US2469555A - Selective radio communication system for a plurality of stations - Google Patents

Description

. y 1949- F. B. HITCHCOCK 2,469,555

- SELECTIVE RADIO COMMUNICATION SYSTEM v FOR A PLURALITY OF STATIONS Filed June 19. 1945 4 Sheets-Sheet 1 3&

. 2.3 EMU outCo 6. cm.u

MW anomg HITCHCOCK SELECTIVE RADIO COMMUNICATION SYSTEM May 10, 1949.

FOR A PLURALITY OF STATIONS 4 Sheets-Sheet 2 Filed June 19. 1945 v N\ a 5 56pm gs 3KB N mv-l May 10, 19 F. B. HITCHCOCK SELECTIVE RADIO COMMUNICATION SYSTEM FOR A PLURALITY OF STATIONS 4 Sheets-Sheet 5 Filed June 19, 1945 mKman s \H P @E 36E uunvs owcw 5 5 2 5 (Emir I .551: ms; 0 68m Aux owe a w m 1 m w w M m mun fi g a uxfioafl vaou 3.4L. M at m mm, Tu fi x m. I 05:19-22 3 5 fi c E m 5 r n B a 133i l I I c0565 May 10, 1949. F. B. HITCHCOCK SELECTIVE RADIO COMMUNICATION SYSTEM FOR A PLURALITY OF STATIONS 4v Sheets-Sheet 4 Filed June 19, 1945 895 SUEzEEQ Patented May 10, 1949' UNITED STATES TENT OFFICE SELECTIVE RADIO COMMUNICATION SYS- TEM FOR A PLURALITY OF STATIONS Forest B. Hitchcock, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

16 Claims.

This invention relates to train communication systems for railroads, and more particularly pertains to a system for providing communication to and from a plurality of trains and a central ofiice, as well as providing communication between the several trains.

In a system involving communication to and from moving trains throughout an area under the direction of a central omce, it is expedient to employ train carried radio transmitting and receiving equipments having an efiective range much less than such area, and employ a plurality of Wayside stations each with a similar effective radio range and coupled with the central office over a wayside communication channel for transmitting messages between the central ofiice and each of the several trains in the area regardless of the limited range of the radio equipments. In such an organization, it will be apparent that the several wayside stations will include radio transmitting and receiving equipments also of a limited range comparable to the range of the various train equipments. The Wayside communication channel interconnecting the central ofiice with the various wayside stations may be of the Wired current type, or any other suitable means for communicating messages between the central office and the several stations at the same time.

One problem involved in a communication organization of this type is, that when a given train is communicating with the central omce, all other trains capable of likewise communicating with that office should know that such communication is occurring, so as not to break-in and interrupt the message then being transmitted over the Wayside communication channel; and, since the train communicating equipment then transmitting may have an effective range sufiicient to reach some particular Way station but not sufficient to reach all train stations in the area, it is proposed to provide for the retransmission of the messages being received from any train station at a particular Wayside station through the agency of the other wayside stations to all other trains within the area. This not only avoids interference with communication between any train and the central office, but also provides communication between trains not within efiective range of each other through the agency of the wayside communication channel.

Generally speaking, and without any attempt to define the exact nature and scope of the present invention, it may be stated that the radio transmitting and receiving wayside equipment is disposed at locations or Wayside stations spaced approximately the effective range of each wayside station radio equipment with a suitable overlap to take care of variations in actual operating range under different conditions. Thus, a train in any position between two adjacent locations or wayside stations is within communicating range with one or the other and perhaps :both stations. Each wayside station preferably comprises a radio transmitter and a radio receiver associated with a directional antenna having such characteristics and operated in such a manner that the transmitter of one field station will have a relatively small effect upon the receiver of the adjacent field station for reasons which will appear hereinafter.

Each wayside radio receiver when supplied with a radio carrier frequency modulated with a particular distinctive tone which is characteristic of transmission from a train, acts to render its own radio transmitter inactive and repeat the message received over the wayside communication channel to all other Wayside stations and the central office. The centralofiioe receives the message, and each of the other Wayside stations cause their respective radio transmitters to retransmit such message to all other trains of the area. While a message is being received by the various trains in the area, either from a wayside station or direct from the transmitting train station, suitable means is operated so that the radio transmitters on such trains cannot be rendered active.

A predetermined distinctive tone is transmitted from each train station along with the voice message on its radio carrier frequency signal to condition the particular wayside station equipment which receives such radio carrier frequency signal for repeating such message over the Wayside communication channel to the other wayside stations for retransmission purposes. As soon as such radio carrier frequency signal having the predetermined tone thereon ceases to be received by a wayside station, the repeat transmission over the wayside communciation channel ceases. This organization is provided in order to avoid a sustained transmission of a carrier current over the wayside channel by reason of feed back action due to signal energy received by its associated radio receiver from the radio transmitter of an adjacent Wayside station. Thus, when communication from a train stops, the predetermined tone frequency on its radio carrier frequency also stops, and conditions at the receiving wayside station for repeat transmission over the wayside communication channel will be cut-out or suppressed.

The directional radio antenna associated with the radio receiver and transmitter at each wayside station is employed to assure complete coverage of the area, and also to minimize the feed back action between adjacent stations causing erroneous or false retransmission conditions.

With such a directional arrangement in a retransmitting organization, it can be seen that a given train at any point on the railroad, when,

communicating with the central ofiice, will cause such communication to exist. on allof the other trains in the area, either by direct radiation for trains within radio range of eachother or by retransmission of the message in caseof trains which are out of radio range of each other.

Without such directional control, it will be found that there are certain positions where a train might be within range of a particular wayside.

station, and yet not be within range of another train within the efiective field pattern of such c particular wayside station, with the result that there would be in effect a blind spot where said another train would not know of existing comniunication to the central ofiice.

Thus, it may be said that one of the objects oi the present invention is to provide a train communication system in which there is suitable means to prevent interference between trains in their communication with the central ofice, and also in such a way that there are no blind spots with respect to the intercommunication between trains both for advising of the busy condition of the Wayside communication channel and also for the purpose of insuring intercommunication between trains.

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings, and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference will be made to the accompanying drawings, in

like letter reference characters provided,

with distinctive preceding numerals, are used for designating corresponding partsthroughout the several views, and in which:

Fig. 1 illustrates in a diagrammatic manner one organization of a central oifice provided with carrier current equipment for associating it with a., carrier current communication channel be,-

tween such central office and a plurality of way-.

side stations;

Fig, 2 illustrates diagrammatically a typical wayside station having carrier current equipmerits. associating it with the carrier current communication channel extending from the central ofice and also having its radio equipment for communication with the moving trains;

Fig. 3 illustrates in a diagrammatic manner the radio equipment employed at a typical train station;

Fig. 4 illustrates in graphical form one organization of the wayside communication channel and the various wayside stations with their directive radio antennas;

Fig.5 illustrates in a graphical form a wayside communication channel interconnecting the wayside stations with the central office and with a modified arrangement of the directive radio antennas for the wayside stations; and

Fig. 6 illustrates in a graphical manner the efiective range and directionof the radio equipmentat a typical train station.

order to simplify the illustrations. in the.

drawings and facilitate in the explanation of the fundamental characteristics of the invention, various parts and circuits have been shown diagrammatically in accordance with conventional symbols. Arrows with associated symbols and are employed to indicate connections of the circuits of the various relays to theopposite terminals of a suitable source of current for the energization of such relays; and the source of current may be of any suitable characteristic for the purpose intended. The various contacts of the relays involvedin the illustrations are shown conventionally as being in a lower or inclined position when the coil or winding of the associated relay is deenergized, and in a raised or horizontal position when the relay is energized; the contacts belonging to any given relay are shown connected to its coil or winding by dotted lines, and these contacts may be either below or above the illustration of the relay winding. The front and back contacts between which the movable contacts are operated by the different relays are shown conventionally as arrow heads, and the movable contacts are ordinarily of the type which have their contacts pulled downwardly by gravity or by spring action.

The specific embodiment of the invention chosen for the purposes of illustration includes.

a carrier current wayside communication system sometimes termed wired radio, and this communication system or channel is of theconventional type for. transmitting messages between the central ofiice and the plurality of Wayside stations.

of any suitable frequency employed for such purposes and may have its carrier frequency modulated in any suitable manner for the transmitting of voice messages.

Each of the wayside and train stations has a suitable radio transmitting and receiving equipment of the conventional type operating on a suitable carrier frequency selected for the particular use of the control area of the railroad under consideration; and in this connection it is contemplated that such frequency will ordi: narily be in the megacycle band since the higher frequencies are more suitable to directional antenna control, although other frequencies may be, employed. For the purposes of the present disclosure, frequency modulation of the radiocarr' rier signals is assumed to be employed for reasons which will be explained as the description pro? gresses, it should be understood that the characteristic principles of the present invention may also be applied to an organization employing,

amplitude or phase modulation.

Since the structure and operation of convening and relationship to the parts necessary for illustrating the characteristic features of the organization embodying the present invention.

In other words, the various component parts of such an organization comprising oscillators;

frequency multipliers, amplifiers, modulating and demodulating organizations, and the like, have been omitted for the sake of simplicity in the disclosure of the novel combination constituting thevarious detailed component invention, Also,

The carrier frequency employed for, such Wayside channel of communication may be parts 'ofthe" conventional units may be varied as desired without altering the operation of the organization as a whole. It is also assumed that the various sources of energization for supplying potential to the plate circuits, for biasing the vacuum tube grids, for effecting the energization of vacuum tube heaters, and the like, may be provided in any of the conventional and well known ways; and certain sources for the plate circuits have been shown only in those cases where it is believed expedient to indicate such sources as being selectively applied to better indicate the conditions under which the invention is intended to operate.

GENERAL ORGANIZATION The extent of the communication area to which the present invention is applied may be determined in any desired manner depending upon the operating characteristics of the particular railroad involved. Such an area, for example may be a traflic dispatching division, a locomotive division, or any other arbitrary section of territory of the railroad throughout which communication is desired between a central ofiice and the trains in that area. For convenience in the disclosure of the invention, it is assumed that the central communication omce CO in Fig. 4 is connected with a plurality of the wayside stations A, B, C, D, etc. by a carrier current system. Such system involves the use of line wires, such as the line wires I and 2 extending throughout the control area as indicated in Figs. 1 and 2 when placed end to end. This line circuit may be used for various other services Without interference with the carrier current system, or may be a line circuit used wholly for the wayside communication channel. In any event, the line circuit should be suitably terminated at each end so as to provide the proper electrical characteristics for such a system. Such a termination is indicated at 3 in Fig. 1.

The carrier current equipment at the central offiice CO (see Fig. 1) and at the various way stations A, B, C, D, etc. (see Fig. 2) is inductively connected to the line circuit through suitable coupling transformers 4 and blocking condensers 5, so as to suitably protect the carrier current equipments from the other services on the line circuit and also from lightning surges and the like.

Central ofiice apparatus Referring to Fig. 1, it will be noted that a suitable carrier current receiver is connected inductively through its line coupling transformer 4 and is normally rendered active by reason of the connection of its plate supply through back contact 6 of the transmit relay T. Thus, any carrier current signal imposed upon the line circuit will be received and the message modulating such carrier current signal will be reproduced by the associated loudspeaker. Associated with the carrier receiver is a suitable carrier responsive relay CR which is normally deenergized but is so related to the receiver that it is energized and picks up its contacts whenever a carrier current signal is received over the line circuit regardless of whether such signal is modulated with a message or not.

The central oflice also includes a carrier current transmitter inductively coupled to the line circuit through the line coupling transformer 4, and this carrier current transmitter is normally inactive. However, the carrier current transmitter can be set into operation, provided no carrier. signal is being received by its associated receiver as indicated by the indicator lamp BK, by merely actuating the self-restoring push button PB. More specifically, the actuation of the button PB closes the circuit for relay T from and including back contact 1 of relay CR, push button PB in an actuated position, windings of relay T to The picking up of the relay T opens back contact 6 to render the associated carrier current receiver inactive, and to close front contact 6 to render the carrier current transmitter active. With the carrier current transmitter active, speaking into the microphone will cause the transmission of a suitable message by the modulation of the carrier. It is, of course, apparent that the operator must hold the button PB actuated throughout the transmission of a message.

In the event that a message is being received, it is apparent that front contact 1 of the carrier responsive relay CR. will be closed thus energizing the busy indicator BK so that the operator is advised against endeavouring to transmit a message. But in the event that he should inadvisedly actuate the button PB, it cannot energize the relay T under such circumstances since the back contact 7 is open.

Typical wayside station apparatus Each wayside station includes a suitable carrier current receiver inductively coupled to the line circuit through its line coupling transformer 4, and is normally rendered active by reason, of

- the connection of its plate supply of energy through back contact 20 of tone responsive relay 2TB.

A suitable carrier responsive relay 2GB is connected to the associated carrier current receiver so as to be energized and pick up its contacts wherever a carrier current signal is received under the line circuit regardless of whether such signal is modulated with a message or not. The closure'of front contact 2| of relay 20R connects through back contact 22 of the tone responsive relay 2TB to the transmit relay 2T thus causing the associated radio transmitter to be connected to its associated antenna through front contact 23 and to be rendered active by the connection of its plate supply through front contact 24. Since the carrier current receiver is directly connected to the radio transmitter through connection 25, any message received by the carrier current receiver is repeated by the associated radio transmitter.

Each wayside station includes a suitable radio receiver which is normally connected to the antenna through back contact 23 of relay ET; and is normally rendered active for the reception of radio frequency signals modulated with any desired messages by reason of the fact that its plate supply is normally connected through back contact 2 -5 of relay 2T. Associated with this radio receiver is a suitable band-pass filter 26 for passing the frequency of the tone carried by the radio frequency signals transmitted from any train station. For the purposes of definiteness in the disclosure this frequency is preferably of a supersonic frequency such as 20 kilocycles, but may be of any suitable selected frequency. The output of the band-pass filter 26 is supplied to the tone responsive relay TR through a full wave rectifier unit 2i.

Whenever the tone responsive relay 2TB is picked up due to the reception of a radio carrier anomalies frequency signal having SUDBIimPOSGdEEthGIEOH the; particular selecteditone, the opening ofvback contact :2 Ztprevents .the inadvertent. energization'.

Qf-reIayLaZT, while. theopening ofback contact .20

disconnects thenplate supply from the carrier. current :receiver and connectsit tot-the carrier current. transmitter. so as to. render it active to repeat the messages .received. by u the associated radio. receiver This .repeater connectionis indi-. catedin the :drawing. of Fig. 2 by the wire 28,. but itniswtorbe understood that. anysuitable means, requiredrfori repeating such messagesmay be employedpandespecially is preferablethatuthe. out: put oflthe. radio receiver include filter means .for preventing the distinctive atone transmitted. .by the. trainrstations. fromubeing repeated over the connection; 28cm: the associated .:carrier current transmitter.

The carrier current transmitter is ,of any conventionaltype: and transmits suitable carrier. currentLsignals' which. are modulated inaccordance withathe-messagesreceived by the associated radio receiver... It is .assumed that the carrier current transmitters and receivers .at the central office and .at.eachlof.1the wayside stations operate on thersame frequency; and similarly, it is also assumed-that :theradio: transmitters and receivers at each of the wayside stations and. the train stations alsooperate on the same radio frequency.

As will be noted by the legend associated with the-antennaof Fig; 2, such'antenna is intended to be 'of a suitable directional. type. giving an efiective radio fieldnas. shown in Figs. 4 .and 5.

The directionalt features .will .be discussed in. greater-detail. in. connection with the. discussion of the directional characteristics. ofrthesystem of the present invention.

Typical train station Withreference to. Fig, 3, a typical train station is; illustrated each including aradio receiver and a.'ra'dio transmitter both of;the conventional type. The. radio; receiver has associatedtherewith a suitablejiloudspeaker andsis. normally rendered.

active for. the receptionlof radiofrequency signals bymeason oftheconnection of its ,plate supply throu h 'bachvcontacttll of transmit; relay 3T,

andrbymeasomcf the connection of the antenna to @such; radio receiver through. back-contact 3| of. transmit relay ST- Whenever. :theradio.receiverureceives a radio carrier-frequency signal, the relayiiCR is caused to the [energized picking, up its front: contact 32 andrenergizing,thebusy lamp 33K. This carrier responsive relay 3CRapicks up regardless. of Whether the carrier signal receivedis modulated with a .message-or not. The opening of back contact. 32. prevents the operation of the selfrestoring push button 3PB from energizing the transmit relay 3T. In this way, whenever the trainstation isreceiving a radio carrier frequency 82?: be. considered: in connection; withithe rdis'closulesog of Fig. 6.:

Whenever the. radio transmitter; iswrenderedn active, it has anassociated tone generatonnwhichm causes the. radio: carrier frequency signaltto. be; modulated in. accordance .with. .S'Ll-Chi distinctive; tone. This ,tone. is impressednuponathe radios frequency regardless of whethermtheroperator'is; speaking into. the associated microphone omnot Thetone generator ispreferably of the. type which. produces a supersonic vtone rthat. does not; inter-a fere withthe eiiicient, and proper-communication of voice messages.

It is believed that the nature of the inventionits advantages and. characteristic features can be best understood with further description being set forth from the standpoint ofoperation: and functioning ofthe system.

ORERATIQN Referring. toFig. 4,,a graphic representation; of the system organizatiomshows acentral oflice CO atxthe left handend of, a wayside communicae.

tion channel representedby asingle line; as. ,con-.-.

nectin the central. oifice CO with a .plura'lit wayside stations A, BQC, D, Erand Fr but it: is :to a be. understood that the. central.office may be y located. at any point alongthe, communicatiomr channel or at, the. righthand end. 7 Thewayside; stations A, B, C, etc. are distributed throughout r. the communication area along-.the right-of-way of anrailroadn Thisrightof way probablywill, not. be inastraightline, but wilhinclude curyesp; of different. degrees. but. this .is; .not material to .1 the. present invention, sinceeachrstation.hasaesc ciated therewith a directional .radio. antenna 40,-, which-has its fielddirected to cover that; portiona of .the railroad'rightsof-way' extending. ,to the; nextadjacent wayside station.

These directional. radio antennas. .4 0-. maybe, ref 1; anysuitable type using corner;refiectors,.,para bolic reflectors, orslmilar reflectors Ontheptherlhand-if it appears advantageousantennaarrays may be employed. The particular, manner.-,in,. which the antennas; 40. are made; directional" is, not material to thepractice-of the present. inven tion; but it is to be understood-that the particular. means employechmust provide a substantial.ratio-., of. .directivity I so that the. majon portion of. "the radiation of. the antenna 1 will extend. toward a the next adjacent station. Such ratio of -directivity... may be in the order of one hundredto one, or it may even be more efficient under certain circumstances.

In Fig. 4, the fieldppatterns of the antennas, 40 :are indicated by dotted lines 4 I; whichrepree sent the points. of equalzlradio -field;.or rsignal;. strength with respect to-their respective antennas;.; These field representations, 4| have not l been drawn to show the. actual;characteristicsbf-smug. particular. type of antenna but, have been; drawn. merely;. to show that antenna bearns, its radia tionsin avparticular general: direction and with. an over-lap. with respect to the. next adjacent. station.-. In this way, the,railroadvrightmf way is: adequately covered with a beam of radio -.fre-,-. quency signal. throughout its length wheneyer all": of thestations are rendered; activa for the;transa mission of a messagetat, the sa m e timen With the eifective radimfield of each antenna overlapping with the effective radio field of, .the next adjacent. station, it; will! bet.appalienltl-thfi lthere will, be a :point in the; over1apping- .po ;tions. of the .fields which will represent equalfield in; tensities of both stations. The particular location...

of this point of equal field intensity is probably not the same from time to time due to the various electrical phenomena characteristic to changes in atmospheric conditions; but in any event with the proper spacing of stations and selection of field strengths for the various stations, it will be possible to obtain a change from the point of equal field intensity to a point where the closest station predominates by a substantial percentage within a relatively short distance. The distance between the two points at which one signal is twice as strong as the other located on opposite sides of the equal strength signal point, may be conveniently termed the fringe zone of reception for reasons presently to be explained.

It is well understood that a radio receiver of the frequency modulation type is capable of maintaining reception of that station which has the highest signal strength to the exclusion of other stations of lesser strength on the same carrier frequency channel. In other words, when two stations operating on the same frequency are received at the same time by a receiver, that station which has the higher signal strength is the one which is received in preference to the other. This selectivity characteristic usually operates to a marked degree whenever the signal strength for the two transmitting stations is of a ratio of two'to one, but will still be somewhat effective upon even smaller ratios. Thus, it will be appreciated that the passage of a train station from the effective radio field of one wayside station to another will receive one station in preference to the other on each side of the point of equal intensity of the two stations. This selectivity of reception is maintained except in passing through the point of equal radio signal intensities, where there is little or no selectivity between the signals, Under such conditions of reception, there are two types of interference in receiving when two transmitting stations are using the same frequency channel.

One type of interference is termed cross-talk, where the voice modulations of both signals are reproduced. This type of radio interference will not be objectionable in the present invention, since both stations transmit the same Voice message and the two modulations of the two different signals will combine to give a single audio reproduction because there will be no substantial time lag in the retransmission operation.

Another type of interference present in receiving two substantially equal radio signals on the same frequency channel involves the production of a beat note in accordance with the difference in the instantaneous frequency deviation of the two carrier signals. When the frequency swings of the beat note are large, this beat note condition is not audible. Thus, this type of interference will have substantially no adverse effeet on the reception of messages in the present organization, because all stations are operating on the same basic frequency to transmit the same message. In other words, the frequency deviation can be kept small between any two adjacent stations by maintaining the basic frequency of the transmitters fairly constant and substantially the same, which taken in connection with the fact that the modulating audio frequencies cause variations in carrier frequency of the two signals to substantially the same extent for both stations because of the identity of the message, will cause the instantaneous frequency deviation for the two carrier signals to remain small. This of course maintains the frequency swing of the beat note condition rather large, so that it will not interfere with the reception of a message, even though such message is being received from two stations simultaneously. Thus, for all practical purposes the passage of a train station from the effective range of radio field of one wayside station into the effective range or radio field of another wayside station will not be detectable in the continuity of message reception.

In this connection, it should be noted that the directional radio antennas M for the wayside stations are all directing their radio beams in the same direction to provide minimum length fringe zones in the overlapping portions of the wayside station ranges. The reason for this is because the present invention takes advantage of the characteristics of attenuation of radio signals. For practical purposes the attenuation of a radio signal may be considered as varying inversely with the cube of the distance away from the transmitter. Thus, the forward signal of a wayside station produces an attenuation curve of signal strength having a rather steep slope near the station which drops off to a rather flat portion as the signal approaches zero at a distance from the station. This is of course also true with respect to the relatively small back-signal in the rear of the directional antenna. In considering two adjacent stations, it will be appreciated that the back-signal curve is shorter than the forward-signal curve, and that the forward-signal curve at its rather fiat portion will cross the backsignal curve at a rather steep portion so that the fringe zone is relatively short compared to what it would be if directional antennas were not employed. It should also be noted that this fringe Zone is reduced to substantially zero the more nearly perfect the directivity of the antenna. Thus, the present invention not only obviates the difiiculties involved in using a common frequency channel for plurality of radio stations but also organizes the arrangement of stations to minimize the conditions under which adverse conditions originate.

For the above reasons, the railroad right-ofway is adequately covered with a field of radio signal so that a train station will receive a radio frequency signal regardless of its location on the railroad, and will receive a suitable signal for continuous reception of a message in passing over the railroad in either direction when all wayside stations are transmitting.

The train station antenna 60 is arranged to have its eifective field directed both in advance of the train and to the rear of the train as indicated in Fig. 6 of the accompanying drawings, so that the direction of train movement does not affect reception. The eifective ranges or radio fields of the train station are indicated by the dotted lines 5!, and the extent of each range substantially corresponds to the range of the wayside stations of Figs. 4 and 5. It should be understood that the train stations may have omni-directional antennas, but since a train is always traveling along the right-of-way in one direction or the other, it is only necessary that such train station receive signals either from the rear or in advance. For this reason, the efficiency of the train station antennas may be increased by making them directional in some suitable manner, such as explained for the wayside antennas.

Central ofiice transmits to trains Let us assume that the operator in the central ofiice. actuates the push-to-talk button PB and a' icasta 1 1 energizes thetransmitrelay T tsee'Fig. 1). This rendersthe associated carrier current active, so that-Whenthe operator speaksinto the associated -microphone, the carrier current signal is modu- =-latcd-in accordance with the frequency of the voice message. This modulated carrier 1 current -is transrnitted over "the wayside communication 54 trans-e1 to each of the wayside-stations A, B," C, D, E and-F shown in'Fig. 4, which" wayside stations are typically illustrated-in Fig. 2.

The-reception of the modulated carrier current "at each-wayside station-causes its associated carrier current receiver to 'efiect'the energization of 'itscarrier responsive relay,- such as relay' 2T.

:12 results'in the reproduction mine associatedlodd- 1 speaker of the message transmitted by the train. Ateachof the other wayside stations, the radio receivers have failed to" receive the'radio carrier frequency signal 1 from the? train station then transmitting because they are not within the efiectiverange of such 'train station. Thus, the -reception 'of'the carriercurrent signal at such "stations, results in the energization of the assolO ciated carrierresponsive relay,"such as relay R,

whichin-turn renders active the associated radio -transmitterby"energizing the associated trans- 'mit relay. Thus, the message :modulatingthe carrier'current signal is. repeated through'the ""Tliusyat each'waysidestationthe associated radiov 15 con t onfii a Causes the associated radio transmitteris'rendered active, and the'radiofrequency signal radiated from the directional i an- -tenna of such station ismodulated in accordance with the audio modulations received by the asixtransmitter toxtransmit the'same message. Thus, all i of the wayside stations, excepirthe station B, retransmit the message originally transmitted irfrom a' train' station within the effective range -sociated carrier current-receiver over the waysidew -of StatlOIrB. It will: be. apparent that any train eernmunicanon connection between stations.

Since each wayside station of the communication area transmits'the message originating at "the central ofiice; suclrmessage'will be received withinvthel range ofthe wayside station 'Bywill alsoi'rbeiwithin range of the transmitting train station, 50 that itx'will receive the message direct, while" each train out oflthe range of such trans- -ateachtrainstatiori inthe communication area-1. itti a s at ny ut n r f s m regardless of 1 its parti-cular location. Referring to Fig. 3 as a typical example, such train station willreceivea radio carrier frequency signal mod- -ulat ed' accordance with the message, and this oudspeaker, Atthe same tiine, "the presence bf the radiocarrief frequency signal will effect the ener'gizationof 'the'associated carrier responsive -r'e1ay, suclras r'e1ay*3CR., and'effect "the enermessage'will be "reproduced 'by the associated :sparticular wayside'fistatiomwill receive the message'from' 'such' particular wayside station. The sireception of a'rradio carrier frequency signal at wany train station, regardless of whether the message' originates at'the. central oflice or some other ".tl2;il1 station,"effects the; picking up of its associatedacarrier responsive relay, such as relay "36R, S01 that such trainstation cannot initiate .wop'eration. of'its associated radio transmitter, and

.fig a sas d'ciated b indicator such as z 5 thus cannotdnterrupt'the" communication of any lamp 3BK,-while atthe s'ametime rendering the associated push-to-talk button such as button "3PB ineffective, Be'catise' thisaction takes place ata1l"thetrain sta'tionsQthey ar'ld0ked-out, so to spe'akfl and cannotdnterrupt the message being! transmitted fromthe centr'al dfiice.

Forthereasonsexplained above, each train "stat'iorr ivill coritinue to receiveits message regardless of its location, and as it moves along the railroad ri'ght oi -way from the effective radio 1;

* field bf-one wayside station into the effective "r-adio field of a difierent wayside station.

. A train transmits to central ofl'ice 'ilxetmsi: assume that a 'train station is located? both-with the'distinctive tone created byits associated tone 'generatormand'l also"by the fre- -.-"quen"cy of the voice message'spokeriinto the asso- :ciated microphone. twfrequency signal is receivedby the wayside sta- This "radiated radio carrier mm Bfand referring to Fig.2, it will be appre- 'ciatedtsthatr the' radio "receiver at such wayside station effects the energization ofits tone respon- 'lsive relay. Z'IRwhichin turn renders the assoiciatedrcarrier-current transmitter active so that '=:-the:connection 28 Will-cause such carrier current transmitter to repeat the voice message received.

'zpartic'ular train station with the central office, nol' the .communication between the central ofiice "and. all train stations in the area.

- Incthis connection,=it sho'u1d be noted that the qortransmit relay at each-"waysidestation, such as 5 dio carrier frequency signal and should endeavour toltransmit to a wayside station, such erroneous transrnission could not be received and eifect the ":interruption of 'the message already on the wayaside. communication channel. In other words,

to therein an effectiveinterlock at-the wayside stations preventing interruption of a message being transmitted over the wayside communication channel; even though the other train stations are inot' actuallyprevented from transmitting. Howsaever, under normaloperation of'the system it is f contemplated that the interlock will be extended 110 the variousitrain stations in such a way as to indicate to suchstations that they cannot transmit because 'thewayside' communication channel It is contemplated that a train station will genera1lyinitiate transmissionwhile within range of 'onlya single waysidestation, and while carrying on its ti'ansmission rill proceed within the range 05"0f thatstation' and perhaps enter the range of w anotherWaysidestation. Since the ranges of the adjacent stations overlap, the wayside station initially' receiving ac'arrier frequency signal from a train station-will continue to receive that signal The carrier. current signal imposed-upon the linewl repeat i e o the wayside wires l. and 2, conveniently termed the wayside communication channel, is transmitted to the centraloffice CO and to each'of the other way- 'sidestations A,"C3D, E and F, At the central :niunicati'on channel, so that the station'into the "rrange 'of which the trainstatiori has just entered v will continue to retransmit the .message. until the owayside station initially receiVin-gthe message has "wincethereception of the carrier current signal wlbecome; outlof' rangez ofz theutraint station. Thus,

should a message from the train station continue even after wholly leaving the range of the receiving wayside station, the wayside station now including the train station then begins to receive the train station radio frequency carrier signal and repeat the message into the wayside communication channel, there being but a slight interruption of the message such as involving the drop away times of a tone responsive relay TR and a carrier relay CR together with the pick up of a tone responsive relay at the station next to transmit. Such an interruption or lapse of time would of course hardly be appreciable since such relays operate in fractions of a second.

Referring to the illustration of Fig. 4, it should be appreciated that the wayside station ranges or radio fields have not been drawn to scale, so that it may be well to point out that the overlapping portion between the radio fields of two adjacent stations is a relatively short distance compared to the distance between stations. For this reason, it is contemplated that a train station will ordinarily initiate its transmission while within range of only a single wayside station, but it may happen that a train station will initiate transmission while within range of two wayside stations. In such an event, both wayside stations receive the message and act to repeat it onto the wayside communication channel. However, this causes no adverse effect in the transmission of the message to the central ofiice, since the carrier current transmitters operate on the same basic frequency and may be of the type which have oscillators of the lockin type tending to maintain the same frequency of the carrier current at both stations and thus cause a proper transmission of the message to the central office because both stations are transmitting the same message.

When all of the wayside stations are transmitting in the case where the message originates at the central office, it will be apparent that the radio receivers at all stations are rendered inactive because of the energized conditions of the transmit relays at such stations (see Fig. 2). Under the conditions where a train station is transmitting to a particular wayside station and the other wayside stations are retransmitting the message, it will be apparent that the reason why the wayside station receives the radio carrier frequency signal from the train station for preference to the wayside station to the left is because the wayside stations are provided with the directional antennas Q0. In other words, it is contemplated that the directional antennas 40 are not affected by transmission of radio carrier frequency signals from the station next in the rear of such directional antenna in spite of the fact that its effective radio field overlaps with such station.

However, under the conditions where a particular wayside station is receiving from a train station and the other wayside stations are retransmitting the message, it might be possible due to radio reflections or the like, that upon the cessation of the transmission from the train station to the particular wayside station then receiving there might be sufficient hangover transmission from some wayside stations as to be received by such particular wayside station. The

hangover transmission after the cessation of a 14 tion just having received a message from a train station, there would be what might be termed a feed back condition through the medium of the wayside communication channel if it were not for the provision of the present invention.

This is prevented in accordance with the present invention since a wayside station cannot repeat a message into the wayside communication channel unless the received radio carrier signal is accompanied by a particular distinctive tone. Thus, any feed back action is obviated in accordance with the present invention.

Referring to Fig. 5 of the accompanying drawings a rearrangement of the wayside stations within the communication area has been shown. It will be seen that the directional antenna of the wayside station A is transmitting its beam toward the right while the directional antenna of the wayside station B is transmitting its beam toward the left. Thus, the two beams may meet at a point 52 representing equal signal intensities.

The radio beams from antennas 50 at 5| are indicated by the dotted lines 53 and 5t respectively. Although the extent of range of such radio stations has not been shown, it is intended that the ranges of these stations will be substantially the sameas shown in connection with Fig. 4 and of course comparable to the range of the train stations, as indicated in Fi 6.

It will be apparent that this arrangement of directional antennas shown in Fig. 5, also provides complete coverage of the railroad right-ofway with a radio field when all of the wayside stations are transmitting a message originating rom the central office CO. The traveling of a train from the range of station A to the range of station B will of course cause such train station to -pass through what has conveniently been termed a fringe zone of signal intensities and including an intermediate portion in which the signals from stations A and B for example will be of equal intensity and contribute towards the reception of the message from the central office CO for reasons described above in connection with the consideration of Fig. 4. It should also be pointed out that similar conditions exist between the back-signals of two adjacent antennas, such as antennas 5i and 55 of stations B and C. In any event, when transmission is taking place with the message originating in the central oilice, all train stations within the communication area receive the message substantially continuously without interruption as the trains pass from the effective radio fields or ranges of successive stations along the railroad right-of-way. When all of the wayside stations are transmitting a message originating in the central oflice, no train station can initiate a message because its transmit relay cannot be energized by the push-to-talk button 3P3 when the carrier responsive relay 3GB is picked up (see Fig. 3).

The only difference in this Fig. 5 is the rearrangement of the directional antennas of alternate stations and the spacing of the Wayside stations. Thus, the characteristic features of the wayside stations and the train stations are the same as described in connection with the preceding Fig. 4. More particularly, while a train station is transmitting to some wayside station, every other train station is prevented from initiating transmission, since such other train stations either receive the message direct from the train station transmitting, or by the way of retransmission of the message through the wayside rs "communication channel; as explained in connectionwith-Fig. 4.

-Mo'respecifically, when a train station trans mits within the range of station A for example (see Fig; the message is received at station A -and repeated into the-wayside communication channe1 which results in the retransmission of such message fromall of the remaining Wayside stations B, C, D, E and F. Thus, a train station .:may communicate with the central ofiice as well as with'all train stations within the communication area. In the event that a train station initiates its transmission within the overlap-ping portion of the effective fields of two adjacent wayside stations, such as wayside station A B, both wayside stations will receive the train transmitted radio signal and repeat its message i into the wayside communication channel in the same manner as explained in connection with Fig. 4, since the carrier current apparatus is con sidered to be of the type having the same frequency characteristics, and may have oscillators "which lock-into synchronism with each other. This results in thereception of the message at .the central ofiice and the retransmission of the message from the other wayside stations.

When a train initiates transmission of a mes sage while Within the range of one wayside station, such as Wayside station A, and then moves into range of Wayside station B, the wayside station B continues its retransmission until the train has moved out of range of the wayside station A, since the retransmission operation at station B prevents any reception of a radio frequency carrier signal at that station. A similar action occurs should a train station initiate transmission within the range of wayside station B and 'then travel into the range of wayside station C. It is thus apparent that a train station may communicate with the central oflice or any other train station without any substantial interruption as it proceeds along the railroad righ't-oi way.

In this Fig. 5 organization of the wayside sta tions Within the communication area, it will be i apparent that should a train station be trans-- mitting to station A for example, the retransmission of such message from the station B'is directed toward the station A. "But since the train station is closer to the wayside station A than is the wayside station B, its radio carrier frequency signal will be the stronger and will be received by wayside station A in preference to the signal from wayside station B. This is of course due to the characteristics of a frequency modulation type of radio receiver as previously discussed. Upon the cessation of the transmission from the train station, the station B might continue its transmission of a radio carrier frequency signal for a few micro-seconds, or what may be termed a hangover time, due to the characteristics of oscillatory -or tuned circuits and the release time of certain relays. If such hangover radiation should be received by the station A, it would cause the wayside communication channel to maintain wayside l station B active which would cause a continued circulatory feed-back action between the stations -A and B, for example. Such a feed-back condition cannot occur in connection with th present invention, since thepresence of a distinctive tone is' required upon the reception of a radio signal in order for the repeat operation of a wayside station to take place. For this reason, the cessation elf-transmission of aradio carrier frequencysignal from a train station immediately renders the associated wayside station then re-.

' ceiving. in'effectivefor "reception from) any other through the medium of-what has conveniently been termed retransmission of the trainrmessage. The present invention has been more particularly described as acommunication system used in connection with railroads for transmitting messages nbetween trains and thelike, but

it1is to be understoodithat the invention may be adaptable for other uses such as in a communication system for. automobile traffic along highways, or in a communication system for air trafno over the airways of an area.

Having described a communication systemand the various characteristic features thereof as embodying the present invention, it is desired to be understood that this form is selected to facilitate in the disclosure of theinvention rather than to limit the number of forms which it may assume; and, it is to'be further understood that various modifications,adaptations and. alterations may be made to the specific form shown to meet the requirements of practice, without in any manner departing from the spirit orscope of the present invention.

What I claim is:

1. In a two-way. train communication .system of the spaceradiationltype for railroads. providing for communication to and from a central oifice and anyone of a number of trains in.a communication area, said system comprising, radio transmitting and receivingmeans on each train manually controllable only if no signalis being receivedito send out a carrier frequency modulated with a message and also a predeter- 4,0 mine-d tone frequency, a, plurality of way stations each including a transmitter and receiver and spaced along the railroad to be within range of a train in any location in the communication area, means including a communication connectionbetweensaid way stations and responsive to the reception from a train on the carrier frequency modulated with said predetermined tone frequency at any given way station for rendering the transmitters for all other way stations effective to send out a signal to all other trains, thereby preventing manual control of the radio transmitters on such other trains to cause transmission and interfere with the transmission of such message.

2. In a system of communication betweena central office and a plurality. of mobile units traveling. through a communication area, a plurality of wayside stations in the communication area each provided with a radio receiver and a carrier current wayside transmitterymeans constituting a communication channel between said way stations and said central office for effecting the transmission of a carrier current signal in response to a message received at any way station and repeated by its wayside carrier current transmitter as received by its associated radio receiver, a radio transmitter on each mobile unit having a limited range less than the extent of the communication area but eifective to transmit messages to the radio receiver of the-nearest wayside station, and means associated with each wayside station responsive to the reception of a carrier current signal received over the wayside communicatiom channel from some other Wayside station for preventing any mobile unit from transmitting to that way station, whereby the initiation of communication with the central office by any mobile unit via a particular way station in the communication area prevents break-in by any other mobile units within range of the other way stations.

3. In a system of communication between a central ofiice and a plurality of mobile units traveling through a communication area, a plurality of wayside stations distributed throughout said communication area, means constituting a communication channel between said wayside stations and said central omce and between each wayside station and every other wayside station, equipment at each wayside station capable of receiving radio messages and repeating such messages over said wayside communication channel to the control ofiice and each of the other stations, a plurality of mobile units each provided with radio transmitting equipment of a limited range less than the extent of the communication area but of sufficient range to always reach the nearest wayside station, and means at each wayside station responsive to a message on said wayside communication channel to prevent its associated equipment from receiving radio messages for repeat transmission over said wayside communication channel, whereby any mobile unit can communicate with the central office over the wayside communication channel without interference from other mobile units within range of other wayside stations.

4. In a system of communication .between a central ofiice and a plurality of mobile units traveling through a selected area, a plurality of t'em for transmitting a message from the central 1 ofiice to all wayside stations and acting at such stations to cause the radio transmitters to repeat such message, and a plurality of mobile units each provided with a radio receiver of the frequency modulation type having means adapted to selectively receive the stronger of two radio signals, whereby each mobile unit can receive a message from said central ofiice Without substantial interruption as it travels into and out of range with the successive wayside station.

5. In a system of communication between a plurality of mobile units traveling through a communication area, a plurality of wayside stations distributed through said communication area, radio transmitting and receiving equipment at each wayside station having a directional antenna for directing its field of radiation and response toward the next adjacent station in one direction but not the other, and each station having an effective range extending to at least the next adjacent station in said one direction, a plurality of mobile units each provided with radio transmitting and receiving equipment of limited range corresponding to said wayside stations whereby each mobile unit is always within range of at least one wayside station regardless of its position in the area, and a wayside communication channel connecting said wayside stations for repeating any message received by the radio receiving equipment at any given way- 18 side station "from a mobile unit within its range to cause the radio transmitting equipment at the remaining wayside stat-ions to retransmit s'uch message for communication to the mobile units within range of the respective wayside stations, whereby each mobile unit can communicate with every other mobile unit in the area regardless of its limited range.

6. In a system of communication between a plurality of mobile units traveling through a communicationarea, a plurality of wayside stations distributed through said area, radio transmitting and receiving equipment at each wayside station, said radio equipment having a limited effective range but which is sufiici'ent to cover the distance from its station to the eifective range of the next adjacent station, whereby a mobile unit is always within range of at least one wayside station regardless of its position in the area, a plurality of mobile units each provided with radio transmitting and receiving equipment having an effective range of a limited distance corresponding to the efiective range of the wayside stations, means associated with each mobile unit for causing the transmission from such unit to be modulated in accordance with the distinctive tone in addition to any voice message transmitted from the mobile unit, a communication channel interconnecting said wayside stations, means at each wayside station for repeating the message received by the radio receiving equipment at that station over said wayside communication channel to the other wayside station only providing said radio receiving equipment at such given station receives a radio signal modulated by said distinctive tone, and means at each wayside station responsive to a message on the wayside communication channel for causing its associated radio transmitting equipment at that station to 4 retransmit the message received over said wayside communication channel, whereby, the cessation of reception at any particular station from a mobile unit causes the cessation of the repeat action of the associated radio receiver upon the wayside communication channel to thereby prevent any retransmitted radio signal from being received by an adjacent station when a mobile unit ceases transmission.

7. In a system of communication between a plurality of mobile units and a central office, a plurality of wayside stations distributed through a communication area, radio transmitting and receiving equipment at each wayside station having a directional antenna for directing its field of radiations and responses toward the next adjacent station, all of said antennas directing their beams in the same general direction and efiective to extend their beams to at least the next adjacent station in said direction, a plurality of mobile units each provided with radio transmitting and receiving equipment of limited range, and a wayside communication connection extending between said wayside stations for repeating the message received by the radio receiving equipment at any given station to the wayside radio transmitting equipments at all other stations for retran'smitting such message, whereby each mobile unit may communicate with other mobile units in the communication area which may be out of range of its own radio transmitting equipment.

8. In a radio communication system for railroads, radio equipment on each of a number of trains including transmitting and "receiving means operating on the same carrier frequency,

said equipments being normally in condition to receive and being manually controllable to transmit provided said carrier frequency is not being received from some other source, and radio equipment at a way station along the railroad including transmitting and receiving means operating on the same said carrier frequency and manually operable to transmit only if no carrier frequency is being received from a train, said way station equipment including a directional antenna for providing transmission and reception substantially in one direction only along the railroad, whereby all trains within range of said way station are within range of each other so that any train communicating with said way station locks out all other trains within range of said way station.

9. In a radio train communication system for railroads, a plurality of train carried equipments each including radio transmitting and receiving means of limited range operating on the same carrier frequency and manually operable to transmit only if no carrier frequency is being received from some other source, a plurality of way stations spaced along the railroad at distances conforming with the range of said train carried equipments, said wa stations having radio transmitting and receiving means operating on the same said carrier frequency, said way stations also having directional antennas for providing transmission and reception substantially in one direction only along the railroad.

10. In a train communication system for railroads, a number of train carried equipments each including radio transmitting and receiving means of limited range operating on the same carrier frequency and manually controllable to transmit only if no carrier frequency is being received from some other source, a plurality of way stations spaced along the railroad at intervals conforming with the range of said train carried equipment, said Way stations having radio transmitting and receiving means operating on the same said carrier frequency and uni-directional antennas directed in the same direction, means constituting a communication channel connecting all of said way stations and a central ofiice, and means operating over said communication channel for causing all of said way stations to transmit carrier frequency when any Way station is receiving a carrier frequency from the transmitting means on a train.

11. In a train communication system for railroads, radio equipment on each of a number of mobile units including transmitting and receiving means of a limited range operating on the same carrier frequency and manually operable to transmit only if no carrier frequency is being received, a plurality of way stations spaced along the railroad at intervals corresponding with the range of said mobile unit equipments, said way stations having radio transmitting and receiving means operating on the same said carrier frequency and uni-directional antennas directed in the same direction, a line circuit connecting all of said way stations with a central office, a carrier current transmitter and a receiver at the control ofiice and at each way station operated by carrier current on said line circuit, the radio receiver means and carrier current receiver at each Way station being normally in condition to receive and the radio transmitting means and carrier current transmitter being normally inactive, means associated with the radio receiving means at each Way station for rendering the carrier current transmitter active, and means associated with the carrier current receiver at each way station for rendering the radio transmitter active.

12. In a communication system for railroads providing communication from any one of a number of trains to any one of a plurality of Way stations and a central office, radio equipment on each train including radio transmitting and receiving means of limited range operating on the same carrier frequency and manually operable only if no carrier frequency is being received from some other source to transmit an inaudible control tone along with a voice message, a plurality of way stations spaced along the railroad at intervals corresponding with the range of said train carried equipments, each of said way stations including radio transmitting and receiving means operating on the same said carrier frequency, means constituting a communication channel between all of said way stations and a central oiiice, and means at each way station responsive only to the reception of said control tone from a train for rendering the radio receiving means at that station effective to send voice messages over said communication channel to said central office.

13. In a train communication system for railroads, radio equipment on each of a plurality of trains including transmitting means and receiving means operated on the same carrier frequency and manually controllable to transmit only if no carrier frequency is being received, a plurality of way stations spaced along the railroad at intervals corresponding with the range of said train equipments, each of said way stations having a radio transmitting and receiving means operating on the same said carrier frequency, means constituting a communication channel for voice messages between all of said Way stations and acentral office, means at each way station for transmitting over said communication channel any voice message received by its respective radio receiving means, and means at each way station responsive to a voice message over said communication channel for rendering the associated radio transmitting means active to transmit a carrier frequency, whereby trains beyond the range of any given train communicating with the central office are prevented from breaking in by reason of the retransmission from all other way stations when a given way station is receiving a voice message from a train.

14. In a system providing two way communication between a central office and a plurality of trains within a communication area, a central oifice, a plurality of Way stations in a communi cation area along a railroad track each including a single channel radio transmitter and receiver, means constituting a communication connection operative over a Wayside channel between each of said way stations and said central oflice for permitting the transmission to the central ofiice of a message received at any wa station or the transmission of a message from the central ofiice to any way station, equipment on each train in the communication area including a single channel radio transmitting and receiving means operable on the same single channel as the way stations and having a limited range less than the extent of the communication area, manually operable means on each train for rendering the associated transmitting means effective to send out a radio signal, means at each Way station acting in response to a radio signal transmitted from a train from within range of that station to transmit a message over said communication connection for rendering all of the other way stations effective to transmit radio signals to all trains within their range, whereby the starting of communication with the central oflice by any particular train in the communication area causes all other way stations to send radio signals reaching all trains in the communication area out of range of that particular train.

15. In a communication system providing two way communication between a central office and a plurality of trains within a communication area, a central ofiice, a plurality of spaced way stations located throughout the communication area each including radio transmitting and receiving equipments having a limited range of directional characteristics but so spaced that their ranges are overlapped to provide a complete coverage of the communication area, a wayside two-way communication channel for communicating between the central oflice and each of the Way stations, means at each way station responsive to a message over said communication channel from said office or an associated way station to cause its associated radio transmitter to be effective to transmit such message to thereby communicate with the trains within its range, and means at each way station responsive to the reception of a radio message by its associated radio receiver for rendering its associated transmitter ineffective and acting to transmit such message over said communication channel to said centra1 oifice and each of the other stations, whereby the transmission from a particular train station to a particular way station effects the retransmission of that message from all other way stations so that the trains within the ranges 16. In a system of communication between a plurality of mobile units travelling through a communication area, a plurality of wayside stations distributed through said communication area, radio transmitting and receiving equipment at each way station operable on a particular frequency and having a directional antennae for directing its field of radiation and pickup toward the next adjacent station in one direction but not the other, and each station having an effective range extending at least to the next adjacent station in said one direction, a plurality of mobile units each provided with a radio transmitting and receiving equipment operable on said particular frequency and of a limited range corresponding to the range of said wayside stations, whereby each mobile unit is always within range of at least one wayside station regardless of its position in the area, and a wayside communication channel connecting said wayside stations for repeating any message received by the radio receiving equipment at any given wayside station from a mobile unit within its range to cause the radio transmitting equipments at the remaining wayside stations to retransmit such message to the mobile units within range of their respective wayside stations, whereby each mobile unit can communicate with every other mobile unit in the area regardless of its limited range.

FOREST B. HITCHCOCK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,393,291 Clark Jan. 22, 1946 2,398,741 Halstead Apr. 16, 1946 2,429,607 Capen Oct. 28, 1947

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