US3829767A - Radio communication system for use in confined spaces and the like - Google Patents

Radio communication system for use in confined spaces and the like Download PDF

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US3829767A
US3829767A US00374654A US37465473A US3829767A US 3829767 A US3829767 A US 3829767A US 00374654 A US00374654 A US 00374654A US 37465473 A US37465473 A US 37465473A US 3829767 A US3829767 A US 3829767A
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cable
outer conductor
casing
impedance matching
terminals
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P Delogne
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B5/00—Near-field transmission systems, e.g. inductive or capacitive transmission systems
    • H04B5/20—Near-field transmission systems, e.g. inductive or capacitive transmission systems characterised by the transmission technique; characterised by the transmission medium
    • H04B5/24—Inductive coupling
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/04—Adaptation for subterranean or subaqueous use
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
    • H01Q13/20—Non-resonant leaky-waveguide or transmission-line antennas; Equivalent structures causing radiation along the transmission path of a guided wave
    • H01Q13/203—Leaky coaxial lines
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H7/00—Multiple-port networks comprising only passive electrical elements as network components
    • H03H7/38—Impedance-matching networks
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B5/00—Near-field transmission systems, e.g. inductive or capacitive transmission systems
    • H04B5/20—Near-field transmission systems, e.g. inductive or capacitive transmission systems characterised by the transmission technique; characterised by the transmission medium
    • H04B5/28—Near-field transmission systems, e.g. inductive or capacitive transmission systems characterised by the transmission technique; characterised by the transmission medium using the near field of leaky cables, e.g. of leaky coaxial cables

Definitions

  • 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.
  • the present invention relates to radio telecommunications in confined spaces, like in underground media such as tunnels, mine shafts, galleries, railroads, motor ways, 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.
  • 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.
  • 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 ya slot with the cable lying on the ground.
  • FIG. 25 is the radiation from two slots spaced m apart, and with the cable lying on the ground.
  • an underground medium like a mine gallery for example.
  • a mine gallery 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.
  • 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 meter.
  • 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.
  • Martin Mining Technol., 52, 7, 1970
  • 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;
  • 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.
  • 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 to decibels, a figure which is high compared with the maximum permissible attenuation of some 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.
  • 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.
  • the inventive system consists in providing the coaxial cable (which is suspended longitudinally in the gallery) with small radiating devices whose function is:
  • 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.
  • 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.
  • a fraction is radiated outside the cable, in all radial directions 6 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 6.
  • 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.
  • 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.
  • their pattern is as shown in FIG. 5.
  • the angle 6 with the axis 13 of the cable amounts to a few degrees and the directivity factor D has a value of 5 to 10 dB. If the thickness of the dielectric sheath is increased, 0 increases and D decreases.
  • 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 seriesconnected 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.
  • 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.
  • FIG. 7 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 l8 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.
  • 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.
  • the impedance 21 of the annular slot 11 of the wavecarrying 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 poweremerging from the slot is also reduced; the choice corresponds to acompromise between the two effects.
  • 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 by i
  • a condenser of 160 pF will reduce the impedance to where j is the imaginary quantity.
  • the value of the self-induction, calculated from th 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.
  • 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 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.
  • 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 sheathed with di-' electric 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.
  • 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.
  • 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, l6, 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, I4, 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.
  • 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.
  • each portion of the severed cable can be held in position by means of a stainless steel stirruppiece 54, after the outer conductor 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. 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.
  • 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.
  • 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.
  • 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.
  • mobile machines carrying a receiver can be radio controlled by fixed transmitters.
  • 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.
  • 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.
  • 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 l0,000 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 m 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • 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.
  • 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.
  • a system as claimed in claim 1, comprising a plurality of said transmitters and receivers being indirectly coupled to said carrier cable.
  • 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.
  • said impedance matching means comprising a capacitor connected to said outer conductor at each end of said transverse gap.
  • 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.
  • 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.
  • 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.
  • 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.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Waveguide Aerials (AREA)
  • Communication Cables (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Details Of Aerials (AREA)
  • Near-Field Transmission Systems (AREA)
  • Details Of Connecting Devices For Male And Female Coupling (AREA)
US00374654A 1970-02-18 1973-06-28 Radio communication system for use in confined spaces and the like Expired - Lifetime US3829767A (en)

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BE85381 1970-02-18
BE120619 1972-08-03

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US (1) US3829767A (OSRAM)
JP (1) JPS49132911A (OSRAM)
BE (1) BE787158R (OSRAM)
DE (1) DE2337949C3 (OSRAM)
FR (1) FR2195126B2 (OSRAM)
GB (1) GB1434975A (OSRAM)
LU (1) LU68137A1 (OSRAM)
NL (1) NL173337C (OSRAM)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4001774A (en) * 1975-01-08 1977-01-04 Exxon Production Research Company Method of transmitting signals from a drill bit to the surface
US4023136A (en) * 1975-06-09 1977-05-10 Sperry Rand Corporation Borehole telemetry system
US4476574A (en) * 1980-09-17 1984-10-09 The United States Of America As Represented By The United States Department Of Energy Radio frequency communication system utilizing radiating transmission lines
EP0272785A1 (en) * 1986-11-06 1988-06-29 Senstar Corporation Intrusion detection system
US4887069A (en) * 1986-11-06 1989-12-12 Control Data Canada Limited Perimeter intrusion detection system with block ranging capabilities
US20100148775A1 (en) * 2008-12-17 2010-06-17 Helmut Greim Standing wave barrier for a magnetic resonance tomography device

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2082842A5 (OSRAM) * 1970-03-27 1971-12-10 Boulore Christian

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4001774A (en) * 1975-01-08 1977-01-04 Exxon Production Research Company Method of transmitting signals from a drill bit to the surface
US4023136A (en) * 1975-06-09 1977-05-10 Sperry Rand Corporation Borehole telemetry system
US4476574A (en) * 1980-09-17 1984-10-09 The United States Of America As Represented By The United States Department Of Energy Radio frequency communication system utilizing radiating transmission lines
EP0272785A1 (en) * 1986-11-06 1988-06-29 Senstar Corporation Intrusion detection system
US4879544A (en) * 1986-11-06 1989-11-07 Control Data Canada Limited Intrusion detection system
US4887069A (en) * 1986-11-06 1989-12-12 Control Data Canada Limited Perimeter intrusion detection system with block ranging capabilities
US20100148775A1 (en) * 2008-12-17 2010-06-17 Helmut Greim Standing wave barrier for a magnetic resonance tomography device
US8207737B2 (en) * 2008-12-17 2012-06-26 Siemens Aktiengesellschaft Standing wave barrier for a magnetic resonance tomography device

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JPS49132911A (OSRAM) 1974-12-20
GB1434975A (en) 1976-05-12
FR2195126B2 (OSRAM) 1977-05-13
DE2337949C3 (de) 1978-11-30
FR2195126A2 (OSRAM) 1974-03-01
NL173337B (nl) 1983-08-01
DE2337949B2 (de) 1974-08-08
DE2337949A1 (de) 1974-02-21
NL7310334A (OSRAM) 1974-02-05
BE787158R (fr) 1972-12-01
NL173337C (nl) 1984-01-02
LU68137A1 (OSRAM) 1973-10-16

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