Patent GB-226227-A (GB)
Title: Improvements in radio communication systems
IPC: H02P9/42
Filed: 1924-12-13
Abstract: 226,227. Soc. Franþaise Radio-Electrique. Dec. 13, 1923, [Convention date]. Transmitting s y s - t e m s ; compounding frequencies by modulation; magnetic modulating systems. - In wireless transmitting systems, the frequency supplied by a main generator, preferably a high - frequency alternator, is modulated by a lower auxiliary frequency, and one of the resulting frequencies is radiated, the other being suppressed. The radiated frequency is kept constant by causing any variation of frequency of the main generator to react automatically on the frequency of the generator. The auxiliary frequency may be obtained by allowing the main frequency to beat with a third frequency supplied by a generator of constant frequency. The undesired frequency may be suppressed by using two-phase or polyphase main and auxiliary generators. In a two-phase system two-phase speech currents may replace the twophase auxiliary frequency, and telephony may thus be effected on a single side band with suppressed carrier. Magnetic modulating devices are preferably employed, which may also serve for multiplying the frequency of the main generator. In the arrangement shown in Fig. 1, the main high-frequency alternator 1 supplies the aerial 3 through a coupling 2. Two modulating chokes 9 are connected across the alternator in series with condensers 10. The auxiliary generator a, shown as a valve energizing a tuned circuit 5, 6, supplies a lower frequency to the chokes 9, and one or other of the sum and difference frequencies is selected for radiation by tuning the aerial 3. At the positive and negative peaks of the auxiliary wave, the two circuits 9, 10 in turn present zero resistance to the alternator, and so modulate the current through the coil 2. Morse signals are sent by a key 14 which when closed short-circuits a resistance 12 connected between the chokes 9 and a direct-current source 11. A choke 13 and condenser 15 restrict the direct current and the auxiliary current to their proper paths. In order to maintain constant the emitted frequency the man frequency is supplied through a coupling 18 to a low-power modulating circuit comprising chokes 16 traversed by the auxiliary frequency supplied through coupling 19. The circuit or filter 21 is tuned to the same frequency as the aerial 3, and any variation of speed of the alternator 1 causes the regulator 7 to act so as to vary the tuning of the circuit 5, 6 and produce an alteration of the auxiliary frequency sufficient to compensate for the initial variation of the main frequency. In place of the auxiliary generator a and chokes 9, modulation may be effected by a cyclically-variable inductance driven by a motor, and the regulator 7 may then act to control the speed of the motor. The chokes 9 may serve in known manner as frequency multipliers in addition to their modulating function. The main high-frequency source 1 mav be an arc or valve, and the low power modulator 16 may be of the valve type. Fig. 2 show a modification in which one of the waves resulting from the modulation process is definitely suppressed by the use of two-phase main and auxiliary generators. Each phase of the main generator 22 is modulated by one of the phases of the auxiliary generator 23, by means of chokes 25 as in Fig. 1. The modulated output of both phases is transferred to the aerial 27 through couplings 26, and the resulting aerial current is either of the sum or difference frequency according to the relative sense of the couplings. The emitted frequency may be kept constant in spite of variations in the frequency of the main source 22 by employing a regulator controlled by the aerial current and acting upon the auxiliary source 23. Modulated C.W. telegraphy may be effected at reduced power by employing only one of the phases. The arrangements of Fig. 2 may be modified for telegraphy on a single side-band by eliminating the auxiliary machine 23 and supplying the chokes 25 with two-phase speech currents, produced by the device shown in Fig. 3. The speech currents in the circuit 29, 30 are supplied to two parallel branch circuits, one of which comprises a high skin-effect resistance 31 and a small inductance 32, the other a high ordinary resistance 33 and a low skin-effect resistance 34. The elements 31, 34 are of iron and owing to incomplete penetration of the wire by the current, their resistance varies as the square root of the frequency for the telephonic range. The impedance of elements 31, 33 predominates over that of elements 32, 34 sq that the currents in the two branches are in phase at all frequencies. Voltages tapped off across the inductance 32 and the resistance 34 are therefore in quadrature, and by connections to amplifiers two-phase modulating currents may be delivered to the chokes 25, Fig. 2. The amplitudes of the various frequency components are maintained the same in both branches, as the current through 31 is proportional to #p, and therefore the voltage across 32 varies as #p, whilst the current through 33 is constant for all frequencies and therefore the voltage across 34 varies as #p, p being the frequency. A condenser may replace the inductance 32. Valves may be employed for the production of two-phase oscillations. For example, the oscillatory circuit of a valve may comprise a resistance in series with a reactance of equal value, and the input circuits of separate amplifiers may be connected across them. In the modification shown in Fig. 4, the frequency of the emission from the aerial 39 is determined by a small oscillator 44 of constant frequency, although the power is supplied by a two-phase alternator 35. Each phase of the alternator is shunted by a circuit coupled to a series coil 41, and the two-phase currents in these shunts are combined with current from the small oscillator 44 to produce beats in the output circuits of detectors 46. The beat currents are in quadrature and are supplied to the respective modulators 37. By suitable arrangement of the couplings 38, a single frequency equal to that of the source 44 is supplied to the aerial 39, any alteration of frequency of the alternator 35 causing a corresponding alteration of the beat frequency which on being passed to the modulators 37 compensates for the original variation. The oscillator 44 may be enclosed in screens, and supplied from accumulators, with the object of maintaining its frequency constant. The system shown in Fig. 4 may be modified by employing a single-phase generator and a single detector 46.