GB556923A - Improvements relating to radio signalling systems - Google Patents

Abstract

556,923. Multiplex wireless signalling. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.). April 21, 1942, No. 5314. [Class 40 (v)] A multi-channel radio signalling system of the kind using directive antennae of the type in which the elevational transmission angle varies with the final carrier frequency and in which each channel comprises a single side band of a frequency modulated by the signal wave, is characterized in that the signal wave of a channel modulates a subcarrier frequency and one of the side bands is used to modulate the transmission or final carrier frequency after one or more intermediate frequency modulations, said final carrier frequency being selected as being the optimum for transmission with regard to the point of reception for the prevailing conditions. Fig. 1 shows an arrangement in which two transmitters No. 1 and No. 2 have each two signal channels A, B and C, D respectively, which are radiated to different points by corresponding directive antennae T1 and T3. In transmitter No. 1, channels A and B feed into first modulators M1, M2 to which is supplied a frequency of about 125 kc. derived by a sub-multiple generator SG from an oscillator O1 Filters F1, F2 select respectively the lower and upper side bands which are then combined together with a controlled amount of the submultiple frequency in an amplifier A1, the output from which is combined in a second modulator M5 with a harmonic of oscillator 01 of about 2500 kc. After amplification at A5, the signal is combined in a final modulator M9 which is fed with carrier frequency current selectively through a switch S1 from one of a set of oscillators 05-08 having a range of 9000 kc. to 18000 kc. After final amplification at A9 the signal is radiated at T1, which is directed towards the receiving station. The signal channels C, D are similarly passed through transmitter No. 2 the oscillator 01 with its sub-multiple and harmonic generators being common to both transmitters, while a switch S3 enables a desired one of the oscillators OS-08 to be selected. For sending two messages from each transmitter to two different points, different oscillators 05-08 will be connected to the final modulators. For sending only one signal from each transmitter using opposite side bands, the same carrier frequency can be used for both transmissions. If desired each transmitter may have two directive antennae directed to two receiving stations corresponding to the two channels. If the stations are in widely different directions a non-directive aerial may be used. In a modification, Fig. 2 (not shown), at the first modulators, the signal channels are modulated on to a series of harmonics of a single oscillator and mixed with one of the harmonics at the combining circuit amplifier before modulation by a harmonic of the oscillator 01. The subsequent stages are the same as for Fig. 1. More than two transmitters may be used.