Patent GB-219998-A (GB)
Title: Improvements in methods of and systems for transmitting radiant energy
IPC: H04B7/12
Filed: 1924-08-01
Abstract: 219,998. Hammond, J. H. Aug. 2, 1923, [Convention date]. Secret and selective signalling systems; thermionic and magnetic modulating systems; heterodyne receiving systems; spark-gap oscillation generators. - Secret and selective systems for the transmission of radiant energy employ two or more waves of different frequency, each of the waves being modulated or divided into groups at the same lower frequency. Each wave is emitted continuously and signalling is effected by varying the phase relationship between the groups or amplitude pulsations, and the receiving-apparatus is responsive only to a predetermined phase relationship. Fig. 1 shows a transmitting circuit employing spark-gap oscillation generators. Two spark-gaps 46, 47 are connected in differently-tuned circuits 44, 45 coupled to separate aerials 31, 34. Each gap is supplied from a direct-current source 50 through rheostats and choke coils. The same group-frequency is imposed on each spark circuit by providing tuned shunts comprising coils 75, 77 and variable condensers 76, 78. The two shunts are coupled inductively to a link circuit comprising coils 101, 102, 110 and a condenser 103 shunted by a second condenser 104 and signal key 105. When the key 105 is open, the groups of waves are in phase, while closure of the key throws the groups into phase quadrature. Thermionic oscillation generators 217, 218 may be employed as shown in Fig. 2, each comprising a back-coupled valve 217, 218 diredtly connected to the Differently-tuned aerials 200, 201. A third back-coupled valve 240 generates a lower frequency determined by the tuned plate circuit 254, which is utilized to modulate the waves produced by the oscillators 217, 218. For this purpose the circuit 254 is coupled to a condenser-shunted coil in the grid circuit of the valve 218, and is coupled to a similar coil 225<1> in the grid circuit of the valve 217 through an intermediata phase-adjusting circuit 255. The circuit 255 comprises coils 257, 258, 262, and a variable condenser 256 shunted by a key 261 and a second condenser 260. As before, the closure of the key 261 puts the modulations on the valve 217 into phase quadrature with those on the valve 218. A third transmitting-arrangement employs separate highfrequency alternators in the aerial circuits, and modulation is effected by magnetic modulators supplied with the lower modulating frequency by a third alternator through phase-adjusting circuits. Fig. 4 shows a receiving circuit for use with any of the above transmitting circuits. The two waves are received on a double-tuned aerial 350, the separate waves being selected bv tuned circuits 358, 359 and applied to valve detectors 360, 361. The plate circuits 366, 367 of these valves are tuned to the modulation or group frequencies, which are applied, jointly with local heterodyne waves from the source 390, to the grids of detectors 370, 371. The plate circuits of valves 370, 371 are tuned to the beat frequency or the difference between the modulation frequency and the local frequency, and the currents in these circuits are of different phase depending on the opening or closure of the transmitting key. Each plate circuit includes a divided coil 373, 3'74, the four coils being set up with their axes radiating from a centre at equal angular distances. Eight similarly-arranged coils 380 connected in series are so placed within the coils 373, 374 that the axes of coils 373, 374 bisect the angle between consecutive portions of the coils 380. No electromotive force is induced in the coils 380 by the static magnetic fields of the coils 373, 374 produced by atmospherics or interfering signals. The rotating magnetic field produced by the excitation of the coils 373, 374 in phase quadrature, due to the reception of the desired signal, induces current of the beat frequency in the circuit 381, which circuit is tuned by the condenser 382. These currents are rectified by the valve 384 and may be rendered audible in the telephone 389 by providing an audio-frequency interrupter 383 in the grid circuit. When using the system for telephony, a microphone circuit 115 is coupled to the phase-adjusting circuit of Fig 1 or Fig. 2, and at the receiver, Fig. 4, the interrupter 383 is short-circuited by a switch 386. The beat frequency of the circuits 376, 377 should be supersonic for telephony. The receiving-circuit, Fig. 4, may be modified by omitting the local oscillator 390, and the valves 370, 371, the coils 364 being divided and used to produce a rotating magnetic field. Fig. 6 shows a transmitting circuit employing three aerials 465 energized by separate alternators 468 of different frequency. Modulation is effected by including in the aerial circuits coils 467 wound on iron cores 470, which also carry magnetizing coils 473 traversed by direct current, and modulating coils 480 supplied through a three-phase transformer 476 from a three-phase alternator 475. Two of the circuits coupling the transformer 476 to the modulating coils 480 contain respectively a phaseretarding coil 481, and a phase-advancing condenser 482. When these are short-circuited the respective modulations are 120‹ apart in phase. Short-circuiting is effected by a key 493 in series with an interrupter 485, so that during the depression of the key the short-circuits are made and broken at an audible frequency. At the receiving-station, the waves are selected and received by three aerials, each connected to a detector so as to produce the modulation frequency in the output circuits. The output circuits are coupled to circuits containing divided coils similar to the coils 373, 374 of Fig. 4, but six in number and arranged 60‹ apart. A sixcoil secondary is placed within these coils and is connected to a further detector having a telephone in its output circuit, in which a note is heard corresponding to the note of the interrupter 485 whenever the transmitting-key 493 is depressed. For telephony, microphones 512 are connected by a switch 514 in place of the key 493, and the interrupter is run at a supersonic frequency, lower than that of the modulating alternator 475. In another transmitting arrangement two pairs of aerials are used, all the aerials being tuned and energized at different wave frequencies. The output of one pair of aerials is modulated at one frequency, and the other pair at another frequency, each modulation frequency being separated into two phases, one for each aerial of a pair. When the signal key is depressed, the phases are brought into quadrature. At the receiving station, each wave is selected by a tuned receiver and is rectified to bring out the modulation frequencies. The coils carrying one modulation frequency are placed similarly to the coils 373, 374 of Fig. 4, and the coils carrying the other frequency are interposed midway between the first coils, thus setting up two rotating magnetic fields of different frequency, whenever the signal key is depressed. A coil similar to the coil 380, Fig. 4, is placed in these fields, and is connected to a detector which has its plate circuit tuned to the difference of the two modulation frequencies. This difference frequency is preferably audible and may be amplified and passed through a telephone. For telephony, the difference frequency should be supersonic, and a further rectifier should be provided at the receiver. The Specification as open to inspection under Sect. 91 (3) (a) comprises the arrangement shown in Fig. 10 (Cancelled), in which the circuit is adapted for receiving signals transmitted as simultaneous modulations or interruptions of two waves of different frequency, no phase-adjusting arrangement being provided at the transmitter. The waves are selected and amplified by the receiving-aerials 675. 676 and amplifiers 679, 680. One wave is passed directly through a divided coil 685, and is passed through a second coil 699, at right -angles to the coil 685, after its phase has been rotated through 90‹ by the coupled circuit 697. The other wave is passed similarly through coils 686, 700 placed intermediate the coils 685, 699. An inner coil 710 is affected by the two rotating magnetic fields, and is tuned by a condenser 712. to the difference frequency. Currents of this frequency appear in the output circuit of the valve 715, are rectified in the valve 726, and produce an audible effect in the telephone 729. In another arrangement for receiving signals sent on a single modulated wave, the modulation frequency is derived by rectification of the incoming wave, and is then used to create a rotating field. A coil is rotated within this field and is connected to a detector having a telephone in its plate circuit. The frequency of the current induced in the rotating coil is the sum or difference of the modulation frequency and the number of revolutions per second of the coil. The circuit of the coil is tuned to one or other of these frequencies. This subject-matter does not appear in the Specification as accepted.