Patent GB-616324-A (GB)
Title: Improvements in radio communication systems
IPC: H04W84/02
Filed: 1946-08-13
Abstract: 616,324. Interconnecting high-frequency subscribers. GENERAL ELECTRIC CO., Ltd., and FAIRBAIRN, E. P. Aug. 13, 1946, No. 23917. [Class 40 (iv)] [Also in Group XL (c)] In a radio commimication system comprising three or more stations which communicate with each other through a relay station, a first station calls a second station by transmitting a characteristically modulated signal which, on receipt at the relay station, causes the latter to transmit a similarly modulated signal at a different frequency which operates indicating means at the second station. Fig. 1 shows three outstations, 1, 2 and 3, each having a receiver operating at a frequency f2 and lowpowered transmitter operating at a frequency f1, and a relay station 4 with a receiver operating at frequency f1 and a high-powered transmitter operating at the frequency f2. Normally, all the receivers, but not the transmitters, are operative. To establish a call, say from station 1 to station 3, the telephone is lifted from the hook switch at station 1 and a calling button associated with station 3 depressed. These operations energize the transmitter at station 1 and cause a carrier modulated by a particular low-frequency which is characteristic of station 3 to be emitted. The reception of this carrier at relay station 4 causes the emission of a carrier of frequency f2 similarly 'modulated by the low or calling frequency of station 3, which operates a calling signal at station 3. The emission of the carrier frequency f2 causes a locking-out action to take place at the remaining outstations so that these are prevented from transmitting until communication between stations 1 and 3 ceases. Release of the calling button at station 1 stops the transmission at frequency f1, and the caller awaits the reply from station 3, after which simplex system communication is established. Restoration of the telephone hook switch at station 1 transmits a clearing signal at a characteristic low-frequency before switching off the transmitter, which switches off the transmitter at the relay station 4. A delay circuit may also be provided at the relay station to switch off the transmitter if the carrier f1 is not received for some predetermined period. Fig. 2 shows a modified arrangement which is adapted for duplex working, the relay station 4 having two receivers each associated with a transmitter. The calling arrangements are similar to those for Fig. 1, but on commencement of the call the responsive frequency of the receiver at the calling station is shifted from f2 to f2+ #f2. The frequencies in the " go " channel are as for Fig. 1, but transmission from the called station in the " return " channel is at a frequency f 1+ #f 1 which is received by the second receiver at relay station 4 and re-transmitted at the frequency f2 + #f2. The difference frequencies #f1, #f2 are large enough to provide satisfactory separation but small enough to avoid retuning of the associated resonant circuits, so that the frequency changes may be effected by merely switching crystals or other means associated with the oscillators in the transmitters and superheterodyne receivers. Outstations 2 and 3 are shown as connected via a hybrid coil and balance 6 to a telephone subscriber or exchange. In this case dial impulses received over the radio link may be used to select a desired station or subscriber.