GB202278A - Improved system of and apparatus for radio communication - Google Patents

Abstract

202,278. Hammond, J. H., (Assignee of Chaffee, E. L.). Aug. 11, 1922, [Convention date]. Secret and selective signalling systems; thermionic homodyne receivers; thermionic modulating systems; multiplex signalling.-In order to prevent unauthorized listening-in, the signal frequencies are first impressed upon an intermediate supersonic frequency, one of the resulting sideband of frequencies being filtered out whilst the other side band is utilized to modulate the highfrequency carrier wave. The supersonic frequency is also normally suppressed, so that radiation takes place only when the microphone is energized. The receiving-station is provided with a local generator of oscillations of the same supersonic frequency as that suppressed in transmission. In addition, the frequency of the main carrier wave may be periodically varied in an erratic manner. More than one supersonic frequency may be combined with the same carrier wave for multiplex working, and the system is also applicable to radio-dynamic control. The valve 20, Fig. 1, generates oscillations having a frequency, say, of 40,000, which are fed through a transformer 22, 24 to the grids of two modulators 33, 34. Signal frequencies from a transmitter 32 are superposed upon the first oscillations through a transformer 31. The plate outputs from the modulators 33, 34 are so opposed across the transformer 35 that the supersonic frequency is suppressed except when the transmitter 32 is active. A filter 36 is designed to pass only the lower side-band frequencies, which are then, in turn, applied to a modulator 41 placed in shunt with the carrier-wave generator 10 across the high-tension supply. The radiated energy is accordingly modulated with those signal components that have passed the filter 36. The aerial-circuit constants are varied in an irregular manner by means of a pair of contacts 51, 52 which intermittently close the circuits of shunt condensers 53, 54, whilst a rapidly rotating vane 55 passes across a stationary condenser plate 56. At the receiving-station, Fig. 2, the grid circuit 63, 64 of a back-coupled rectifying-valve 66 is tuned to the main carrier-wave. The rectified current is fed by a transformer 74 to a circuit 78, 92 tuned to the unsuppressed side band of the modulated intermediate supersonic frequency. This circuit forms the input side of the final detector 100, which is also fed through a coupling 83 with oscillations from a valve 80 of the same frequency as the initially suppressed supersonic frequency.