Description: FIELD OF THE INVENTION

This invention relates to improvements in communication systems of inductive-carrier type and, more particularly, this invention relates to communication systems of inductive-carrier type in which a plurality of radio-frequency carrier signals having various modes of modulation to accomplish a number of discrete functions are impressed on a cable of special design or other suitable conducting media extending in proximity to highways, railroad right of ways or other delineated areas in which one-way or two-way communication services are to be established.

BACKGROUND OF THE INVENTION

This invention has particular applicability in the field of highway or other roadway communications and in providing a restricted-range broadcast service in small communities where conventional broadcast transmitters cannot be used because of lack of availability of AM broadcast channels in the standard broadcast band, now almost fully occupied in many sections of the United States.

Many systems of the inductive-carrier type, including those of the applicant, have been employed in the past for highway, railroad and other uses. However, these have presented serious technical problems when operated at relatively-high carrier frequencies, such as those in the AM broadcast band. Radiation of electrical wave energy, which is an inherent characteristic of inductive-carrier systems when operated at radio frequencies, often extends over distances far in excess of the permissible limit specified by the Federal Communications Commission for low-power radio devices of restricted range type. While it has been possible, by careful adjustment of the radio frequency (r.f.) carrier level to comply with the Commission's rules in certain localized applications, such as the highway radio system installed by the applicant on the George Washington Bridge in 1940, experience in most cases has demonstrated that it is extremely difficult, and in some instances impossible, to comply with the FCC rules over any substantial period when unattended transmitters are employed and, at the same time, to maintain a sufficiently strong induction field at broadcast frequencies to enable good reception in radio-equipped cars traveling over lengths of highway served by the system.

Experience with roadside conductors of various types, including single and dual-conductor transmission lines has indicated that the strength of the induction field about these conductors is subject to substantial variation along their length. Near the transmitter source, for example, the field strength may be too high to comply with FCC rules at broadcast frequencies if a useful, noise-free signal is to be provided in cars on all lanes of the highway served by the system. In addition, if the cable is ground-laid or is in the surface of the right-of-way, as required on turnpikes and thruways where above-surface installations are not desired, variations in the inductive-signaling field due to changes in soil conductivity under different weather conditions and other irregularities in environmental conditions have been found to present difficulties over a substantial period of time in maintaining a reasonably-constant field strength and restriction of radiation within limits set by the FCC.

Moreover, experience with conventional forms of cables, or wires, when employed along the roadside as r.f. signal conductors for the purpose of producing an induction-signaling field as a means of impressing carrier-signal energy on the vertical whip antenna system of radio broadcast receivers carried by motor vehicles indicates that the coupling loss between the vertically disposed vehicle antenna and the horizontally-polarized signals from the roadside cable system, whether in the form of a single longitudinally-extending transmission line or in horizontal loop configuration, encompassing the roadway area, is unnecessarily high. This results in requirement of substantially more r.f. power in the roadside cable system than would be required if a vertically-polarized or convolutive field, having vertical and horizontal polarization characteristics, were provided. The present system incorporates as an important element what are believed to be unusual and novel means for developing such a convolutive field to produce a signal of maximum strength in receiving systems of motor vehicles carrying conventional antennas of vertical whip type. This, in turn assists in meeting the requirements of the FCC with respect to restricted-range radio devices.

An additional, and serious problem, is presented in applying inductive-carrier methods at AM broadcast frequencies in the vicinity of large metropolitan areas, such as New York City and environs, where the AM broadcast band is fully occupied. This is of primary importance insofar as applications of inductive-carrier methods in the field of highway communications is concerned since one of the most valuable functions in these urban areas is in providing information to drivers on such matters as traffic congestion, hazardous or unusual road conditions on the route ahead, routing instructions and other intelligence that will assist motorists on major, and often overcrowded, traffic arteries in the vicinity of large cities.

To illustrate the latter problem and to indicate the nature of the difficulty that is involved, it is pointed out that in the New York City area the lower frequencies in the AM broadcast band, where inductive-carrier systems at broadcast frequencies may most effectively be applied in highway communication services, are fully occupied. For example, 540 kilocycles, a preferred frequency for operation of inductive-carrier systems in areas where this channel is available, is used by a suburban station, employing a 250-watt transmitter in daytime service. The next channel that can be employed for conventional broadcast service in the New York City area under the Commission's allocation plan is 570 kilocycles, occupied by a 50-kilowatt metropolitan-class station. Signals from both stations can be heard throughout the area. If conventional AM broadcast equipment were to be used for the highway service on the frequency of 555 kilocycles, midway between the 540 KC and 570 KC channels assigned to local stations, mutual interference would be produced, assuming that as in standard broadcast operation modulation sidebands would extend to 10 kilocycles above and below the carrier frequency, since sideband areas would overlap. An additional communications problem is presented on parkways, turnpikes and new interstate highways with respect to hazards presented by disabled cars and inability of drivers to quickly summon aid, since conventional wayside telephones often are widely spaced and not locally available. Also, many turnpikes have no wayside telephone circuits to permit installation of telephones at reasonably spaced intervals, within easy walking distance from disabled cars.

Practicable solutions to the problems as set forth above are incorporated in the present invention. These solutions also produce a substantial improvement in the quality and intelligibility of received signals as reproduced by typical AM broadcast receivers now in general use in the majority of motor vehicles; relative uniformity and stability of operation of unattended roadside transmitters is provided; minimization of radiation of wave energy to areas remote from the roadway is attained while maximum intensity and uniformity of the induction field may be maintained over long distances on a common carrier frequency; unwanted transfer of signal energy to roadside electric-power or telephone lines, with the interference potential that such coupling may produce, is minimized; heterodyne beats between adjacent roadside transmitting zones is avoided; and in preferred emvodiments of the invention relaying of signals to vehicles traveling throughout the length of a highway is accomplished without demodulation and remodulation of carrier signals, thus greatly simplifying equipment, minimizing distortion and eliminating over-modulation difficulties that otherwise would exist at remote, unattended highway transmitting points along the roadway system. By use of self-powered carrier telephones that may be located at half-mile intervals along the roadside cable and coupled thereto, together with use of multiple carriers, a distresscalling system of value to motorists is provided. These and other improvements presented by the system of the invention are described in subsequent pages.

OBJECTS OF THE INVENTION

It is, therefore, an objective of the present invention to provide an inductive-carrier communication system of a type that will provide a received signal of maximum strength and uniformity that is applicable to highway, railroad and other restricted-range communication services where it is desired to effect communication without physical contact with conductors extending throughout the length of the system from a terminal point or between terminal points where signals originate.

It is an additional object of the present invention to provide an inductive-carrier communication system in which maximum inductive-signaling field is developed by the cable system of the invention with minimum radiation of electrical wave energy at points removed from the area in which localized inductive-carried communications is to be established.

It is a further object of the present invention to provide an inductive-carrier communication system that can be adapted readily to highway, railroad, airport and other communication services by use of new and improved cable structures that incorporate coaxial trunk circuits and inductive-signaling conductors within a common protective jacket, said cable structure being such that it may be buried in roadway surfaces of any type or configuration and is relatively insensitive to the conduction charactertistics of the medium in which or on which the cable may be installed.

It is another object of the present invention to provide a new and unique cable structure for roadway communication services of inductive-carrier type that will provide a signal of maximum intensity in radio receiving equipment carried by vehicles employing conventional forms of vertical "whip" antennas by providing an induction field having a vertical polarization characteristic as contrasted with the horizontal polarization produced by conventional transmission lines extending in a horizontal direction along roadways or horizontal loops encompassing the roadway area that have been disclosed or employed in the prior art.

It is an additional object of the present invention to provide an inductive-carrier communication system that will provide a useful signal of maximum strength and uniformity along the length of the zone or zones served by the system with minimum inductive transfer of signal energy to power or telephone lines that may extend in proximity to and along the zone or zones within which inductive communication is desired.

It is a further object of the present invention to provide a new and improved coaxial cable structure incorporating trunk coaxial feed circuits and inductive signaling conductors that may be installed readily above the ground, on the surface or underground with minimum attentuation of the induction field with respect to the location of the cable or the characteristics of the medium on which or within which the cable may be located.

It is an additional object of the present invention to provide an inductive-carrier communication system in which modulation methods are such that relay of signals over long distances, as along a highway or railroad, may be accomplished on a common carrier frequency, with relay repeaters or translators of such design that demodulation and remodulation processes are not required at repeater or relay points where trunk carrier signals of relatively low frequency are converted to an R.F. carrier at a frequency common to the entire system and applied at intervals along a trunk circuit of coaxial type to supplementary inductive-signaling conductors, each of which provides a useful inductive communication zone, each zone serving an individual length of highway, railroad or other facility and in contiguous sequential relationship to adjacent zones.

It is a further object of the present invention to provide an inductive-carrier system that will serve a multiplicity of functions, including control and monitoring of individual roadside transmitter units in order to check on operation and quality of signals at a remote central control point; remote control of wayside signs and signals, with monitor check-backs at the central control points on a fail-safe basis; data transmission by multiple sub-carriers on the trunk portion of the cable provided by the system; two-way point-to-point and mobile communication services via the cable system; distress calling, location-identifying and communication facilities for use by occupants of disabled vehicles and other communication and signaling facilities useful on highways and on railroads.

It is an additional object of the present invention to provide a coaxial trunk and inductive-signaling cable structure and associated supporting and/or protective means enabling the cable to be installed in the beds of new highway or railroad construction or on existing roadways in such manner as to withstand without damage the pressures or temperatures that are involved in construction and maintenance procedures.

It is another object of the present invention to provide a coaxial cable system and supporting and/or protective structure therefore that will enable the installation of inductive-signaling and intercity or other multi-channel communication facilities of sub-surface type to be installed in or along highway or railroad rights-of-way in such manner that cable may readily be installed and thereafter be protected against damage.

DESCRIPTION OF THE DRAWINGS

Other objects of the present invention will be readily apparent from the following description and drawings in which:

FIG. 1 is a diagrammatic view of one embodiment of the inductive-carrier communication system of the present invention;

FIG. 2 is a schematic view of one form of signal attenuating and line-coupling means that may be used in the inductive-carrier communication system of the present invention;

FIG. 3 is a schematic view of another form of a signal attenuating and line-coupling means that may be used in the inductive-carrier communication system of the present invention;

FIG. 4 is a schematic view of an inductive-signaling line termination unit that may be used in the inductive-carrier communication system of the present invention;

FIG. 5 is a perspective view of one embodiment of the cable structure of the present invention;

FIG. 6 is a perspective view of another embodiment of the cable structure of the present invention;

FIG. 7 is a perspective view of yet another embodiment of the cable structure of the present invention;

FIG. 8 is a perspective view of still another embodiment of the cable structure of the present invention;

FIG 9 is a perspective view of a further embodiment of the cable structure of the present invention;

FIG. 10 is a schematic view of an inductive-carrier communication system of the present invention utilizing the cable structure shown in FIG. 5;

FIG. 11 is a partially perspective, partially schematic view of an inductive-carrier communication system of the present invention utilizing an induction signaling cable separate from the trunk coaxial cable;

FIG. 12 is an enlarged perspective view of the embodiment of the cable structure of the present invention shown in FIG. 8;

FIG. 13 is a partially sectional perspective view of a portion of a two-direction highway showing a combined coaxial trunk and inductive signaling cable buried in the dividing strip thereof;

FIG. 14 is a partially sectional perspective view of a portion of a two-direction highway showing the coaxial trunk cable buried in the dividing strip thereof and the inductive signaling conductors buried along the outer edges of the roadway surface;

FIG. 14A is a partially sectional view showing a preferred manner of burial of the inductive signaling conductors of FIG. 14;

FIG. 15 is a partially sectional, perspective view of a portion of a two-direction highway showing the coaxial trunk cable buried in the dividing strip thereof and the inductive signaling conductors buried along the inner edges of the roadway surface;

FIG. 15A is a partially sectional view showing a preferred manner of burial of the inductive signaling conductors of FIG. 15;

FIG. 16 is a partially sectional perspective view of a portion of a two-direction highway showing a combined coaxial trunk and inductive-signaling cable buried in the center of each of the roadways of the highway;

FIG. 16A is a partially sectional view showing a preferred manner of burial of the cable of FIG. 16;

FIG. 17 is a partially sectional perspective view of a preferred form of structure for protecting buried cables used in the inductive-carrier communication system of the present invention;

FIG. 17A is an enlarged partially sectional perspective view of the structure of FIG. 17;

FIG. 18 is a diagrammatic view of another embodiment of the inductive-carrier communication system of the present invention;

FIG. 19 is a schematic view of one form of loop configuration that may be used in the embodiment of the present invention shown in FIG. 18;

FIG. 19A is a modification of the loop configuration of FIG. 19;

FIG. 20 is a diagrammatic view of an inductive-carrier communciation system according to the present invention in which there is included signal relaying means for relaying signals over long highways;

FIG. 21 is a plot of relative field strength versus distance along the cable shown in FIG. 20;

FIG. 22 is a diagrammatic view of an inductive-carrier communication system according to the present invention in which there is included a preferred form of signal relaying means for relaying signals from a central point;

FIG. 23 is a diagrammatic view of an alternate form of signal relaying means that may be used in the system of FIG. 22;

FIG. 24 is a diagrammatic view of an inductive-carrier communication system according to the present invention in which there is included signal relaying means employing frequency or phase modulation methods;

FIG. 24A is a plot of the pre-emphasis characteristic curve of the pre-emphasis network of FIG. 24;

FIG. 24B is a plot of power loss versus frequency at the loudspeaker circuit of a typical motor vehicle AM broadcast receiver;

FIG. 24C is a modified form of line-coupling attenuator unit that may be used with the system of FIG. 24;

FIG. 25 is a diagrammatic view of a roadway communication system of the type shown in FIG. 20, in which automatic visual indicating means are provided to show the operative or inoperative conditions of roadside transmitting and relay equipment;

FIG. 26 is a diagrammatic view of an inductive-carrier communication system according to the present invention in which means are included for automatically and continuously monitoring the program characteristics of the entire system;

FIG. 26A is a diagrammatic view of a modified form of transmitter that may be used in the system of FIG. 26;

FIG. 27 is a diagrammatic view of another embodiment of the inductive-carrier communication system according to the present invention;

FIG. 27A is a diagrammatic view of remote control sign means that may be used in the system of FIG. 27;

FIG. 27B is a diagrammatic view of the sign of FIG. 27A showing change in message as provided by the system of FIG. 27;

FIG. 28 is a diagrammatic view of a roadside carrier system for distress signaling and communication purposes, utilizing the coaxial trunk cable shown in previous illustrations;

FIG. 29 is a diagrammatic view of a roadside carrier telephone which may be used in the present invention;

FIG. 29A is a detailed view of the telephone of FIG. 29;

FIG. 30 is a diagrammatic view of another embodiment of the present invention;

FIG. 30A is a diagrammatic view of a telephone equipment which may be used in the present invention; and

FIG. 31 is a diagrammatic view of another embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Description of FIG. 1

An illustrative application of one form of the invention is shown in FIG. 1 in which a carrier transmitter 10, in this case operating at a broadcast frequency of 540 kilocycles, is connected by coaxial cable 11 to a roadside coaxial cable 12-12A extending parallel to and intermediate traffic lanes 13A and 13B carrying vehicle traffic in opposite directions. In coaxial cable 12-12A, the center conductor is denoted by 12 and the ground sheath conductor is denoted by 12A. At intervals along coaxial cable 12-12A, preferably installed below the surface of the roadway or the adjoining area thereof, a controlled amount of radio frequency (r.f.) carrier energy is applied by means of coaxial branch connections 15, 16 and 17 and adjustable coupling and attenuating means 18, 19 and 20 to longitudinally-extending conductors 24, 24A, 26 and 27, respectively, which serve as the inductive-signaling elements of the system.

As will be described hereinafter, these inductive-signaling conductors may be incorporated as an inherent part of the roadside coaxial cable 12-12A and contained within the same cable structure or jacket 25, or the inductive-signaling elements may otherwise be associated with the coaxial cable 12-12A in fixed circuit and spacial relationship. The ends of inductive signaling elements 24, 24A, 26 and 27 are connected through termination units 28, 29, 30 and 31 respectively to the common metallic ground circuit provided by the sheath 12 of coaxial cable 12-12A. Inasmuch as the inductive-signaling elements 24, 24A, 26 and 27 have a fixed and uniform impedance relationship with respect to the common ground sheath 12 of the coaxial cable, the inductive transmission line formed by each of these elements and ground sheath 12 can be terminated readily in such manner as to match the characteristic impedance of each line section at the broadcast carrier frequency employed throughout the length of roadway system.

As illustrated in FIG. 1, inductive signaling elements 24, 24A, 26 and 27 are disposed along the coaxial cable 12-12A in contiguously sequential manner to provide a continuous and substantially uniform induction field at a common carrier frequency in order that signals as received in radio-equipped vehicles traveling throughout the length of the roadway served by the system will be uninterrupted and of substantially-constant strength as the vehicles pass through the individual signaling zones created by the inductive fields from the conductors 24, 24A, 26 and 27. A vehicle traveling from west to east on traffic lane 13B would, for example, hear the transmitted signals on 540 KC first from inductive-signaling conductor 26, then from conductors 24, 24A and 27 in sequence without material change in received signal level or break in reception. Objectional change in strength of the induction field extending throughout the length of roadway shown in the illustration is prevented by minimizing any reflection from the terminal units 30, 28, 29 and 31. Such reflection otherwise would result in standing waves along the conductors 26, 24, 24A and 27, causing variation in the field and undesired radiation of wave energy over distances in excess of limits designated by the Federal Communications Commission for unlicensed low-power radio devices.

An important advantage of the arrangement as shown in FIG. 1 is that a substantial amount of carrier energy may be impressed on coaxial cable 12-12A in order to serve a relatively long stretch of roadway, but by means of the attenuators 19, 18 and 20 the amount of carrier energy applied to each individual inductive signaling conductor 26, 24, 24A and 27 may be regulated so that the inductive field surrounding each conductor may be controlled within desired limits. Thus, the system can be adjusted to provide a desired field strength, such as 5,000 microvolts per meter, at different points along the center of traffic lanes 13A and 13B without objectionable radiation of wave energy to points removed from the right-of-way.

The roadside transmitter 10 may be connected with a remote control or program center 32 by means of a telephone line 33 or any other suitable wireline or radio communication circuit. Alternatively, the transmitter 10 may be connected by any well-known type of switching means, 34 locally or remotely controlled, with a local program source 35 at the roadside location. The latter may be any well-known type of repeating magnetic-tape reproducing and/or recording device on which messages addressed to motorists can be recorded and continuously repeated, a microphone, or any other suitable source of information or signals to be transmitted to receiving equipment carried by vehicles traveling along the traffic lanes served by the system.

Description of FIG. 2

One arrangement of r.f. signal attenuating and line-coupling means is shown in FIG. 2 wherein r.f. carrier energy from the center conductor 12A of coaxial trunk cable 12-12A is applied through coaxial branch connection 15 and adjustable or fixed coupling capacitor 36 to adjustable attenuator 37, of any suitable well-known type, such as the resistive "T" network shown, which presents a substantially constant impedance at input and output terminals with variation of the attenuator. The output terminal 38 is connected with inductive signaling elements 24 and 24A, forming a part of wayside cable 25 comprising the coaxial trunk cable 12-12A and the inductive signaling elements held in fixed spacial and impedance relationships as will be more fully described hereinafter. It will be noted that by use of the "T" connection of the output terminal 38 with inductive signaling conductors 24 and 24A, signal energy may be carried in two directions along the roadway from line-coupling and attenuator unit 18, thus minimizing the number of coupling-attenuator units required along a given length of roadway. In addition, this arrangement produces two induction fields of equal strength and opposite direction at any given instant, hence tending to cancel signal voltage that may be induced on wayside electric-power or telephone lines extending adjacent conductors 24 and 24A thereby extending the range of the signals beyond the desired limits of the right-of-way and presenting a potential source of interference with other systems or services at points remove from the roadway. The coupling capacitor 36 preferably has a small capacity value in order to minimize loading and voltage-attenuating effect on the trunk circuit presented by coaxial cable 12-12A.

Description of FIG. 3

Referring now to FIG. 3, there is shown an r.f. line-coupling and attenuator unit such as 20, FIG. 1, which provides signal energy at its output terminal 41 in only one direction. As shown signal energy from the center conductor 12A of coaxial cable 12-12A is applied through an adjustable or fixed coupling capacitor 39 to adjustable attenuator 40, of resistive type. Output terminal 41 of attenuator 40 is connected to inductive signaling element 27 which may, as shown, be contained within the same cable structure 25 as the coaxial trunk cable 12-12A.

Description of FIG. 4

Referring now to FIG. 4, there is shown in greater detail the inductive-signaling line termination unit such as 29 of FIG. 1. As shown, termination unit 29, to which conductor 24A is connected, comprises an adjustable or fixed resistor 42, preferably of non-inductive type 43 to match the characteristic impedance of the R.F. transmission line at its operating frequency (this line comprising inductive-signaling conductor 24A and ground sheath 12 of coaxial cable 12-12A) thus preventing reflection of signal energy back along the line with consequent possible formation of standing waves and attendant radiation.

Description of FIG. 5

Referring now to FIGS. 5 to 9, there are shown alternative embodiments of a new and improved cable structure which may be employed in the inductive carrier communication system of the present invention. The embodiment of the cable, as shown in FIG. 5, comprises a center conductor 12A and coaxial sheath 12 separated by dielectric sleeve 12B. This coaxial portion of the cable is employed for trunk-circuit use in transmitting carrier or other signals for long distances along the roadway served by the system. An inductive-signaling conductor 24, fabricated of copper, aluminum or other suitable conductive material in solid or stranded form is supported within dielectric sleeve 44 at a fixed distance from coaxial ground sheath 12 by means of a common protective insulating jacket 25-25A. The dielectric sleeve 44 is fabricated of polyethylene or other suitable insulating material possessing good dielectric properties at the radio frequency or frequencies employed in the system. Jacket 25-25A may be of any suitable and commonly-used insulating material such as vinyl plastic. As the inductive-signaling conductor 24 is held at a fixed impedance relationship as a part of the transmission line in which sheath 12 is the ground conductor and the transmission line has a given impedance value, a combined coaxial trunk relay and inductive-signaling cable of this type may readily be installed and provided with proper terminations to minimize radiation. At the same time, such cable structure minimizes difficulties that would be presented in supplying r.f. energy from the center conductor 12A of coaxial cable 12-12A to conductor 24 at different points along the cable.

Description of FIG. 6

A second embodiment of a combined coaxial trunk and inductive-signaling cable structure is shown in FIG. 6 wherein center conductor 12A and coaxial sheath 12 are similar to those shown in FIG. 5. However, in this cable structure the inductive-signaling conductor 24 is in the form of a coaxial copper sheath in order to present maximum skin surface and thereby minimize losses in the conductor at broadcast frequencies. Within sheath 24 are dielectric sleeve, 45, of polyethylene or other suitable insulting material, and center conductor 46 which is held at ground potential. (The same reference numeral 24 is used throughout this application to identify the inductive signaling conductor; the same reference numerals 12-12A also are utilized throughout the specification to denote the coaxial trunk cable employed for trunk relay and to supply r.f. energy to the inductive signaling conductors). Both the inductive signaling line 24-46 and the coaxial cable 12-12A are held within a common insulating jacket 25-25A, inductive-signaling element 24 being supported within jacket 25A by means of dielectric sleeves 45 and 47 of polyethylene or other suitable dielectric material.

Description of FIG. 7

A modification of the inductive-signaling cable shown in FIG. 6 is illustrated in FIG. 7 in which center conductor 12A and sheath conductor 12 of coaxial cable 12-12A are enclosed in insulating protective jacket 25. The inductive-signaling element, sheath conductor 24, dielectric sleeves 45 and 47, and center ground conductor 46 are held in an insulating protective jacket 25A which is removably attached to jacket 25 to facilitate circuit connections. In effect, however, the arrangement forms a single cable which may be laid in the ground, in roadway surfaces or otherwise installed with minimum of difficulty.

Description of FIG. 8

An additional embodiment of a combined inductive signaling and coaxial trunk cable is shown in FIG. 8. As shown, coaxial elements 12 and 12A are similar to those illustrated and described heretofore. As in the case of FIG. 7, the inductive signaling element 24, as in FIG. 7, is in the form of a conducting sheath which presents maximum skin surface to minimize losses at radio frequencies in the AM broadcast band. A suitable dielectric sleeve 48, such as polyethylene, is used between induction-signaling conductor 24 and coaxial ground sheath 12, both in coaxial relationship. A dielectric sleeve 49 having a wall thickness substantially greater than that of inner sleeve 48 is employed to minimize losses when the cable is buried in earth or in physical contact with conducting materials such as metal surfaces of bridges or tunnels, railings on which the cable is supported and the like. A protective insulating jacket 25, fabricating of suitable material such as vinyl plastic, is employed as shown. The inductive transmission line in this cable structure is formed by outer sheath 24 and inner ground sheath 12, establishing the impedance of the circuit.

Description of FIG. 9

A further embodiment of a combined inductive-signaling and coaxial trunk cable is shown in FIG. 9. Center conductor 12A and coaxial ground sheath 12 are held in dielectric sleeve 48 about which is positioned in convolutive manner a conducting strip 24 of copper, aluminum or other suitable conductor which forms the inductive signaling element of the cable. As shown in the illustration, the spiral conducting strip 24 is held within a relatively thick-walled dielectric sleeve 49. A protective insulating jacket 25, of vinyl plastic or other suitable material surrounds dielectric sleeve 49. The inductive signaling line in this case is formed by conducting strip 24 and coaxial ground sheath 12, with fixed impedance presented by the line.

Description of FIG. 10

Referring now to FIG. 10 there is shown in schematic form the use of an inductive signaling cable of the type shown in FIG. 5. An R.F. carrier modulated by audio signals from program source 32 is supplied by transmitter 10 at a designated frequency in the broadcast band to the roadside coaxial cable formed by inner conductor 12A and ground sheath 12, extending along traffic lane 13A. A relatively small amount of r.f. carrier energy is applied from coaxial center conductor 12A through coupling capacitor 55 and adjustable attenuator 57 to inductive signaling conductor 24 supported within jacket 25A and positioned in fixed relationship with respect to ground sheath 12 as illustrated in FIG. 5. The inductive transmission line formed by conductor 24 and ground sheath 12 is terminated by resistor 58, assuming inductive or capacitive reactances have been balanced out. At a given distance along the cable, such as 1/2 mile, coupling capacitor 59 and r.f. attenuator 60 enable a desired amount of r.f. signal voltage from center conductor 122 of coaxial cable 12-12A to be applied to inductive signaling conductor 24A, serving its individual section by roadway, and extending to termination resistor 62, connected betweenn conductor 24A and ground sheath 12. In similar manner, r.f. signal energy from center conductor 12A of coaxial cable 12-12A is applied through coupling capacitor 63 and adjustable attenuator 64 to inductive signaling element 24B. By proper adjustment of attentuators, 57, 60 and 64, the induction field extending along the cable system may be established in such manner that a substantially uniform and strong signal is received in radio-equipped cars traveling along the traffic lane 13A throughout the length of that portion of the system shown in the illustration.

Description of FIG. 11

FIG. 11 illustrates one preferred form of induction signaling cable which may be separated from the coaxial trunk cable 12-12A and at the same time present a fixed transmission-like impedance so as to facilitate proper termination to avoid radiation. The induction signaling cable is of such a structure as to minimize losses at AM broadcast frequencies when the cable is installed below the surface of roadways as required on throughways or interstate highways where overhead or above-surface cables are not permitted. In the illustrative arrangement shown in FIG. 11, r.f. signal energy at a designated carrier frequency in the AM broadcast band is applied from carrier source 10 through coaxial trunk cable 12-12A and coaxial branch connection 17 to coupling capacitor 39 and adjustable attenuator 40, of coupling and attenuator unit 20, to the inductive transmission line formed by conductor 24, formed in convolutive manner as shown, disposed in coaxial relationship to center conductor 50, held at ground potential. Conductor 24 is separated from center conductor 50 by a dielectric sleeve 48, formed of polyethylene or other suitable insulating material. To minimize effect of the medium in which or on which the cable is laid, a relatively thick-walled dielectric sleeve 49, such as polyethylene, surrounds the inductive signaling conductor 24, while an insulating protective jacket 25, fabricated of vinyl plastic or other suitable material, comprises the outer shell of the cable.

As indicated by the illustration, the wall thickness of the inner dielectric sleeve 48 is preferably substantially less than that of the outer dielectric sleeve 49. This arrangement permits the impedance of the inductive transmission line formed by spiral conductor 24 and center conductor 50 to be established primarily by the relationship between these two conductors, with minimum changes in line characteristics or losses because of variations in soil conductivity or other extenal factors. The inductive signaling cable shown in FIG. 11 may be employed on roadways where it may be desirable to utilize separate inductive-signaling cables fed by r.f. signal energy from a conventional coaxial cable, such as 12-12A, for trunk relay between terminal points.

Description of FIG. 12

FIG. 12 is an enlarged detail of a modified form of the combined coaxial trunk and inductive-signaling cable shown in FIG. 8 and illustrates the use of a spiral conductor strip 24 in lieu of the sleeve form of conductor 24 as shown in FIG. 8. This illustration also more clearly shows the relatively-large wall thickness of the outer r.f. dielectric sleeve 49 employed in this illustrative form of cable as compared with the inner coaxial dielectric sleeve 48 that separates inductive signaling conductor 24 from inner coaxial ground sheath 12.

The illustration of FIG. 12 also emphasizes the difference between this inductive-signaling cable structure and that of conventional coaxial cables that have as basic purpose the confinement of all signal energy within the outer ground sheath in order to minimize transmission loss in carrying signal energy from one terminal to another. Conventional coaxial cables have no provision for establishing means whereby the signal energy carried by the cable may also be employed to establish an external inductive signaling field of substantially uniform and controlled nature for use in communicating with radio equipment carried by vehicles traveling parallel to the cable and at a substantial distance therefrom.

The cable shown in FIG. 12 also differs basically in design and function from double-shielded coaxial cables such as employed in community television systems to minimize radiation from the cable in order to prevent unauthorized viewers to intercept the programs for which subscribers pay. In these double-shielded cables, the both conducting sheaths are at ground potential and in direct electrical contact. There is no dielectric between the two ground sheaths, and except for a protective jacket there is no thick-walled dielectric such as polyethylene sleeve 49 disposed between the outer ground sheath and the jacket. All available types of coaxial cable having an outer insulating jacket employ the latter only for protective purposes, and the wall thickness of the jacket is determined by mechanical rather than radio-frequency transmission-loss factors.

Description of FIG. 13

FIG. 13 illustrates the use of a combined coaxial trunk and inductive-signaling cable, with outer jacket 25, such as shown in FIGS. 5 through 9 and in FIG. 12, as installed in the dividing strip 13C of a two-direction highway having separated traffic lanes 13A and 13B. The induction field surrounding the cable thus is effective in reaching receiving equipment carried by vehicles traveling in either direction along the roadway.

Description of FIG. 14

FIG. 14 illustrates the use of a conventional coaxial cable 12-12A, as installed in the center strip 13C of a roadway on which vehicles move in opposite directions on traffic lanes 13A and 13B, to supply carrier signal energy through junction box 94, the latter recessed in the ground and containing line-coupling capacitors such as 36 (FIG. 2) and R.F. attenuators such as 37 (FIG. 2) to coaxial branch cables 95 and 95' and inductive-signaling cables 70 and 70' serving traffic lanes 13A and 13B respectively. Cables 70 and 70' are inductive signaling cables of the type shown in FIG. 11 and are designed in such manner, as hereinabove explained, to produce a maximum strength of induction field of substantially uniform nature along the cable which is terminated as previously explained to eliminate formation of standing waves on the line and attendant radiation beyond specified limits. The inductive signaling cables, in this illustrative arrangement, are installed below the surface of roadway 13A and 13B and along their outer edges between the roadway surface and shoulders 13D and 13D'.

Description of FIG. 14A

FIG. 14A is a detail of FIG. 14 showing the use of a narrow channel, 96, between the roadway pavement 13A and shoulder 13D in which the inductive-signaling cable 70 is recessed. Channel 96 may be filled with any suitable protective material such as epoxy or cold-flow rubber sealing compound which will adhere to the outer edge of the roadway and cause the cable to be held securely in position, as well as protect it from damage from passing vehicles, road maintenance machinery and effects of weather or sunlight.

Description of FIG. 15

FIG. 15 illustrates an arrangement in which a conventional coaxial cable 12-12A and junction box 94 are located in protected position below the surface of center strip 13C. Junction box 94 contains line-coupling and adjustable r.f. attenuators as described in foregoing paragraphs relating to FIG. 14 and by means of branch coaxial cables 95 and 95' applies a controlled amount of r.f. carrier energy from the coaxial cable 12-12A to inductive-signaling cables 70 and 70', which may be similar to the structure shown in FIG. 11. In this instance, the inductive signaling cables are installed below the roadway surface along the inner edges of pavements 13A and 13B, between the roadway and inner shoulders 13E and 13E'. This is shown in detail in FIG. 15A wherein the inductive signaling cable 70 is buried in the shoulder 13E at a point in proximity to pavement 13A to minimize effects of weather and to provide protection from passing vehicles and road maintenance machinery.

Description of FIG. 16

FIg. 16 illustrates an arrangement in which the combined coaxial trunk and inductive-signaling cables comprised within jackets 25 and 25', as shown in FIGS. 5 through 9 and in FIG. 12, are installed in channels 13F and 13F' cut or formed in the center line of each roadway 13A and 13B carrying traffic in opposite directions and separated by division strip 13C. A detail of a cross-section of one of the roadways at the point where the cable is installed is shown in FIG. 16A wherein 13F represents a longitudinally-extending expansion joint normally used in many concrete pavements and 96 represents a channel cut or formed in the upper surface of the roadway 13A to permit installation of the cable 25 below the surface. After or during installation of the cable, the channel 96 is filled with epoxy or cold-flow sealing compound of suitable type to provide mechanical protection from vehicles, maintenance machinery and effects of weather and sunlight.

Description of FIGS. 17 and 17A

FIGS 17 and 17A show a presently-preferred structural arrangement by means of which the coaxial trunk and inductive-signaling cable within jacket 25, having a structure as shown in FIG. 12, together with additional coaxial cables of conventional types, 12'-12A' and 12"-12A", may, if desired, be positioned in new highways beneath the roadway surface within a partitioned metallic structure. This structure comprises a plurality of contiguous "V"-shaped members 97, 97A and 97B with horizontal closure members 98 and 98A and 98B, the whole forming a unitary structure mechanical strength to protect the cables from damage when the structure is positioned in the bed of a roadway during construction between the foundation of crushed rock 13G and layers of asphalt 13A' and 13A or other surfacing material such as concrete. The open construction of the "V" members before closure strips 98, 98A and 98B are installed permits cables to be laid easily and quickly.

After the cables are in place, the closure strips are positioned as shown in FIGS. 17 and 18. On completion of the roadway, each "V" member in effect forms a closed conduit in which cables may be added or removed at appropriately-spaced junction points. Use of a non-metallic closure strip 98 for the channel 97 in which the induction-signaling cable 25, is installed permits establishment of an external induction over the roadway area field for vehicle-communication, signaling and control purposes.

Description of FIG. 18

Referring now to FIG. 18, there is shown an inductive communication system in which trunk coaxial cable 12-12A, extending along a roadway 13A - 13B carrying traffic in two directions, is supplied with carrier signals from zone transmitter 10 through line-coupling unit 105 of any well-known and suitable type. Signals from a program or other signal-originating center 32 may be carried by coaxial cable 12 - 12A or other suitable circuit through low-pass filter 106 and coaxial branch circuit 107 to the input of a low-pass filter (60-5000cps) and audio amplifier 108 whose output is connected to the signal input of transmitter 10, operating at an AM broadcast frequency such as 540 KC. In this illustrative example, coaxial cable connection between program source 32 and low-pass filter amplifier 108 is indicated as the signals from source 32 may, if desired, be one or more low-frequency carrier signals below 100 kc. In this event the low-pass filter and amplifier unit 108 would be replaced by a band-pass filter and carrier receiver (not shown).

Carrier signals at a broadcast frequency such as 540 kc. as well as carriers of lower frequency thus can be carried along cable 12 - 12A. Carrier energy at the illustrative frequency 540 kc, as well as at lower carrier frequencies if desired for use with special communication receivers carried by vehicles, may be applied through line-coupling and r.f. attenuator unit such as 20 to inductive-signaling cable 70 such as that illustrated in FIG. 11, which in this case forms a transmission line in the form of horizontal loop extending from line-coupling and attenuator unit 20 around both traffic lanes 13A and 13B for a substantial distance such as 1/2 - 1 mile as indicated by the illustration. The far end of cable 70 is connected through termination unit 29 to the metallic ground circuit provided by ground sheath 12 of coaxial trunk cable 12 - 12A for reasons previously set forth. Such a loop configuration of the transmission line can present advantages when compared with use of separate cables along each roadway as tests have shown that a terminated transmission line arranged in loop configuration as shown will produce an induction field of maximum intensity within the area of the loop, in this example concentrating the most effect portion of the induction field within the roadway area.

Such loop configuration of the transmission line 70 also enables strong signals to be received in vehicles traveling along both traffic lanes 13A and 13B contained within the loop structure. However, unlike a conventional loop antenna designed to radiate carrier wave energy, the loop structure shown in FIG. 18 is a terminated two-conductor transmission line on which no standing waves appear, thereby it does not function as an antenna in the commonly-accepted sense. Also, carrier energy at the AM broadcast carrier frequency and at the low carrier frequencies can effectively be received within the loop area since, unlike a loop antenna intended for radiation of carrier wave energy at a specific carrier frequency to which the loop is tuned in order to radiate wave energy to remote receiving points, the transmission line employed in the loop configuration shown in FIG. 18 is aperiodic and is not resonated in any manner. Exact impedance-matching of the line at the termination point is established at the broadcast frequency where suppression of radiation is an important factor.

At a given distance (such as 1-2 miles) from line-coupling unit 20 along the coaxial cable 12-12A, a second line-coupling and r.f. attenuator unit 20A permits a regulated amount of signal energy at broadcast as well as at lower carrier frequencies to be applied to a second horizontal loop, formed by inductive-signaling cable 70A and encompassing the section of roadway 13A-13B, the roadway area served by the second loop being adjacent the roadway area served by the first transmission-line loop. Cable 70A is connected at its far end through termination unit 29A to the metallic ground provided by coaxial ground sheath 12.

Description of FIG. 19

A schematic diagram of the inductive-carrier transmission cable 70 formed in loop configuration is shown in FIG. 19. Carrier signal energy at broadcast and low carrier frequencies is applied through line-coupling capacitor 39 and attenuator 40 of line-coupling attenuator unit 20 to inductive-signaling conductor 24 which may be in the form of a coaxial sheath as shown or in spiral configuration as shown in FIG. 11. The center conductor 50 is held at ground potential. The far end of conductor 24 is connected through termination resistor 42 to the ground sheath of coaxial trunk cable 12-12A. As illustrated, current flow along conductor 24 toward termination resistor 42 causes the electro-magnetic lines of force at any given instant to have the same polarity as related to direction of current flow at different points along the line, assuming that there is no wave reflection. If there are roadside power or telephone lines extending along the traffic lanes and in proximity thereto, as represented by overhead wires 109, FIG. 19A, a substantial amount of carrier energy will be induced on the overhead wires, which may lead to interference with other systems on the same carrier frequency or frequencies in other area removed from the roadway that are served by the overhead lines. To minimize this coupling effect, a configuration of transmission line and circuit connections as shown in FIG. 19A may be employed. As in the illustrative arrangement of FIG. 19, carrier signal energy is applied from coaxial trunk cable 12-12A through line-coupling and attenuator unit 20 to the conducting sheath 24, formed as a split-loop with current flow in sections 24a and 24b in opposite direction from that in sections 24c and 24d at any given instant, thus causing opposite polarity of the electro-magnetic lines of force as indicated by the circular arrows in sections 24a and 24c, or 24b and 24d. The center ground conductor 50 of the coaxial cable of which sheath 24 is a part is connected to the ground sheath of trunk coaxial cable 12-12A. At the mid-point of the loop between sections 24b and 24d the inductive-signaling conductor 24 is connected to ground conductor 50 through termination resistor 42. As current flow from line-coupling and attenuator unit 20 along signaling conductor 24 is in two directions.

Assuming a perfectly balanced and terminated loop of this type, equal and opposing signal voltages will be induced on the roadside power or telephone lines 109 by loop sections 24b and 24d hence minimizing inductive transfer of signal energy.

Description of FIG. 20

FIG. 20 is illustrative of the operation of the system of the invention in relaying signals over long highways, retaining the same broadcast carrier frequency throughout lengths of roadway served by a plurality of relay or repeater transmitting units, with means for providing a relatively uniform induction field throughout the system. Audio program signals from a program source 32 are carried by line connection 33 to the signal inputs of (1) an AM transmitter 10 operating at a broadcast frequency such as 540 kc. and (2) a very los frequency (e.g., 30-kilocycle), FM transmitter 110 of narrow-band type (such as provided by deviation ratio of less than unity). The carrier signals from the two transmitters are impressed on coaxial trunk cable 12-12A through line-coupling unit 113 of any well known diplexer type having two inputs and a common output. Carrier energy at 540 kc is applied from coaxial cable 12-12A through high-pass filter or coupling unit 114 and adjustable r.f. attenuator 40 to inductive signaling conductor 24 extending parallel and in proximity to coaxial trunk cable 12-12A or forming a part of a combined coaxial trunk and inductive-signaling cable of the types shown in FIGS. 5 through 9 and FIG. 12. Conductor 24 is terminated at its far end by means of impedance-matching resistor 42 to the ground sheath 12 of coaxial trunk cable 12-12A thereby producing as inductive field extending throughout the length of the conductor 24, designated as Zone 1A. At the beginning of Zone B, carrier energy at 540 kc again is applied from coaxial trunk cable 12-12A through high-pass filter or coupling unit 114A and adjustable attenuator 40A to inductive-signaling conductor 24A, the end of which is connected to ground sheath 12 of coaxial cable 12-12A through termination resistor 42A, thereby forming an induction signaling field extending along Zone B. In similar manner, r.f. carrier energy at 540 kc is applied at the beginning of Zone 1C from trunk cable 12-12A through high-pass or line coupling unit 114B and attenuator 40B to inductive signaling conductor 24B, the end of which is terminated by resistor 42B connected to ground sheath 12 of coaxial cable 12-12A. Providing an induction field extending along Zone 1C.

Description of FIG. 21

The attenuators 40, 40A, and 40B at the beginning of each zone may be adjusted in such manner that the maximum field strength is kept at a desired value such as indicated at 120, FIG. 21, which is below the prescribed radiation limit of the FCC. The length of each inductive-signaling conductor 24, 24A and 24B is kept such that normal attenuation of the signal with length of line in each zone is held within limits such that the minimum field strength at the end of each zone 1A, 1B and 1C is well above the value, indicated at 123, needed to fully stabilize the automatic volume control circuit of automobile receivers, thereby providing a received audio signal of substantially constant level as the car travels through the length of each zone.

Inasmuch as the line-coupling and attenuator units such as 114 and 40 respectively are passive devices, requiring no external source of power other than the radio frequency signal voltage that they transfer from coaxial line 12-12A to the inductive-signaling conductors such as 24, no maintenance problems such as tube or transistor replacements are involved. Sufficient carrier power can be provided at terminal transmitter 10 to feed a substantial number of zone signaling conductors, such as 24, 24A and 24B, without involving a radiation problem.

At the end of illustrative Zone 1, shown as 3-5 miles, the very low frequency (VLF) FM signal at 30 kc is applied from coaxial trunk cable 12-12A through low-pass filter 115 to the signal input of a VLF FM receiver the audio output of which is applied through connection 117 to the signal input of a second AM transmitter 10A operating at a carrier frequency of 540 kc. As automatic limiter circuits of the FM receiver provide a relatively uniform output signal with respect to level changes in the audio signals applied to the input of transmitter 10A the low-frequency FM channel provides a means for interconnecting a plurality of roadside AM transmitters with a central programming point 32 in lieu of use of telephone lines or other circuits for this purpose. It is assumed that the audio input circuit of transmitter 10A would have an automatic limiting or compression amplifier circuit of any well known type to minimize possibility of overmodulation by relayed program signals.

The 540 kc carrier signal at the output of transmitter 10A is applied through line coupling unit 119 of diplex-input type to coaxial trunk cable 12'-12A'. The low frequency carrier from transmitter 110 at the terminal point also is applied through line coupling unit 119 for continued transmission along coaxial trunk cable 12'-12A'. It will be noted that coaxial cable section 12'-12A is isolated electrically from cable 12-12A with respect to the 540 kc carrier frequency from transmitter 10A. Both the diplex coupling unit and the low-pass filter 115 effectively prevent any 540 kc signal energy from transmitter 10A from being fed back along line 12-12A, thus eliminating phasing or heterodyne problems caused by inter-action of the carriers used in the different zones.

Carrier signals at 540 kc from zone transmitter 10A are applied through high-pass filter or line coupling means 114C and attenuator 40C to inductive signaling conductor 24C, the beginning of highway transmitting zone 2, in the same manner as heretofor described.

Description of FIG. 22

FIG. 22 illustrates one preferred means that may be employed in relaying signals from a central point such as remote program center 32, local program source or amplifier 35, or other suitable signal source to provide communication with radio-equipped vehicles or other radio receiving points within a localized signaling area. As shown in FIG. 22, the localized signal area is formed by traffic lanes 13A and 13B, carrying vehicular traffic in opposite directions, served by the coaxial trunk and inductive-signaling cable comprising coaxial cable 12-12A and inductive-signaling conductors 24, 24A, 24B, 24C, 24D, 24E, 24F and 24G, extending over a total distance of 12-20 miles in this illustrative system. The inductive signaling conductors are connected to coaxial line 12-12A in manner previously described by line-coupling units 20, 20A, 20B, 18, 20C, 20D and 20E as shown and to the common ground sheath 12 of coaxial cable 12-12A by terminal units 29, and 29A through 29G respectively.

Low-frequency carrier transmitter 110 feeds signal energy at a frequency such as 30 KC through any well-known diplex line-coupling means 113 to coaxial trunk cable 12-12A. In like manner, carrier signal energy at a designated frequency in the standard AM broadcast band, such as 540 KC, also is applied through diplex filter 113 to coaxial trunk cable 12-12A, which may be of the structure shown in FIGS. 5-9, inclusive, combining the coaxial trunk conductors 12-12A and inductive-signaling elements 24, 24A through 24G. In this illustrative arrangement of the system, the frequency of trunk carrier transmitter 110 is one of the subharmonics, 30 KC, of the broadcast frequency 540 KC.

By means of line-coupling units 20 and 20A a regulated amount of carrier signal energy at 540 KC (and 30 KC if desired for use in reaching special receivers used by vehicles) is applied to inductive signaling conductors 24 and 24A, the ends of which are connected through termination units 29 and 29A, respectively, to common ground sheath 12, thereby forming an inductive signaling field extending laterally across traffic lanes 13A and 13B and longitudinally for the lengths of the two conductors 24 and 24A -- a total distance of 3-5 miles in the illustrative example. At the end of this first 3-5 mile zone, it is assumed that the power level of the 540 KC carrier has been reduced by transmission losses in trunk cable 12-12A to the point where it cannot supply further effective signal energy to additional inductive-signaling conductors such as 24B and 24C, thus requiring a repeater or other relay means to extend the transmission range of the system at 540 KC.

Since the transmission losses of the 30 KC carrier have been appreciably less than the losses of the 540 KC carrier, signal energy from the former is utilized to produce a new 540 KC carrier signal to bring about this extension of range. This is accomplished by use of a repeater at a point, B, along the cable. As shown, repeater B comprises a 30 KC relay receiver 116 and an associated 540 KC relay transmitter 10A. Carrier energy at a frequency of 30 KC as supplied by terminal transmitter 110 is applied by coaxial branch connection with trunk coaxial cable 12-12A through line-coupling unit 114 of any well-known type and low-pass filter 106A (with cutoff above 40 KC) to the signal input of relay receiver 116. Receiver 116 demodulates the 30 KC carrier and applies the derived audio program signals to the signal input of a 540 KC relay transmitter 10A of any well-known crystal-controlled or automatic frequency-control (AFC) type. The 540 KC signal output of transmitter 10A then is applied through line coupler 114, of any suitable and well-known diplex input type, to coaxial trunk cable 12-12A. To prevent the original carrier signal at 540 KC from being transmitted forward along the same section of trunk cable 12-12A that carries the 540 KC signal from transmitter 10A, a los-pass filter 106 is inserted in the coaxial trunk circuit at the point where termination unit 29A is situated, blocking forward passage of the 540 KC signal from transmitter 10 and backward passage of 540 KC carrier signals from source 10A along trunk cable 12-12A beyond the tone within which transmitter 10A is associated. However, the 30 KC trunk carrier from terminal transmitter 110 is passed forward through filter 106 without any marked attenuation. Low-pass filter 106A prevents feedback of the locally-produced 540 KC carrier from zone transmitter 10A into the relay receiver 116.

In the same manner as hereinabove described, with respect to the first signaling zone, carrier signals at 540 KC from transmitter 10A (as well as the 30 KC signals, if desired) are supplied from coaxial trunk cable 12-12A to inductive signaling conductors 24C-24D, 24B and 24E through line-coupling attenuator units 18, 20B and 20C, respectively. The ends of conductors 24C, 24D, 24B and 24E at connected to common ground sheath 12 of coaxial cable 12-12A through termination units 29C, 29D, 29B and 29E, respectively. The inductive signaling field in this zone thus extends from termination unit 29B to termination unit 29E over the illustrative distance of 6-10 miles. As a 540 KC carrier frequency is employed throughout the two zones extending from line-coupling and attenuator unit 20 to termination unit 29E, and as a relatively uniform signaling field is maintained along the cable for this distance, vehicular receivers will provide a uniformly-strong audio signal without change in tuning or volume controls as the vehicles proceed throughout the length of the cable served by that portion of the system that has been described.

At the end of the useful service range of zone transmitter 10A, a low-pass filter 106B is inserted in the trunk coaxial cable 12-12A to block forward passage of the 540 KC carrier along the cable. The 30 KC trunk carrier, however, is passed without any significant attenuation and at subsequent repeater points, such as at points "C" (not shown) and "D" along the cable, is utilized in the same manner as described in connection with explanation of the functions of relay receiver 116 and transmitter 10A.

At relay point "D", for example, the 30 KC trunk carrier from terminal transmitter 110 is applied from coaxial trunk cable 12-12A through branch coaxial cable 118, line-coupling unit 114A, and low-pass filter 106C to the r.f. signal input of 30 KC receiver 116A. The demodulated program signals then are applied to the audio-frequency signal input of 540 KC relay transmitter 10B, modulating its carrier. It is assume