Radio Communication System For Use In Confined Spaces And The Like

Abstract

A radio communication for use within confined spaces including at least one coaxial-type cable in which electric signals are propagated at a reduced attenuation, such signals being screened by the outer conductor of the cable, annular transverse gaps in the outer conductor forming interruptions used to facilitate passage of radiated electromagnetic waves and provide communication with mobile or possibly fixed radio transmitters and receivers not directly connected to the cable. Low insertion loss of the gaps is achieved by impedance matching elements being positioned thereon. A rigid low-loss dielectric material casing may house the impedance matching elements and encompass the transverse gap. The casing may be longitudinally split and formed of detachable halves to permit access thereinto so as to facilitate replacement and exchange of the impedance matching elements.

Parent Case Text

The application is a continuation-in-part of Ser. No. 109,367, filed Jan. 25, 1971, now abandoned for Wireless Telecommunication System for Use in Confined Spaces.

Description

FIELD OF THE INVENTION

The present invention relates to radio telecommunications in confined spaces, like in underground media such as tunnels, mine shafts, galleries, railroads, motorways, and the like.

A primary object of the invention is to provide a system for propagating radio waves designed for communication and remote control within a confined space so that radio contact between movable and possibly with fixed transmitters and receivers can be obtained over long distances at any point of the cross-section of the confined space, without the aid of amplifying repeaters.

Another object of the present invention is to provide radiating sources adapted to be connected to a coaxial cable in order to convert part of the energy propagating in the cable into radio waves propagating in the confined space and thereby to regenerate these waves when they are attenuated.

Still another object of the invention is to provide a rigid case made of low dielectric material in two parts which, when assembled, contains the radiating source and a cavity wherein is housed a plate which bears the elements of an impedance matching network and connections for connecting the said network to the conductors of the wave-carrying cable, the case having at least one removable partition which enables one to insert, inspect and replace the impedance matching network if necessary without having to release the two parts of the case.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the system possessing the features, properties of elements and the relation of components which are explained in the following detailed disclosure, and the scope of which will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWING

For a fuller understanding of the nature and objects of the invention, reference may now be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is the distribution of currents and the electrical field for the monofilar mode of propagation of radio waves when a wire or cable is suspended longitudinally in the center of a gallery according to the prior art;

FIG. 2 is the distribution obtaining when the wire or cable of FIG. 1 is positioned close to a wall;

FIG. 3 is the specific attenuation of waves propagated in the monofilar mode according to the prior art as a function of frequency and the distance from the cable to the walls;

FIG. 4 is a coaxial cable with a slot or gap formed according to the present invention;

FIG. 5 is the radiation pattern of the waves radially radiated from the slot of FIG. 4;

FIG. 6 is the circuit equivalent to the slot of FIG. 4;

FIG. 7 is a sectional view of a slotted coaxial cable according to the present invention arranged to reduce the reflecting factor and the insertion loss;

FIG. 8 is an improved impedance matching device;

FIGS. 9 and 10 show, respectively, a top view and a side view of the two portions of a cut coaxial cable inserted in a rigid case constructed according to the invention;

FIGS. 11 and 12, respectively, represent perspective views of the lid and the bottom of the case;

FIG. 13 shows a perspective view of a support adapted to be housed in the interior of the case;

FIG. 14 is a top view of the upper part of the support of FIG. 13;

FIG. 15 is a top view of the lower part of the support of FIG. 13;

FIGS. 16 and 17 are, respectively, side and front views of FIG. 13;

FIG. 18 is a radio communication system constructed according to the present invention;

FIG. 19 is the electric field of the monofilar mode on either side of a slot;

FIG. 20 is the interference of monofilar waves between two slots;

FIG. 21 is the monofilar wave formed by a directional coupler comprising two slots;

FIG. 22 shows monofilar waves along a line comprising two directional couplers;

FIG. 23 is the monofilar wave formed when the coaxial cable is close to a wall;

FIG. 24 is the radiation from a slot with the cable lying on the ground; and

FIG. 25 is the radiation from two slots spaced 30m apart, and with the cable lying on the ground.

DETAILED DESCRIPTION

The use of radio transmission equipment for communication and remote control in subterranean or confined locales such as mines, quarries, tunnels, rail- and motorways and the like, is of growing importance and has now come to be considered a necessity for reasons of increased productivity, safety and convenience. A conventional wire transmission mode and apparatus like the telephone lacks flexibility, since it requires the apparatus to be physically coupled to the cable, thereby generating reasons as to why attempts have been made to create radio communications.

However, the propagation of radio waves is seriously impeded in an underground medium like a mine gallery for example. Such a medium can be regarded as a form of hollow waveguide the cut-off frequency of which is of the order of several tens of megacycles per second. Lower frequency waves propagate through the ground as if there were no gallery at all and accordingly they are quickly damped. Higher frequency waves can be guided by the gallery space, but they are strongly perturbed by the various obstacles present therein, like men, machines, variations in the cross-section and orientation of the gallery, and the like.

In an attempt to overcome these limitations, it has been proposed to suspend a single isolated wire longitudinally within a confined space like a gallery, and it was discovered that the cut-off frequency effect has disappeared. This is easily explained by the fact that the gallery now resembles more a coaxial cable than a hollow waveguide. In most operating underground mines there are service lines like water pipes, electric cables and the like which produce a similar effect but this is more complex because the lines are numerous and often earthed in the most unorthodox fashion at their anchoring points. As a result only few of the available literature reports on this are useful to the researchers in the field.

A notable exception is, however, the report of Gabillard et al. (The London IEE Congress, November 1969). It presents a complete study of the specific attenuation of this type of propagation, which is called the "monofilar mode." These researchers found that the system acts as a kind of "bad" coaxial cable, with the electric current flowing along the suspended wire and then returning along the gallery walls, and with the major part of the losses occurring in the latter.

It becomes obvious that the larger the effective cross-section of this conductor, the smaller the resistance of this return conductor, since the current associated with the wave then travels along a much easier path. This obtains when the cable is suspended in the center of the cross-section of the gallery, for the currents use then the whole perimeter of the wall, as shown in FIG. 1, where reference numbers 1 and 2 refer to the walls and floor respectively of a gallery 3 and 4 is a single suspended cable wherein waves travel away from the observer at right angles to the plane of the figure, the arrow line 5 designating the lines of force of the electric field, and the circles 6 designating the current returning inside the rock 7 towards the observer. It may be noticed that lines of force 5 are distributed fairly evenly throughout the cross-section.

When the monofilar cable 4 is close to a wall of the gallery, however, as is clearly shown in FIG. 2, the current uses only a limited area of the return rock conductor; whereas the specific attenuation accordingly increases markedly. This becomes particularly apparent in the five curves of FIG. 3 which give the values of the specific attenuation of the monofilar mode .alpha. (in decibels/100 meters) as a function of the frequency f, and of the distance of the cable to the gallery wall.

Gabillard et al., moreover, were able to confirm that specific attenuation depends also upon the conductivity of the rocks in which the gallery had been bored, and it is also known that this varies with the nature and types of the rocks.

Furthermore, since the primary objective of the study was to supply communications to portable radio apparatuses situated at any point within the cross-section of the gallery, it was established that it is advantageous for the lines of force to be distributed fairly uniformly; whereby, when the cable is close to a wall, the lines of force crowd between the wall and the cable, and therefore the coupling loss, which is the ratio of the power radiated by a portable emitter to the power supplied to the monofilar mode, can be extremely high. This effect will be discussed hereinbelow with reference to the detailed description of the invention (FIG. 23).

In summation, the monofilar mode is difficult to assess in real galleries. However, if it is desired to supply communications to mobile apparatuses anywhere in the cross-section of the gallery, one is nevertheless compelled to use the monofilar mode and to accept losses in the walls. Therefore, this mode of propagation, used per se, only enables one to reach propagation ranges which are barely in the order of up to one kilometer.

Several field researchers and manufacturers have used coaxial cables (i.e., cables wherein an inner conductor is placed axially inside an outer conductor) the outer conductor of which comprises openings through which energy is permitted to escape. Thus, Martin (Mining Technol., 52, 7, 1970) uses loosely braided coaxial cables; while in the Munich and Brussels underground railways radio communications are achieved by using coaxial cables whose outer conductor is split longitudinally, in effect with a slot extending from one end of the conductor to the other; and finally, the Anderw Corp., a United States corporation (Telecommunications, 6, 44, 1972) has recently commercialized a cable of the type used in television distribution, having a helically corrugated outer conductor which has been planed to create a series of small holes. In every instance, the coupling loss, which is now defined as the ratio of the power radiated by a mobile transmitter to that penetrating inside the coaxial cable, is of the order of 75 to 105 decibels, a figure which is high compared with the maximum permissible attenuation of some 140 dB with respect to the best types of portable radio-sets. This handicap of the leaky coaxial cables is so great that such systems are only slightly better than the Monk and Winbigler line (IRE. Trans., PGVC-7, 21, 1956) by virtue of their smaller specific attenuation and their better resistance to atmospheric agents. It is also of note that the manufacture of these cables involves a specialized and frequently costly technology, and that the coupling, which is supposed to be controllable by varying the size of the openings of the outer conductor, is really determined in finality during the manufacturing stage and cannot be adjusted later on to suit particular requirements arising from peculiarities of the confined site to be supplied with communication and/or remote control means.

Hence, since the true mechanism whereby radio waves propagate in coaxial cables of the leaky type has not yet been satisfactorily elucidated, it can be stated that all practical embodiments so far experimented with are merely aleatory. It is therefore highly desirable to provide for a method or system for imprisoning the energy of the wave carried by the coaxial cable, while still permitting energy exchanges to take place between the interior of the cable and any point situated within the cross-section of the space defined by the confining medium.

It has been inventively ascertained that if a source of radiation is placed outside of a coaxial cable it will simultaneously generate two types of waves, namely, on the one hand, radiated waves which propagate radially away from the source, and on the other hand, waves which are guided along the outer surface of the cable. If the outer conductor of the coaxial cable, which is suspended longitudinally in the confined space so as to transport the waves emitted by the sources, is covered by a dielectric material, the guided waves are of the Goubau type; and if it now occurs that the effective radius of these Goubau waves is equal to or grater than the cross-section of the confining medium, by virtue of their frequency, these waves are actually of the monofilar mode described by Gabillard et al.

Both radiated and guided types of electric fields may be coupled to the aerials of mobile transmitter or receiver stations but are subject to all hazards of propagation in a confined space, whereas the propagation of the internal fields generated within coaxial cables avoids these hazards. The exchanges between the cable interior and exterior must be controlled in dependence upon the conditions of each specific case. Thus, there is a distinct need for a simple, sturdy, inexpensive and small-sized radiating device of a well-defined and controllable technical performance.

The inventive system consists in providing the coaxial cable (which is suspended longitudinally in the gallery) with small radiating devices whose function is:

a. to remove a small portion of the power of the wave transported by the coaxial mode without serious disturbing the propagation of said mode;

b. to convert into radiation, in the same manner an aerial does, a fraction of the removed power, said fraction being available for radio signals in case the coaxial cable should be placed near a wall;

c. to convert the remaining fraction of the removed power into monofilar mode usable when the coaxial cable is clear of walls, and

d. to carry out the above energy conversions in the reverse direction so as to enable mobile transmitters to excite the coaxial mode, said reversal being automatically achieved by virtue of the well known reciprocity principle of the electro-magnetic theory.

According to the inventive system, each radiating device is obtained by completely removing a narrow annular strip of the outer conductor of the coaxial cable, whereby the system constituted by the thusly exposed inner conductor and the neighboring ends of the severed outer conductor, also referred to as slot or gap, can act as source of radiation so as to fulfill the four hereinabove defined functions.

This sytem is depicted, partly in cross-section, in FIG. 4, wherein a coaxial cable 8, consisting of an inner conductor 9 held inside an outer conductor 10 which may be covered by a dielectric material (not shown), has a narrow transverse slot or gap 11 formed by completely removing an annular strip of outer conductor.

Theoretical studies and experiments have been conducted with a view to determining the effects of the gap 11 on a wave 12 travelling inside the cable (from left to right, for example), such a wave being referred to as the incident wave. The following effects were observed:

a. A fraction is reflected towards the left inside the cable; and a reflection factor was established.

b. a fraction is transmitted towards the right, inside the cable, past the gap. A transmission factor was established or, in more practical terms, an insertion loss which is the loss caused by the gap in the transmission within the coaxial cable.

c. a fraction is converted into two waves of equal amplitude, which are guided by the outer surface of the cable and travel away from each other in an opposite direction.

d. a fraction is radiated outside the cable, in all radial directions .theta. from the gap, in the same way as from an aerial. The spatial distribution of the radiated power is characterized by a directivity factor D which is a function of the angle .theta..

In general, the following conclusions can be drawn from these studies:

i. The thickness of the dielectric sheath is virtually without influence on the reflection factor and the insertion loss, that is to say, the parameters characterizing the propagation within the cable, but it determines the manner in which the energy passed by the aperture is divided into guided waves and radiated energy. A thick sheath favors the Goubau waves to the detriment of the radiated waves. It is well known that the concentration of the Goubau waves increases with dielectric permittivity, thickness, and with frequency, but at frequencies lower than 100 megacycles/sec. the effective radius of these Goubau waves is always larger than the distance of the cable to the gallery wall so that the thickness of the covering is immaterial. At higher frequencies, however, the concentration of the waves can be turned to account so as to reduce the effect of obstacles present in the gallery, and it is sufficient to make sure that they are placed where the fields are weak. Regarding the radially radiated waves, their pattern is as shown in FIG. 5. The angle .theta..sub.max with the axis 13 of the cable amounts to a few degrees and the directivity factor D.sub.max has a value of 5 to 10 dB. If the thickness of the dielectric sheath is increased, .theta..sub.max increases and D.sub.max decreases.

ii. The effect of frequency is rather less than was thought at first, and could be calculated exactly assuming that the slot is in the air (no gallery). At frequencies of the order of 30 megacycles/sec. the power of the incident wave is distributed as follows:

75 percent are reflected inside the cable

2 percent are transmitted inside the cable (insertion loss: 17 dB)

23 percent are transmitted outside the cable.

At 300 Mc/s, these figures are:

61.5 percent reflected towards the left

7.5 percent transmitted (insertion loss: 11.2 dB)

31 percent transmitted outside the cable.

These typical values demonstrate the small effect exerted by frequency. Frequencies of several Gc/s must be attained to reduce the insertion loss to 2 or 3 dB. The influence of frequency on the directivity factor is even smaller.

A simple gap in the external conductor of the cable thus provides an excellent radiating device as regards the utilization of power issuing from the gap, but on the other hand a device which is rather disappointing as regards transmission with the cable. The inventive studies have shown that, from the latter point of view, the gap is the equivalent of an impedance which is series-connected in the outer conductor and the high value of which explains why a large fraction of incident power is reflected. This circuit equivalent to slot 11 of FIG. 4 is represented in FIG. 6 and comprises a resistance 14 and a capacity 15 in parallel.

By adding adequate impedance-matching elements to the gap it is possible to design radiating devices by means of which the power of the incident wave can be utilized in a well-defined manner.

The simplest way of reducing the reflection factor and the insertion loss is to lower the slot impedance by connecting a capacitor between the two sides of the slot.

At frequencies higher than 100 Mc/sec., only a small capacity is required and this can be produced through the arrangement shown in FIG. 7. This shows a coaxial cable whose outer conductor 10 has been cut, thereby producing two ends 16 and 16'. The inner walls 17 of these ends are threaded and are mechanically connected to correspondingly threaded metal adaptor elements 18 and 18', the ends 19 and 19' of these elements each having a slightly different diameter, so that the concentric ends 19 and 19' overlap without touching, thereby creating the required small capacity.

At lower frequencies however, a conventional capacitor can be used.

This simple matching system can be further improved by compensating the residual capacitive effect by inserting a coil in series with the inner conductor as shown in network of FIG. 8, wherein the capacitor 20 is connected between the ends A and B of the slot 11 and a self induction coil 22 is inserted in the inner conductor 9 while 21 represents the impedance of the slot. It is seen that this network acts as a series resonant circuit which, however, is heavily damped by the characteristic impedance of the cable.

In a confined space like an underground gallery, the impedance 21 of the annular slot 11 of the wave-carrying coaxial cable depends on the frequency and the position of the cable, but at frequencies lower than 100 Mc/sec. resistive values of 400 to 1,000 Ohms are typical. The choice of the capacity of the condenser 20 (FIG. 8) varies with the lowering of impedance which is desired. Thus, if the value of the capacity is high the insertion loss is reduced but at the same time the power emerging from the slot is also reduced; the choice corresponds to a compromise between the two effects.

As indicated above, the impedance of the slot shunted by the condenser is capacitive and the residual capacity can be compensated for and the impedance matching improved by the insertion of a self-induction coil 22. The value to be chosen in order to achieve a correct compensation at the working frequency f is related to the capacity C.sup.1 by

L = 1/(2 .pi.f).sup.2 c.sup.1

where C.sup.1 is given by

WC.sup.1 = 1+W.sup.2 C.sup.2 R.sup.2 /WCR.sup.2

For example, if the working frequency is 30 Mc/s and the impedance of the slot is 400 Ohms, a condenser of 160 pF will reduce the impedance to

Z = R/1+j 2 .pi. f CR = 2.72 - j 33 Ohms

where j is the imaginary quantity.

The value of the self-induction, calculated from the above relation is L = 0,177 u H. As the coil removes the effect of the reactive 33 Ohms, the impedance matching is considerably improved. Of course, there would be no objection to inverting the coil and the condenser.

At frequencies lower than 300 Mc/sec., the width of the slot 11 may exceed somewhat the diameter of the cable without causing any substantial disturbance. In practice I choose a width of a few centimeters.

The above theoretical considerations on impedance matching of the slot are embodied in a practical design which have been found successful at frequencies lower than about 100 Mc/sec. Indeed in the course of industrialization of the method, I realized that the gaps in the outer conductor of the wave-carrying coaxial cable could advantageously be carried out by cutting the cable altogether and inserting the ends of the cut conductors into a rigid case made of a low loss dielectric material like nylon, the case acting as a junction-box wherein are housed the matching impedance elements inserted between the portions of the outer conductor and the portions of the inner conductor respectively.

The case is made in two hollow parts which, when assembled, ensures the fixing of the portions of the cable situated on either side of the slot produced by cutting the cable. The case comprises a cavity wherein is housed a plate which bears the elements of a matching impedance network and connections for connecting the network to the conductor of the wave-carrying cable. At least one of the two parts of the case comprises a removable partition which enables one to insert, inspect and replace the impedance matching device if necessary without having to release the two parts of the case.

The impedance matching network comprises a condenser and a self-induction coil arranged in series, the former with the outer conductor, the latter with the inner conductor.

This is illustrated, by way of example, in FIGS. 9 to 17 wherein the design is a rigid case 23 made of a low loss dielectric material for fixing the two portions of a coaxial cable 8 which has been completely severed, the cable comprises an inner conductor 9 sheathed with dielectric 9' and an outer conductor 10 sheathed with dielectric 10' (FIGS. 9 and 10).

The rigid case 23 comprises a bottom part 24 and a lid 25 provided with grooves 26 and 27 for receiving the two portions of the sheathed cable when the lid is screwed to the bottom by screws 28 and bolts 29 placed in holes 30 and cavities 31 designed to that effect. Transverse openings 32 made in the lower part of the bottom enable one to suspend the closed case along the general direction of the cable. The bottom comprises a rectangular cavity 33 capable of receiving a rectangular support 34, made of a material analogous to that of the case, provided at its lower part with a cavity 35 (FIGS. 15 and 17) for receiving the impedance matching network, and provided at its upper part with connecting elements for linking the network with the inner and outer conductors of the severed coaxial cable. After the impedance matching elements have been fixed, the cavity 35 may be filled, if desired, with a low loss dielectric liquid material which solidified afterwards.

The lid 25 comprises a cavity analogous to the cavity 32, however, that is not shown in the drawing.

The bottom and the lid, or either one of them, comprise a removable partition 36 (FIGS. 9-12) formed of a material analogous to that of the case to which it is secured by screws 37; whereby in view of this partition, the rectangular support 34 can be introduced, inspected and replaced as desired without the two parts of the case having to be loosened.

The top part of the support 34, which matches the lower cavity 35, is raised with relation to the lateral sides 38 (FIG. 13).

A metal plate 39 (FIGS. 13, 16, 17) is secured at either end between a bolt 40 and a screw 41 which traverses the support and the plate vertically upwards and passes beyond the bolt. The screw 41 also carries, at the lower part of the support, a plate 42 provided with an eye terminal 43 (FIG. 15), the two plates of a condenser 44 being soldered to the eye terminals. The raised portion of the support comprises two symmetrically arranged cavities 45 (FIGS. 13, 14, 17) capable of each receiving a metal block 46. Each block can be secured between a screw 47 and a bolt 48 which extends through the support from the bottom to the top. The bolt 48 carries in the cavity 35, which corresponds to the raised portion of the support, a plate 49 (FIG. 15) provided with an eye terminal 50, the ends of a self induction coil 51 being soldered to the eye terminals. Each block 46 also comprises a vertical slot 52 wherein one end of the inner conductor 9 can be secured by means of a screw 53 after the sheathing 9' of the inner conductor 9 has been removed; and ends of the inner conductor being thus properly connected through the coil 51.

Moreover, each portion of the severed cable can be held in position by means of a stainless steel stirrup-piece 54, after the outer conductor 10 has been stripped of its insulating sheathing 10'. The two flat portions 55 of the stirrup-piece having been provided with a hole 56 can be pressed (FIGS. 13-17) against the bolt 40 by screwing to the lower bolt 41 a hollow 57 which is provided with an inside thread; the two ends of the outer conductor being consequently properly connected to the condenser 44 (FIG. 15) and, in addition, the two fixed (inner and outer) conductors remain parallel to each other due to the difference in levels which exists between the edges 38 and the raised portion of the support 34.

Naturally it is possible to conceive many other networks for matching impedances, especially designed to achieve a wider band-width, comprising several coils and condensers interconnected to form a circuit having adequate electrical characteristics. These and the present circuit need no further elaboration since they are based on standard network theory.

The use of radiating devices of the type hereinbefore disclosed, spaced in more or less regular fashion along a coaxial cable which is arranged in a gallery or some other restricted path of transmission, allows the safeguarding of the electro-magnetic waves against propagation difficulties encountered in such surroundings. As shown in FIG. 18, a transmitter 58, situated near such a radiating device 23A, is capable of setting up, through the latter, two waves travelling inside the cable in opposite directions 59 and 60. Each identical radiating device 23 B, 23 C, etc., causes a small fraction 61 of these waves to escape. This fraction comprises two parts: on the one hand, the monofilar mode, i.e. two Goubau waves of equal amplitude which are guided at the outer surface of the cable and travel away from each other in an opposite direction and, on the other hand, the coaxial mode, i.e. waves which are radiated outside the cable in all radial directions from the slot of the radiating device. Both types of waves can be intercepted by receivers 62 situated within the working range of the device. If these working ranges overlap, a continuous radio communication is obtained all along the length of the cable. Obviously by virtue of the reciprocity principle already referred to, these radio links are reciprocal.

It will be understood that the above-mentioned transmitters and receivers are by no means coupled to the cable.

For the sake of convenience, some of these transmitters and/or receivers may be physically connected by appropriate means to the inner conductor, so as to constitute therefore fixed stations of the communication system.

The system with a fixed receiver can be used to radio control machines like winches, monorails, etc., by means of a portable transmitter. Conversely, mobile machines carrying a receiver can be radio controlled by fixed transmitters.

It is obvious that the above radio communication system is a direct application of the hereinabove mentioned theoretical studies and experiments, and in particular of the observed mentioned (c) and (d) effects.

In order to assess the efficiency of the proposed radio communication system it is convenient to refer to the practical results obtained in an experimental tunnel of about 1 mile in length, dug in tuff, at Lanaye, Belgium. Since this tunnel is free from pit props and other obstacles it enabled one to determine the relative usefulness of both the monofilar and coaxial mode, respectively, depending upon the position of the cable and the slots. It was discovered that the monofilar mode is usable even if the cable is suspended only 10 cm away from the wall.

The graph of FIGS. 19 to 25 represent the detected electric field of the monofilar mode a meter away from the cable when the coaxial mode is injected inside the cable with a power of 30 mW at the frequency of 30 Mc/s. In the present series of experiments the power was injected by galvanically connecting an adequate transmitter at one end of the cable, the coaxial cable being terminated by its characteristic impedance, and by holding at a distance of one meter from the cable all along the gallery, the tip of the aerial of a field meter instrument, the latter instrument consisting of a sensitive voltmeter provided with an aerial 30 cm long. The error with respect to the calculated voltage, does not exceed 2 to 3 dB. Of course, by virtue of the reciprocity principle, the position of the apparatuses can be interchanged, i.e. the field meter be connected to the cable and an adequate portable transmitter moved along the cable, whereby the same results are obtained. The arrangement is depicted schematically in the upper part of the figure. wherein C is the coaxial cable with slots S according to the invention, E is the transmitter fixed at the beginning of the cable and I the characteristic impedance at the end of the cable, the distances of cable being measured in meters from E; the detected voltage of the monofilar mode, plotted in the ordinate (in microvolts) against distance in the abscissa (in meters) is represented in the lower part of the figure.

In FIG. 19 a cable 260 m long was suspended longitudinally in the experimental tunnel one meter away from a wall; it had a single inventive slot S situated a hundred meters away from the transmitter E. The graph shows the voltage did not fall below 4,000 uV at any point on either side of the slot; at about 2 meters from the slot, it reached a peak of about 11,000 uV; at the slot itself it dropped slightly under 3,000 uV; beyond the slot it rose again above 10,000 uV and dropped gently over the last 40 meters to reach the value of 4,000 uV.

FIG. 20 illustrates what occurs between two slots 100m apart from each other, the first being situated 100 m from the transmitter and the second 60 m from the end of the cable. The slots excite the monofilar mode with equal amplitudes in both directions thereby producing appreciable standing waves, and radio link may be broken.

In order to prevent the establishment of this undesirable effect, the slots should excite the monfilar mode in one direction only. This can easily be achieved by replacing each single slot by a directional coupler D, consisting of a pair of identical slots S a quarter wavelength apart. The waves excited by one directional coupler are shown in FIG. 21.

FIG. 22 shows how these D couplers placed every 100 m regenerate the monofilar mode. It is clear from the high values of the recorded electric field that in this case the two couplers could have been placed further away from each other; these values mean that the attenuation of the monofilar mode was particularly small due to the fact that the cable C was suspended at a fair distance away from the wall.

It was seen from the study by Gabillard et al (FIG. 3) that the coupling loss in the monofilar mode is extremely high if the cable is close to a wall.

The earlier experiment was repeated in the present cycle of studies using a coaxial cable C provided with two directional couplers D 100 meters apart, according to the present invention, the cable being suspended close to a wall W of the tunnel, as shown in FIG. 23, wherein the series of numbers along the spikeline W define the distances of the cable from the wall, in cm. It is evident that attenuation of the monofilar mode can become very important. It is ascertainable that the correlation between the values of the field and distances from the wall is due to deformation of lines of field, as was explained with reference to FIGS. 2 and 3.

At the limit, when the cable is resting on the floor, a wall or hanging close to the roof of the tunnel, the monofilar mode cannot be used and one can only rely on the direct radiation of the radiating slot devices with which the cable is provided. Now, since the radiation from a slot does not propagate very far it is necessary to place these device fairly close to each other.

This is illustrated in the case of a coaxial cable lying on the ground and having one or two slots as shown in FIGS. 24 and 25 respectively which demonstrate that a radio link can be assured if there is a radiating device every 20 or 30 m. It is always advantageous to place the cable free from obstacles, at least when this is possible.

The hereinabove described radio communication system offers great advantages over those of the prior art. The coupling loss which is defined here as the ratio of the power radiated by a mobile transmitter to the fraction of that power which actually enters the coaxial cable is commonly as low as 25 dB, compared to the 75 to 105 dB reported for other coaxial systems. Moreover the coupling loss is dependent mainly upon the distribution of lines of force in the confined space and not on the frequency so that one can use low working frequencies at which the specific attenuations of the monofilar and coaxial modes are low.

Radio communications have been made in this fashion between walkies-talkies situated both at arbitrary locations of the cross-section of mine galleries at distance of up to 5 miles.

An additional advantage of the system is that the working range can be calculated very accurately because the total attenuation of the radio path is the sum of well known quantities, namely coupling losses and attenuations in the coaxial and monofilar modes.

While there has been shown what is considered to be the preferred embodiment of the invention, it will be obvious that modifications may be made which come within the scope of the disclosure of the specification.

Claims

What is claimed is:

1. A radio communication system for use in confined spaces and the like, comprising at least one carrier cable having at least two conductors; a first one of said conductors forming an outer conductor coaxially encompassing said other conductor and being longitudinally coextensive therewith, said outer conductor having at least one annular transverse gap formed therein and extending entirely therethrough so as to segment said outer conductor and provide short interruptions for the passage of radiated electromagnetic waves, said cable providing an aerial exhibiting a high directivity factor extending in directions proximate to the cable axis; and at least one transmitter and receiver being only indirectly coupled to said carrier cable.

2. A system as claimed in claim 1, comprising a plurality of transverse gaps being formed in said outer conductor at predetermined locations along the longitudinal length thereof.

3. A system as claimed in claim 1, comprising a plurality of said transmitters and receivers being indirectly coupled to said carrier cable.

4. A system as claimed in claim 1, said transmitter and receiver being mobile and movable relative to said carrier cable.

5. A system as claimed in claim 1, said transmitter and receiver being stationary and in predetermined fixed position relative to said carrier cable.

6. A system as claimed in claim 1, said carrier cable comprising impedance matching means extending across and bridging said transverse gap in said outer conductor so as to reduce reflection factors and insertion losses formed by said interruptions.

7. A system as claimed in claim 6, said impedance matching means comprising a capacitor connected to said outer conductor at each end of said transverse gap.

8. A system as claimed in claim 7, comprising electrical self-induction coil means connected in series with the inner conductor of said carrier cable in axial alignment with said transverse gap in said outer conductor so as to compensate for residual capacitive effects and forming an impedance-matching series resonant circuits.

9. A system as claimed in claim 6, comprising a hollow casing housing said impedance matching means, said casing being constituted of a low-loss dielectric material permitting passage therethrough of said radiated electromagnetic waves, said casing forming a junction box for said coaxial cable.

10. A system as claimed in claim 9, said casing comprising terminal means proximate the opposite ends thereof, said carrier cable being cut through and connected to said terminals so as to have the space between said terminals define said transverse gap, said severed outer conductor being connected to respectively two of said terminals; capacitor means extending between and connecting said terminals, said severed inner conductor being connected to respectively two further of said terminals; and self-induction coil means interconnecting said last-mentioned terminals.

11. A system as claimed in claim 9, said casing comprising longitudinally-split separable casing portions; and means for detachably assembling and fastening said casing portions so as to facilitate access to the interior thereof for interchanging said impedance matching means in conformance with the operative requirements of said system.

12. A system as claimed in claim 9, said casing being formed of a rigid plastic material.

13. A system as claimed in claim 9, said casing comprising an impedance matching network.