United States Patent 3740488

A transmitter having a long line antenna grounded at both ends transmits audio signals through the earth to one or more receivers in a mine. Each receiver has a simple audio amplifier coupled to a loop antenna which compensates for greater attentuation of the higher audio frequencies being transmitted.

Linfield, Robert F. (Boulder, CO)
Farstad, Arnold J. (North Glen, CO)
Allen, James W. (Longmont, CO)
Ball, Lawrence (Boulder, CO)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
381/79, 381/124
International Classes:
H04B5/00; (IPC1-7): H04B5/00
Field of Search:
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US Patent References:

Foreign References:
Primary Examiner:
Brown, Thomas W.
We claim

1. A communications system comprising:

2. A system according to claim 1 wherein said transmitter means includes:

3. A system according to claim 2 wherein:

4. A system according to claim 2 wherein:

5. A system according to claim 2 wherein:

6. A system according to claim 2 wherein:

7. A system according to claim 2 wherein said coupling means includes:

8. A system according to claim 1 wherein said transmitter means includes:

9. A system according to claim 1 for mine use, which includes:

10. Receiver apparatus for receiving through the earth transmission of an audio signal of a band of frequencies within the audio range, comprising:


1. Field of the Invention

The invention generally relates to communication systems, and particularly to a through the earth voice communication system.

2. Description of the Prior Art

Various instances exist for which through the earth voice communication links are desired. A prime example is in the field of mining wherein a reliable communication system is required for communicating voice instructions from surface rescue operations to miners trapped in a mine to reduce the problems normally associated with the survival and rescue of the trapped miners.

At the occurrence of a mine disaster, injured and uninjured men will leave the vicinity of the explosion and fires, and will seek refuge in pre-existing refuge chambers at various locations within the mine. Smaller groups of men may be trapped anywhere within the mine and may not have access to these refuge chambers or other special equipment previously placed in designated areas. These men will likely have no equipment or means of communication other than that which was part of their attire at the time of the disaster.

Following a disaster it is probable that power to sections of the mine will be disrupted, or disconnected for safety reasons, and accordingly all previously existing hard wire communication systems will be inoperative. A need exists therefore for a relatively simple, lightweight, portable and rugged receiver for receiving voice communications.

Various through the earth voice communication systems have been proposed wherein a carrier signal is modulated by the voice intelligence to be transmitted, such modulation being in the form of frequency modulation, amplitude modulation or pulse position modulation, for example.

For through the earth communication, the higher frequencies are attenuated more and accordingly the carrier signal is attenuated after a relatively short distance through the earth. Alternatively much more expensive and higher powered transmitting equipment is required to compensate for the attenuation.

Another proposal, such as in U.S. Pat. No. 1,470,430, suggests the transmission of an audio frequency by varying a direct current circuit with a microphone to cause fluctuations in the current. The transmitting antenna is connected to the earth at both ends and the receiver has a similar receiving antenna. Since the receiving antenna must be connected to the earth it is not readily carried by a miner and is subject to destruction in the event of a disaster. Additionally, with any transmission of audio frequencies through the earth, the higher audio frequencies are attenuated more than the lower frequencies transmitted thereby resulting in a distorted output from the receiver.

The present invention provides a through the earth voice communication system which allows for great separation between transmitter and receiver, and wherein the receiver may be carried on a person, and wherein voice reproduction is of extremely high quality.


A transmitter means is provided for transmitting an audio signal through the earth to a receiver. The signal transmitted is a band of frequencies within the audio range. The transmitter and receiver may be at the same, or different levels with one, or both being located within the earth. The receiver means includes an audio amplifier, a loop antenna coupled to the audio amplifier and an output, such as a speaker or earphone for converting the audio amplifier output to an audible signal. For use where a great distance separates the transmitter and receiver, the loop antenna is designed such that its self resonant frequency is above the band of frequencies being transmitted so that its lineraly increasing output voltage with increasing frequency, compensates for the increased attenuation of the higher frequencies transmitted.

For operation where the transmitter is relatively close to the receiver, the loop antenna is designed to have a relatively flat response in the vicinity of the self resonant frequency which, in the latter instance, is within the band of frequencies being transmitted.


FIG. 1 illustrates, with portions broken away, a mine environment in which the present invention may be utilized;

FIG. 2 is a block diagram of the transmitter means;

FIG. 3 is a block diagram of the receiver means;

FIG. 4 is a curve illustrating the attenuation of various frequencies in through the earth transmission;

FIG. 5 is a curve illustrating the loop antenna response;

FIG. 6 is a curve illustrating the resultant signal obtained through the use of the loop antenna of FIG. 3, in conjunction with the transmitter means of FIG. 2;

FIG. 7 is a plan view, and FIG. 8 is a side elevational view of the coverage obtained with a long line antenna;

FIG. 9 is a block diagram illustrating a transmitter means for obtaining greater coverage;

FIGS. 10 and 11 illustrate other antenna configurations;

FIG. 12 illustrates another type of antenna which may be used in the transmitting means;

FIG. 13 illustrates the invention in conjunction with a miner's helmet;

FIG. 14 illustrates another use for the present invention; and

FIG. 15 is a curve illustrating the loop antenna response for the embodiment of FIG. 14.


In FIG. 1 there is illustrated a mine site with a portion of the earth broken away to show a section of the mine 10. Located at the surface is a transmitter means 12 having a long line, insulated wire transmitting antenna 13 grounded at both ends 15 and 16 by connection to the earth. To increase transmission efficiency the ends 15 and 16 may be grounded by a technique utilizing a plurality of grounding stakes driven into the ground in one or more groups with the stakes having, for example, salt water poured on them, and each being electrically connected to an end. The transmitter means is operable to transmit an audio signal, directly and without modulating a carrier, through the earth to one or more receiver means. A receiver means may be carried by each miner 20 and will also be located in pre-existing refuge chambers 22.

The transmitter means 12, as illustrated in more detail in FIG. 2, includes an audio frequency power amplifier 25 for amplifying the audio signal provided by an input means such as microphone 27 in response to a speaker. If desired, the audio amplifier can be driven with a preamplifier 29 of the type that is typically used for normal audio frequency amlification and a suitable power supply 31 is provided to supply necessary operating potentials.

The output of the audio frequency power amplifier 25 is an audio signal within the audio range. The audio range occupies a band from approximately 20 to 20,000 Hz and speech occupies the band primarily from 50 to 7,500 Hz although most of the information is contained within the 300 to 3,000 Hz range. The transmitter means 12 may be designed to provide an output band of frequencies substantially within this latter range and herein termed the voice band. In order to couple the output of the amplifier 25 to the antenna 13, there is provided a coupling and matching network in the form of a transformer 32 having a primary and secondary winding 33 and 34 respectively for matching the output of the amplifier 25 to the impedance of the antenna 13. The transformer 32 may incorporate a secondary winding 34 which includes multiple taps 35 selectable, such as by a switch 36, to permit matching a wide variety of antenna impedances.

In setting up the transmitter means after the occurrence of a mine disaster one section of antenna 13 may be coupled to one end of the transformer secondary winding and the other section of antenna 13 may be coupled to the switch 36. The two antenna sections are preferably on respective reels carried by vehicles which may then drive off in opposite directions to antenna grounding positions. Alternatively one end may be grounded and a single long length of antenna wire, reel mounted and vehicle carried, may be connected to the switch (or vice versa). For permanent insulating the antenna should be buried just below the surface to avoid damage.

The receiving means 38, as illustrated in FIG. 3, includes an audio amplifier 40 designed for amplifying the band of frequencies being transmitted, and suitable filter means may be included for rejecting noise signals outside the transmitted band. The audio amplifier 40 includes means for connection to a source of operating potential in a form of power supply 42. If the receiving apparatus is located within a refuge chamber, the power supply 42 may be a rechargeable battery within the chamber; if the receiving means 38 is carried by a miner, the power supply 42 may be from the battery normally carried by the miner to power his helmet carried light.

A receiving antenna in the form of loop antenna 46 is provided to receive the transmitted signal, and more particularly the magnetic field thereof, to provide an output voltage, herein termed the terminal voltage of the loop, to the audio amplifier. The output of the audio amplifier 40 is connected to an output transducer means 49 for converting the output signal to an audible signal. The output transducer means may, for example, be a speaker or a set of earphones.

One of the problems associated with transmission of the audio frequencies through the earth is that all of the frequencies are not attenuated by the same amount. This is illustrated by way of example in FIG. 4.

In FIG. 4 the horizontal axis is a log scale of frequency, and the vertical axis represents the strength of the magnetic field received, measured in decibels (db), relative to 1 amp per meter, a typical designation for magnetic field strength. By way of example, FIG. 4 illustrates the situation for a thousand foot depth with an earth conductivity of 10-2 mhos per meter. Curve 52 is the resultant response with a transmitter antenna of a 1,000 meter length and a 1 ampere antenna current and curve 52', having a similar slope, is the response with an antenna of a 100 meter length and a 1 ampere antenna current. It is seen that as the frequency increases, the magnetic field strength decreases and this is due to the fact that increased attenuation occurs with increased frequency. Since the field strength is not uniform across the voice band, any amplification thereof will result in a distorted and perhaps unintelligible output. The loop antenna 46 is designated to compensate for this non-uniform attenuation.

The loop antenna 46 consists of winding 55 having a number of turns of wire disposed about a core which may be air, or for example, a ferrite core 57 as illustrated in FIG. 3. A typical loop antenna response is illustrated in FIG. 5 wherein the horizontal axis is a log scale of frequency and the vertical axis represents the loop terminal voltage induced, due to a given magnetic field, the units being in decibels relative to one volt. Up until a point at which self resonance is approached, the induced RMS terminal voltage is given by the relationship:

Vi = 2πf N AμH (1)


Vi is in volts

f is the frequency in hertz

A is the area of the coil in square meters

N is the number of turns of wire in the coil

μ is the effective permeability of the core

H is the RMS magnetic field in amperes per meter.

The area of the antenna may be calculated knowing the diameter D (A = πD2 /4` ) and since there may be many layers wound around the core 57, D is taken to be the mean diameter. Associated with the winding is a distributed capacitance whose capacitive reactance increases with increasing frequency while inductive reactance decreases with increasing frequency. When the inductive and capacitive reactances are equal at a particular frequency, the loop antenna is self resonant and the particular frequency is herein termed the self resonant frequency fr. The curve of FIG. 5 is the output voltage vs. frequency characteristic, for a given magnetic field H of 1 amp per meter with 800 turns of wire, an area of 0.69 square meters and a core of free space. The curve illustrates that the loop terminal voltage increases linearly to a peak point 60 and thereafter decreases linearly. Peak point 60 occurs at the self resonant frequency fr. The linear response is approximately 6 db per octave, that is, for each doubling of the frequency the terminal voltage doubles. By proper choice of the number of turns in the loop (N), the area of the coil (A) and winding pattern which affects the distributed capacitance, the resonant frequency for the loop antenna of FIG. 3 is designed to occur above the band of frequencies being transmitted. In this manner the antenna is operated in the linear range 62 encompassing the voice band transmission. Accordingly, as the higher frequencies result in lower field strengths and as the loop antenna provides a higher output voltage for these frequencies the combined action being illustrated in FIG. 6 results in a terminal voltage which does not vary greatly with frequency within the band of interest.

In FIG. 6, curve 65 represents the magnetic field strength response such as previously illustrated in FIG. 4 and curve 66 represents the linear portion of the receiving loop response curve of FIG. 5. If the two curves are multiplied by one another (or added logarithmically), that is if, for each frequency, the actual value of H be utilized in equation (1), there results a substantially constant output voltage as illustrated by curve 68. It is to be noted that the transmitted audio signal is actually varying in amplitude; the voltage referred to is the RMS voltage. The amplification of the signal represented by curve 68 which is substantially linear in response to the receipt of the band of frequencies being transmitted insures a substantially distortionless output from the audio amplifier 40, FIG. 3, and an intelligible output signal from the output transducer 49. Intelligible communications for a system is a necessity, particularly in a mine disaster situation since its use may be by injured, frightened, or confused personnel who must respond to given instructions. Because of transmitter power limitations in the mine and strong atmospheric noise levels at the surface, the available signal to noise ratios at the surface will be far less than those obtained with the transmission from the surface to the mine. However by using a narrow band code system with a low data rate as a miner response unit, the signal to noise ratio can be made sufficiently high to obtain reliable message transmission. These response units may be placed within refuge chambers with each being assigned a slightly different frequency from the others for identification purposes.

For a miner unable to reach a refuge chamber, a seismic response may be made in accordance with received instructions by hammering on the mine wall or other mine structure.

FIG. 7 is a plan view of a long wire antenna 72 of length L, illustrating an approximate area of coverage 74 and FIG. 8 is a side elevational view illustrating the area of coverage 76 penetrating into the earth. The coverage obtained with the system is dependent upon various factors such as earth conductivity, the length L of the wire antenna and the current coupled into it from the audio amplifier. In order to increase the coverage an arrangement such as illustrated in FIG. 9 may be utilized.

In FIG. 9 there is provided a coupling means 80 for coupling the output of the audio frequency power amplifier to a plurality of antennas 83 and 84. Alternatively, a plurality of audio frequency power amplifiers could be coupled to the microphone to drive respective antennas. Although not illustrated, the coupling means of FIG. 9 as well as other coupling means, to be described, could incorporate the multiple tap arrangement of FIG. 2.

Other predetermined antenna placements are illustrated in FIGS. 10 and 11, FIG. 10 showing a crossed pair, and FIG. 11 showing a radial array, by way of example.

FIG. 12 illustrates an alternative to the long wire antenna grounded at both ends, as previously described. Coupling means 86 couples the output of the audio frequency power amplifier to a large loop antenna 89 which may, in a practical embodiment, have sides which are many thousands of feet in length.

The simplicity of the system, and particularly the receiver means minimizes expense such that each miner may be provided with the receiver means. Fabrication may be made utilizing integrated circuits and the receiver means may be easily carried on the miner or may be built into the miner's helmet as illustrated in FIG. 13. The helmet 92 carries a miner's light 93 operated by a miner-carried battery 95. A receiving loop antenna may be positioned conveniently on the miner or, as illustrated by numeral 98, may be positioned around the inside of the helmet 92 and an output speaker or earphones 100 may be positioned on the brim 102 of the helmet 92. The remainder of the receiving means is contained in a package 104 also mounted on the brim 102. Obviously the small size of the package 104 permits its placement in any other desired position on his helmet 92 or elsewhere on his person.

For non-disaster situations, the present invention provides a voice communication link not only from the surface to the mine but additionally from the mine to the surface. For mine to surface types of communication a transmitter means such as illustrated in FIG. 2 may be placed within the mine with the antenna ends grounded in the mine, and a receiver such as illustrated in FIG. 3 may be utilized at the surface. Since power requirements need not be kept to a minimum at the surface, the receiving means may be operated at increased power levels to insure higher signal to noise ratios.

The transmitting antenna could also be of the type illustrated in FIG. 12 wherein the loop could be positioned around the walls of a tunnel. In general, the transmitter and receiver means are for spaced apart operation with at least one being for location within the earth.

The invention has been described with respect to mine rescue work or mine operations. The system finds application for other areas where through the earth voice communication is desired. One such application is the transportation area as illustrated in FIG. 14. A transmitter means 110 includes an audio frequency power amplifier 112 receiving signals from an input means 115 which may be a microphone for real time voice transmissions or a recorded message to be transmitted, via a long wire antenna 118 buried beside, or as illustrated in FIG. 14, buried beneath a roadbed 120. The transmitter means 110 includes the power supply 122, preamplifier 123, and the coupling means 125, as previously described.

A vehicle such as car 129 includes a receiving means 132 having a loop antenna 133 the receiving means being of the type described with respect to FIG. 3. The vehicle thus could receive information or warnings from such transmissions which could for example, upon approaching an intersection or road hazard, warn the driver of the approaching hazard and the action he should be prepared to take.

In the mine situation, the transmitter and receiver means were separated by a relatively long distance such that attenuation occurred as described in FIG. 4. Since in FIG. 14 the antenna 118 is relatively close to the receiving means 132 the design of the loop antenna 133 must be modified. FIG. 15 shows the designed output voltage versus frequency characteristic for the loop antenna 133. The antenna is designed such that the response is relatively flat in the vicinity of the self resonant frequency fr and the self resonant frequency is within the band of frequencies being transmitted. This may be accomplished by adding resistance to the loop antenna to lower its Q thereby flattening the curve. In addition to choice of winding pattern, number of turns (N) and area (A), the self resonant frequency fr may be shifted downward by the addition of capacitance to the loop antenna such as across the loop antenna terminals. Thus operation of the loop antenna 133 is within the range from point 135 to point 136. Since the field strength will be relatively constant for the band of frequencies transmitted, the relatively constant portion of the loop response curve will result in a substantially constant output RMS voltage as heretofore described.