US2341224A - Two-way radio telephone system - Google Patents

Description

Feb. 8, 1944. MCDONALD-HAL 2,341,224

TWO-WAY RADIO TELEPHONE SYSTEM Filed Oct. 31, 1940 7 Sheets-Sheet 1 v nwmzrons JOSEPH F. McDONALD AND HERSCHEL a. MARTIN ATTORNEY Feb. 8;, 1944.

J. F. MO DONALD ETAL TWO-WAY RADIO TELEPHONE SYSTEM Filed Oct. 31, 1940 7 Sheets-Sheet 2 INVENTORS JOSEPH F. MCDONALD AND HEgiCgHEL ,5 MA RTIN I W A HORNE! I Feb. 8, 1944. J. F. M DONALD ETAL 2,341,224

TWQ-WAY RADIO TELEPHONE SYSTEM 7 Sheets-Sheet 5 INVENTORS JOSEPH F. MCDONALD AND HERSC EL 5. MART/N BY 7 W' ATTORNEY.

SY ms Feb. v 1944 F. MCDONALD ETAL I 2,341,224.

TWO-WAY RADIO TELEPHONE SYSTEM Filed Oct. 31, 1940 7 Sheets-Sheet 4 v V INVENTORS JOSEPH F. MCDONALD AND ,Q} HERSLZL 8. MARTIN A TTORNE Y Feb. 38, 1944. J. F. M DONALD ETAI. 2,341,224

TWO-WAY RADIO TELEPHONE SYSTEM Filed Oct. 3l, 1940 7 Sheets-Sheet a INVENTORS JOSEPH F. MCDONALD AND B. MARTIN m ATTORNEY W. E x a? u E HERSC EL mw w M w h NMJ W 4 wwlmm v 7 MW Q QT QT; m

mm zwk w w t wzwmk 1944- JfFyM DoNALD ETAL 2,341,224

TWO-WAY RADI LEPHONE SYSTEM Filed 0cd '3 l 1 9 40 7 Sheets-Sheet '7 REIC'IEIVIING TRANSMITTING 8 \"ANTENNA ANTENNA 1 mausmrmva A a? a E lV/N g g DESKSET 4 i APPARATUS $55512 L i alt BRIDGE IN RAD/0' ROOM 439 ROOM Q RADIO TELEPHONE 4 RECEIVE TRRNSMIT .PUSI'ITO RECEIVE WEATHER mom wnn SPEAKER USE IN'W'POSITI ION USE IN "'8" POSITION FOR SELECTIVE CALLING FOR TWO-WAY CON- ALWAYS RETURN T0"B" VERSATION ORTO CALI. AFTERDIAUNG NUMBER 368 Q BY WHISTLE SIGNALS CHANNELSELECTORS CHANNEL SELECTORS "men Q;

2' 3 t Fig. 7 Fly. Fi .4

' INVENTORS l JOSEPH F. MCDONALD AND F 32 F y-5 F g.6 I BY 7'??? MART/N Patented Feb. 8, 1944 2,341,224 I TWO-WAY RADIO TELEPHONE SYSTEM Joseph F. McDonald, New York, and Herschel B; Martin, Brooklyn, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application October 31, 1940, Serial No. 363,628

Claims.

This invention relates to two-way radio telephone systems. The apparatus herein shown and described has many novel features, the construction being such as to attain certain specific objects, the more important ones of which are as follows:

It is an object of our invention to provide a two-way radio telephone system having transmitting and receiving equipment which is adapted to operate on a selected choice of any predetermined number of radio frequency channels. During conversation, the transmitter and the receiver at a given station may be automatically tuned to the same or different radio frequencies.

Another object is to provide a convenient control unit having a hand-set which includes both microphone and earphone. The control unit is to be provided with a cradle for the hand-set and its cabinet should house or support suitable indicator lamps and push buttons, a volume control device for the earphone, a toggle switch and a selector dial for making selective calls to another station.

It is another object of our invention to provide radio telephone equipment having a plurality of receiving channels, all of which may be normally maintained in operation for the reception of incoming calls, and to provide means for deadening the receiving circuits on all but one channel, this channel being selected for reception of incoming speech signals.

It is another object of our invention to provide radio telephone equipment suitable for the transmission of outgoing calls on any selected one of a plurality of transmitting channels either by two-tone selective code signals, or by a special low frequency tone signal corresponding to Whistle signals, or by voice signals.

It is another object of our invention to provide an automatic control of the transmitting and receiving circuits for alternate use thereof, as when transmitting and receiving speech signals. In carrying out this object we preferably provide a speech-responsive relay for short-circuiting the receiving antenna and for impressing a modulated carrier wave on the transmitting antenna. This feature is such that when the microphone ceases to pick up voice signals the receiving circuits are automatically restored to operative condition and the carrier of the transmitter is removed from the transmitting antenna.

It is another object of our invention to provide for the convenient reception of weather reports through a loud-speaker connected to any one or more of the receiving circuits or through a handset connected toithe same receiving circuits.

I It is a further'object of our invention to provide radio telephone transmitting equipment which shall bepowered by a motor generator, this generator being automatically started up in response to the lifting of a hand-set off its cradle.

It is a further object of our invention to provide for the-alternative control of receiving and transmitting telephone equipment from either of two remote points such as may conveniently include, for example, the radio operators cabin and the ships bridge.

It is a further object of our invention to provide a radio telephone system which is equipped with suitable channel indicating pilot lamps for showing selected channels on which transmission and reception is to be carried on.

It is another object ofour invention to provide a station selector dial for initiating call signals, and a convenient call indicator havingindicator lights which will identify the channel of an incoming call, this equipment being inclusive also of a selective bell ringer and channel indicator so that when a particular telephone station is 'called only its particular bell will respond, while the bell ringer at other stations having difierent appropriate call signals will be unresponsive. At the called station thechannel indicator is arranged to beactuated by the call selector so as to notify the called party as to the intended channel to be used.

In carrying out our invention we, of course, make usev of structural parts and assemblies which toa large extent are Well known in the art. The novel features of our invention, however, will be best, appreciated by reference to the claims which follow a detailed description of a typical embodiment. This typical embodiment will now be described under various-captions, although it 'should'be borne in mind that cooperation between the elements mentioned under different captions is essential to the convenient and ex- .peditious use of the system as a whole.

In the accompanying drawings we have illustrated the invention by means of different figures as follows:

Figures 1 to 6, inclusive, are fragmentary circuit diagrams which should be fitted together in accordance with the layout diagram of Figure 7 in order to show comprehensively the entire system; v

. a Figure 8 shows schematically the different components of a complete transmitting and receiving cated in Figures 1 and 2.v

stood that the circuits for a number of different relays will need to be described and where the contact pairs or contact groups of these relays are referred to they will be indicated by the letters. a, b, c and d, following the reference number for the relay itself. The contact pair nearest the relay coil will invariably be referenced a, and succeeding contact pairs or groups will be given the letters b, c and (1. Furthermore, it should be generally observed that our receiving system is designed to operate under standby conditions, so that it is necessary to-provide a permanent source such as the ships power linefor operation of the receivers and also for certain relay operations incident to the making of a call. a

On the other hand, in the carrying out of our invention it is provided that a motor generator shall be started up automatically upon lifting either one of two hand-sets oif its cradle;'and the motor generator then provides the necessary auxiliary voltages for operation of" certain relays as well as for the power supply to the code signalling and voice transmitting tubes. The motor generator is automatically shut down upon restoring the hand-set to its cradle.

The general arrangement of operating units Referring to Figure 8,'we show therein a cabinet I, whichhouses both transmitting and receiving apparatus, located preferably in the radio room of aship or in another convenient location. The receiving antenna 2 and the transmitting antenna 3 are suitably connected to appropriate apparatus within this cabinet, all as will be better understood upon reaching a detailed description of the circuits. Two control units 4 are shown connected to the transmitting and receiv ing equipment within the cabinet I each of these units being provided with a' telephone hand-set and with suitable push-buttons, pilot lights and calling dial, all as more clearly pictured in Figure 9. Figure 8 also shows an annunciator box 439 which houses a bell ringer and a number of pilot lights for showing the channel on which an incoming. call' is to be responded to. These pilot lights are selectedand the bell is rung in accordance with the operation. of an automatic selector mounted within the apparatus cabinet I, the selector beingrendered responsive normally to a number of different receiving channels which are maintained in a standby condition. The bell A38 is shown mounted on the outside of the box 439 and its operation will be hereinafter described. The loud-speaker 315 is also shown connected to an output-transformer 300 (Figure 3) within the cabinet I.

The control unit One of the'control units is illustrated in Figure 9 and a circuit diagram thereof appears dupli- The reason for this duplication is that, while it is presumed that two control units may be placed at separate points and. wired therefrom to the cabinet which conasinaee tains the transmitting and receiving apparatus, it is necessary to indicate wherein certain of the circuits are in parallel and where at least one of these circuits forms a series connection from one control unit to the other.

The hand-set 312 comprises a microphone Mil and an earphone AM. When not in use the hand set rests in a cradle 402 having control of a double-pole single-throw switch 3' which comprises contact pairs a. and b. A toggle switch 368 is also provided for the purpose of actuating two pairs of contact springs c and d. A call selector dial 364 is provided with one pair of off-normal springs 403 and a pair of impulsing springs 404. Pilot lamps 36.1. are provided to indicate the particular channel which is selected for a conversation. In the circuit diagram only three of these pilot lamps have been shown at each control unit,

whereas in practice it has been found convenient to provide selectivity with respect to any one of six communicating channels. The principles of operation, however, are well illustrated in the'circuit diagram by means of the alternative arrangements shown in respect to three channels.

A speech responsive relay will presently be described, the operation of which is indicated by pilot lamps designated R and T on the control unit. When the microphone does not receive speech waves the pilot lamp R normally remains lighted to indicate that the system is inreadiness for reception. In response to speech waves in the microphone, however, a relay 42 (Figure 5) releases and a break-in relay I93 operates for the purpose of impressing a carrier waveon the transmitting antenna; for short-circuiting the receiving antenna; and for causing the pilot lamp T to light up while simultaneously extinguishing the pilot lamp B. This operation is entirely automatic as will hereinafter be shown.'

For the reception of weather reports with the aid of a loud-speaker a push-button 365 may be momentarily depressed and thereafter a pilot lamp 405 will light up to indicate that the necessary conditions have been established for rendering the loud-speaker responsive to a selected communication channel. This condition will persist until" the hand-set 312 is replaced in its cradle.

A toggle switch 368 is used in its two positions A and B in accordance with the legends applied to these letters on the panel of the control unit. That is to say, the switch is thrown to the A- position for selective calling as by the use of the dial selector. The switch is thrown to the B- position after dialing, so as to establish the proper conditions for two-way conversation or to tion from the negative side of the D. C. motor generator source 42! (Figure 5') may be traced from the movable contact 432 on the left hand button 430 of the control unit shown in Figure 1 to the back contact of the right hand pushbutton 4339 in this control unit, thence to the movable contact appropriate to the left hand push-button 43 in the control unit of Figure 2. The back contact 13i of the right hand pushbutton $38 in the control unit of Figure 2 is in circuit with a conductor 405 interconnecting two resistors liil and [50, the purpose of which will be set forth under the caption Vodas relay system and will also be mentioned in, connection with the description of operation of certain other relays.

.5) shown in response to speech currents, and when the movable contact 1 closes with its companion Channel arrangement This two-way radio system is adapted for op-- eration in accordance with regulations imposed by the Federal Communications Commission of the United States. Certainchannelsare available for ship-to-shore communications. Other channels are available for ship-to-ship communications. One particular channel is available in the Great Lakes region, for example, as a calling channel and for distress signals, and should not be used. except in cases of necessity.

In accordance with these requirements the control units and the circuit arrangements of our system are designed to operate automatically on the calling frequency 5 without stopping to depress the-channel selector button 430 (indicated in Figure 9 as #5 button). The means for rendering the receiving and transmitting channels operative at this calling frequency will hereinafter be described.

The receiver circuits ed. The secondaries are tuned by means of-the usual variable capacitors 401. A connection from the receiving antenna 2 is also made to a relay contact 12 appropriate to relay I93 (Figure This relay operates as will hereinafter be t the receiving antenna is grounded.

The receiving apparatus comprises a number of different radio frequency amplifier circuits each tuned to a difierent frequency. These are indicated by blocks 408 and 409. Individually these R. F. amplifiers are designated as tuned to . channels 1, 2, 5 and 6, respectively. Corresponding amplifiers may be provided for channels 3 and 4, but for the sake of simplicity these have not been indicated; Each R. F. amplifier is in circuit with a converter oscillator unit 4 I 0 or M I. The oscillators of units 4| 0 are tuned to different frequencies so as to obtain the sameintermediate frequency from either channel 1 or channel 2. This intermediate frequency is then fedgto an I. F, amplifier, second detector, and an A. F. amplifier M2. Similarly, the outputs from the converter oscillator units 4 are fed to a common input circuit for a corresponding 1. F. am- .plifier which is represented in combination with a second detector and A. F. amplifier in the block 4I3.

For receiving a conversation at the earphone the amplified speech currents from either of the units 512 or 4 I3 is impressed across a transformer 28! to an earphone circuit, the two ends of which are-grounded. This circuitma-y be traced from the secondary of transformer 281 through contacts h of either one of the relays I44 or I45 and it thence to the earphone M of the hand-set 312 which happens to be in use.

I A. loud-speaker 315 is provided especially for unit 4" is a push-pull arrangement so that the output transformer 300 has a primary which is center-tapped, this center-tap being fed with +90 volts from the ships D. C. line. The secondary of this transformer 300 is grounded at one terminal, while the other terminal connects through a conductor 5I5 which may be traced through relay contacts 358d (Figure 4) and thence to a volume control rheostat 316 leading to the voice coil of loud-speaker 375. As will be seen, both ends of this loud-speaker circuit are grounded and hence the loud-speaker itself responds only to the currents which are induced across the transformer 300.

When neither relay I43 nor relay 358 is actuated the loud-speaker circuit is nevertheless completed through relay contacts I430. This is the stand-by condition for the reception of incoming calls by voice or by whistle signals. When using the hand-set, however, relay I43 will be actuated, as will be hereinafter shown, and hence with relay 35B unenergized the loud-speaker circuit will be opened and no response will be ob-- tained therefrom.

Crystal oscillator circuits Three of the oscillator circuits for respectively different transmitting channels are shown illustratively in Figure 6. Each of the transmitting channels uses a separate oscillator circuit comprising a discharge tube I9, a piezo-electric crystal 80, and a tuned plate circuit which includes an inductance 52, and a variable capacitor 58. The refinements of the circuit arrangement include also a resistor 68 interconnecting the tuned circuits 52-58 and the anode of the tube I9. The control grid of this tube is connected to one electrade of the piezo-electric crystal and also to a feed-back coupling means 69. The feedback coupling means is indicated in Fig. 6 by means of blocks in order to imply that any suitable feedback coupling device may be employed whether inductive or capacitive. For example, as taught in a Patent #2,085,444, granted June 29, 1937,, to

, L. M. Perkins, the oscillator control grid may be connected to an open-ended coil which constitutes a capacitive coupling means if disposed in close proximity to the conductor of the anode circuit. Inductive coupling on the other hand would be provided if the anode circuit for the oscillator tube were to include the primary winding of a transformer, the secondary winding of which would be connected between ground and the control grid of the oscillator tube I9. Applicants, therefore, make no claim to any specific form of feedback coupling means. A grounded grid leak resistor I4 provides self-biasing of the control grid with respect to the'cathode.

Filament heating potential is supplied by a center-tapped secondary on transformer I29, this transformer being energized by alternating current from the motor generator 4I1 (Figure 5). The center-tap on the filament transformer winding for the oscillator tubes is grounded through contacts w-z of relay I93 when the latter is energized in response to speech currents from the microphone. The terminals of this transformer winding are connected with the conductors M5 and M6. High frequency potentials are shunted around the filament transformer secondary by means of grounded capacitors 6|. Conductor M5 is connected to each tube filament at one end thereof, while conductor 4 I6 connects to front contacts a, of relays II], II and I2. These relays are selectively actuated for energizing the filaments of the oscillator tubes in accordance with the channel'to be used.

A 400 volt D: C. source from the motor generator 4I'l 'is lead through resistor M9 tothe screen grids of the tubes 19, and also through the i'nductances 52 of the tuned anode circuits to each of the anodes of these tubes.

The relays Iii, I I and I2 are each provided with a second pair of contacts b for connecting the selected oscillator tank circuit across a coupling condenser 38 to the control grid of a power amplifier tube 8I3. This tube is, therefore, caused to deliver a carrier wave under control of the selected oscillator circuit.

Power amplifier circuit The power amplifier tube 8I3 is connectible to a tank and antenna loading circuit by means of relays 4', 5 and 6, which are operated in dependence upon the channel to be used. The coils of these relays are connected in parallel with correspending relays It, I I andIZ in a selector circuit which is controlled by the push buttons on the control units. The tanks and loading coils-are preadjusted to the frequencies at which the transmitter is to be operated. These tuning elements are indicated'illustratively as including inductance i5 and capacitor I8 in circuit with the transmitting antenna 3. Other elements included in the circuit arrangement are shown as variable inductances 2!, 23 and 24 which are provided with movable taps connecting through contacts b of the respective channel relays l, 5 and I3 to the coupling condenser across which the amplified carrier wave is fed from the anode of the power amplifier tube M3. The parallel inductance and resistance unit 33 is provided for the purpose of choking'parasitic oscillations.

' The filament of the tube M3 is fed from its own secondary winding on the A. C. filament transformer I29. This secondary winding is not shown in the drawing (Figure 5) since it would unduly crowd the diagram and furthermore would be shown as having a grounded center tap the same as another secondary which is so represented.

In fact, the transformer I29 may have as many as siX independent secondary windings each having a specific filament circuit to be independently controlled or to have its center tap either grounded or connected to the negative terminal of an independent power source. The control grid of the tube H3 is biased by means of a volt lead to the motor generator M1. A choke coil 39 and a resistor 40 are introduced into this lead for filtering purposes, while the resistor 31 having a value of preferably 15 ohms or thereabouts makes 'for smoother application of the oscillator voltage to the control grid.

Since the channels are tuned to widely different frequencies, it is preferable that the tuning the particular installation.

Relay 9 is provided in order that the antenna 3 may be automatically grounded when the transmitter is shut down; that is to say, when the motor generator MT is stopped. This relay 9 is, therefore, energized by direct connection of its winding across the 50'volt leads from the motor generator 4H, and remains energized as long as the generator runs.

Voice modulation circuits Voice modulation is accomplished as follows:

The microphone 400 on either of the hand-sets is connected to one of the contacts 0 of relay I44 or I45. As will hereinafter be shown, one o'i' these relays is energized upon lifting a hand-set off its cradle. The front contacts a of relays I44 and I45 are connected to front contact a of relay I42. This relay is also closed when using the hand-set. Following this circuit further, from the movable contact a 0n relay I42 it is shown traversing. contacts (1 of relay I40 and thence to the primary winding of transformer (Figure 5). The other terminal of this transformer primary is grounded through resistor 94. The secondary of the transformer 95 is included in the input circuit of a class A amplifier tube 4 I 8. The output circuit of this tube includes the primary of transformer IM. This transformer has a grounded center-tapped secondary and the, two terminals of its secondary winding are connected respectively to the grids of a push-pull amplifier system comprising two amplifier triodes MB. .A stage of class B amplification of the voice modulation currents is obtained by inductive coupling across the transformer H2 to the grids'of the push-pull amplifier tubes 420. The output trans former I I4 has its primary connected to the anodes of the tubes 42!), and this transformer has two secondaries MI and. 422. The secondary winding 42I feeds modulation currents across resistor 46 and the inductive and resistive unit 3311 to the screen grid in the power amplifier tube BIS. The secondary winding 422 modulates the anode of the power amplifier tube. the circuit therefor including a choke 3| and a resistor 32.

Tune signalling oscillator circuit In order totransmit a two-tone signal for calling another station, the dial 364 on the control unit is used. This dial has a pair of offnormal contacts 403 by means of which a relay I46 is actuated during the times of impulsing a code signal as by means of the impulsing contacts 4%. The circuit for relay I46 may be traced as follows:

From ground, which is the positive terminal of the 50 volt motor generator source 4I'I (Figure 5), current transverses the relay winding and thence through contacts 403 and contacts a of the hand-set cradle switch 3'" to the 50 volt'terminal of the motor generator. Depending upon the digits of the calling code number to be transmitted, that is to say, depending upon the man,- ner in which the dial 364 is manipulated, a predetermined series of impulses will be produced by the impulsing contacts 404, thereby causing relay 'I4I to respond impulsively. The circuit for this purpose may be traced as follows:

From the grounded +50 volt motor generator supply, current traverses the winding of relay MI and thence through contacts 01 of one of the relays IM or I45, depending upon which handset has been lifted. Assuming that it is the hand-set shown in Figure 1, this will be the circuit through relay M411 and thence to the impulsing contacts 604, one of which is connected to contacts d of the toggle switch 368, these contacts being closed when the switch is in the A- position. Therefore, the circuit is completed through contacts a of the hand-set cradle switch former 88.

tube 423 may, therefore, be traced as follows:

From ground, which is cathode potential, the input circuit includes the selected portionof' the secondary winding of transformer 88, the front m to the minus terminal of the tor source. V p y The functions of relay I40, termed a dialing relay are as follows, it being understood that for dialing a call-selector number this relay remains energized during the entire time, while for dialing whistle signals it is energized by dot-and-dash impulsing in accordance with the closing of the ofi-normal springs 403 of the dial: Contacts I a place ground on the center-tap of a filament transformer secondary which isnot shown, but is one of a plurality of similar second-- aries all wound on transformer I29, thisparticular secondary being used exclusively asthefila ment source for a tone frequency oscillator tube 423. This grounding of the, secondary enables tube 423 to draw current. V

Contacts I40b open in order to disconnect the circuit between the anode of a speech responsive tube 424 and its associated relay 42. When this relay de-energizes a. short-'circuiting ground is removed from the coil of a break-in relay, I93. This relay I93 is then actuated for the purpose of putting carrier current on the transmitter circuits and for other purposes to be hereinafter explained. l Contacts I40cclose for the purpose of supplying a tone signal to the grid of transmitter modulator tube 4I8. Contacts I40d open in order to disconnect the microphone circuit from theprimary of transformer 95. l Tone signal is derived from the two-toneoscillator 423, the output circuit of which includes the limiting resistors 83 and 84 in circuit with the +400 volt motor generator feeder. The oscillations from the tube 423 are impressed across the capacitor 85 to the transformerBfi, both windings of which are grounded at one end. By means of taps e and j on thesecondary of transformer volt genera- 88, the inductance of the secondary is altered,

under control of relay I39, this relay being actuated by current fed thereto when toggle, switch 368 is in the A-position. During the entire time of dialing the number of a called station, relays I39 and I40 are actuated in; response to: thesetting of toggle switch 308 to the A-position so that the input circuit for tube 423 includes only a portion of the secondaryv turns on the trans- The input circuit for the oscillator contact a, of relay I39, the grid leak condenser ,lcombination 3B, 81' and thence to the grid ofdischarge tube 423., The inductance of this circuit l.:is such that, when combined with certain capacitances, either of two tone-frequency signals are impressed upon the modulator tube 4| 3; 7 One "of, these frequencies is determined by the capaci-= tor I30 which is normally in shunt withthe sec- ."ondary winding of the transformer 88 when relay MI is not actuated, while the other frequency is determined by the capacitor I3I which is additionally introduced when relay MI is actuated. This variation in tone frequency is explained as follows:

Let it be assumed that the two tone frequencies are 1500 cycles and 600 cycles respectively. Be-

former 88. This capacitor circuit may be traced PUQ F From ground through capacitor I30 to front contact I39!) and thence across-the grid leak ca-v pacitor combination 86, 81 the circuit goes to the grid of oscillatortube 423.

,. When the impulsing relay I4I energizes in response to the closing of i'mpul'singfconta'cts 404, then the supplementary capacitor I3I is added to thecapacitorfl30. This additional capacitance changes the tuning of the input circuit soas to cause jthe tone frequencyto become 600 cycles. Preferably a resistor '89 is placed in seriesm'th the capacitor I3I'. When the station to be called is not provided with a two-tone selector and alarm bell it is then necessary to send out so-called whistle signals which may be responded to by the loud-speaker of the called station. These whistle signals may consist of dots and dashes and may be sent out'by.means of the dial 364. In this case a dot is transmitted by. fingering the #1 hole in the dial andthedash b fingering the #4 hole in the dial. 3 During the operation of whistle siglowers the tone of the oscillator to preferably 300 cycles.

Relay contacts l39c are used to" adjust the amplitude level of the output from the oscillator 423. The 300 cycle tone when impressed on the grid of modulator amplifier 41 0 needs to'bereduced in amplitude compared with the 600 cycle and 1 500 cycle tones. For the latter (when movgable contact springv I390 engages with its front companion) theuseful signal circuit is shunted by a series path throu'ghresistors I54 and I55 to ground. When, however, relay I39 is de-energized for 300 cycle signalling, then'resistor1l54 is traversed by the total output. from the oscillator 423 and a larger portion of this output is shunted off to ground through resistor I55.

. As previously stated, the-tone frequency. oscillator tube 423 possesses an exclusive filament source (not shown) consisting of a center-tapped secondary on transformer' 'l29 (Figure 5). The

center-tap is connected tofront contact I400 and the companion contact is grounded. Relay I40 isenergized by the closing of the 'offenormal gle switch 368 to the A-position. For sending springs 403 on the dial 364 or by setting the tog- 7 code combinations of dots and dashes th''togand 4 for a dash.

gle, switch is set to they B-position, so as to use dial springs 403 for keying the oscillator 423 on and off; the dial being fingered 1 for a dot The transmitter power supply I The power supply unit 4H hasbeen' incidentally mentioned heretofore. It comprisesa motor 425 for driving three separategenerator units as. follows: ,1 I l 1 -The unit 426 provides an .A. d epen of prer- -'erably .65 volts for energizing filament. transformer 129 havinea number ofs'econdary windmgs for different portions of the transmitter equipment. A 50 volt D. C. supply unit is'indic'ated at 421, its positive brush being grounded. The third generator unit 428 has two sets of commutators, one fordelivering a D. C. supply of 400 volts, and the other an'additional 600 volts. The 400 and'600, volt commutators are in series. This provides'400 volts for audio oscillator plate power amplifier screen, crystal oscillator plates and screens, and microphone amplifier plate. The 1000 volt terminal feeds to the plates of modulator tubes "M9 and 42B and to the power output tube 813. Filtering means (not shown) are provided for each of the output circuits from the motor generator, and the fram of the motor generator set is kept free from ground in order to maintain the effectiveness of this filter.

Hand set relay system hand set 312 (Fig. 1) is raised, the

cradle contacts *3-IIa-and 31 II) are closed. Contacts 'b supply 90 volts from the ships line to the coil of relay I44, the'other terminal .of which is connected through a limiting resistor I46 to the +90 volt ships line. Relay I44 remains energized, therefore-"as long-as the handset 312 remains off the cradle. The'cradle contacts a, complete *a 5'0 volt D. C.-circuit to the dial contacts-4'33 and toggleswitch contacts 3680 and 368d,'as well "as'toone of the'contacts of a pushbutton 365 which is used in connection with the reception of weather reports. 1

The actuation of relay I44 provides the following conditions:

"Contactsa' connect +90 volts (ships line) to I a-"starter'relay 429 '(Fig. 5) by means of which the motor generator unit 4| I is started up. -Contacts I44a also supply +90 volts through 'a'resistor I'4'8 to"actuate relay I43'and thence to-the -=90 volt side of the ships line. Contacts 17 close-"a-circuit'from the earphone 40I to the sec- "ondary'of receiver output transformer- 281, thus enabling theoperator to listen to a conversation 'asgreceived by'anyone'of the receiving sets.

Contacts I440 connect the microphone-400--with "frontcontactfl-42a-and thencethrough back contact Mild to the primary of microphon'e trans- "for'mer 95 at theinput end of the modulator cir- "cuit. Contacts I44d are used only when dialing, 'sin'c'efthey close a circuit from the impulsing con- "tacts"404 on'thedialto the impulsing relay "I 4 I.

The "functionswf relay I45 are identical to those ofrelay I44, but relay I45 is actuated by liftingthe other hand set shown in Fig. 2, and

"it servesto connect the.microphone andearphone "of that" hand set with their respective transmit- :tihg and receiving instruments.

Itwas stated that relay I43 is actuated re- "spon'se to the actuation of relay I44 (01- relay 145). The conditions thereby obtainedare as follows:

Contacts 143a connect the 50 volt motor gen- "erat'orfeeder-with the coil of relay I42,- this cir- "cuit traversing a resistor I49, the relay coil, "and thence to ground. The purpose of the resistor I49 is to delay the action of this relay I42 by an appreciable amount, as will be hereinafter explained. Contacts I43a also supply a locking voltage to relay 358,1provided 'this'relay is first actuated in response to the de- -'pression 'of push-button36'5. Contacts =I43b-are used to complete a locking circuit foranynneof the relays 136,131 and I38, providedone of these relays is selected for actuation by a channel- '"selectingpush button- 430. --'I'he'-movable contact.

'I43b is connected through resistor I50 to back contact 43I which is appropriate to the push. button 430 furthest to the right on the .control unit shown in Fig. 2.. Normally this contact :is in series with other contacts of all of thepush-buttons of the two control units, finally ending at movable contact 432 on the push-button 4:33 furthest to the left in the control .unit'shown in Fig. l. The last mentionedcontact is connected to the 50 volt line from the motoregenerator. The purpose of the series circuit through all the push-buttons is to open this locking circuit when.- ever a new channel selection is made. It thus will be seen thatany previously locked channel relay I36, I31 or I38 will be unlocked at .the time of making anew selection.

Relay contacts I430 open .the loud speakercire cuit in order that reception may be obtainedcn the earphoneAUI only. The loud speaker circuit has been previously traced under the caption of The receiver circuits.

of'modulator amplifier transformer 95. Contacts |42b open the .volt circuit from contacts I93 u, m to the back contactsc of relays.I36, I31 and I38. Contacts I420 open,in.or der to discone nect the automatic selection.circuit:f.or,r,e1 y I33 since these contacts momentarily close a5!) volt circuit through the winding of relay 36 onlyduring the time between starting up :of themotor generator and thedelayedaction of relay1I42. If, however, when lifting the.hand set, a difierent channel selection is to .be made, this can be done with relay I42 energized. inasmuch ,asjthe locking circuit for relay 1'33 willuthen be opened in the same manner assfor disconnecting any other used channel relay in response to thebreaking .Of the '50 volt connection through the push-button 4 series.

Contacts I-42d close in order .to apply the 9.0

' volt source to the ringer. restoring mechanism in the call selector .431, as will .be hereinafter ex- ..Channel selection "When. responding toan incoming call itis frequently the case that this call will be madeon .the calling channel"#5. It is only necessary then tofliftfthe receiver andcommence a conversation 'for the .reason'that, as soon as themotor generator 45'? starts up, current is supplied through an already closed circuit from the ---50 volt motorgenerator, lead; through relay contacts I'42c and'thence through 'the'coil ofrelay-H6430 the grounded positiveside of the 50 volt motor generator source. Due to the'impedance of' resistor I49 inseries with'the coilofrelay I42 this relay. does not open its 0 contactsuntil after relay I36 .pulls .upand establishes its own locking circuit through contacts I361).

Thefunctions, of relay. I36 (appropriate to the selectionof channel #5) are as follows:

Contacts 'I'36a close a circuit 'fromthe -'-50 volt motor generator lead to each 01 the'parallelconnected windings of two relays Band I2 (Fig.

6). Relay Gpulls up in order to extend the transmitter power amplifier circuit through capacitor 30, relay contacts 6b, antenna coil24, relay contacts 6a and thence to the transmitting antenna Relay I2 also.- energ'izes. or the ,purpQSe. of

. of the selected oscillator tube 19. Thus only one of the transmitting oscillators is put into service for controlling the carrier wave frequency delivered by the power output tube M3, the other oscillator tubes 19 being deprived of filament current.

An additional function of relay contacts I36a is to supply current to the appropriate one of the channel indicating pilot lamps 361 on the control unit. The circuit for this pilot lamp leads through a resistor 442, the filament on pilot lamp and thence to ground. 3

Contacts I331) are associated with a locking circuit for the relay itself. This circuit may be traced from the -50 volt motor generator lead which is carried to the movable contact 432 oi push-button 430, and thence through all of the push-buttons of the two control units to back contact 43| of the push-button furthest to the right on control unit of Fig. 2 and thence through a, circuit which may be traced to conductor 405 intermediate between resistors I9I and IE0. This 50 volt source is then extended through resistor I50, contacts I43b-, contact I36!) and thence through the, coil of relay I36 to ground. It will beapparent from the tracing of this locking circuit that relay I36 will be de-energized upon depressing any of the channel selecting'pushbuttons 430 other than its own appropriate pushbutton. I

Front contact I360 serves tov connect the cathodes of the channel receiver oscillator con verter tubes through contacts u and :c of relay I93 to the 90 volt ships line. Thesereceiving tubes for the channel #5 receivervare thus energized to the exclusion of corresponding receiver oscillator converter tubes in the other receivers, whereas the back contacts I350, I310 and I380 serve to maintain each of the receivers operative when they are to remain so for standby requirements. The disconnection of unwanted receivers during a telephone conversation is obtained by energizing relay I42 and opening its contacts 12 between the ---90 volt ships line and the aforesaid parallel connected back contacts I36c, i310 and I380.

. The control ,of relay I31 and I38 is similar to that of relay I33, except for the fact that pushbutton operation is required in all cases in order to-make the desired channel selection. If, for example, relay I31 corresponding to channel #2 is to be operated, then the appropriate pushbutton 430 is depressed momentarily. As soon as relay I31 pulls up, its locking circuit through contacts I311) is established and will be held by virtue of the current flowing through resistor I50 as previously described. In this case the appropriate transmitter oscillator 19 will be put in service by'the actuation of relay I I. Relay 5 will also be actuated to connect the transmitting antenna 3 with its antenna tuning coils which are tuned to'the carrier wave of the #2 channel. Finally the cathode of the oscillator and converter tubes in the #2 channel receiver will be connected to the 90 volt ships line to the exclusion of-similar connections with the oscillator and converter tubes of the other receivers.

The vodas relaysystem Among the objects of our invention we men tioned that we preferably provide a speech-responsive relay for short-circuitin the receiving antenna and for impressing a modulated carrier wave on the transmitting antenna. Among those conversant with radio telephone practice such a speech responsive relay is sometimes termed a vodas relay. Relay 42 is here shown (Fig.5)

performing such a function in cooperation with relay I93. The operation of this system is as follows:

Speech currents from the push-pull tubes M9 in the microphone amplifier circuit are impressed across resistors I03 to the primary of a transformer I05. The secondary of this transformer is connected at one terminal to the 90 volt ship line and the other terminal feeds across resistor I06 and capacitor I01 to the grid of a discharge tube 424. The cathode of this tube is heated by an appropriate secondary (not shown, but one of the plurality previously mentioned) on filament transformer I29, this secondary having a center tap which is connected to the 90 volt ships line. The anode of tube 424 connects through contacts M0?) to the winding of relay 42 and thence to the +90 volt ships line.

Speech currents impressed across the transformer I05 have the effect of blockin tube 424 so that relay 42 de-energizes. Accordingly the grounded volt) armature of relay 42 rests upon its back contact and completes a circuit from the +50 volt motor generator lead through resistors 433 in the two control units. The leakage currents through these resistors serve no useful purpose at this time, however, but 'the groundedarmature of relay 42 diverts this cur-- rent from the pilot lamps R so that the two filament terminals of these pilot lamps are maintained at the same +50 volt potential and the lights go out. The transmitting pilot lamps T,

however, are lighted by this grounded +50 volt potential through their filaments to conductor 434 and thence through resistor I9I to back contact 43I of the push-button series. The other end of this series is connected by contact 432 (Figure 1) to the 50 volt lead. Accordingly the transmitting pilot lamps are lighted. At the same time relay I93 operates, since one of its terminals is supplied with current from the grounded +50 volt source, and the other terminal is in series with resistor I9I the push button series and the +50 volt lead to contact 432 (Figure 1).

7 When speech currents in the transformer I05 cease, then discharge tube 424 becomes conductive, relay 42 is actuated and ground potential is placed on both sides of relay I93 as well as upon both sides of the filaments in the pilot lamps T. Under these conditions a telephone conversation may be received, and pilot lamps R will so indicate, because their filaments are no longer shorted out by a ground potential on the back contact of relay 42.

The functions of relay I93 will now be explained:

This relay is of a special type wherein, if the relay is energized, contacts u, a: first open, secondly, contacts 11, y close and thirdly, contacts w, a close.

The opening of contacts u, :1: removes the volt source from the receiver oscillator and converter tubes so that the receiver in use goes dead. This circuit may be traced from the 90 volt ships line through contacts a: and u to the particular front contacts c of the selected relay I36, I31 or I38 appropriate to the channel in use, and thence to the receiving oscillator and converter tubes in use.

Contacts 12, y close in order to ground the receiving antenna 2. Finally contacts 112, a close in order to place ground on the center taps of filament source secondaries on transformer I29. This grounding of the secondaries removes blocking voltage from the bias circuit between the cathode and the first grid of the transmitter oscillator 19 of the particular transmitter in use. Blocking voltages are also removed from the grids of tubes e20 in the transmitter modulator circuit. The'oscillator and modulator tubes, therefore, become normally biased and are conditioned for transmission of modulated speech waves.

The vodas relay system is quick-operating in response to the impress of speech currents on the modulator circuit, since the relay I93 becomes energized as soon as the armature of relay 42 releases to open the grounding circuit at the lower end of resistor IQI. However, when speech is momentarily interrupted, as between the words of a'sentence, the relay 52 is made slow-acting by virtue of the time constant of the grid leak condenser combination Its-4&1 on the tube 424. With reasonable continuity of speech, therefore, the relay 32 remains de-energized and relay I33 remains energized. After a moments delay, however, in an outgoing conversation the receiving circuit is restored to its operative condition while the transmitter carrier is cut off by the blocking of the oscillator tube 19, as when its cathode becomes connected to ground through the resistor 436. A steady potential drop is developed across this resistor by virtue of its connection through resistor 35 to the +400 volt generator source. Keying of the transmitter is thus obtained by closing the contacts w, 2 of relay I93 so as to short-circuit resistor 436.

The weather relay Weather relay 338 is energized by a momentary impulse impressed thereon through push-button 365 on the control unit. Toggle switch 368 is placed in the B-position. The hand set 312 is removed from its cradle. The push-button 365, therefore, receives a 50 volt current from the motor generator (now started up) through contacts a of the cradle switch 3! I. The stationary contact of push-button 365 feeds this current to the coil of relay 353 and thence to ground. The functions of this relay are explained as follows: Contacts 353a close a holding circuit for the coil of relay 358, current traversing these contacts being derived from the 50 volt generator line through relay contacts Hlfiia.

Movable contact of group 3581) is connected to the 90 volt ships line and when it contacts with its back stationary companion a filter 290 is in circuit between this -90 volt lead and the control grid of a first stage amplifier tube 25H in the loud speaker amplifier H4. Tube 29I has its cathode-to-anode circuit energized by the 90-volt ships line, which is insulated from ground. The purpose of filter 230 is to intensify the 300 cycle tone which is used for whistle signalling when the loud speaker is'in a standby condition. Hence the 300 cycle calling tone is more readily heard above the noise level of miscellaneous transmissions received on the various channels. During reception of weather reports, however, this 300 cycle filter 2% produces a certain amount of distortion which adversely effects the clearness of reception of weather reports. Accordingly the closing of movable contact 3581) against its sta; tionary front companion shunts out the filter 233 in the circuit from the volt lead to the grid of tube 29I and in place of the filter 290 anoninductive resistor 292 is introduced which affords substantially the same D. C. impedance as the inductanceof the filter.

Contacts 3580 remove ground from thew con}.

tact on .relay I93, thereby disabling the transmitter. When the movable contact 358:: engages with its front contact, ground potential is impressed upon one side of the microphone circuit, the other side of which is permanently grounded at the hand set. This avoids accidental actuation of the break-in relay I93 by release of relay {2 due to noise impressed upon the microphone cire cuit. Contacts 358d close in order to complete the loud speaker circuit from transformer 281 to the voice coil 315 of the loud speaker.

It will thus be seen that all the necessary conditions are established by the operation of relay 358 for the reception of weather reports on the loud speaker.

Signing 017 procedure When a conversation has been finished, Or when the listening to weather reports is finished, the hand set 312, which has been removed from its cradle, is hung up. The opening of contacts 31Ib de-energizes relay I44. or I45 (whichever one has been actuated). The relay in the motor generator starter 429 is released, thereby shutting down the motor generator system. All relays which were energized by the 50 volt generator power supply are released. The standby conditions for reception on all of the receiving circuit channels is restored. The transmitter circuits are entirely de-energized.

The call selector and ringer Three methods of making a call have been de-- scribed, one by dialing a code number to which the call selector in the receiving station responds, to the exclusion of call selectors at other stations; a second by manipulating the dial to produce dot and dash impulses of a 300 cycle tone frequency which is to be heard as a whistle signal on the loud speaker of the called'station; and a third-by talking into the microphone when using thec'alling frequency channel #5, the intention being to have the speech heard in the loud speaker of the called station.

Since the loud speakers at all stations not engaged in a conversation are assumed to be maintained in a standby condition of reception, either the second or third methodsof calling may be successful but they are not as desirable as the first method, provided the station'to be called-is equipped with 'a call selector such as indicated at 431 (Fig. 3).

Various types of call selectors are known in the art but the type particularlyadapted to our system is one wherein a step-by-step switch may be actuated in response to trains of impulses Of'dif? ferent counts-dependent upon the code number of the called station. We have found that a so-called 104A selector set manufactured by the Western Electric Company meets our requirements. vIn such a selector set a step-by-step switch .is equipped with a code wheel adapted to render the selector responsive'only to its own designated code signal. The call selector is, therefore, adapted to be tripped and set back to its initial position whenever a difference occurs between.

the-code signal used and the code signal for which the call selector is adapted. to respond, If

the call selector at the station to be called follows the two-tone dialing impulses through to the com-- a responsive device on the selector is actuated bya local circuit through contacts l42d which are closed in consequence of lifting the hand set 321 off its cradle.

The call signal may comprise two portions, one for selecting the intended station, and the other for indicating the channel on which a conversation is to proceed. After the calling signal has been dialed, therefore, the code wheel will be set to any one of four positions in response to the last operation of fingering the dial. These positions preferably indicate channels 1, 2, or 6 and may be indicated on the annunciator 439 by means of pilot lamps each in circuit with a given one of certain contacting elements which are associated with the code wheel in the call selector. The details of construction of the call selector itself and of the code wheel and its associated contacts are not shown because applicants make no claim tonovelty in respect to this construction.

The call selector 431 is actuated in response to tone signals which are derived from either of the audio frequency amplifiers shown in the blocks M2 and 413. The signals are impressed across resistors 3H3 or 3|! and couplin condensers 320 or 32!. Th impulses are then amplified by a special ringer amplifier 440, the output lead from which is shown connected to the call selector.

Recapz'tulation of operating procedure While the uses of our invention will be more or less apparent to those skilled in the art after reading the foregoing description of the structure of a preferred embodiment, the advantages of our system may perhaps be better understood by reviewing the operating procedure as follows:

Our system is designed to receive incoming calls by means of a selective bell ringing system and also by means of a loud speaker. When the station is called by the selector and bell, the calling station dials a radio telephone number which rings the bell. When the station is to be called by loud speaker it is necessary to assume that the volume control of the loud speaker of the called station is turned on and that the officer on watch is prepared to recognize his own whistle signal. With either calling system, in order to reply, it is essential that the channel of conversation should be understood. On lifting the hand set, therefore, to answer a call one may immediately listen in on channel #5 if this channel is indicated by the pilot light on the annunciator 439, otherwise the push-button for selecting the indicated channel must first be depressed.

If the whistle signal is used, the presumption is that the call is made on channel #5. The voice of the calling operator or the calling tone will,

therefore, presumably be heard on this channel.

The calling operator may then give instructions verbally as to the channel to be used for further conversation.

Even with whistle signals it is possible to designate the channel on which this further conversation is to be made. In this case 1, 2 or 3 short tones at the end of the whistle signal will indicate that reply should be made on one of the channels so designated.

Reception of weather reports may be provided on any of the assigned channels. After depressing the push-button for reception of weather signals, therefore, the receiving operator may press channel buttons 1, 2 or 3, using the chanml which gives the best reception- .The broadcast may be heard either from the loud speaker or from the hand set. 7

To call a station on channel #5 one should lift the hand set and listen in the telephone receiver to determine that no one else is using this calling channel, then it is possible to proceed to make a call either by talking or by dialing the whistle signal or by dialing the radio telephone number, all of which methods have been. de-.

scribed.

The signing off procedure usually viously explained, the opening of the cradle contacts 31m and 3'Hb accomplishes. the shutting down of the motorgeneratorset and the restora tion of all actuated relays to normal.

Various modifications of ourinvention willbe understood by those skilled in the art as non-essential variations from the embodiment herein shown and described. The. scope of the inven-.

cradle on said control unit, said cradle having.

circuit closing means operable upon liftingthe hand-set therefrom, means responsive to the operation of said circuit closing means for applying powerto said transmitting apparatus and .for. automatically selecting a predetermined set. of. said pre-tuned circuitsin said transmitting and receiving apparatus, other circuit closing means for manually selecting a different setof said pretuned circuits, and means in said transmitting and receiving apparatus for conditioning the same to initiate or to answer a call on an assigned one of said carrier frequencies.

2. Ina two-way radio telephone system, transmitting and receiving apparatus including pretuned circuits for selective use at different carrier frequencies, a control unit remotely situated with respect to said apparatus, a telephone handset cradled on said control unit when not in use, a power supply unit for said transmitting apparatus, a starter switch for said power supply unit, means operable in response to the-lifting of said hand-set for controlling said starter switch, thereby to energize said transmitting apparatus from said power supplyunit, a set of push-button switches mounted-on said control unit, one for each ofa plurality of assigned carrier frequency channels, relay means operable by said push-buttonswitches for rendering operative an optionally selected set of said pre-tuned circuits corresponding to a desired carrier frequency channel, furthermeans-operable in response to the lifting of said hand set forv automatically selecting a predetermined set of said pre-tuned circuits, means on said control unit under control of said relay means for indicating the selected channel while said hand-set is in use.

3.-A radio telephone system having transmitting equipment including a carrier wave generator, a power amplifier, and a three-function includes. the announcement of station identification. and then the handset is replaced'on itscradle. As pre-.

modulator cooperatively in circuit between the output side of said generator and the input side of said amplifier, said equipment being combined with radio receiving equipment including code call responsive apparatus and speech-responsive reproducing means, means including a, microphone connectable to said modulator for causing it to perform a speech modulating function, means including. a tone oscillator and an impulsing dial for causing said modulator to perform a second function by applying an interrupted monotone call signal to said power amplifier, means including a switch for producing a twotone output from said oscillator in response to a dialing operation, thereby causing said modulator to exercise its third function by applying selectiVe code signals to said power amplifier, and means in said radio receiving equipment for conditioning the same to effectively respond to any oneof the types of signals corresponding with the three functions of said modulator when originated by remotetransmitting equipment of the type herein defined.

4. A radio telephone system according to claim 3 and including in said receiving equipment a plu rality of receiving circuits each pre-tuned to operate on a different carrier frequency channel, means effective at times for rendering said circuits simultaneously operative, and effective at other times for rendering a predetermined one of said circuits solely responsive during a telephone conversation.

5. A radio telephone system according to claim 3 and including in said receiving equipment a call selector responsive to selective code signals of the type produced by dialing a number.

6. A radio system according to claim 3 and including in said call responsive apparatus a loud speaker responsive to interrupted monotone call signals.

7. A two-way radio telephone system comprising transmitting and receiving circuits, said transmitting circuits having amplifier tubes, and having means for varying the potential difference between their cathodes and control grids from a blocking bias to an operating bias, said receiving circuit having an antenna with a connection closeable therefrom to ground, a hand-set including a microphone and ear-phone connectable respectively with said transmitting and receiving circuits, a modulation amplifier in circuit between said microphone and said transmitting circuit, means including a discharge tube external to the modulation amplifier and transmitting circuit for sensing the presence of speech currents in said modulation amplifier, a relay the winding of which is in the output circuit of said discharge tube, and means operable by said relay in response to said speech currents for removing said blocking bias from the tubes ofsaid transmitting circuit, thereby causing a speech-modulated carrier wave to be sent out, and for simultaneously rendering said receiving circuit unresponsive by closing said receiving antenna ground connection.

8. In a. radio telephone system having a transmitter and a receiver, a hand-set including a microphone and an ear-phone, a two-position switch operable by spring tension in opposition to the weight of said hand-set for supplying operating potentials from a given source to said system, means for modulating the output of the transmitter by voice currents through the microphone, a loud speaker, a loud speaker amplifier circuit having a low frequency-selective filter ing the generation of microphone and carrier wave currents by said transmitter, and still other relay contacts for connecting said loud speaker to its amplifier circuit; and means operable by said two-position switch when subjected to the weight of said hand-set, for unlockin said relay.

9. In a two-way radio telephone system, transmitting and receiving apparatus including 'pretuned circuits for selective use at different carrier frequencies, a plurality of control unitsseparately and remotely situated with respect to said apparatus, a telephone hand-set cradled on each said control unit when not in use, means operable in response to the lifting of any one of said hand-sets for conditioning said transmitting apparatus to radiate a communication, a single series circuit through back contacts of different sets of push buttons, there being one set for each control unit, and means selectively and remotely operable by each of said push buttons for conditioning the pre-tuned circuits of said system to carry on a two-way conversation over a desired carrier frequency channel.

10. A radio telephone system comprising trans-' mitting and receiving apparatus including station and channel selecting devices; a telephone hand-set adapted to be cradled in a control unit when not in use, said control unit being remotely situated with respect to said apparatus; a selector dial and channel-selector-push-buttons 40 mounted on said control unit; means including a low frequency oscillator for modulating a carrier wave to be radiated by said transmitting apparatus; means for tuning the circuits of said transmitting and receiving apparatus to a desired carrier frequency under control of an appropriate one of said push-buttons; relay means under control of said selector dial for keying the output of said low frequency oscillator, thereby to send out a call signal; and means including a handset cradle switch settable by the lifting of said hand-set from its cradle, and including a relay system controlled by said switch for empowering said transmitting apparatus, also for supplying operating potentials from a suitable source to said relay system and thence to said low fre-- quency oscillator.

11. A system according to claim 10 and including means operative under control of said selector dial for varying the frequency of said oscillator.

12. A system according to claim 10 and including off-normal contact springs controlled by said selector dial for keying the output of dot and dash signals.

13. A system according to claim 10 and including a double-pole-single-throw switch operative in one position for conditioning said dial to be so manipulated as to modulate said transmitting apparatus by alternate pulses of different tones from said low frequency oscillator, and in the other position for conditioning said dial to be so manipulated as to key said transmitting apparatus by dot and dash impulses of a single tone from said low frequency oscillator.

14. The method of remotely controlling the paratus, disabling said transmitting apparatus while rendering said receiving apparatus operative on a selected channel during periods of silence at said microphone, and manipulating one of said push buttons to select a desired communication channel on which to transmit and to receive a telephone communication.

15. The method according to claim 14 and including the step of visually indicating at said control unit the selected one of the available communication channels.

JOSEPH F. McDONALD. HERSCHEL B. MAR'I'IN.

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