US2417555A - Radio carrier telephone system - Google Patents

Description

March 18, 1947. L. A. KELLEY 2,417,555

RADIO CARRIER TELEPHONE `SYMEM Filed March l, 1944 3 Sheets-Shes?I l IN VEN TOR. leo/SLM@ BY v-MQ L AHORA/fr INVENTOR.

L. A. KELLEY RADIO CARRIER TELEPHONE SYSTEM 'Filed March 1, 1944 siv/rcf/ March 18, 1947.

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` March 1s, 1947.

L. A. KELLEY 'RADIO CARRIER TELEJHONE SYSTEM Filed Maren ,1, 1944 s sheets-smet sl WNS JM IN V EN TOR.

ze@ A .zfezge kmwv l /WTOR/YEY Patented Mar. 18, 1947 RADI() CARRIER TELEPHONE SYSTEM Leo A. Kelley, New York, N. Y., assigner to The Bauland Corporation, Chicago, Ill., a corporation of Illinois Application March 1, 1944seria1 No. 524,819

This invention relates to new and useful improvements in radio carrier telephone systems.

The object of the invention is to provide a multiplex radio communication system which is simple and inexpensive to install and to operate.

With this object in View telephone equipment at a station, e. g. a loud speaker is so connected with a twoway channel that by the simple opera'- tion of a switch (manually or automatically operated) the station may be used to speak or to listen. A plurality of such stations and channels may be provided at each terminal. Preferably, one of the channels utilizes voice current fre- 8 Claims. (Cl. Z50-9) quencies and the other various carrier frequencies.

The drawings diagrammatically illustrate an embodiment, Fig. 1 showing the terminal station, Fig. 2 the significant circuits of one of the telephone sets, and Fig. 3 a radio relay station.

Fig. 1 Vshows in diagrammatic form the electrical circuit of one terminal of a carrier-radio telephone system. A simple system comprises two such terminals, alike in every respect except that the radio frequency employed by the radio transmitter of one terminal is different from that employed by the other terminal radio transmitter, the radio receiver at each terminal being tuned to the radio frequency transmitted at the opposite terminal. Where the distance between terminals is too great to be spanned by such a simple system, cr where natural conditions are such as to render the received radio signa-1 too weak for satisfactory communication, one4 or more radio relay stations may be placed at suitable locations intermediate the terminal stations.

One relay station comprises two radio transmitters and two radio receivers; One radio receiver is tuned to receive the adjacent (but not at the relay station being described) radio transmitter, for example the one to the west of it, feeds its output into one of the radio relay transmitters which transmits the received signal on a different radio freque'ncy toward the next adjacent (but not at'the relay station being described) receiver to the east of it, in the eiiample chosen. The remaining radio transmitter and radio receiver at the relay station are used in an exactly similar manner te accomplish relaying in the east to west direction. Carrier apparatus maybe employed at any or all oi the relay stations but the present description is not intended to cover arrangements of this sort.

Furthermore, more complicated radio networks may be considered but this description is intended primarily to cover only aradio linie-'comprising two terminal stations each equipped with carrier for executive use, paging, etc.

'or table, for example, near the user.

2 telephone apparatus and one or more radio relay stations not so equipped. The complete equipment at one typical terminal station will now be described. Referring to Fig. 1, the strictly radio portion of the terminal equipment consists of a transmitter T and a receiver R, each with a suitable antenna. The circuit details of these units, except for the relay TR which will be described later, are not of particular interest since the proposed carrierradio telephone system may be used in conjunction with any type of radio telephone apparatus. The transmitter and receiver may, for example, utilize amplitude modulation or frequency modulation, high radio frequency or low radio frequency, etc. The only special technical requirement on these units in the present instance is that the frequency band width of the radio signal channel provided by them shall be sucient to accommodate the total frequency band employed by the ordinary voice and carrier telephone channels connected to them. This, however, is an ordinary matter of design and is easily fulfilled. The transmitter T is provided with input terminals in and receiver R with output terminals out These are the terminals which are connected to the ordinary voice and carrier telephone apparatus next to be discribed. n

As illustrated in Fig. 1, four independent telephone circuits are provided, indicated by the four branch circuits 4converging on transmitter T and receiver R. To put it another way, one twoway radio telephone channel is arranged to furnish service to and from four independent two- Way voice and carrier telephone channels. The kactual number of carrier telephone channels is not important,` a greater or smaller number being used as desired.

Attention is `now directed to the boxes designated Iv, I1, etc., at theleft-hand part of Fig. 1. These boxes are all alike, so a description of one will suice. Each one, in the present system, .contains apparatus similar to one station of an intercommunication system, a private system of communication used in many oices and factories One of these boxes, ,which will be referred to hereafter as an intercom station, is located conveniently on a desk This is where theuser talks and listens in carrying on a conversation.

In Fig; 1 the apparatus in thel circuit between terminals` Cv, C1,etc., and the inand out terminals of the' radio transmitter and receiver is intended to be mounted as a unit in close prox'- imity to T and R. However, the intercom stations Iv, I1, etc., may be located where it is desired to use them. They may be connected to terminals Cv, C1, etc., by a small gage balanced pair cable for the voice circuits and a single wire per intercom station for energizing the associated TLR relays. In a practical case the pair of wires interconnecting an intercom I and a pair of terminals C may be several thousand feet long.

Pertinent details of the intercom station are shown in Fig. 2 together with the connections from it to the apparatus mounted in close proximity to the radio transmitter and receiver. The main parts comprise a voice frequency amplifier, the box at the bottom of the figure, a, dynamic type loud speaker LS and a talk-listen switch of the well-known rotary type.

The switch has three layers. Each layer consists of two flat arcuate conducting pieces which rotate about the center of their arcs when the switch is actuated, and several stationary con tacts of conducting material. The arcuate pieces of all three layers are rigid through insulating material to a common shaft which is rotated through a small angle when the switch lever is actuated. Thus, all the arcuate pieces move simultaneously and through the same angle or circumferential distance. The switch has two po sitions. As may be seen by reference to the small diagram in the upper right-hand part of the fig- 1 When the user desires to talk he depresses the l' switch lever to the talk position and speaks. When he has finished speaking he releases the switch lever which returns automatically by a spring return to the listen position. What happens to the circuit connections will be pointed out next.

The circuit connections as shown in Fig. 2 are with the switch in the listen position. It is seen that this connects the line terminals of the intercom station to the in terminals of the amplifier through the switch front layer. It also connects the loud speaker LS t the out terminals of the amplifier through the switch middle layer. The switch rear layer in its lower part makes volume control potentiometer PI, located between amplifying tubes V1 and V2 active to the exclusion of P2, whilein its upper part it keeps the winding of the TLR relay, located with the carrier apparatus in close proximity to the radio transmitter and receiver, de-energized by breaking the circuit to the energizing battery. It is clear that with the switch in the listen position any Voice currents entering on the line terminals of the intercom station will be amplified under the control of volume control Pl and reproduced on loud speaker LS at the desired volume. At the same time the TLR relay remains de-energized.

When the switch lever is depressed to the talk Y position, the circuit is Vas follows: The loud speaker LS is connected through the switch front layer to the in terminals of the amplifier. The line terminals are now connected to the out terminals of the amplifier through the switch middle layer. At the same time the upper part of the switch rear layer completes the circuit for energizing the TLR relay and the lower part `makes volume control potentiometer P2` active to the exclusion of Pi. Therefore, with the switch in the talk position words spoken into the loud speaker are converted into voice currents thereby, the voice currents are amplied under the control ci P2, and the amplified voice currents at the desired amplitude are supplied to the line terminals. At the same time the TLR relay is energized.

It may be noted in passing that the arrangement of Fig. 2 just described does away with the necessity for placing an instrument to the mouth or ear as the talking and listening may be per formed some distance away from the intercom station. Of course, one must be close enough to manipulate the switch. It is contemplated that a telephone handset might be used which is equipped with a push button switch to connect the receiver or transmitter portion of the handset to the voice line in much the same way as described for the intercom station. This push button switch would also energize and de-energize the TLB. relay. It is further contemplated that the TLR relay may be energized by the operation of a voice operated relay well-known in the art.

The battery employed for energizing the TLR relay may take any one of a number of forms. For instance, if the intercom station is supplied with electrical energy from an alternating current lighting circuit, the power transformer may have a low voltage winding connected to a full wave rectifier of the copper oxide type and a capacity-resistance filter for this purpose.

Returning to a consideration of Fig. l, it is apparent that the currents passing through any pair of terminals C to or from the associated intercom station I are ordinary voice frequency currents. The apparatus in circuit between the terminals C and the in and out terminals of the radio transmitter and receiver is a simplified car rier telephone terminal station. It comprises one channel of ordinary voice, the component parts of which are designated with a subscript v, and three channels of superimposed carriers, the component parts of which are designated with a number subscript depending on the carrier `channel number.

The carrier telephone system illustrated and preferred is of the so-called single side band carrier suppression type. Such a system has certain advantages which are especially desirable. In the rst place, it is economical in the frequency range required per carrier channel because no provision has to be made for one entire side band and a small portion of the other. Another important advantage is that the radio transmitter is not loaded with the carrier frequency energy which, being an unmodulated component, does not serve to transmit intelligence in any event. A further advantage is the relative freedom from interference gained by not transmitting the carrier frequency. The reason for this is that when the carrier frequency is transmitted it is exposed to extraneous interference just as much as the side band frequencies. Since the carrier demodulator functions by the interaction of the carrier frequency and side band, it follows that the ef fect of the interference is worse where the carrier frequency is itself affected by the interference.

' In the carrier suppression system the carrier frequency for demodulation is supplied locally at the carrier receiver and, therefore, not exposed to interference. Supplying the carrier frequency locally for demodulation imposes a special requirement on the carrier frequency oscillators in respect to frequency stability. However, oscillators having the requisite frequency stability have been constructed and are entirely practical.

four channe'ls, The yordinary voice channel has no carrier 6515111atp;and f cupies this frequency range from (i to 2800 Carrier cljiannel Not `1 a 'carrier oscillatorfrequency offQQO `C. P. S. `and ccupies a freq'ency range Dfrom sito to57oo 'crt-s. `c riet channel No. z has ro Aa carrier oscillator frequency Jof*3f .l '0() 0 P5. S. and ocupies aireque'ncy range frornuGlOQ to 8700 o. i?. s. Carnet chantre; a has a cerner "oscillator frequency of 11,90() C. P. S. and occupies a frequency range from 9 10Qto 11 7 00 C. vP. S. This recitation of carrier scillatorjfrequencies 'and frequency ranges occu ed, illustratesone of the adyantages of thpreferred carrierv system previously mentioned, namely, that it is economi-y cal rin the frequency range required per carrier channel. In carrier channel No. 1H, for example, the entire upperside band, the` carrier frequency, and that part of the lower sideband from 5700 to 5900 C. P. S. are suppressed leaving the lower side band from 31coto'57oo o. P. s. to be transmit'ted. Upon deniodulation with the same car- 'rier frequency a reprodiicedvoice range of. from 200 to 2800 C. P. Stvis secured.4 In order to achieve this result transmitting `both side bands,

a. frequency range from 3100 to'8700 C. P. S.

would be required, more thandouble that actually employed with the preferred system.

Mention has already been Iiiade of the TLR relays., one of which isiassciated with each intercorn station the sense that is, energized when the switch' is in the talk position and deenergized in the listen position. All the TLR relays .at a terminal are yconv eniently located with the carrier apparatus. Referring to Fig. 2 it may be seen that eachTLR, relay'whas two armatures. The lower one has twocontacts, Aone normally (i. e., when `de`energiaed)' closed and the other Vnmrn'ally open'. The upper one hasfarsingle nor.. 4mally open contact. lifig. `1 'shows that the lower armature connects tnejchapttel with which' it is associated to the' output ter'jrniriialsY Voi radio re ceiver R through its normally closedpontact when the relay is deenergized,':andy tothe input terminals of the radio' transmitter I through its normally open AContact ii/ heny tlie relay is energized. In order to" simplify thedrawing of Fig. 1, the upper armatures are not shown in proximity to their respective energizing coils butinstea'd they appear in lagroupjust belowthe radio transmitter T where they designated That, rrLRl", etc., one forv each T1115?,v relay.` All' o'lj vthe armatures in this group are Stra'lpd. together and connected to' ground. All `or their normally open contacts are` strappedtogether and connected toone terminal o f the energizing coil for the transmitter relay(`4 which relay may be conveniently located iithinfthe housing oi the radio transmitter. frite cthljtehninai of the energizing coil foi theV 'IE`?." re l"a`.ya is connected `to a' SOur Qf rllr'cifl'l@al` nsrarplfeseiied',by a battery, the othersideof which source is con nected to ground; It is evid nt4 that when rthe switch ofanyonef di th u t rn-stations is' held inthe "talk" posit on, ergizing" thaassociated TLR relay, the 'IRrelay 1s also energized through the closure'A ofthe? upper armatureoijthe ILR relay with' its normallyperrlcontactgl The latter operation completes the energizing ,circtut` ofV the TR relay'tljirdiiglrgrurclY VBeczatiisijj: of tlie'parallei connection ofthev upprarmatur tants' of the f mail reiaysgf the rn; reia'y remains feriergien when more than. one of gtheintr om "sltiionsis .tened to the *talk* position. ioniy the, smic-nes .orali the inte'roomstatmns the listen position is the TR relay de- "eilersized- Theil?. vrelay has `one armature which coopcrates with one normally open Contact. In Fig, 1 no connections are shown for this a'rmature and Contact as they can b ereadily describedin words. ,Generallyjspeakioa thejIR relay s,des ienie.d U0 save-jon the .consumption of electrical, energy by the radio transmitter those 'intermittent periods in th'ecourseo operation when allofthe channels. ab the. local terminal happen lnot to vte efrlbyed `for talkins. Radio, transliitterfr 'at the present day win employ various radio tubes .which require that. .electrical ,energy be supiilied to them in order that they mayjfunction, Someoi this energy goes toward heating thecathodes of the tubes. A large partis expended in supplying the operating `voltages for the other electrodes, particularly the screen and plate voltages in theradio frequency power output stages. Theoretically al1 power could be cut off from the transmitter whenever it is not being used at random intervals yduring a period lof service. Practically it is often not feasible to do this with the energy supplied for heating cathodes for many tubes used in ,transmitters because of the time lag in bringingthe cathode to the operating temperature and the `danger ofharming the cathode suriaceif ahigh potential is applied to another electrode in the same tuoe before a suitablenegative space charge has been built up in the inter-electrodespace within the tube. However,` itis always practical to cut on all power other than that required for cathode heating during the temporary periods mentioned and efiect considerable saving thereby, It is now clear what connections are to be made to the armature and contact of the TR relay: they are connected in series With the entire source of electrical yenergy which supplies .the transmitter if that is permissible or else with that part of the source other than that required for heating. Elements, such as crystals, Vwhich are temperature con.- trolled also should not be switched on and off at odd intervals during .la period oi service. The net result of the operations of the TR,` relay is that at least a substantial part of the electrical energyto thet'ransrnitter is disconnected whenever the TR relay is de-energized and this happens automatically every time all the local intercom' stations happen to be switched` to the listenj position' during a period of operation. 'I hus the transmitter is fully energized only when it is actually needed. In this way considerable electrical energy may be saved and the life of expensive tubes extended.

lt should be noted that the functions to be performed in. the ordinary voice channel are fewer than those in the carrierchannels, so a description will 4loe first given of typical operations over the ordinary Voice channel. Operations over a carrier channel will then be described which will be the same for all carrier channels.

rss'iirneQ` as a starting point, that the intercom stations on the ordinary Voice channel at .the local and-distant terminals are standing by., Frornthe description thus far given, this means that each intercom switch is at the listenr position and consequently that the following situatonexists at'eacnterniinal. The Voutput of the radiorec ev'er is connected through the lt'mver `armature f of theTL-Ri'relay and its normally closcd'contact and the apparatus in the ordinary voice channel via terminals Cv to the input terminals of the intercom amplifier. Also the loud speaker is connected to the output terminals of the intercom amplifier. Consequently, each intercom station Iv (one at the local terminal and one at the distant terminal) is in readiness to receive a call from the other.

Suppose the local Iv makes the oall. The person at the local Iv depresses the switch to the talk position, makes his call by speaking to the loud speaker and then releases the switch allowing it to return to the listen position. Referring to the detailed description already given of an intercom station, the speakers voice is converted into voice currents which are amplified the desired amount and fed to terminals Cv. From terminals Cv the voice currents now through the left-hand winding of the balanced and shielded audio transformer Tv.

Transformer Tv performs two main functions. First, because it is balanced and shielded it preserves the balance of the circuit to its left despite the fact that its right-hand Winding is connected to an unbalanced circuit. Second, it provides for any desired change in impedance level in going from one of these circuits to the other. These functions, and how they are effected in the design of the transformer, are well-known in the art.

The circuit from Iv to the left-hand terminals of Tv is preferably of the balanced type to secure freedom from cross talk and extraneous interference. The circuit to the right of Tv includes the electric wave filter LPv which, for reasons of economy in construction, is preferably of the unbalanced type. The desirability of providing also for a change in impedance level arises from the ,fact that in many practical cases the impedance level of the circuit to the left of Tv may be greatly different from the most favorable impedance level for designing the lter LPv.

The voice currents in passing through filter LPv (in this instance from left to right) are transmitted only in the frequency range from to 2800 C. P. S. Any voice currents having frequencies above 2800 C. P. S. entering LPv on the left are practically extinguished so far as the currents leaving LPv on the right are concerned. Therefore, the voice currents which go on to the radio transmitter T are restricted to the frequency range 0 to 2800 C. P. S. Voice currents of frequencies exceeding 2800 C. P- S. are not necessary in a system, such as the one being described, where intelligbility is the main consideration. As to the lter LPv itself, sufiice it to say thatl it comprises a network of inducta'nce coils and condensers and that its design and construction are well-known in the art.

After leaving filter LPv, the voice currents are conducted to the input in terminals of the radio transmitter T via the lower armature of the TLRv relay and its normally open contact. It will be recalled that holding the switch at Iv in the talk position also energizes the winding of the 'ILRv relay. This explains why the right-hand terminals .of LPv are connected to the in terminals of 'I' at this time. It will also be recalled that energizing the TLR-v relay also energizes the radio transmitterT unless it has already been energized by reason of one or more of the otherlocal intercom stations being operated to the talk position. Therefore, the voice currents from the ordinary voice channel now entering the radio transmitter, find it in functioning condition. They modulate the radio frequency carrier wave in the transmitter and the modulated radio wave is radiated from the associated antenna.

At the distant terminal the radio wave modulated by the ordinary voice channel is intercepted by the receiving antenna and conducted to the radio receiver R. (In describing what takes place at the distant terminal reference is also directed to Fig. 1 which represents local and distant terminals equally well.) Radio receiver R demodulates the received modulated radio wave and delivers at its output terminals "out a practical replica of the voice currents with which the radio Wave was modulated at the local radio transmitter.

Since the intercom Iv at the distant terminal is assumed to be in a listening' condition, its TLRv relay is now de-energized. As a result, the lower armature of this relay is touching its normally closed contact thereby conductively connecting the out terminals of R with the right-hand terminals of LPv. Filter LPv, just as in the case of the local terminal, transmits only currents in the frequency range 0 to 2800 C. P. S. But it is rereiving at its right-hand terminals, from the out terminals of R, voice currents in this same range so it passes them through to the right-hand winding of transformer Tv.

Filter LPv transmits currents in the 0 to 2800 C. P. S. rango equally well in either direction. At the local terminal where these particular voice currents originated, transmission through LPv is from left to right, but here at the distant terminal it is from right to left. The voice currents go only where they are supposed to go by reason of the transmission and impedance characteristics of the other electric wave filters BF, BF2, and BFs.

From the left-hand terminals of transformer Tv the voice currents are delivered to Iv via the terminals Cv. Since this is the Iv at the distant terminal which is assumed to be in a listening condition, the voice currents received by it are amplified the desired amount and supplied to a loud speaker. Thus the person at the distant Iv hears the call spoken by the person at the local Iv. It is clear that the description is the same, intfrchanging the terms local and distant, if the call originates at the distant Iv and that the ensuing conversation is carried on in the same manner as in making a call, the talker holding his switch in the talk position and the listener allowing his switch to remain in the listen position.

A description will now be given of operations over a typical carrier channel. Only that part which is different from what has been given for the ordinary voice channel will be covered in detail. Carrier channel No. 1 is taken as typical. It is assumed that I, at the local terminal is about to originate a call and that I. at the distant terminal is in a listening condition. As in the case of the ordinary voice channel, the person at the local I, holds his switch in the talk position and makes his call by speaking toward his loud speaker. The resulting voice currents, amplified the desired amount, are fed to the terminals C1 of the No. 1 carrier channel apparatus. They then pass through the balanced low pass lter LP1, the components of frequencies above 2800 C. P. S. being thereby suppressed. Such components are not necessary for the practical operation of the present system, as previously explained, and if they were not suppressed at this point they would load the modulator MD1 and the band filter EF1 with components of unnecessary frequencies. LP1 is of the balanced type becausev it is connected to balanced circuits on both sides, i. e.y the balanced line to I1 and the balanced modulator MB1. f

As will appear further on, in the description, MB1 functions both as modulator and demodulator, hence the designation. At this point it functions as a modulator in'conjunction with carrier oscillator O1. It comprises four units of dry disk copper oxide rectiers, or similar rectiiiers. As shown in Fig. 1 they are connected to form a lattice network and are so poled as to their conducting direction that when the closed circuit comprising the rectiers only is traced, they are all conducting in the same direction. This is not the only arrangement that could be used, butit is illustrated here as the preferred one.

The carrier oscillator O1 is desirably of the frequency stabilized vacuum tube type. Suitable oscillators are well-known. In the No. l channel O1 is set to have a frequency of 5900 C. P. S. Its outputis connected to the midpoints of the balanced circuits to the left and right of MD1, as illustrated, and is adjusted in amplitude' so as to be much larger than the voice currents with which it interacts in the modulator. Stated another way, the oscillator amplitude should be sufficient to produce a substantial voltage bias on the top and bottom rectifier units areconducting and the two diagonally connected rectifiers are non-conducting. During the `next halfcycle of the oscillator, the polarity` of its output i leads is reversedcausing the two diagonally connected rectifiers to be conducting and the top and bottom rectiers to `be non-conducting. Thus thereotier units, under the control of the carrier oscillator, act like a reversing switch on the connections from the output of LP1 totheinput winding of T1. the voice currents in the left-hand winding of They reverse the directionof ow of A itself. ls inthe ordinary voice channel it permits. the connection ,of a balanced circuitfto an unbalanced one and effects any desired change in impedance level. of .a well-knowntype designed to transmit cur.- rents in the frequency range 3100 to 5700 C. P. S. and suppress currents in the frequency ranges above and below these limits. Subtraction of `this frequency rangefrom the 59000. P. S. carrier shows that only the lower side band corresponding tc modulation of the carrierby voice currents in the range 200 to` 2.800 C. P. .S. is transmitted by. EF1. Since5900 C. P. S. is outside the transmitting range of Bliifany residual carrier current isxfurther reduced to negligible magnitude.

Rememberingthat the'person usingI, is holding the switch in the talk position Whilemakng his call, the TLR1relay is thereby energized, connecting the output of the BFi filter to the in T1 at every half cycle of the carrier frequency.

It may be shown that this operation supplies the transformer T1 principally with only the upper and lower side bands, sum and difference frequencies, equivalent to modulating the carrier frequency current in amplitude by the voice currents.

Bearing in mind that this is a carrier'suppression system, it should be noted that the reversing operation by itself produces no current of carrier frequency in the transformer T1. However, if the oscillator output is notexactly connected to the electrical midpoints of the modulator circuit, a small unbalance current of carrier frequency may iiow in transformer T1. As is well-known, this may be made as small as desired by improving the balance of the modulator circuit. A balance suflicient for practical purpose may be readily obtained.

T1, therefore, passes on to theleft-hand terminals of BF; currents consisting of the upper and lower side bands of the 5900 C. P. S. carrier current modulatedl by the voice currents together with a residual amount-of the carrier current terminals of. radio transmitter T and putting it on the air'in 'case it is not already in this condition byreason ofi talking operations on one or more. of theA other local channels.

Carrier. channels Nos. 2 and 3 arepreci'sely the same as carrier channel No. 1 withthe exception of the carrier oscillator frequency andthe re.- sulting frequency range occupied by the lower sideband in each` case. Assuming that all the local intercom stations are being used for talking, then the following situation exists. In the case of the ordinary voice channel voice frequency currents in the frequency range 0 to 2800 C. P, S. are supplied to the in terminals, of radio transmitter T. ,In the case of al1 the carrier channels the frequency range of the voice currents is translated by the carrier apparatus to `occupy a range higher in the frequency scale than that occupied by the original voice. Thus, channel No. l supplies the` .in terminals of the radio transmitter T in the range 3100 to 5700 C. P. S.; channe'lNo. 2, 6100 to 87000. P. S.; and channel No. 3, 9l00to 11,700 C. P. S. Because each channel utilizes a 4different frequency range to actuate the radio transmitter,V it may be operated independently of every `other channel using the same radio transmitter. Sc far as LPv and the band filters are concerned, the independent operation is practically secured byl designing them' totransmit in. the appropriate yfrequency ranges `with little loss, to utilize efliciently the frequency ranges between channels for changing from transmission to suppression so that in the frequency ranges employed by all other channels adequate suppression is achieved. and to arrange that the impedance looking into theA right-hand terminals (as shown in Fig. l)V is high in, all `frequencyranges outside the proper transmitting range.

Continuing with the description of channel No. 1, the lower side band currents (3100 to 5700 C. P, S.) resulting from the call being made at `I1 modulate, in the apparatus of the radio transemployed. l Regardless of the particularl type of rad-io system, there will appear at the=out ter- `BF1 is an electric wave filter minals vof R a practical facsimile of the lower side band (3100 to 5700 C. P. S.) currents which entered the in terminals of the local radio transmitter. The switch at the distant I1 is assumed to be in the listen position and so TLRi is de-energized connecting the right-hand terminals of BF1 with the out terminals of R. Since BFi transmits currents in the frequency range 3100 to 5700 C. P. S., it receives and transmits to its left-hand terminals the currents now coming from the out terminals of R. Selection by the proper distant carrier channel is assured because even if other distant carrier channels happen to be connected to the out terminals of R at this time they would reject currents in the frequency range appropriate to channel No. 1.

The lower side band currents from the lefthand terminals of BFi are transformed by T1 which performsthe same function as at the local terminal, the only difference being in the direction of transmission. Therefore, the lower side band currents are fed to the distant MD1 which, together with O1, now acts as a demodulator, As previously explained, MB1 operates equally well in either direction. At the local talking terminal it receives the voice currents to 2800 C. P. S.) from LP1 and delivers to. T1 mainly currents of frequencies equal to 5900 C. P. S. plus 0 to 2800 C. P. S. and 5900 C. P. S. minus 0 to 2800 C; P. S. At the distant listening terminal it receives the lower side band currents (3100 to 5700 C. P. S.) and delivers to LP1 mainly currents of frequencies equal to 5900 C. P. S. plus 3100 to 5700 C. P. S. and 5900 C. P. S. minus 3100 to 5700 C P. S. The explanation already given for MDi acting as a modulator is the same, except for the diiferent frequency details, as when it acts as a demodulator.

The resulting currents of frequencies 9000 to 11,600 C. P. S. together with the small amount of unbalance carrier frequency (5900 C. P. S.) current are suppressed by LP1, and the resulting currents of frequencies 200 to 2800 C. P. S. (facsimile of the original voice currents) are transmitted with little loss by LP1 and delivered via terminals C1 to the intercom station I1 at the distant terminal. Since this I1 is in a listening condition, the voice currents are amplified and applied to the loud speaker which reproduces the call made at the local I1.

This complete-s the description of the operation over carrier channel No. 1 in one direction. It is, of course, the same in the opposite direction, the local terminal becoming the distant terminal and Vice versa. Likewise, the operation is the same over any other-carrier channel, the only difference being in the frequency of the carrier oscillator and the frequency range occupied by the corresponding lower side band.

Fig. 3 shows in diagrammatic form a radio relay station which may be interposed between the local and distant radio terminals whenever direct communication between terminals is not feasible. The designations West and East have no other significance than convenience in referring to the direction of transmission. It comprises two radio receivers R1, R2 and two radio transmitters T1, Tz, each with associated antennas. They are preferably the same as those used at the terminals. For west to east operation, Rl receives on its antenna the radio wave from the next radio transmitter to the west, which may be either a terminal or relay station, and produces at its out terminals currents of the same frequencies as have already been describedfor Vthe terminal station. These currents are connected to the in terminals of the transmitter T1 where they modulate a radio wave of frequencyr different from thatreceived by the associated R1. The new modulated radio wave is radiated from the associated antenna and is picked up by the next radio receiver (terminal or relay) to the east.

The operation from east to west is similar, the frequencies of the radio waves being different from those employed for west to east operation. This is for the purpose of securing independent two-way radio operation.

Each radio vrelay transmitter has a TR relay which has the same energy-saving feature already described for the radio terminal transmitters. The only difference is in the details of its energizing circuit. In this respect, eachvradio relay receiver has a relay RR with one armature and its normally open contact. Relay RR1 is arranged to be energized whenever the receiver R1 is actually receiving a radio signal of its proper frequency. A portion of this radio signal is rectified, as by radio tube VR1, and the rectified current caused to energize the relay RRl. When RR1 is energized its armature and contact close the energizing circuit of the associated TR1 relay, thus putting the associated transmitter T1 on the air. The overall effect is that if for any reason during a service period all the channels at a terminal happen temporarily to be in listening condition, the radio transmitter at that terminal and all relay transmitters (if used) operating in the direction of the opposite terminal will be off the air.

What is claimed is:

l. In a carrier radio telephone system, a plurality of terminal stations each having a radio transmitter and receiver, and a plurality of transmission media constituting two-way channels, a communicating set for each channel, each set having a telephone, an amplifier, and a twoposition switch connecting in one position the associated channel with the output and the telephone with the input of the amplifier, and in the second position the telephone with the output and the channel with the input of the amplifier, a relay for each set controlled by the associated switch, and contacts for each relay connecting the associated channel in one condition of the relay to the output of the receiver and in the -other condition of the relay to the input of the transmitter at the terminal station.

2. The system according to claim 1, and in which one channel is arranged to transmit signals at voice frequency, and each of the other channels at a different carrier frequency.

3. The system according to claim 1, and in which a plurality of the channels contain each a band pass filter passing a different band of frequencies, and a low pass filter in another one of the channels.

4. In a carrier radio telephone system, a plurality of terminal stations each having a radio transmitter and a radio receiver, and a plurality of transmissionr media 'constituting two-way channels, each containing a low pass filter and some containing band pass filters, a communication set for each channel, each set having a telephone, an amplifier, and a two-position switch connecting in one position the low Apass lter of the associated channel with the output and the telephone with the input of the amplier, andin the second position` the telephone with the output and the low pass filter with the switch, and normally closed and normally open contacts for each relay connecting the low pass filter of one and the band pass filters of the other channels, respectively, to the output of the receiver and the input of the transmitter at ythe terminal station.

5. In a carrier radio telephone system, a plurality of terminal stations each having a radio transmitter and receiver, and a plurality of `transmission media constituting two-way channels arranged to be connected in multiple to the radio transmitter and receiver, an intercom set for each channel, each set having a loud speaker, an amplifier, and a two-position switch connecting in one position the associated channel with the output and the loud speaker with the input of the amplier, and in the second position the loud speaker with the output and the channel with the input of the amplifier, a relay for each set energized in the second position of the associated switch, normally closed and normally open contacts for each relay connecting the associated channel, respectively, to the output of the receiver and the input of the transmitter at the terminal station, a power control relay in each transmitter, and normally open contacts for the relays of all sets controlling in multiple the power control relay.

6. In a telephone system, a terminal station, transmitting equipment and receiving equipment at said station, a plurality of two-way communication lines at said station, normally open multiple connections between all said lines and the transmitting equipment, and normally closed multiple connections between all said lines and the receiving equipment.

14 7. The telephone system according to claimV 6, and means associated with each line for controlling the opening and closing of said connections to the line.

8. The telephone system according to claim 6, relay means associated with tne line controlling the opening and closing of said connections to the line, and relay means controlling the transmitting equipment controlled by the first-mentioned relay means.

LEO A. KELLEY.

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

UNITED STATES PATENTS Number Name Date 1,544,910 Kendall July 7, 1925 1,548,260 Espenschied Aug. 4, 1925 1,313,483 Heising Aug. 19, 1919 1,559,867 Griggs Nov. 3, 1925 1,771,700 Alexanderson July 29, 1930 1,814,956 Ohl July 14, 1931 1,519,626 Nichols Dec. 16, 1924 2,111,023 Dixon Mar. 15, 1938 2,181,457 Peek et al. Nov. 28, 1939 2,337,878 Espenschied Dec. 28, 1943 1,501,104 Espenschied July 15, 1924 1,752,346 Kelley et al Apr. 1, 1920 1,690,227 Heising Nov. 6, 1928 2,289,794 Martin July 14, 1942 1,709,044 Smythe Apr. 16, 1929 1,447,204 Espenschied Mar. 6, 1923 2,388,906 Corderman Nov. 13, 1945

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