CA1050616A - Inductive carrier communication systems - Google Patents

Abstract

INDUCTIVE CARRIER COMMUNICATION SYSTEMS A communication system for the transmission of the sound portion of motion pictures or the like in drive-in theaters directly to automobile radios by means of an induction cable buried beneath the ground in the reception area in which automobiles viewing the motion picture are to be parked. The cable is of such design that when properly positioned and combined with an amplitude-modulated radio frequency signaling means, a relatively uniform amplitude-modulated electromatic field of the desired strength and of a frequency which is within the normal reception range of car radios (i.e. the standard broadcast band) is established throughout only the area where the automobiles are parked. To limit signal reception outside the designated parking area of a drive-in theater an interfering signal is transmitted by coupling audio-modulated radio-frequency energy to nearby electric power lines of overhead type or by means of a transmission line or cable adjacent and outside the theater area. The interfering signal employs the same radio frequency as is used by the induction cable in the reception area of the theater.

Description

BAC~ ROUND OF THE INVENTION
This invention relates to improvements in communication systems of inductive-carrier type and, more particularly, this invention relates the use of such communication systems for the transmission of the sound portion of motion pictures or the like in drive-in motion picture theaters directly to the radios of . automobiles parked within the theater area.
:; The drive-in motion picture theater is a theater designed for the showing of motion pictures wherein the audience drives their automobiles into the theater area and parks in designated parking spots in view of a large screen. Normally, sound is .- ~

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tran~mittf~d to fach c~r by means of a portable ~peaker which is detachably moun~ed on ~ stanchion poct near each design~ted parkin~ spot. The speaker is removed from the post and placed in each automobile during the viewing of the film. Sound signals are transmitted by means of underground wires from the projection booth -to each post~ through wires in each post and thereafter through wires from the post to the speaker. The signal is transmitted from the sound amplifier in the projection room to the speaker in each automobile in much the same manner as a signal is transmitted in a home sound system.
There are many inherent disadvantages to such a system.
Each car must be exactly positioned in close proximity to the speaker posts. Each of the individual speakers must be maintained by the theater. It is common for cars to attempt to leave the theater at the end of the show without removing the speaker and thus cause damage to the post, the speaker and the car itself.
Additionally, drivers commonly hit the speaker posts by accident during manuevering in parking. As can be seen from the above, the maintenance of such an exposed system is comparatively expensive. Also, the sound quality is much inferior to that in enclosed theaters because of the use of very small speakers employed in drive-in theaters in order to accommodate the speakers to the stanchion posts that support the speakers.
As will be more fully discussed hereafter, the present invention comprises the use of an inductive~carrier electro-magnetic transmission system o~ localized type whereby the sound portion~of the motion picture is transmitted to the car radios of the automobiles parked within the theater area. Many systems of the inductive-carrier type, including those of the applicant, have been employed in the past for localized restricted-range transmission on highways, at airports and on railroads. To applicantts knowledge no such system has ever been adopted and

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employed for use with automobile radios in driye~in motionpicture ~he~ters and the like. I'he pas-t uses of such inductive-carrier type systems have presented serious technical problems when operated without license under the low-power rules of the Federal Communications Co~mission (FCC) of the United States and when using relatively-high carrier frequencies, such as those in the standard ~medium wave) broadcast band. Radiation of electrical wave energy which is an inherent characteristic of inductive-carrier systems when operated at radio frequencies, often extends over ~listances far in excess of the permissible fiel~strength limit specified by the U.S. Government and others for unlicensed radio devices of restricted-range type. While it has been possible, by careful adjustment of radio frequency tr.f.) carrier level to comply with the Government 13 rules in certain localized applications, such as the highway radio ~ystem installed by the applicant on the George Washington Bridge, New York City, in 1940, experience in most cases has demonst~ated that it is extremely difficult, and in some in~
stances impossible, to comply with the Government's rules over any substantial period when unattended transmitters are employed and, at the same time to maintain a sufficiently strong induction f~e;~ at broadcast frequencies to enable good reception in radio equipped cars travelling over lengths of highway served by the system.
Experience with roadside conductors of various types, including single and dual-conductor transmission lines has in-d~cated that the strength of the induction field about these conductors is subject to substantial variation along their leng'~h, Near t~e transmitter s.ource, for example, the field .stren~th may ~e too ~gh to comply with the Governmon~'s rules .at ~'r~adcast fre~uencies if a strong, noise-free signal is to ~e pro~i~ded in t~e desired reception area. In addition, if

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the cable is installed below th~ ~,urface of the gou]ld as would be required , , ~; :

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for US~? in clrive-in ttle~ters W~ bove-s~-~r~e~ ins~all~tions would not b. ~esir;lbl~, v~ria~ions in the inductive-siynaling field due -to chan~es in soil conductivity under different weather con-ditions and o~her irregularitles in environmental conditons would present difficulties over a substantial period of time in maintaining a reasonabl~-constant field strength and restriction of radiation within limits set by the Government.
Moreover, experience with conventional forms of cables, or wires, when employed as r.f. signal conductors for the purpose of producing an induction-signaling field as a means of im-pressing carrier-signal energy on the an-tenna system of radio broadcast receivers carried by motor vehicles indicates that the coupling loss between the vertically disposed vehicle antenna and the horizo~tally-polarized signals ~rom the cable system is un-necessarily high. This results in requirement of substantially more r.f. power in the cable system than would be required if a convolutive field, having vertical and horizontal polarization characteristics, were provided. The present system as employed in drive-in theaters 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 r~ceiving systems of motor vehicles carrying conventional antennas. This, in turn, assists in meeting the requirements of the Governm2nt with respect to restricted-range radio devices.

SUMMARY OF THE_INVENTION
The present invention eliminates the inherent problems of the post and speaker system commonly employed in drive-in theaters by the use of an inductive cable or cables buried beneath the ground in the reception area in which automobiles viewing the motion picture are to be parked. The cable is of such design that when combined with an inductive amplitude modulated radio-*requency signalin~ means~ a relatively uniform ~4-- .
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amplitude-modulated electromagnetic field of desired strength and of a ~requency which is within the normal reception range of car radios (i.e. the standard broadcast band) is established through-out only the area where the automobiles are parked. The cable design as shown hereafter offers p:ractical solutions to the problems previously had with induction radio as set forth above.
By use of the present cable positioned as disclosed herein, it is possible to produce a uniform signal over a restricted area.
Outside the designated reception area, si~nal strength quickly diminishes. Thus, at mo~t locations, the signal cannot be re-cei~ed ~y those who park without fee near the theater but out-s~de the designated reception area. Reception outside the designated area can be furtner limited by use of a security transmitter that feeds a small amount of radio frequency power to a cable positioned around the designated area which transmits ~n interfering signal at the same frequecy as the signal in the designated reception area. To prevent unauthorized reception outside of the designated reception area because of signals induced on power lines and the ~i~e, an interfering signal may be impressed on all such lines passing outside and near the theater area. These precautions are, of course, necessary to ~nsure that people are not able to hear the theater presentation without payment of entrance fee. These and other improvements as presented by the system of the invention are described in su~sequent pages.
rt i therefore~ an ob~ect of the present invention t~ provide an inductive-carrier communication system for use in dr~ve-in theaters which will eliminate the need for the inferior post-and~speaker system of the prior art and provide 3~ a uniform ampl~tude~modulated signal of high audio ~uality thr~ughout only the viewing area, which may be received on the standard hroadcast ~and of automobile car radios.

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It is an additional object of -this present invention to provide means whereby drivers of cars on the outside of the drive-in theater may be prevented from hearing the sound associ-ated with the film being shown by the theater, especially along roacds in the immediate vicinity of the theater where the picture can be seen outside of the drive,in premises.

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X~ is addltiotlally .In object of this present inverltion to make such in~uc~ion system so th~t it is highly efficietltJ mee~s Government requlr~ments as to field strength and will provide a signal of maximum in~ensity in radio receivers in vehicles emp~oying conventional antennas by providing an induction field having vertical and horizontal polarization characteristics.
DF.SCRIPTION OF_THE DRA~INGS
Other objects of the present invention will be readily apparent from the following description and drawings in which:
Figure 1 is a diagrammatic view of one embodi~ent of the inductive-carrier communication system of the present invention as employed in roadside use;
Figure 2 is a schematic view of one ~orm of signal attenua-ting and line-coupling means that may be used in the inductive-carrier communication system of the present invention;
Figure 3 is a schematic view of another form of a signal attenuating and line-coupling means that may be used in the in-ductive-carrier communication system of the present invention;
Figure 4 is a schematic view of an inductive-signaling line~termination unit that may be used in the inductive-carrier communication system of the presen~ invention;
Figure 5 is a perspective view of one em~odiment of the cable structure of the present invention;
Figure 6 is a perspective view of another embodiment of the cable structure of the present invention;
Figure 7 is a perspective view of yet another embodiment of the cable structure of the present invention;
Figurè 8 is a perspective view of still a~other embodiment of the cable structure of the present invention;
Figure 9 is a perspective view of a furth~r e~bodiment of the cable structure of the present invention;
Figure 10 is a schematic view of an inductive-carrier communication system of the present invention utilizing the --6-- .

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cable structure stlo~rl in ~i~ure 5;
~ igure 11 is a partially perspectlve, partially schematic view of an inductive-carrier communication system of the present inven~ion utilizing a modification of the induction cable;
- Figure 12 is an enlarged perspective view of the embodiment of the cable of the present invention shown in Figure 9;
Figure 13 is a partially sectional perspective view of a portion of a drive-in motion picture theater showing an inductive-; signaling cable buried in two of the parking ramps of the theater, which ramps position the cars to properly view the screen;
Figure 14 is a diagrammatic view of an inductive-carrier communication system of the present invention as employed in use in drive~in motion picture lheaters;
Figure 15 is a diagrammatic view of a portion of the inductive-carrier communication system and projection room e~uipment of the present invention as employed in use in drive-in ~ motion picture theaters; and :. Figure 16 is a diagrammatic view of an alternative arrange-ment of the colQmunication system of the present invention as employed in drive-in motion picture theaters.

DETAILED DESCRIPTION OF THE EMBOnIMENT
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-. . An illustrative application o~ the carrier system of the prese.nt in~ention employed in roadside use is shown in Figure l in which a carrier transmitter 10, in this case operating at a ~ broadcast frequency of 540 kilocycles, is connected by coaxial .
: ~ cable ll to a roadside cable 12-12A extending parallel to traffic lanes 13A and 133 carrying vehlc e traffic in opposito directions.

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In co~lxial c~ble 12-12~, the c~:~nter concluctor is denoted by 12 A
and the yround she.~th conductor is clenoted by IZ. A-t intervals along coaxial cab]e 12-12.i~,, preferably installed below the surface of the roadway or the adj~ining area thereof~ a con-trolled amount of radio frequency ~r.E.) carrier energy is applied by means of coaxial branch connections 15 J 16 and 17 and adjustable coupling and attenuating means 18, 19 and 20 to longitudinally-e~tending conduc-t:ors 24,24A, 26 and 27,respective-; ly, which serve as the inductive-signaling elements of the system.
As will be described hereinafter, these inductive-signaling conductors may be incorporated as an inheren-t 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 ter-mination units 28, 29, 30 and 31 respectively to the common metallic ground circuit provided by the sheath 12 of coaxiaI
cable 12-12A. Inasmuch as the inductive-signaling elements ,~ 24,24A, 26 and 27 have a ~ixed and uniform impedance relations~iip ~ith 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 a manner i as to match the characteristic impedance of each line section at the broadcast carrier frequency employed throu~hout the length of roadway system.
~' As illustrated in Figure 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 ~ubstantially uniform induction ~ield at a common carrier fre-quency in order that signals as received in radio-equipped vehicles traveling ~hroughout the length of the roadway served by the system will be unin~errupted and of substantially-constant ~, .

strell~t~l a; ttl~ vellicles pa-s throucJh ~h~ individual signaliny ~ones cr~at~ bv the inductiv~ fiel~ Lrom the conductors 24,2 26 and 27. A vehicle trav~ling from west to east on traffic lane 13B would, ~or example, hear the transmitted signals on 540 ~ilocycles first from inductive-signaling conductor 26, then from conductors 24, 29A 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 lenyth of roadway shown in the illustration is prevented by minimizing any L~ection 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 Government ~or unlicensed lower-power radio devices of restricted-range type.
An important advantaae of the arrangement shown in Figure 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 r 18, and 20 the amount of carrier energysupplied to each individual inductive-signaling conductor 26, 24, 24~ and 27 may be regulated so that the inductive field surrounding each conductor may be ~ontrolled 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 ~ave 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 suitab:Le wireline or radio communication circuit.
Alternatively, the transmitter 10 may be connected by any well-known type oi- switching means, 34 locally or remotely controlled _g_ ~5~
wit~ a loc~l program source 35 at the roadside loca~ion. The latter may b~ any .~ell-known type of repeatin~ ma~netic-tape reproducing and/or recording device on which messages addressed to motorists can be recorded and continuously repeated, a micro-phone, or any other suitable source of inEormation or signals to be transmitted to receiving lequipment carried by vehicles traveling along the traffic lanes served by the system.
DESCRIPTION OF FIGURE_2 One arrangement of 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 ~he 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 comprisi~ the coaxial trunk cable 12-12A and the lnductive-signaling ele~ents held in fixed spacial and impedance relationships as will ~e more fully des-~ribedhereinafter. It will be noted~that k~ use of the "T"
connecting of the output termi~al 38 with in~uctive-signaling conductors 24 and 24A, signal energy may be carried in two directions along the roadway from line-coupl~g and attenuator unit 18, thus minimizing the number of coupl~ng-attenuator units required along a given length of roadway. I~ 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~-o3nductors 24 and 24A
thereby extending the range of the signals b~yond the desired limits of the right-of-way and presenting a p3tential source of interference with other systems or servic~ at points remote ~,`, ' , ' .
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from the roadway~ The coupling capacitor 36 preferably has a small capacity value in order to minimize any vol-taye-attenu-ating effect on the truck circuit presented by coaxial cable 12-12A.

~ eferring now to Figure 3, there is shown an r.f.
line-coupling and attenuator unit such as 20, Figure 1, which provides signal energy at its output terminal 41 in only one d~rection. As shown signal energy from the center conductor 12A of coaxial cable 12-12A is applied through an adjustable or fl~xed coupling capacitor 39 to adjustable attenuator 40, of resistive type. Output terminal 41 of attenuator 40 is connected to ~nductive signaling element 27 which may, as shown, be con-; tained within the same cable structure 25 as the coaxial trunk caBle 12-12A~

Referring now to Figure 4, there is shown in greater detail the inductive-signaling line termination unit such as 29 of ~igure 1. As shown, termination unit 29, to which -~ 20 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) ','5 thus preventing reflection of signal energy back along the line with consequent possible formation of standing waves and attendant radiation.
DESCRIPTION O~ ~IGURE 5 Referring now to Figures 5 to 9, there are shown ~lternative embodiments of a new and improved cable structure w~ich may ~e employed in the inductive carrier communication ~- system of the present invention. The embodiment of the cable :`, .

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as shown in Figure 5, comprises a center conductor 12A and coaxial sheath 12 separated ~y dielectric sleeve 12B. This coaxial portion of t~e cable is employed for trunk-circuit use in transmitting carrier or other signals for long distances along the roadway served by the system. An induc-tive~signaling conductor 24, fabricated of copper, aluminum or other suitable conductive material in solid or stranded form is supported with-in dielQctric 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 dielec-triC 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 a vinyl plastic.
~s the inductive-signaling conductor 24 is held at a fixed ~mpedance 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 2Q ~e ~nstalled and provided with proper termination to minimize radIation. At the same time, such cable structure minimized 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.

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A second embodiment of a combined coaxial trunk and inductive~signaling cable structure is shown in Figure 6 wherein center conductor 12A and coaxial sheath 12 are similar to those s~own In ~igure 5. Howe~er, In this cable structure the in~
duct~ve~signaling conductor 24 is in the form of a coaxial copper sheath in order to present maximum skin surface and - t~ereby minimize losses in the conductor at broadcast frequenies.

, Within sheath 24 are dielectric slee~e, 45, of polyethyle~e or other suitable însulating 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 numberals 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 ~thin 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~

A modification of the inductive-signaling cable shown in Figure 6 is illustrated in Figure 7 in which center ~; conductor 12A and sheath conductor 12 of coaxial cable 12-12A
are enclosed in insulating protective jacket 25. The in-,~
ductive-signaling element, sheath conductor 24, dielectric sleeves 45 and 47, and center ground conductor 46 are held in - an ~nsulating protective jacket 25A which is removably attached -tb jacket 25 to facilitate circuit connections. In effect, however, the arrangements forms a single cable which may be laid in the gxound, in roadway or drive-in theater surfaces or otherwise insta:Lled with minimum of difficulty.

An additional embodiment of a combined inductive-signaling and coaxial trunk cable is shown in Figure 8. As shown coaxial elements 12 and 12A are similar to those illus-trated and described heretofore. As in the case of Figure 7, the induct~ve-signal~ng element 24, as in Figure 7, is the orm of a conducting sheath which presents maximum skin surface :: ~
~13-`~'':'' to minimize losses at radio frequencies in the standaxd broad-cast ~and. A suita~le dielectric sleeve 48, such as polyethylene is used ~etween inductive-signalinq conductor 24 and coaxial ground sheath 12, both in coaxial relationship. A dielectric sleeve 49 having a wall thickness substantially greater than t~at of inner sleeve 48 is employed to minimize losses when the cable is buried in earth or in physical contact with condu~ting materials such as metal surfaces of bridges or tunnels, railings on which the cable is supported and the like.
A protective insulating jacket 25, fabricated of suitable - material such as vinyl plastic, is employed as shown. The ~'nductive transmission line in this cable structure is formed ~y outer sheath 24 and inner ground sheath 12, establishing impedance of the circuit.

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further embodimen-t of a combined inductive-signaling and coaxial trunk cable is shown in Figure 9. Center conductor 12A and coaxial ground sheath 12 are held in dielectric sleeve ; 48 a~out 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 ~n the illustration, the spiral conducting strip 24 i5 held w~thin 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-s~gnaling line in this case is formed by conducting strip 24 and coaxial ground sheath 12, with fixed impedance present by the -- line.
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DE~CRIpTION O~ ~IGURE 10 ".
Re~err~ng now to ~gure 10 there is sho~n in schematic ~orm the roads~de use of an inductive-signaling cable of the t~pe s~own ~n ~igure 5. An r~f. carrier modulated by audio ~:. .
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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 ..~
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: ...... ~ . , :~ . , energy is dpplied r rom coaxial ce~er conductor 12A through coupling capacitor 55 and adjustable attenuator 57 to inductive-signaling conductor 24 supported within jacket 25A and positioneci in fixed relationship with respect to ground sheath 12 as illustrated in Figure 5. The inductive transmission line formed by conductor 24 and ground sheath 12 is terminated by resistor 58, assuming inductive or capacitive reac~ances have been balanced out. At a given distance along the cable, such as 1/2 mile, coupling capaci-tor 59 and r.f. attenuator 69 enables a desired amount of r.f. signal voltage from center conductor 12A of coaxial cable 12-12A to be applied to inductive-signaling conductor 24A, serving its individual section of roadway, and extending to termination resistor 62, connected between conductor 24A and ground sheath 12. In similar manner, r.f. signal anergy 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 attenuators, 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.

In the illustrative arrangement shown in Figure 11, r.f.signal energy at a designated carrier frequency in the standard 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 a-ttenua-tor 40, of coupling and attenua-tor unit 20, to the inductive transmission line formed by conductor 24, dlsposed in coaxial relationship to c~nter conductor 50, held at ground potential. Conductor 24 is separated from center eonductor 50 by a dielectric sleeve 48, formed of polyethylene or o-ther suitable insulating material. To minimize effeet of the medium in which or on Wh-ieh the cable is laid, a relatively thick-walled dieleetric sleeve 49, such as polyethylene, surrounds the inductive-signaling conductor 24, while an insulating proteetive jaeket 24, fabrieated of v;nyl plastic or other suitable material, eomprises the outer shell of the eable.
As indieated by the illustration, the wall thiekness of the inner dielectric sleeve 48 is preferably substantially less than that of the outer dielectric sleeve 49. This arrangement permits the impedanee of the induetive transmission l~ne formed by spiral eonduetor 24 and eenter conduetor 50 to ~e established primarily by the relationship between these two conduetors, with minimum ehanges in line eharaeteristies or losses because of variations in soil eonduetivity or other external factoxs. The inductive~signaling cable shown in ~igure 11 may be employed on roadways where it may be desirable to utilize separate inductive-signaling eables fed by r.f.
signal energy from a conventional coaxial eable, such as 12-12A
for truck relay between terminal points.

~igure 12 is an enlarged detail of the eombined co-ax~al trun~ and ~nd~ctive-signaling eable shown in Figure 9.
T~s ~llustration also more elearly shows the relatively-large ~0 wall th~ckness of the outer r.f. dielect~ie sleeve 49 employed ~n th~s illustrat~ve form of ca~le as compared with the inner ~16 coaxial dielectric sleeye 48 that separates inducti~e~signalin~
conductor 24 from inner coaxial ground sheath 12.
This illustration of Figure 12 also emphasizes the difference between this induc-tive-signaling cable structure and that of conventional coaxial cables that have as basic purpose the con~inement 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 carxied 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 travelling parallel to the cable and at a substantial distance therefrom.
~` The cable shown in Figure 12 also differs basically ~n design and function from double-shielded coaxial (triaxial) cables such as employed in community television systems to m~nimize radiation from the cable in order to prevent unautho-~zed viewers from intercepting the programs for which sub-~scribers pay. In these double-shielded (triaxial) cables, both conducting sheaths are at ground potential and in direct electrical contact. There is no dielectric between the two :
~xoup 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 thick-ness of thQ jac~et is determined ~y mechanical rather than rad~o~requency transml'ssion~loss factors.

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DESCRI~'T[~ OF FIG~ RF 13 Wnile the inductive-carrier communication system of the present invention has been shown in conjunction ~,rith highways for roadside use, it can be used in drive-in motion picture theaters to transmit the sound portion of motion pictures to the radios of the autornobiles in attendance.
In a drive-in motion picture theater, the audience in their automobiles ~00, after payment of the entrance fee, are admitted into the the~ter area and allowed to park in designated parking spaces in view of the motion picture screen 201. The parking spaces are positioned on and along continuous ramps 202 in front of the screen 201. The ramps 202 raise the front ends of the automoblles to enhance the automobile occupants' view of the screen. Since the spaces are along the ramps 202, they fc~rm rows in front of the screen 201. Normally in prior forms of drive-in motion picture theaters the sound is trans-mitted to each car by means of a portable speaker system detachably mounted on a post adjacent in each designated parking spot. The use of the communication system oiF the present inven-tion eliminates the need for use of such speakers and posts.
The inductive signaling conductor 203 accordin}g to the present invention is positioned underneath at least some o~ the ramps 202 in proximity to the car antenna 204 of each r~dio equipped vehicle.
The inductive signaling conductor 203 has impressed on it a carrier signal modulated to transmit the sound portio~ of the motion picture shown on the screen. The signal is received by the antennas 204 of each of the automobiles in at~endance and is thereaEter played through each of the automo~le's radios.
In Figure 13~ the inductive-signaling co~Dductor 203 may be buried in the ramp 202 or otherwise disposed ~ithin a short distance from the car antenna 204.

To form inductive-signaling conductor 203, an~ o~ the cable structures described heretofore may be ~sed including the ~50~
em~odiments of Figures 5,6,7,8,9,:Ll and 12. Equally, any ofthe coupling and terminating structures prevlously disclosed may ~e used to connect and ground the cables in carrying forward t~e use of the present inductive-carrier communication system in drive-in motion picture theaters.
Because of the configul-ation of the cable of the present invention, particularly as shown in Figures 11 and 12, and Because of the relative positioning of the automobiles and t~e inductive-signaling conductor 203, a high level of efficien-cy of transmission of the signal between the inductive-signaling conductor 203 and each automobile antenna 204 may be accom~
plished. The configuration for positioni.ng the inductive-s~gnaling conductor 203 in the theater area which produces the ~est reception for all of the automobiles in the reception area, ~s when the inductive-signaling conductors 203 are positioned as s~own along each of the rows of parking spaces formed along t~e ramps 202, as shown in Figure 14. ~.
DESCRIPTION OF FIGURE 14 and 15 , ; Figure 14 shows in diagrammatic form, a drive~in . 20 motion picture theater according to the present invention. The screen 201 upon which the motion picture is projected is pos~tioned for easy v~ewing throughout the theater parking area.
The ramps 202 (A-F) are arranged in concentric arcs in front of the motion picture screen 201~ Opposite and some distance :
- from the screen is positioned the projection room 205, con taining projection equipment 220 for displaying a picture upon t~e screen 201. The projection .room 205 also contains film sound equipment 221 and a radio transmitter 215 for modulating ~ ;

t~e carr~er wa~e ener~y from said transmitter in accordance W~t~ t~e sound port~on of t~e motion p~c:ture ~e~ng shown. The r. f . generator is coupled ~y means of a power divider 216 of an~ well~kno~n type to induction cables 203 and 203A which run . ~
-19- : , .. .. .

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in an "S'l or zigzag pattern along -the rows of parking spaces.
As seen in Figure 14, :`

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thOSO indUCtiV~ S~ 3 C~ C1VC7;~S 203 arld 203~ mLIY be formed by combination ca~les o~ ttle type show~-l in the embodiments of Figures 5,6,7,8,9 or 12. .~s disc1Os~d previously, .he trans-mitter 215 is op~ratively connec-ted to the center portion 12A of each such combination cable. Each center conductor 12A is shielded by means of a cond-lcting sheath 12 surro~nding itJ held at yrou~d potential.
As seen in Figure 15, at the end of the inductive-signaling conductors 203 and 203A, farthest from the projection room 205, the inductive-signaling elements 24 of each of the combination cables forming inductive signaling conductors 203 and 203A7 is operatively connected to the corresponding central shielded portion 12A of the combination cable. The inductive-signaling element 24 of each of the inductive-signaling conductors 203 and 203A is terminated through a load resistor 223 to the sheath 12 which is held at ground potential.
Alternatively, the embodiment of cable shown in Figure ll may be used to form inductive-signaling conductors 203 and 203A.
As seen in Figure ll, the inductive-signaling element 24 would be operatively connected to the transmitter 2l~ at one end and and would terminate at the other, as previsouly explained, by coupling to ground potential.
In large installations more than two inductive-signaling conductors may be used to cover the parking area or in the alternative, when combination cable such as in Figure 12 is used, æeveral inductive-signaling elements along each combination ~able may be used.
~ hile, with the cable structure and the positioning as disclosed heretofore, signal strength falls away rapidly when outside the theater area, other measures may be taken to insure that parties who have not paid the entrance fee may not park their automobiles outside the theater area and still receive the signal on their radios. One or more inductive-signaling cables 230, of -the type disclosed in Figure 11 or its equivalent may be positioned around an outside portion of the theater area where protection against unauthorized reception is desired. If necessary, such cable or cables may be used to almost completely surround the t~eater area. The cable 230 should be coupled to an r.f. transmitter 217 located at the outer edge of the drive-in theater and may be terminated through load resistor 218 to ground 219 at the other end. The security transmitter 217 should emit a carrier signal at the same frequency as that being Impressed on inductive-signal conductors 203 and 203A. The carrier wave from security transmitter 217 should be modulated with an interfering signal of white noise or other sounds diferent than that being impressed on inductive-signaling con-; ductors 203 and 203A. Accordingly, one positioned outside the designated theater area would receive the white noise or other protective signal which would cause interference with the signals received from the inductive-signaling conductors 203 and 203A within the theater area. This interference would ~nh~b~t unauthorized listening to the sounds associated with t~e picture.
Some of the signal transmitted by inductive-signaling conductors 203 and 203A may be carried outside the designated area ~y undesirable coupling to power transmission lines, other conductors and the like located outside but in the immediate vicinity of the theater area. To prevent power lines and the like from transmitting an intelligible signal outside the theater area, an lnterfering signal at the same frequency as th`e ~ignal ~n said inductlve~signaling means 203 and 203A of w~ite noise and ot~er appropriate sound may ~e directly or 3Q ~nd~rectly ~nduced on such line to interfere with reception ~uts~de t~e t~eater prem~ses.
s preferable to locate the inductive-signaling :~5~
conductor along each row o~ designa-ted parking spaces in order that t~le signal ~e ln close proximity to the car antennas. This insures a comparatively noise-free signal even when electrical storms exist in the vicinity of the theater. It is possible, however, to position the inductive signaling conduc-tor along only some of the rows of designated parking spaces and provide acceptable reception.

~igure 16 shows an alternative arrangement of the cable system in a drive-in motion picture theater according to the present invention. The screen 201 upon which the motion p~cture is projected is positioned for viewing throughout the theater parking area. The ramps 202 (A-F) are arrang~d in concentric arcs in front of the screen 201. Opposite and some distance from the screen is positioned the projection room 205 containing, as shown in Figure 15, projection equipment 220 ~or dtsplaying a picture upon the screen 201, film sound equip-ment 221 and a radio transmitter 215 for modulating the carrier ~ave energy from said transmitter in accordance with the sound . ~ , portion of the motion picture being shown. The radio transmit-ter is coupled by means of a power divider 216 of any well-~nown type to coaxial cables 225 and 225A. Cables are comprised of a center conductor 226, fed by power divider 216, and ground sheath 227 held at ground potential, serving to confine all ; signal energy within the coaxial cables 225 and 225A. The co-ax~al ca~les 225 and 225A connect with the input circuits o - coupling means 231, 232, 233, 234, 235 and 236 as shown n The outputs of coupling means 231 through 236 feed r.f. energy to ~` inductive-signaling conductors 210 tA~F]~ The inductive~
s~gnaling conductors comprise center conductor 12A, ground s~eath 12 and radiating element 24 as shown in more detail in ~gures 9 and 12. The inductive-signaling cables 12A, 12 and ; ....
:-: , , ,. :
, .

24 as shown in ramp section 202A, o ~igure 16, also are comprised in ramp sect~ons 202B through 202F. As sho~n in ramp section 202A, the center conductor 12A is connected with ~nductive-sicJnaling element 24 at the far end 222 of each length of cable, and is connected to ground at coupling means 231. The ground connection is as shown at 223 and 224 of Figure 15. As described in connection with Figure 14, an inductive-signaling cable 230 is formed around the periphery of the drive-in theater to prevent unauthori~ed listening to t~e motion picture sound on streets outside of the theater area~ Cable 230 is fed by a security radiO transmitter 217 as earlier described in connection with the description of ~F~gure 14. The far end of the security cable is connected through load resistor 218 to ground 219. As described in connection with Figure 14, white noise or other sound at the same radio frequency as employed by the cable system within the drive-in theater is used to modulate radio transmitter 217~ The purpose of resistor 218, Figures 14 and 16, is to ~atch the .impedance between resistor and cable, thereby m~nimizing radiation beyond the limits specified in the Government's rules relating to restricted range devices.
- Likewise, resistor 223 in Figure 15 serves the same purpose of impedance matching and minimizing radiation beyond the Go~ernment~s limîts relating to restricted range devices.
The term coaxial cable as used herein may refer to the central shielded portion 12 and 12A of the combination cable such as is found in Figure 12 as well as a separate : coaxial ca~le.
While t~e cable structures such as shown in Figures 5, 6, 7, 8, 9, 11 and 12 represent presently-preferred cable em~odiments, it is understood that other forms of radiating coaxial cables may ~e utilized in providing localized radio ~ ~ ~23~

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signals for the purpose of reaching radio-equipped cars within drive-in theaters.
While the foregoing specification is descriptive of certain embodiments of the system of the present invention incorporating in a single integrated system a number of motion picture drive-in theater applications, the scope of the in-vention is not in any sense restricted to the illustrative embodiments as shown, and other embodiments evident to those s~illed in the art are considered to be within the scope of the present invention, said scope to be determined from the iollowing claims:

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A communication system of the inductive-signaling type for use in drive-in motion picture theaters and the like having rows of designated parking spaces therein comprising a carrier wave generator which generates a carrier wave within the designated broadcast band which can be received by conventional automobile radios, means for modulating the carrier wave energy from said generator, means coupling the output of said generator to the center conductor of a coaxial cable having a conducting sheath around said center conductor which conducting sheath is held at the same ground potential as said carrier wave genera-tor, at least one inductive-signaling conductor ex-tending across more than one of said designated park-ing spaces along one of said rows of designated park-ing space, said inductive-signaling conductor having a coupling means connecting one end of said inductive-signaling conductor to said center conductor whereby a controlled amount of carrier wave energy may be transferred from said center conductor to said induc-tive-signaling conductor which inductively transmits said modulated carrier wave energy for receipt by the conventional automobile radios of automobiles parked in said designated parking spaces and terminating means connecting the other end of said inductive-signaling conductor to the conductive sheath of said coaxial cable.

2. The system of claim 1 where there is a plurality of inductive-signaling conductors, each one positioned along at least part of one row of said park-ing spaces.

3. The system of claim 2 where each row of designated parking spaces has a ramp for properly posi-tioning an automobile in view of a motion picture screen within which ramps said inductive-signaling conductors are mounted.

4. The system of claim 3 wherein said ramps form concentric arcs in front of said screen and where-in the inductive-signaling conductors are connected to-gether by said coaxial cable means which extends across said ramps to said carrier wave generator.

5. The system of claim 1 wherein said induc-tive-signaling conductor extends along and between more than one of said rows of designated parking spaces.

6. The system of claim 5 wherein each row of designated parking spaces has a ramp for properly posi-tioning the automobiles parked thereon in relationship to a motion picture screen and within which ramps that portion of said inductive-signaling conductor, which extends along said rows of parking spaces, is mounted.

7. The system of claim 1 wherein said sig-naling conductor extends along at least a portion of said coaxial cable.

8. The system of claim 2 wherein said co-axial cable and said inductive-signaling conductor are for at least a part of their length combined in a unitary cable structure.

9. The system of claim 1 wherein along the outside of at least a portion of the theater area form-ed by the rows of designated parking spaces, is posi-tioned a second inductive-signaling conductor which is operatively connected to a transmitter which generates a carrier wave at the same frequency as the signal im-posed on the said first inductive-signaling conductor positioned along at least one of said rows of desig-nated parking spaces and a means for modulating the carrier wave differently from that in said first in-ductive-signaling conductor whereby a signal is pro-duced in said second inductive-signaling conductor that interfers with the reception of the signal pro-duced by the first inductive-signaling conductor out-side the theater area.

10. The system of claim 1 wherein a radio transmitter which generates a carrier wave at the same frequency as that imposed on said inductive-signaling conductor is operatively connected to means for modu-lating said carrier wave differently.