Patent GB-884418-A (GB)
Title: Improvements in radio communication systems
IPC: H04B7/02
Filed: 1958-05-27
Abstract: 884,418. Radio receiving systems. GENERAL ELECTRIC CO. May 27, 1958 [May 27, 1957], No. 16841/58. Class 40(5). Relates to a radio communication system for use along roadways, waterways, or pipelines, in which a mobile transmitter cooperates with several fixed receivers spaced along the route Which relay the signal either by wire or radio link back to a central station. The receivers normally operate in a predetermined order of priority, the one providing the most reliable link being assigned top priority, but a control element is provided at each receiver whereby the fixed priority is over-ruled if one of the receivers of lower priority is providing a stronger signal. The system comprises a mobile F.M. transmitter 8, Fig. 1, a plurality of base receivers 10a. .10d spaced along the route and provided with control elements 17a. .17d, a control network 12, and a central station 14. The control elements 17a &c. include a carrier energized relay and variable delay means which initiate control functions after a delay period which is inversely proportional to the strength of the received signal. The control network 12 is a switching matrix responsive to control signals from the elements 17a &c. along the lines 24a, 26a, 28a, 30a, &c. to establish a transmission path to the central station 14 for the best of the information signals received from the receivers along lines 20a, 20b, 20c, 20d. The control network comprises two banks of relays 40a..40d and 50a..50d, Fig. 2. The relay contacts shown in dotted blocks are operated by voting relays located at the receivers, but are shown in Fig. 2 for convenience. If all the base receivers receive a signal of equal amplitude the receiver 10a is connected to the central station, second priority being accorded to receiver 10b, and so on. Assuming that receivers 10a, 10b, receive signals of equal amplitude, and receivers, 10c, 10d receive signals of lower amplitude, the voting relays of the control elements 17a, 17b will be energized at the same time. Contacts 25a, 27a, will close to energize relay 40a, but the opening of contacts 29a, 31a, will prevent the energization of any of the relays 40b, 40c, 40d. The operation of relay 40a causes relays 50b, 50c, and 50d, to be energized and break the connections 20b, 20c, and 20d between the corresponding receivers and the central station 14. At the same time the circuit of relay 50a is opened, thus allowing the output from the receiver 10a to pass via the line 20a to the central station. If, however, the receiver 10c, for example, receives the strongest signals, the relay 40c is energized via contacts 25c, 27c, resulting in the operation of relays 50a, 50b, and 50d to break the connections 20a, 20b, and 20d, and to open contacts 51a, 52a, and 51b, 52b, to prevent the operation of relays 40a and 40b. Relay 40d is prevented from operating by the opening of contacts 29c, 31c. Each base receiver 10, Fig. 3, comprises a F.M. receiver 15, a negative signal being taken from the limiter grid return of the receiver 15 to the control grid of D.C. amplifier valve 90 in the control element 17 to provide an amplified signal which is directly proportional to the strength of the carrier signal. A normally non-conducting triode 98 is primed to conduct by energization of the carrier energized relay 68 and when conducting, operates the voting relay 123 to feed the audio signal over line 20 and the control signals over lines 24, 26, 28, 30, to the control network 12. The conduction of the valve 98 is dependent on the potential of its control grid which is coupled via a delay control 141 to the anode of valve 90. With no carrier being received, when the tap 88 of the transformer 72 is at a lower potential than the lead 84, current flows through diodes 108, 112, and potentiometer 110, to establish a negative potential with respect to earth at the anode of valve 90. Since the cathode of the valve 98 is at a more negative potential the capacitors 142, 146 are charged via the gridcathode space of the valve 98. Also during this half cycle the capacitor 118 is charged to a negative potential with respect to earth, this potential being arranged to be equal to the potential at the limiter of the receiver 15 when minimum threshold signals are received. This serves to filter out any transients which may occur from temporary change or loss of signal. During the next half cycle the triode 90 conducts but due to the charge established on the capacitors 142, 146, the triode 98 is biased below cut off. When a sufficiently strong carrier is received energization of the relay 68 connects the cathode of the triode 98 to earth, and a positive signal appears at the anode of the valve 90 which is directly proportional to the strength of the received signal. Since the potential of the control grid of the valve 98 is the difference between the voltage drop across the delay control 141 and the anode potential of the valve 90, the time between the receipt of the carrier signal and the raising of the control grid of the valve 98 to conduction potentional is dependent on the amplitude of the anode voltage of the valve 90 and the rate of discharge of the delay control 141, the rate of discharge being the same for each control element 17.