US2411786A - Communications system - Google Patents

Description

Examiner 5 Sheets-Sheet l w. s. HALsTl-:AD

Filed Aug. 11, 1944 COMMUNICATIONS SYSTEM Nov. 26, 1946.

Examine 5 Sheets-Sheet 2 W/L/AM 5. HALTf/ID INVENTOH ATTORNEY A Examnel` 935 Nov. 26, 1946. w. s. HALsTl-:AD 2,411,786

COMMUNICATIONS SYSTEM Filed Aug. ll, 1944 5 Sheets-Sheet 3 l n 9 m W/LL /A M 5. HALSTEAD INVENTOR BY AW ATI'OHNEY Examm U9, ELEPHUm.

NOV- 26, 1946- w. s. HALSTEAD COMMUNICATIONS SYSTEM Filed Aug. 11, 1944 5 Sheets-Sheet 4 D r f am/.MM Hw A 5. /1 M, M u W if Hom,

Nov. 26, 1946. w. s. HALSTEAD 2,411,786

COMMUNICATIONS SYSTEM Filed Aug. 11, 1944 5 sheds-sheet 5 f l 250] 257) f4? f MAsTm A pon/fp 1 asc/Lum@ 4m/Hf@ v 24s 248 f AUD/0 AMPL/F/Q W/LL/AM 5. HALSTEAD HVVENTOR ATToRN'gY A U9, ltLtvhum Patented Nov. 26, 1946 Examiner,

COMMUNICATIONS SYSTEM William S. Halstead, Purchase, N. Y., assigner,

by mesne assignments, to Farnsworth Television & Radio Corporation, Fort Wayne, Ind., a

corporation of Delaware Application August 11, 1944, Serial No. 548,962

12 claims. 1

This invention pertains to carrier wave communications systems in general, and more particularly to a system for utilizing a frequencymodulated transmitter at a central station for signal distribution over relatively large areas by means of public utilities lines or other conductors, and receiving said signals in predetermined signal areas or zones on frequency-modulated zone receivers, and re-transmitting the signals at each particular localized signaling zone on an amplitude-modulated zone transmitter.

By utilizing the system of the present invention, the high level of noise interference ordinarily encountered on power lines and other conductors may be substantially eliminated because the frequency modulation employed at the central station has a greatly improved signal to noise ratio over that provided by an amplitudemodulated system.

My Patent No. 2,389,257, dated Nov. 10, 1945, for a Carrier Wave signaling system, discloses in general a system utilizing power lines, wayside conductors, or any available conductors in general for establishing a distributing network so that a central station transmitter may be used in conjunction with repeater stations. The central station in the above-mentioned patent relies generally on utilizing a central transmitter operating on a comparatively low frequency while the repeater station receives the signalling impressed upon the distributing network and redistributes it in localized zones on a different frequency, such as on frequencies ordinarily incorporated in the broadcast spectrum.

The present invention, however. differs in general from that of said patent in that the present invention employs frequency modulation at the central station transmitter, the signals of said transmitter being received in localized signaling zones and re-transmitted on amplitude-modulated transmitters so that the conventional home type receivers may be employed for receiving signals which originated at the central station as frequency modulated signals.

Throughout the application the term distributing network or wayside conductors is understood to mean any or all types of conductors such as power lines, telephone or telegraph lines, cables, rails, pi sul ble conductor which.. permits a e operation of the m.

It is an object of the present invention to provide a carrier wave communication system utilizing power lines, telephone lines, telegraph lines, or any other suitable conductors for establishing communications coverage over a relatively large area, wherein a central station or master transmitter impresses frequency-modulated signals on power lines or other conductors. The frequency-modulated signals may be received in predetermined signaling zones or areas on frequency modulation types of receivers, the outputs of which are impressed upon local zone transmitters which in turn impress amplitudemodulated signals on localized conductors (which may or may not be part of the conductor system over which the frequency modulated signals are carried) for establishing local signaling zones or areas.

A further object of the invention is to provide a system of the class described wherein frequency-modulated signals are impressed upon public utility lines or other wayside conductors which embrace a relatively large area, and having said signals received on frequency-modulated zone receivers and retransmitted on amplitude-modulated zone transmitters in localized zones, with the frequency of adjacent amplitudemodulated zone transmitters being different from one another to restrict interzone interference.

A further object of the invention is to provide various means of coupling a central station or master transmitter to a distributing network embracing a relatively large area so that the amplitude-modulated transmitters of repeater stations may retransmit, over localized areas a signal which originally was transmitted from the central station on a frequency-modulated transmitter. By this means conventional amplitudemodulated broadcast receivers may be employed for receiving intelligence which originated at the central contact station as frequency-modulated signals. 4

A further object of the invention is to provide a repeater booster comprising a receiver and transmitter with a relay control system which is employed at a point on the power lines or other conductors where there is appreciable attenuation, said attenuation usually being caused by excessively long lines or caused when said conductors run for a considerable distance underground or through metal conduit.

Further and other objects may be and may become apparent from a perusal of the present disclosure, since the present showing is by way of illustration only and is not to be construed as a limitation except as defined by the supporting claims.

In the drawings:

Fig. 1 is an overall block diagram of an induction radio system.

Fig. 1A is a modification of Fig. 1, showing an alternative method of coupling the equipment to the distribution network.

Fig. 2 is a block diagram of a frequency-modulated transmitter.

Fig. 3 is a schematic diagram of a frequencymodulated transmitter as represented in block diagram Fig. 2.

Fig. 4 is a block diagram of a frequency modulation zone receiver having means for coupling to a zone transmitter.

Fig. 5 is a schematic diagram of the frequency llrpiiodillated zone receiver shown in block diagram Fig. 6 is a block diagram of an amplitude modulated zone transmitter.

Fig. I shows the schematic diagram of the ampiitude modulated transmitter shown in Fig. 6.

Referring to the drawings, and more particularly to Fig. 1 thereof, the numeral 8 designates a high voltage line, as for instance a 23.000 volt line, on which is a 100 kilocycle frequency modulated carrier and 23.000 volt feeders extending throughout the service zone.

A microphone I5 or dual turntable with suitable amplifiers |4 is connected to a 100 kilocycle frequency modulated central station transmitter I3. Said transmitter is coupled to the 115 volt line I2 and feeds through its associated transformer II, 2300 volt line I0 and then through transformer 9 into the 23000 volt feeders 8 extending throughout the service area.

There are here shown zones I and 2, and as the description of one zone will sumce for both or any duplication thereof, the apparatus in zone I will be described.

The transformers I6 and I8 of zone I connected by lines |1 and I9 feed the 100 kilocycle FM carrier to the 100 kilocycle FM zone receiver 20. The output of zone receiver 20 is coupled to a 590 kilocycle amplitude modulated zone transmitter 22. This zone transmitter 22 is automatically controlled by a FM carrier operated relay circuit 23. which operates the transmitter 22 whenever the central station transmitter I3 is in operation. The output of said transmitter 22 is coupled back into the power lines I9, I1, and 26 through its associated transformers |8 and 25 into a standard broadcast receiver 21, any number of which similar receivers can be operated on various sub-circuits in the zone.

In Figs. 2 and 3, the central station transmitter is shown in greater detail, Frequency-modulation is employed in this transmitter and also in the zone receivers in order to minimize disturbances, due to noise.

As will be seen in the block diagram Fig. 2, a microphone 21, which may be any type of signal originator, is connected to a reactance tube modulator 28, which is coupled to an oscillator 29, :iid a power amplifier 30, attached to an antenna In Fig. 3, this apparatus is set forth in greater detail and may be taken as indicative of one form that this may assume. In this figure oscillator tube 34 is coupled through variable resistor 82 and capacitor 42 supplying radio frequency energy to the reactance modulator tubes 32 and 33 and their associated circuit which functions in a conventional manner when a modulated input is impressed at input 15. Oscillator tube 34 is also coupled through coupling capacitor 4.1 t0

power amplier tube 35 and its associated circuit. 'I'he power amplifier tank circuit consists of in ductance 64 tuned by the variable capacitor 52 which is in turn shunted by xed capacitors such as 53, 54, 55, and 56. These xed capacitors may be cut in or out of the circuit for tuning by means of switch 94. Inductance 65 is inductively coupled to inductance 64 and couples to the 115 volt power line through attenuator 90 and capacitors 51 and 58, at terminals 88 and 89, or these terminals 88 and 89 can be connectedto a radio frequency transmission line 9|, said line to run parallel and in close proximity to a power line 92 and ending in a terminating unit 93. 'Ihis latter method of coupling is shown in Fig. 1A and description thereof. Voltage regulator tubes 36 are required for stable operation of tube 32, 33, and 34.

Rectifier tube 38 is a full wave rectier which with its associated power transformer 12 supplies high voltages to the plates and screen grids of the transmitter tubes. Filter chokes 92 and 93 in conjunction with lter capacitors 59, 60, and

. 6| produce a smooth direct current output from the power supply.

The frequency modulated zone receiver is illustrated in Figs. 4 and 5, particular reference being made to Fig. 4, where the same is shown in block diagram, consists of an antenna 95, a R. F. ampliner 96, a converter 91, an I. F. amplifier 98, a limiter 99, a discriminator |00, and an A. F. amplier |0I, which leads to the amplitude-modulated transmitter. Leading from the limiter 99, is a D. C. amplifier |02 operating relay |03, which in turn controls said amplitude modulated transmitter when the central station transmitter is in operation.

The detailed circuit as shown in Fig. 5 is for the most part a conventional superheterodyne receiver except for the addition of limiter tubes |09 and ||0 and discriminator tubes III and |I2 and consists of two radio frequency tube stages |04 and |05, a converter tube |06, followed by two intermediate frequency tubes |01 and |08. The signal is then applied to the cascade limiter consisting of tubes |09 and I|0 so that the amplitude of the applied signal is materially reduced thereby suppressing noise disturbances to a large degree. The FM signals are then applied to a phase-shift type of discriminator, tubes and ||2 and associated circuit Where the frequency deviations are converted into amplitude variations. The resulting audio frequency signal is then amplified by tube I3 and fed through transformer 228 where it then goes to the output receptacle 244, Receptacle 244 connects to the amplitude modulated zone transmitter. Gain control 2|5 regulates the volume of the signal to the desired amount. A conventional power supply and lter with tube I6 supplies plate and heater current for this receiver.

As a result of the limiter actionvoccurring in tube |09 a D.C. voltage is developed across resistor 202. This negative voltage is applied to a D.C. amplifier comprising tubes ||4 and ||5. The output of tube I|5 is connected to relay 240 which leads to output terminals 24|. Contacts on relay 240 control the plate voltage supply of the A. M. transmitter. Relay 240 operates only when suflicient negative voltage is obtained from resistor 202 and this occurs only when the carrier frequency of the FM central station transmitter is being received.

Fig. 6 is the block diagram of the amplitude modulated zone transmitter operating on some 179, iELEPHONY.

Examiner established frequency in the range limit of standard broadcast receivers employed, and usually at a predetermined frequency found to be signal free, and in this instance 590 kilocycles. In this instance, for example, master oscillator 250 is amplified by power amplifier 25| and the R. F. output is then fed to the antenna 241. The power amplier 25| is modulated by audio amplifier 249 which can be operated by any type of signal source such as microphone 248 or connected to the output terminal 244 of the frequency modulated zone receiver described in Fig. 5.

In Fig. 7 is a detailed circuit of Fig. 6. Oscillator tube 252 is tuned by oscillator tank 253 and coupled to power amplifier tube 254 and its associated circuit through capacitor 255. The modulated output of this tube 254 is coupled through its tank inductance 253 to output coupling inductance 251. The modulating input 246 to the audio system is coupled through the speech input transformer 258 and the output of this transformer connects to audio driver tube 259. A class B driver transformer 260 couples tube 259 to the push-pull class B modulator tube 26| which in turn connects to the modulation transformer 262. This transformer 262 supplies class B modulation to the power amplifier tube 254.

A conventional power supply for the transmitter consists of plate transformer 263, rectifier tube 264, filter chokes 265 and 266 and filter capacitors 261 and 268. Heater current for all tubes is supplied by transformer 269. Switch 210 controls the input from the 115 volt line to the power supply while switch 280 controls the operation of the plate supply transformer. Terminals 212 permit the transmitter to be controlled from output terminals 24| controlled by relay 240 of Fig. 5, so that signals from the 100 kilocycle central station transmitter will be re-broadcast automatically.

In operation, this amplitude-modulated zone transmitter has its frequency determined by setting the master oscillator tuning condenser 213, the proper setting being determined from the calibration curve. With the R. F. output disconnected, the power amplifier coarse adjustment switch 214 is rotated until a dip is obtained on the plate milliameter 28|. The power amplifier fine adjustment tuning condenser 215 is then adjusted to give a minimum defiection on the plate milliameter 28|. The R. F. output 216 is plugged into a 115 volt source with the R. F. output attenuator 211 set approximately in half-way position. Then the coupling is varied between amplifier tank 256 and output coil 251 until a maximum desired deection is obtained on the plate meter 28|, or output terminals 216 may be coupled to a tuned transmission line as disclosed in Fig. 1A. The audio-frequency attenuator 219 is now adjusted so that the modulation indicator 218 fiashes only on severe peaks, and should the indicator 218 remain lighted, there will be severe distortion.

Fig. 1A shows an alternate method of applying induction radio signaling to a power line circuit. Microphone I5 or dual turntable with amplifiers I4 is connected to a 100 kilocycle frequency modulated central station transmitter I3. The -output from transmitter I3 is then fed into a tuned line |20 which extends for a distance of from 500 to 1000 feet substantially parallel and in close proximity to a conventional 2300 volt primary power circuit |22. Tuned line |20 is terminated at its extreme end by a suitable terminating unit 2| to avoid standing wave radiation, substantially conning the signals to the power line and its immediate field.

The illustration indicates that the circuit |22 may run for a distance in underground conduit |23 before coming to the surface at zone where the FM induction radio signals are picked up by receiving antenna |24 which connects to zone receiver |21 as previously described and illustrated in Figs. 4 and 5. Zone receiver |21 in turn is connected to the A. M. transmitter |28 and with identical relay control circuit |29 as previously described in detail in reference to Fig. 5. Ihe A. M. zone transmitter operating on 590 kilocycles is then fed into tuned line |30, said line being terminated by a terminating unit |3| to eliminate standing waves in the same manner as described in reference to FM antenna |20. This line |30 is run parallel and preferably in close proximity to the 2300 volt power. circuit for a distance of from 500 to 1000 feet thereby imposing the 590 kilocycle A. M. signal back into the local zone circuit and its secondary power circuits such as circuit |32 to which may be connected a conventional broadcast receiver |35. Any number of similar receivers can be used to pick up the signal on this and other circuits in the zone. The broadcast receiver |35 picks up the signal from the power circuit inductively by antenna |36.

From the foregoing descriptions taken in connection with the drawings, it should be evident that an induction radio signaling apparatus is provided.

inasmuch as the transmission of the amplitude-modulated transmitter must be of a type that transmits only to a restricted zone, it will be noted that by the adjustment of the several elements thereof, and more particularly the attenuator 219, which not only acts as a radio frequency attenuator but also a line coupling device, such result is attained, and that only receivers within the restricted zone will be affected thereby.

Also that it is specially useful in army camps, and similar places where broadcasting of a more or less secret nature is desirable. Also that with this system receiver sets of conventional types can be utilized in the houses or homes within the selected area; and that with this system the signals are confined within such area for the system permits establishment of a purely localized broadcasting service in the broadcast band with transmission of effective signals being coni-ined substantially within limits of, as for instance, the cam area. This system thereby affords a relative degree of security in such transmissions as compared with those of normal space-radio systems and permits, as before noted, of the use of conventional broadcasting receivers in all buildings within the entire camp area which may extend many square miles.

It will also be understood that a frequencymodulated central station transmitter sends out the broadcasting signals from a central studio located for example, at an army camp, then through the distributing network extending throughout the camp or other area. The induction radio field, extending laterally from these conductors reaches out for a distance of several hundred feet in actual use, dropping away so rapidly that at a distance of 300 to 400 feet, the signal ordinarily is below the noise level. Also the signal intensity of' the induction field, as measured with a eld strength meter located at a given lateral distance from the conductors at the edgeof the selected area, may be regulated to a desired field strength value, and which in this country would be specified by the Federal Communications Commissions regulations.

While certain specic illustrations of the invention are shown, it is to be understood that they are merely illustrative of the invention and that many other modifications may be employed without departing from the spirit of the invention as defined by the subjoined claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a carrier wave communications system, means for generating a frequency-modulated carrier, means for coupling said frequency-modulated carrier to a distributing network, means for coupling said network to a receiver responsive to frequency modulation, an amplitude modulated transmitter, means for coupling said frequency modulated receiver output to an amplitude modulated transmitter operating on a frequency different than said first transmitter, means for coupling the output from the amplitude modulated transmitter back into said distributing network, a second receiver operating in the local zone area of the distributing network and tuned to the frequency of said amplitude modulated transmitter, and means for reproducing into sound the output of the second receiver.

2. In a restricted zone signaling system, means for generating frequency modulated radio frequency signals, means for coupling, superimposing, and adjusting the frequency modulated radio frequency signals to a predetermined level on an electric transmission line, means for coupling said transmission line to a receiver located at a remote point, coupling means for re-impressing the received intelligence of said radio frequency signals on a second transmitter, a second transmitter of the amplitude modulation type operating on a frequency different than said first transmitter, 'means for coupling, superimposing and adjusting to a predetermined level the second frequency on said electric transmission line, a receiver disposed within the restricted signaling zone established by the second transmitter and adjusted to said second frequency, and means connected to the output of said receiver for translating the intelligence of said second frequency.

3. In a restricted zone signaling system, means for generating frequency modulated radio frequency signals, means for coupling, superimposing, and adjusting the frequency modulated radio frequency signals to a predetermined level on an electric transmission line, means for coupling said transmission line to a receiver located at a remote point, coupling means for re-impressing the received intelligence of said radio frequency signals on a second amplitude modulated transmitter, said second amplitude modulated transmitter operating on a frequency different than said first transmitter, the frequency between said second transmitter and said rst transmitter being such as to eliminate interaction therebetween, means for coupling, superimposing and adjusting to a predetermined level the second frequency on said electric transmission line, a receiver disposed within the restricted signaling zone established by the second transmitter and adjusted to said second frequency, and means connected to the output of said receiver for translating the intelligence of said second frequency.

4. In a restricted zone signaling system, means for generating frequency modulated radio frequency signals, means for coupling, superimposing, and adjusting the frequency modulated radio frequency signals to a predetermined level on an electric transmission line, said radio frequency being of such a value as to pass through line transformers with minimum attenuation, means for coupling said transmission line to a receiver located at a remote point, coupling means for reimpressing the received intelligence of said radio frequency signals on a second amplitudemodulated transmitter, said second amplitudemodulated transmitter operating on a frequency different than said first transmitter, the frequency difference between said second transmitter and said iirst transmitter being such as to eliminate an interaction therebetween, means for coupling, superimposing and adjusting to a predetermined level the second frequency on said electric transmission line, a receiver disposed within the restricted signaling zone established by the second transmitter and adjusted to said second frequency, and means connected to the output of said receiver for translating the intelligence of said second frequency.

5. In a restricted zone signaling system, means for generating frequency modulated radio frequency signals, means for inductively coupling said frequency modulated radio frequency signals to an electrically conducting line of said system, a termination unit connected to said last means to restrict the formation of standing waves thereon, means for inductively coupling said electrically conducting line to a receiver, coupling means for re-impressing the received radio frequency signals on a second amplitude modulated transmitter, said second amplitude modulated transmitter operating on a frequency different than said first transmitter, means for inductively coupling the second frequency signals t0 an electrically conducting line of said system, a termination means connected to said last named means to restrict the formation of standing waves thereon, a receiver disposed within the restricted signaling zone established by said second transmitter and adjusted to said second frequency, and means connected to the output of said receiver for translating the intelligence of said second frequency.

6. In a restricted zone carrier wave signaling system having at least one electrically conducting line, a frequency-modulated transmitter for transmitting modulated radio frequency signals, means for coupling said modulated radio frequency signals to an electrically conducting line of said system, a plurality of frequency modulation receivers, means for coupling said electrically conducting line of said system to said plurality of receivers, a plurality of amplitude modulated transmitters, coupling means for re-impressing the received radio frequency signals on said plurality of amplitude modulated transmitters, one of said plurality of amplitude modulated transmitters being complemental to one of said plurality of receivers, each of said plurality of transmitters operating on a frequency different than said first frequency-modulated transmitter, and means for coupling each of said complemental transmitters to an electrically conducting line of said system.

7. In a centrally controlled carrier wave distribution system having at least one electrically conducting line, a central station frequency modulated transmitter, means for modulating said central frequency modulated station transmitter, a radio frequency attenuator and line coupling unit connected to the output of said transmitter, the output of said radio frequency 179, lELEFHONY, Examnel attenuator and line coupling unit being connected to said electrically conducting line, a secondary communications distributing system including a frequency modulated zone receiver and an 10 termination means connected to the coupling means to restrict the formation of standing waves on said coupling means, a plurality of frequency modulated zone receivers, means for couamplitude modulated zone transmitter, means pling` said electrically conducting line of said for automatically initiating operation of the system to said plurality of frequency modulated amplitude modulated zone transmitter when the receivers, a. plurality of amplitude modulated carrier from the frequency modulated zone rezone transmitters, coupling means for re-impresceiver is impressed upon said amplitude modulatsing the received radio frequency signals on ed zone transmitter, and a zone radio frequency said plurality of amplitude modulated zone transattenuator and line coupling unit connected to mitters, one of said plurality of amplitude modthe output of said zone transmitter, the output ulated zone transmitters being complemental t0 of said zone radio frequency attenuator and line one of said plurality of amplitude modulated recoupling unit being connected to said electrically ceivers, each of said amplitude modulated zone conducting line to establish a localized amplitransmitters in adjacent zones operating on diftude modulated signaling zone. ferent frequencies, all of said amplitude modu- 8. In a centrally controlled carrier wave dislated zone transmitters operating on a frequency tribution system having at least one electrically different than said frequency modulated central conducting line, a frequency modulated central station transmitter, means for inductively coustation transmitter, means for modulating said pling each of said complemental amplitude modfrequency modulated central station transmitter, ulated transmitters to an electrically conducting a radio frequency attenuator and line coupling line of said system, and termination means conunit connected to the output of said transmitnected to each of last said coupling means to ter, the output of said radio frequency attenurestrict the formation of standing Waves on each ator and line coupling unit being connected to of last said coupling means. an electrically conducting line of said system, a 1l. In a centrally controlled carrier wave dissecondary communications distributing system tribution system having at least one electrically including a frequency modulated zone receiver conducting line, a frequency modulated central and an amplitude modulated zone transmitter, station transmitter, means for modulating said means for automatically initiating operation of frequency modulated central station transmitter, the amplitude modulated zone transmitter when a radio frequency attenuator and line coupling the carrier from the frequency modulated zone unit connected to the output of said transmitter, receiver is impressed upon said zone amplitude the output of said radio frequency attenuator modulated transmitter, and a zone radio freand line coupling unit being connected to said quency attenuator and line coupling unit conelectrically conducting line, a secondary comnected to the output of said amplitude modulatmunications distributing system including a freed zone transmitter, the output of said zone radio quency modulated zone receiver and an amplifrequency attenuator and line coupling unit betude modulated zone transmitter, means for autoing connected to an electrically conducting line matically initiating operation of the amplitude of said system. modulated zone transmitter when the carrier 9. In a restricted zone carrier Wave signaling from the frequency modulated zone receiver is system having at least one electrically conductimpressed upon said amplitude modulated zone ing line, a frequency modulated central station transmitter, said automatic initiating means intransmitter for transmitting modulated radio cluding a carrier-operated relay responsive to frequency signals, means for coupling said modoperation of said frequency modulation zone reulated radio frequency signals to an electrically ceiver, and a power supply relay responsive to conducting line of said system, a plurality of operation of said carrier-operated relay to supfrequency modulated zone receivers, means for ply plate voltage for said amplitude modulated coupling said electrically conducting line of said zone transmitter, and a zone radio frequency atsystem to said plurality of frequency modulated tenuator and line coupling unit connected to the receivers, a plurality of amplitude modulated zone output of said amplitude modulated zone transtransmitters, coupling means for re-impressing mitter, the output of said zone radio frequency the received radio frequency signals on said pluattenuator and line coupling unit being connectrality of amplitude modulated zone transmitters, ed to said electrically conducting line to estabone of said plurality of amplitude modulated zone lish a localized signaling Z011@- transmitters being complemental to one of said 12. In a communications system of the class plurality of frequency modulated receivers, each described utilizing a distributing network, a freof said amplitude modulated zone transmitters quency modulated master transmitter, means for in adjacent zones operating on different frequenmodulating Said master transmitter, a distribcies, au of said zone amplitude modulated transo utins network, means for coupling Said master mitters operating on a frequency different than transmitter to said distributing network, a plusaid frequency modulated central station transrality of frequency modulated zone receivers, mitter, and means for coupling each of said commeans fOl' Coupling Said Zone reCeVers to the plemental amplitude modulated transmitters to distributing network, a plurality of amplitude an electrically conducting line of said system. modulated zone transmitters to operate on sub- 10. In a restricted zone carrier wave signaling stantially identical frequencies, means for cousystem having at least one electrically conductpling each of said frequency modulated zone reing line, a. frequency modulated central station ceivers to a complemental amplitude modulated transmitter for transmitting modulated radio zone transmitter, and means for coupling said frequency signals, means for inductively couzone transmitters to the distributing network.

pling said modulated radio frequency signals to an electrically conducting line of said system,

WILLIAM S. HALSTEAD.

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