thirty inch ~iameter loop antennas are used with the portable radios and repeaters. ~he personal-carried radio can ; be used with a pLurality o~ loop antenna designs including a loop antenna which is incorporated into a suspenders/belt combination which can be worn by a ~2~6g: ~i3 ~ 368-3~
miner. The mobile radio utilizes a horizonta:L loop antenna sandwiched between two boards and mounted on the outside of a mine vehicle.
The invention may be summarized, accordiny to one aspect, as a method for communicating in mines which comprises:
a. inductively coupling a first radio and a second radio to a transmission line electrical conductor that extends into a mine, the first radio and the second radio each includiny a receiver for receiving one frequency F1 and a transmitter for transmitting on at least one frequeney E'2 different from said frequency F1 with the frequencies F1 and F2 both lying in the medium frequency range; b. inductively coupling at least one access transceiver to said transmission line electrical conductor, the access transceiver having a receiver for said frequency F2 and a transmitter for transmitting a frequency F3 ; different from said frequencies F1 ancl F2; c. inductively coupling at least one local transceiver to said transmission line electrical conductor within said mine, the local transceiver having a receiver for receiving said frequency F3 and a trans~itter for transmitting said frequency F1;
d. transmitting a first signal carrying information to be : communicated from the first radio on the frequency F2;
e. receiving said first signal at said access ~ransceiver;
~. retransmitting said first signal from said access transceiver on the frequency F3 as a second signal carrying said information to be communicated; g. receiving said second signal. at at least one of said local transceivers; h. retrans-mittiny said second signal from said local transceiver on the irequency F1 as a third signal carrying said information to be communicated; and 1. receiving said third signal at said receiver of said second radio whereby said information to be communicated is conveyed fxom said first radio to said second 6936~-33 radio.
According to another aspect, the invention provides a method for alerting a person in a mine to contact a station which comprises: a. using a ~uned loop antenna to inductively couple a pager transmitter loca~ecl a~ a station to a transmission line electrical conductor extendin~ into a mine, said pager transmitter adapted to ~ransmit a frequency F5;
b. inductively coupling at least one repeater to the transmission line electrical concluctor within said mine by use of at least one tuned loop antenna, said repea~er adapted for receiving the frequency F5 and transmitting a frequency F4;
c. inductively coupling a pager to the transmission line ` electrical conductor within said mine by use of a tuned loop an~enna, sa.id pager including a pager receiver for receiving the frequency F4; d. transmitting a first siynal at the frequency F5 from saicl pager transmitter to said repeater;
e. receiving the first signal at said repeater; f. retrans-mitting the ~irst signal from said repeater as a second signal at the frequency F4; g. receiving the second signal at the pager receiver; and h. using the second siynal to activate an alarm means on the pager for notifying a person in said mine to contact said station.
According too ~et another aspect, the in~ention provides a personal-carried vertical tuned loop antenna comprising: a one-piece harness including a pair of ~lexible shoulder straps which loop over a person's shoulders, means for attaching the shoulder straps to a belt worn about the person'~
waist and a pair of cxoss straps which run perpendicular to the shoulder straps, along a back face of the harness, with each of the cross straps being connected to each of khe shoulder straps and a space "w" being left between the two cross straps such that the two cross straps form a rectanyular area; a continuous 8a ~Z~3i~

wire loop attached to -the outer face of the rectangular area formec1 by the cross straps; and a :Loop antenna tunlng box electrically connected to ~he wire loop and attached to one of the shoulder straps.
An advantage of the present invention is that the repeaters can communicate with the base stations using MF radlo signals.
Another advantage of the present invention is that MF
radio signals can be transmitted over long distances from a ~0 radio located remotely to an electrical conductor to a surface base station.
Another advantage of the present invention is that vertical and horizontal loop antennas are used for inductively coupling MF radio signals onto electrical conductors.
Another advantage of the present invention is tha~
the vertical loop antennas also inductively couple the ~F radio signals onto natural waveguides.
Another advantage of the present invention is that an improved current limiter circuit is used to prevent incinerary conditions from developing in radios within the mine communication system.
Another advantaye of the present invention is that the radio transmitters are optimized for generating a loop magnet,c moment.
Another advantage of the present invention is that it includes a portable radio ~hat can be used during mining ; emeryencies.

8b _9_ Another advantage o~ the present invention is that it includes a personal-carried radio that can be used with a plurality of tuned vertical loop antennas.

Another advantage oE the present invention is that it includes a paginy system for calling or warning miners.

Another advantage of the present invention is that it includes an improved tuned vertical loop antenna that can be worn by a miner and which maintains a constant loop area.

Another advantage of the present invention is that it includes a tuned horizontal loop antenna that can be mounted on the outside of a vehicle.

These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after havin~ read the following detailed description of the pre~erred embodi~ent which is illustrated in the various drawing fi~ures.

~ rs Fi~. 1 is an idealized view of a medium ~requency mine communication system of the present invention;

Fig. 2 is a block diagram of a portable radio and vertical loop antenna of the communication system of Fig. l;

:

Fig. 3a is a diagram of a suspender 1Oop antenna and a personal-carried radio of the communication system of Fig. l;

Fig. 3b shows an alternative embodiment o~ the suspender loop antenna of Fig. 3a;

Fig. 4 shows the pa~er of the communication system of Fig. l;
Fig. 5 is a diagram of the mobile vehicular radio and mobile horizontal loop antenna of the communication system of Fig. l;

Fig. 6a is a block diagram of a pair of repeaters of the present invention;

Fig. 6b is a block diagram of a pager repeater of the present invention;
Fig. 7a is a circuit diagram of a conventionally designed intrinsically sa~e battery protection circuit; and Fig. 7b is a circuit diagram of a current limiter circuit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to Fig. l, there is shown a medium frequency mine communication system referred to by the geneeal reference numeral 20. The communication system 20 comprises a portable radio 22, a mobile vehicuLar radio 24, a personal-carried radio 26, a pager 27, a plurality of repeaters 28, a plurality of .'`63 payer repeaters 29, a surface base station 30 and a pager base station 31. It is understood that there may be more or fe~er than one of any of the radios 22 24 or 26 and of surface base station 30, pager base station 31 or pager 27.

A plurality of transmission line electrical conductors 32 exist within a mine 34~ ~The conductors 32 may be telephone cables, AC power cable, monitor cable, rails, steel pipelines, etc.) Magnetic coupling between the transmission line electrical conductors 32 in an entry way 35 enables radio signal current flow in one conductor to induce signal current flow in a nearby conductor. The portable radio 22 is installed at a working face 36 of the mine 34 directly below the transmission line electrical conductor 32.
A portable radio vertical loop antenna 38 attached to radio 22 is held in close proximity to the electrical conductor 32 by a connector 40~ The mobile vehicular radio 2~ is mounted inside of a mine vehicle 42 and is connecteA to a vehicular horizontal tuned loop antenna 44, mounted on the outside of vehicle 42, via a cable 46. The personal-carried radio 26 is a compact, battery powered transceiver designed to be mounted on a miner's belt. A personal-carried vertical tuned loop antenna 47 worn by the miners, is connected to the ra~io 26. The pager 27 can al~so be carried by a miner and would also be connected to the personal-carried vertical tuned loop antenna 47. The plurality of repeatecs 28 are transceiver units designed to receive two frequenices F2 and F3 and to transmit two frequencies Fl and F3. The repeaters 28 are located in close physical proximity to electrical conductor 32. The surface base station 30 includes a medium ~requency transceiver capable of transmitting the ~29~~ ~3 frequencies F2 and F3 and receiving thq ~requencies Fl and F3. Base station 30 is located at the mine surface portal or at any other central dispa~ch or monitoring point. The surface base station 30 and the pager base station 31 are individually coupled to the plurality of transmission line electrical conductors 32 via a sur~ace base station vertical loop antenna 49 and a pager vertical loop antenna 50 which are located in close physical proximity to conductors 32. The pager base station 31 includes a pager transmitter 51, a pager encoder 52 and a pager computer 53.

Reerring now to Fig. 2, there is shown the portable radio 22 in more detail~ The connector 40 is a nylon tie wrap which encircles the conductor 32 and the portable radio tuned loop antenna 38 holding antenna 38 in close proximity to conductor 32. The antenna 38 is a vertical tuned loop antenna having a diameter '~d" o~ about thirty inches, and is connected to the portable radio 22 through wire 54. The radio 22 comprises a medium frequency (300-800KHz) transceiver 55, capable o~ receiving one ~requency Fl at a receiver 56 and transmitting two ~requencies Fl and F2 from a transmitter 57; a stand-by battery pack 58, protected by an intrinsically .safe current limiter circuit 59; a charging and power regulation circuit 60; a speaker 62; a squelch control knob 64; a noise cancelling microphone 66, a time-out circuit 68, and an external speaker 69. A remote power su~ply unit 70 with intrinsically sa~e output, is connected to radio 22 through a cable 71. Under normal conditions, the portable radio 22 is mounted on a wall and serves as a stationary radio in the mine wide communications system. In emergencies, portable radio 22 can be ; 35 disconnected ~rom the power supply unit 70 and ~L2~ 63 ~13-conductor 32 and removed from its wall mount for portable use.

FigO 3a shows the personal-carried radio 26 and personal-carried vertical tuned loop antenna 47 in more detail. The radio 26 is a small, approximately 1 7/8" thick x 4 1/8" wide x 8" tall, medium frequency transceiver capable of transmitting two ~requencies, Fl and F2, feom a transmitter 80 and receiving one ~requency, Fl, at a receiver 82, The radio 26 contains a squelch control knob 84 and is powered by a battery pack 86 which is equipped with an ; intrinsically safe current limiter circuit 88. The personal-carried tuned loop antenna 47 is a vertical tuned loop antenna that can be worn by a miner. In the pre~erred embodiment, the antenna 47 is a wire loop incorporated as part o~ a suspender loop antenna designated by the general re~erence numeral 92. The suspender loop antenna 92 is a one-piece harness comprising a pair of flexible shoulder straps 100 which loop over a miner's shoulders like suspendersO
A pair o~ ceoss-straps 104 run perpendicular to shoulder straps 100, on the back of suspender loop antenna 92 (i.e., the pair o cross-straps 104 would be situated on the miner's back). A space "w" exists between the two cross-straps 104. ~ plurality of slots 106, on the lower ends of the shoulder straps 100, provide a means ~or securing the suspender antenna 92 to a belt 107 worn around the miner's waist. The belt 107 could also be permanently attached to the suspender loop antenna 92. Alternatively, the slots IOG could be any other suitable means for securing the suspender antenna 92 to the belt 107 such as a plurality oE
buckles, snaps or buttons. The antenna 47 is attached to the outside surEace oE the rectangle Eormed by the cross straps 104 and shoulder straps 100. A loop antenna tuning box 108 is securely ~astened to one of the shoulder straps 100. A series tuned circuit, located inside of tuning box 108, switches the antenna 47 between receiving and transmitting modes. A
connecting wire 109 connects radio 26 to the tuning box 108. An antenna plug 110 allows connecting wire 109 to be plugged into the tuning box 108. The antenna plug 110 also allows radio 26 to be connected to antennas of other designs. A pair o clips 112 attached to radio 26 provide a means Eor attaching radio 26 to an ordinary belt worn around a miners waist.

Fig. 3b shows an alternative embodiment o~ the suspender loop antenna 92 designated by the general reference numeral 1l4. Elements in suspender loop antenna L14 which are analogous to elements in suspenders 1Oop antenna 92 are designated by the ori~inal number followed by a prime designation. In suspender loop antenna 114, a pair of cross straps 104' run perpendicular to shoulder straps 100' on the front ~ace of the suspender loop antenna 114. A wire loop antenna 47' is attached to the outside surface of the rectangle ~ormed the cross straps 104' and shoulder straps IOO~o ~ loop antenna tuning box 108' is securely fastened to a Eront sur~ace oE one oE the shoulder straps 100'. The personal-carried radio 26 is connected to the loop antenna tuning box 108' by the connecting wire 109 and the antenna plug 110 in the same manner as was described in Fig. 3a.
Similarly, a plurality o~ slots 106' provide a means for securing suspender loop antenna 114 to a miner's belt as shown in Fig. 3a. The embodiment depicted as suspender loop antenna 114 is useEul, Eor example, in mine rescue operations where rescue team members carry an oxygen tank on their backs. In that situation, the .

6~

--1 s -oxygen tank would detune the loop antenna if it were also located on the miner's back.

Fi~. 4 shows the pager 27 attached to the suspender loop antenna 92 of Fig. 3a. The pager 27 is attached to a front surEace of one of the shoulder straps 100 and comprises a receiver 116, capable o~
receiving the frequency F4, a decoder 118 and a signal light 120. The pager 27 ls powered by a battery pack ; 10 122 which is protected by an intrinsically safe circuit 124. The pager 27 is connected to the loop antenna tuning box 108 by a connecting wire 125 and an antenna plug 126. The antenna plug 126 allows the pager 48 to be connected to antennas of other designs.

Fig. 5 shows the mobile vehicular radio 24 in more detail. Radio 24 is a medium frequency transceiver capable o~ receiving one ~reguency Fl at a receiver 130, and transmitting two Erequencies, Fl and F2, from a transmitter 132. A squelch control knob 134 is located on the face of radio 24. The radio 24 is mounted inside the cab of vehicle 42 and is protected by an intrinsically saEe limiter circuit 25 136. The cable 46 links an antenna connector 137 to the vehicular tuned loop antenna 44. The antenna 44 is a long piece of wire Eashioned into a rectangle lying horizontal to the bed of vehicle 42. The antenna 44 encircles a plurality oE steel rods 138 coming up from the bed of vehicle 42. A pair oE
plywood boards 140 lie above and below antenna 44.

Fig~ 6a shows the plurality of repeaters 28 in more detail. Each repeater 28 includes an access medium Erequency transceiver 150 and a local medium frequency transceiver 152. The access transceiver 150 is capable of receiving a signal at ~requency F2 at a receiver 154, amplifying and replicating the F2 signal at the frequency F3 and transmitting the F3 signal from a transmitter 158. The local transceiver 152 is capable o~ receiving a signal at frequency F3 at a receiver 165, amplifying and replicating the F3 signal at the frequency Fl and transmitting the Fl siynal fro~ a transmitter 170. The receiver 154 is tightly coupled to conductor 32 by a repeater vertical tuned loop antenna 174 and an antenna cable 176 which links antenna 174 to receiver 154. The transmitter 158 is also tightly coupled to conductor 32 by a repeater vertical tuned loop antenna 180 and an antenna cable 181 which links antenna 180 to transmitter 158.
Similarly, receiver 166 and transmitter 170 are tightly coupled to conductor 32 by a pair o~ repeater tuned loop antennas 182 and 183 respectively, and a pair o~ antenna cables 184 and 186, respectivelyO The . 20 access transceiver 150 and the local transceiver 152 are protected by a pair of intrinsically safe (IS) limiter circuits 192 and 196, respectively. A pair o~
sealed lead acid batteries 198 and 200 are connacted to : the IS circuits 192 and 196, respectively.
FigO 6b shows one o~ the plurality of pager repeaters 29. Each repeater 29 includes a transceiver 201 which ~omprises a receiver 202, capable o~
~: receiving the frequency F5, and a transmitter 204, capable of transmltting the ~requency F4. The receiver 202 is tightly coupled to the transmission line conductor 32 by a pager repeater vertical tuned loop antenna 206 and an antenna cable 208 which links ::
antenna 206 to a receiver 202~ The trans~itter 204 is also tightly coupled to the transmission line conductor : :
:, $i~`3 32 by a pager repeater vertical tuned loop antenna 210 and an antenna cable 2l2 which links antenna 210 to transmitter 204. The transceiver 201 is powered by a sealed lead acid battery 214 which is protected by an S intrinsically safe limiter circuit 216.

In the pre~erred embodiment o~ the present invention, the frequencies Fl, F2 and F3 are chosen to be 400K~z, 520KHz and 300KHz, respectively. The basis for this choice is the empirical observation that the optimal frequency for propagating signals in underground mine transmission line electrical conductors is 300KHz. This is because the attenuation rate ~or electromagnetic signal propagating on the transmission line electrical conductors, decreases with frequency of propagationO At 300K~z the attenuation rate is only 2dB/1000 ~t., whereas at 520KHz, the attenuation rate is 4-5dBjlOOO ft.
Additionally, ~or frequencies below 300KHz the mine generated electcical noise increases by 6dB for each "halving" of frequency. Thus, 300KHz represents an optimal propagation frequency. In contrast to propagation e~iciency on the transmission line electrical conductors, however, remote loop antenna to transmission line coupling improves with Erequency.
Thus, 520KHz signals are more e~iciently coupled between a remote antenna and a conductor than are 300KHz signals.

The functioning o~ the mine communication system 20 shown in Figs. 1 and 5 can now be explained. The portable radio 22, the mobile radio 24 and the personal-carried radio 26 all use their respective tuned loop antennas 33, 44 and 47, to magnetically induce signal current flow in nearby conductors 32.
Because tlle antennas 38, 44 and 47 are o~ten four to fifteen feet from conductors 32 (remote), they induce only weak signal currents in conductors 32. To increase the opera~ing range of the system 20, the repeaters 28 are u~sed to receive weak radio signals, ampliEy the signals and then reinduce stronger current flow in the conductors 32. For example, when communication from the mobile vehicular radio 24 is desired, a signal is transmitted at frequency F2.
This frequency allows efficient coupling between vehicular loop antenna 44 and electrical conductor 32 even when they are not physically close to each other.
When the F2 signal, propagating in conductor 32, encounters one of the repeaters 28, the F2 signal is picked of by loop antenna 174 and relayed to receiver 154. The F2 signal is amplified, replicated (i.e.
changed to the frequency F3) and retransmitted by transmitter IS~ at requency F3. Because antenna 180 is tightly coupled to conductor 32, the F3 signal is eEficiently coupled back onto conductor 32 and propagates to every repeater 28 in system 20 and to the base station 30. At every repeater 28, the F3 signal is received by the receiver 166 via loop antenna 182.
The F3 signal is then amplified, replicated and retransmitted at frequency Fl from trans~itter 170 through loop antenna 113 back onto conductor 32. Since every mobile vehicular radio 24, personal-carried radio 26 and poetable radio 22 is always tuned to frequency Fl, they receive the signal.

The radios ~2, 24 and 26 can also communicate directly with one another, at short range, without the use of repeaters 28, by transmitting directly on frequency Fl.

~2~

The base station 30 can communicate with radios 22, 24 and 26 by transmitting a message to the repeaters 28 on ~requency F2. This message is then - replicated by the repeaters 28 and transmitted to the S radios 22, 24 and 26 on frequency Fl. The base station 30 can also communicate through the repeaters 28 by transmitting on ~requency F3 and receiving signals at Fl.

The pager 27 functions by alerting the person wearing the pager to contact the sur~ace~ The pager computer 53, contained within pager base station 31, can be programmed to initiate calls, periodically, until the person wearing the pager is reached. The computer S3 would initiate the call by generating a digital code, ~rom the pager encoder 52, which would be modulated in a frequency shift key (FSK) format. The digitally coded call would then be transmitted by the pager transmitter Sl at the frequency F5. The call is transmitted from the pager loop antenna S0 onto the transmission line electrical conductors 32 and to the pager repeaters 29. When the F5 signal, propagating in a conductor 32, encounters one o~ the repeaters 2g, the F5 signal is picked off by loop antenna 206 and relayed to receiver 202. The FS signal is amplified, replicated and retransmitted by transmitter 204 at frequency F4. Because antenna 210 is tightly coupled to conductor 32, the F4 signal is coupled back onto conductor 32. When a person wearing pager 27 and suspender loop antenna 92 comes close to a conductor 32, the F4 signal is received by receiver 116, decoded by decoder 118 and used to activate the signal light 120. This alerts the payer wearer to contact the surface. Once the pager wearer has contacted the surEace, the pager computer 53 is instructed to cease .

~Z"3~~i3 sending calls. In the preferred embodiment, the frequencies F4 and F5 are chosen to be 450KHz and 250KHz, respectively.

The use of tuned loop antennas (such as antennas 38, 44, 47, 174, 180, 182, 183, 49 and 50 in the present invention) is important to the Eunctioning of system 20 for three reasons. First, loop antennas are very effective electromagnetic couplers in both the transmitting and receiving modes. In the transmit mode, loop antennas produce high current flow in nearby conductors and loop antennas do not change inductance (saturate) when transmitting. Additionally, loop antennas have the capability of being either tightly electromagnetically coupled to a conductor (i.e. being coupled in close physical proximity to the conductor) or of being remotely electromagnetically coupled to a conductor (i.e. achieving coupling to a conductor even when the loop antenna is 1-20 feet away from the conductor). Second, loop antennas have the ability to couple both transmission line elec~rical conductors and natural waveguides. Natural waveguides are formed when a layer of less conductive material (such as coal, trong or potash) is bounded above and below by more conductive rock. It is well known that the electrical field component oE an electromagnetic wave is vertically polarized while the magnetic ~ield component is hori~ontally polarized. Thus, in the mine 34, the loop antenna 3a, hanging in a vertical plane below conductor 32, can e~ficiently electromagnetically couple the electrical conductor 32 and is also correctly positioned to receive the magnetic component of electromagnetic waves traveling in the natural `~ waveguide mode in working face 36.

~L~96$63 ~21 The importance of the natural waveguide coupling mode is that it enables communication links to be established through more than 1000 feet of solid coal and 300 ~eet oE rock where no electrical conductors exist. Because of the ability of loop antennas to couple both el~ctrical conductors and natural wave guide ~odes, the portable radio 22 would have the following operating ranges in a mine 34:

Tight Coupling Operating Range (Conductor Mode) Type_oE Conductor _ Ran~ (feet) Unshielded Wire Pair 33,000 Shielded Wire Pair 20,000 Remote Coupling Operation Range Mode __ Distance (f~et) Conductor 8,000 Seam 1,000 (radius~

Finally, the ~hird functional advantage of loop antennas is that they are easy to install and are much less expensive than other coupling devices such as errite or air core torroid couplers. In Eield testing, it has been determined that the loop antenna `~ 38 in Fig. l can be suspended from conductor 32 using a connector 40 which can be simply a piece o~ nylon string~

he design oE the trans~itters 57, 80, 132, 158, 170 and 204 in Figs. 2-6 is also important to the 3S ~ùnctioning of the present invention. The : ~:
,~

~:

:~

~22-transmitters 57, 80, 132, 158, 170 and 204 are designed to yield optimization of the magnetic moment o~ the transMitting loop antenna The magnetic moment (M) is given by the equation:
M = NIA
where N = the number of turns in the loop antenna;
A = the area of the loop antenna in square meters; and I = the peak current in the loop.
Optimization of the transmitters is achieved by recognizing that for the transmitting loop antenna, M = (Po/BW)1/2 where PO = the power dissipated in the loop, and BW = the bandwidth of the FM carrier signal. So, in a series tuned circuit, Q = ~L/RL = Po/BW
where Q = qualify factor (loaded Q) = radian ~requency L = inductance (henry) RL ~ series resis~ance To maximize the magnetic moment, the loop bandwidth (BW) is made as small as possible while still being wide enough to accommodate the occupied bandwidth of the FM carrier siynal. Thus, in the present invention, the ratio Po/BW is seen as the electrical optimization parameter and not as just the ~aximum power to load resistance.

The receivers 56, 82 and 130 in Figs. 2, 3 and 5 have been designed to include a means for measuring the received signal to noise tS/N) ratio. The squelch controL knobs 64, 84 and 134 on the radios 22, 26 and 24 are calihrated so that each click of the squelch control knob indicates a change in the S/N ratio by l0d~. This Eeature circu~vents the necessity of having radio maintenance personnel carry ~ield strength meters to determine the high voice quality communication range (Eade margin).

The portable radio 22, shown in Fig. 2, serves two important functions in the mine communication system 20. First, under normal conditions, radio 22 functions as a stationary radio transceiver in system 20. A plurality of radios 22 would be placed in various locations in a mine 34 such as at the working face 36 and in saferooms, belt loading points and central control or communication points. The external speaker 69 provides a high volume audio capability so messages can be heard in the near vicinity of the unit.

Second, under emergency situations, such as a fire, explosion or cave-in, the portable radio 22 can be re~oved from its wall mount and carried by a miner enabling him to receive evacuation instructions and information.

Referring now to Fig. 7a, there is shown a ~ conventionally designed intrinsically safe (IS) - battery protection circuit 220. Circuit 220 includes a wirewound resistor 222 and a fuse 224 connected in series with a battery 226. A pair of contacts 228 and 229 provide a means for drawing current Erom circuit 220.

Fig. 7b shows an IS current limiter circuit 230 of the present invention. The circuit elements enclosed within the two dashed boxes o~ Fig. 6b form a current trip circuit 232 and a redundant current trip circuit 234. The circuit 232 includes a EieLd efEect transistor (FEr) 236 connected in series be~ween a ;

~2~6~t ~,3 node 238 and a node 240. A branch of node 238 contains a resistor 242, a transistor 244 and an FET
246 connected in series between node 238 and a ground 248 connected to the source terminal of FET 246. The emittec of transistor 244 is grounded. A node 250 is located between transistor 244 and FET 246. A
resistor 252 is connected between node 250 and the gate terminal o~ FET 236. A node 254 is located between transistor 244 and node 250. A resistor 256 is connected between node 254 and a anode lead 258 which is connected to the anode of a twelve volt battery 260. A resi~tor 262 is connected between node 240 and the gate terminal of FET 246, A node 264 is located between resistor 262 and FET 246. A resistor 266 is connected between node 264 and a ground 268.

The redundant trip circuit 234 has an electrical structure identical to that of circuit 232 and includes a pair of FET's 270 and 272 and a transistor 274. The emitter o~ transistor 274 is grounded. The FET 270 is connected in series between the node 240 and a node 278. A lead 279 is connected between a node 280, which lies between resistor 242 and node 238~ and a node 281. A branch o node 281 contains a resistor 282, the transistor 274 and the FET 272 connected in series between node 281 and a ground 283 connected to the source terminal o~ FET 272. A node 284 is located between transistor 274 and FET 272.
resistor 286 is connected between node 284 and the gate terminal of FET 270. A node 288 is located between transistor 274 and node 284. A resistor 290 is connected between node 288 and the anode lead 258.
A resistor 292 is connected between node 278 and the gate terrninal of FET 272. A node 294 is located ~2~ `3 between resistor 292 and FET 272. A resistor 296 is connected between node 294 and a ground 298.

The area to the right o redundant trip circuit 234 in Fig. 6b includes a current limiting FET 300 connected in series between the node 278 and a wire-wound inductor 302. A feedback control operational ampliEier 304 is connected to the anode lead 258 at a node 305 by an output lead 306. A
resistor 301 is connected in series between the node 305 and ampli~ier 304. The ampli~ier 304 includes a current limit voltage comparactor 312 and a case temperature limiter 314. An output lead 316 forms a rectangular loop 318 which is connected between comparator 312 and the node 281, The loop 318 includes a resistor 322 connected in series be~ween node 281 and comparator 312. An input lead 330 of comparator 312 is connected to a grounded resistor 332. Another input lead 334 of amplifier 304 is grounded. An input lead 336 of the amplifier 304, which includes a resistor 338, is connected to the anode lead 258. A node 3~4 is located on input lead 330 between grounded resistor 332 and comparator 312.
A lead 346, which includes a resistor 348, runs from : 25 node 344 to anode lead 2580 An input lead 349 connects limiter 314 with lead 346 at a node 350. A
lead 352, which includes a resistor 354, connects limiter 314 with anode lead 258. A node 3S6 is located on lead 352 between limiter 314 and resistor 354. A grounded heat responsive thermistor 358, :~ located near the current limiting FET 300, is connected to lead 352 at node 356. A thermal connection 359 is made between thermistor 24~ and F~'r 300 using thermally conductive epo~y. The gate terminal of an FET 360 is connected to the lead 306 at a node 361. The drain terminal of FET 360 is connected to the anode lead 258 at a node 362 by a lead 363. The lead 363 includes a resistor 364. A
ground 365 is connected to the source terminal of FET

3~0. An output lead 366 is connected between the limiter 3l4 and a node 367 lying on lead 363. A
resistor 368 is connected between the gate terminal of FET 300 and a node 369 lying on lead 366. A grounded capacitor 372 is connected to lead 346 at a node 374 lying between node 344 and node 3S0. An anode terminal 380 is located at the end of anode lead 258 furthest removed from battery 260. A cathode terminal 382 is located at the free end of i~ductor 302. The terminals 380 and 382 provide a means for connecting electronic equipment to the circuit 130.

The area to the left of current trip circuit 232 in Fig. 6b includes a cathode lead 384 which is connected to the cathode of battery 260. A plurality of battery charging diodes 386, connected in series, join cathode lead 384 at a node 388 near the cathode of battery 260. .Cathode lead 384 branches at a node 390. One branch of cathode lead 384 is connected to a sense resistor 392. Along the other branch, a precision wirewound resistor 394 is connected between :~ node 390 and node 238. A sense connector 396 is located at the eree end of resistor 392. A cathode charge connector 398 is located at the Eree end of the series of diodes 386. An anode charge connector 400 is located on the free end of a lead 402 which connects to anode lead 258 at a node 404 near the : anode of battery 260.

; The Eunctioning of the IS current limiter circuit ; 35 230 of the pcesent invention can llOW be explained.
:`' :~

The current limiter circuit 230 is designed to replace the conventional IS battery protection circuit 220 shown in Fig. 6a. The circuit 230 would be used with radios 22 and 24 and repeaters 28 of Fig. 1. For example, Fig. 2 shows the portable radio 22 equipped with a battery pack 58 and an intrinsically safe limiter circuit 59. The personal-carried radio 26 is equipped with a smaller IS li~iter circuit 88 having the same design as circuit 230.
The circuits 59, 136, 88, 192, 196, 216 and 214 are needed when using the radios 22, 24 and 26 and repeaters 28 and 29 and the pager 27 in gaseous atmospheres~ .such as are found in coal mines, to prevent explosions. The current trip circuit 232, shown in Fig. 6b, emulates the fuse 224 of Fig. 6aO
The feedback control operational ampli~ier 304 in Fig.
6b emulates the resistor 222 in Fig. 6a. The redundant trip circuit 234 of Fig. 6b serves as a back-up to current trip circuit 132. The operation of current limiter circuit 230 limits the instantaneous demand current flow to an intrinsically sa~e level.

The initial condition of the ~ET's 236, 270 and 300 is a low channel resistance condition of about 0.18 ohms. The current Elow through resistor 394 produces a voltage Vtl) by Ohm's Law. The current limit voltage com~arator 312 has a reference voltage V(2), which is normally greater than voltage V(l), established by the biasing resistors 348 and 332. As long as the voltage V(l) remains less than voltage V(2), the channel resistance of FET 300 remains at the low value of about 0.18 ohms. However, in the event an exce~sive deman~ of c~rrent flow is caused by a ; 35 fault in the equipment connected between terminals 380 ~:;
~::
-~L2~ 3 and 382 or in the current limit voltage comparator 312, the volta~e V(l) rises above voltage V(2). This drives the output lead 306 to a low level causing the channel resistance of FET 300 to increase and thus limiting the current flow through FET 300. Transistor 244 then turns on causing the FET 236 channel resistance to increase. Simultaneously, the FET 246 channel resistance goes to a low ohmic resistance state, for the purpose of latching FET 236 in its high resistance state, thereby peemanently opening the demand current path. By opening terminals 380 and 382, the latch condition in circuit 230 can be removed. The redundant trip circuit 234 backs up the current trip circuit 232.
To further insure the fuse-like nature oE circuit 230 and to prevent overheating of the FET 300, the heat responsive thermistor 358 is attached with thermally conductive epoxy to FET 300 at connection 359. If FET 300 heats up, the temperature increase is tranferred to thermistor 358. Whenever the temperature of thermistor 358 exceeds a limit set by V(2), the gate of FET 300 is driven to a low state, thus increasing the channel resistance o~ FET 300.
The function of case temperature limiter 314 is to prevent excess heat build-up in FET 300 which could cause incendiary conditions to develop.

When equipment is connected across the terminals 380 and 382, a transient demand current ~lows to charge capacitors in the equipment. This current is slowed down by the wire-wound inductor 302. The energy of this current transient is limited to less than 0.2 millijoules.

,~

-2~-The plurality of diodes 386 and the charge connectors 388 and 400 provide a means for recharging battery 260. The sense resistor 392 and the sense connector 396 provide a means ~or determining the battery charging states. Initially, this is a high charge current rate ~ollowed by a flat charge rate.

The design o~ IS current limiter circuit 230 insures that a fault in any circuit (such as a short circuit) will not cause incinerary conditions to occur in the circuit. The entire circuit 230 is potted to prevent coal dust from accumulating on the component parts.

Although the present invention has been described in terms of the presently preferred embodiment, it is ; to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art aEter having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modi~ications as fall within the true spi~it and scope of the invention.

~:
:

: ' ~, ~ '

Claims

1. A method for communicating in mines which comprises:
a. inductively coupling a first radio and a second radio to a transmission line electrical conductor that extends into a mine, the first radio and the second radio each including a receiver for receiving one frequency F1 and a transmitter for transmitting on at least one frequency F2 different from said frequency F1 with the frequencies F1 and F2 both lying in the medium frequency range;
b. inductively coupling at least one access transceiver to said transmission line electrical conductor, the access transceiver having a receiver for said frequency F2 and a transmitter for transmitting a frequency F3 different from said frequencies F1 and F2;
c. inductively coupling at least one local transceiver to said transmission line electrical conductor within said mine, the local transceiver having a receiver for receiving said frequency F3 and a transmitter for transmitting said frequency F1;
d. transmitting a first signal carrying information to be communicated from the first radio on the frequency F2;
e. receiving said first signal at said access transceiver;
f. retransmitting said first signal from said access transceiver on the frequency F3 as a second signal carrying said information to be communicated;
g. receiving said second signal at at least one of said local transceivers;
h. retransmitting said second signal from said local transceiver on the frequency F1 as a third signal carrying said information to be communicated; and i. receiving said third signal at said receiver of said second radio whereby said information to he communicated is conveyed from said first radio to said second radio.

2. A method for communicating in mines as recited in claim 1, further comprising:
inductively coupling said first radio and said access transceiver to a seam of coal or rock whereby a natural waveguide mode of propagation for said first signal through the seam to the access transceiver is established.

3. A method for communicating in mines as recited in claim 1 wherein, the frequency F2 is approximately 529 Khz and the frequency F3 is approximately 300 Khz.

4. A method for communicating in mines as recited in claim 1 wherein, said second signal is received at a third radio tuned to the frequency F3 and inductively coupled to said transmission line electrical conductor.

5. A method for communicating in mines as recited in claim 1 wherein, the frequency F3 has a lower attenuation rate on said transmission line electrical conductor than the frequency F2.

6. A method for communicating in mines which comprises:
a. using a first tuned loop antenna to inductively couple a first radio to a transmission line electrical conductor that extends into a mine and a second tuned loop antenna to inductively couple a second radio to said transmission line electrical conductor the first radio and the second radio each including a receiver for receiving at least one frequency F1 and a transmitter for transmitting on at least one frequency F2 different from said frequency F1 with the frequencies F1 and F2 both lying in the medium frequency range;
b. using at least one access tuned loop antenna to inductively couple at least one access transceiver to said transmission line electrical conductor, the access transceiver having a receiver electrically connected to at least one of the access tuned loop antennas for receiving said frequency F2 from said transmission line electrical conductor and a transmitter electrically connected to at least one of the access tuned loop antennas for transmitting a frequency F3 different from said frequencies F1 and F2 on said transmission line electrical conductor;
e. using at least one local tuned loop antenna to inductively couple at least one local transceiver to said transmission line electrical conductor within said mine, the local transceiver having a receiver electrically connected to at lest one of the local tuned loop antennas for receiving said frequency F3 from said transmission line electrical conductor and a transmitter electrically connected to at least one of the local tuned loop antennas for transmitting said frequency F1 on said transmission line electrical conductor;
d. transmitting a first signal carrying information to be communicated from the first radio on the frequency F2;
e. receiving said first signal at said access transceiver;

f. retransmitting said first signal from said access transceiver on the frequency F3 as a second signal carrying said information to be communicated;
g. receiving said second signal at at least one of said local transceivers;
h. retransmitting said second signal from said local transceiver on the frequency F1 as a third signal carrying said information to be communicated; and i. receiving said third signal at said receiver of said second radio whereby said information to be communicated is conveyed from said first radio to said second radio.

7. A method for communicating in mines as recited in claim 6, further comprising:
using the first tuned loop antenna and at least one of the access tuned loop antennas to inductively couple said first radio and said access transceiver to a seam of coal or rock whereby a neutral waveguide mode of propagation for said first signal through the seam to the access transceiver is established.

8. The method of claim 6 for communicating in mines wherein, the frequency F3 is approximately 300 Khz.

9. The method of claim 6 for communicating in mines wherein, the frequency F1 is approximately 400 Khz.

10. The method of claim 6 for communicating in mines wherein, the frequency of F2 is approximately 520 Khz.

11. The method of claim 6 for communicating in mines wherein, said transmitter of said first radio and said second radio can also transmit on said frequency F1.

12. The method of claim 6 for communicating in mines wherein, said transmitter of said first radio can also transmit on said frequency F3.

13. The method of claim 6 for communicating in mines wherein, said receiver of said first radio can also receive said frequency F3.

14. The method of claim 6 for communicating in mines wherein, said first tuned loop antenna is a vertical tuned loop antenna physically attached to said electrical conductor by a connecting means for holding said first tuned loop antenna in close proximity to said electrical conductor.

15. The method of claim 6 for communicating in mines wherein, said second tuned loop antenna is a vertical tuned loop antenna physically attached to said electrical conductor by a connecting means for holding said first loop antenna in close proximity to said electrical conductor.

16. The method of claim 6 for communicating in mines wherein, said first tuned loop antenna is a horizontal tuned loop antenna mounted on a mobile vehicle.

17. The method of claim 6 for communicating in mines wherein, said second tuned loop antenna is a horizontal tuned loop antenna mounted on a mobile vehicle.

18. The method of claim 6 for communicating in mines wherein, said first tuned loop antenna is a vertical tuned loop antenna physically attached to a garment worn by a miner.

19. The method of claim 6 for communicating in mines wherein, said second tuned loop antenna is a vertical tuned loop antenna physically attached to a garment worn by a miner.

20. The method of claim 12 for communicating in mines wherein, said first radio is located outside of said mine.

21. A method for alerting a person in a mine to contact a station which comprises:
a. using a tuned loop antenna to inductively couple a pager transmitter located at a station to a transmission line electrical conductor extending into a mine, said pager transmitter adapted to transmit a frequency F5;
b. inductively coupling at least one repeater to the transmission line electrical conductor within said mine by use of at least one tuned loop antenna, said repeater adapted for receiving the frequency F5 and transmitting a frequency F4;

c. inductively coupling a pager to the transmission line electrical conductor within said mine by use of a tuned loop antenna, said pager including a pager receiver for receiving the frequency F4;
d. transmitting a first signal at the frequency F5 from said pager transmitter to said repeater;
e. receiving the first signal at said repeater;
f. retransmitting the first signal from said repeater as a second signal at the frequency F4;
g. receiving the second signal at the pager receiver;
and h. using the second signal to activate an alarm means on the pager for notifying a person in said mine to contact said station.

22. The method for alerting a person in a mine to contact a station as recited in claim 21, further including the steps of:
a. digitally encoding said first signal prior to transmission from said pager transmitter, and b. decoding the second signal prior to activating the alarm means.

23. The method of claim 21 for alerting a person in a mine to contact a station wherein, said frequency F4 is approximately equal to 450 Khz.

24. The method of claim 21 for alerting a person in a mine to contact a station wherein, said frequency F5 is approximately equal to 250 Khz.

25. A personal-carried vertical tuned loop antenna comprising:
a one-piece harness including a pair of flexible shoulder straps which loop over a person's shoulders, means for attaching the shoulder straps to a belt worn about the person's waist and a pair of cross straps which run perpendicular to the shoulder straps, along a back face of the harness, with each of the cross straps being connected to each of the shoulder straps and a space "w" being left between the two cross straps such that the two cross straps form a rectangular area;
a continuous wire loop attached to the outer face of the rectangular area formed by the cross straps; and a loop antenna tuning box electrically connected to the wire loop and attached to one of the shoulder straps.

26. The personal-carried vertical tuned loop antenna of claim 25, wherein, the pair of cross straps run perpendicular to the shoulder straps along a front face of the harness.

27. The personal-carried vertical tuned loop antenna of claim 25, wherein, the loop antenna tuning box contains a series tuned circuit for switching the continuous wire loop between a receiving mode and a transmitting mode.

28. The personal carried vertical tuned loop antenna of claim 25 further including:
a. a pager electrically connected to the loop antenna tuning box, said pager including a receiver and a means for alerting the person that a signal is being received by the receiver, b. adjusting the loop bandwidth to the smallest value possible that will still accommodate the bandwidth of the frequency modulated carrier signal.