Patent GB-713093-A (GB)
Title: Improvements in and relating to combined transmitter and receiver arrangements
Inventor: FLEMING-WILLIAMS BRIAN CLIFFOR
IPC: H04B1/50
Filed: 1951-10-25
Abstract: 713,093. Combined radio-frequency transmitting and receiving systems. COSSOR, Ltd., A. C. Oct. 27, 1952 [Oct. 25, 1951], No. 25023/51. Class 40 (5) A local station, when its transmit-receive switch is in transmit position, can only transmit back to a headquarters station after a predetermined call signal has been received by the local receiver. As shown, when the local station switch 38 is in receive position and a stand-by switch 40 is closed, a relay 41 pulls up and connects a modulation tone detector 17 of the receiver to energize a relay 21, so that when a call tone is received the relay 21 closes the circuit of a call indicator 27 and connects HT to the transmit position of a transmitter switch 39 coupled with the switch 38. If the local station wishes to transmit the switches 38, 39 are connected to transmit so that the relay 41 is deenergized and the tone device 17 disconnected, but a further holding contact 45 is then made to the relay 21 through the still pulled up contact 46 and a timing device 47 so that transmitting is permitted only for a finite time afterwards determined by the device 47. Further, the local transmission is preceded by a brief tone modulation from the impulse modulator 49. After the switch being returned to receive position the local station can again transmit by switching to Transmit position provided the timing device 47 is still closed and the stand-by switch 40 has not been reset. Headquarters station.-On receipt of the reply signal (on a different carrier frequency f2) a tone sensitive detector pulls up a relay disconnecting the headquarters tone modulator from its transmitter (on frequency f1) so that the call signal is discontinued once the channel has been seized by a local station. Further, a device responsive to decreased noise on the carrier f2 being reecived operates through a delay device and a second coil on the headquarters station relay to hold the latter pulled up after the reply tone ceases, and for a short time even after the carrier f2 ceases (to prevent false actuation during fading). A switch may also be used to apply current to the relay to maintain the call tone inoperative indefinitely during a reply from a distant local station accompanied by fading. Alternatively the switch 40 and relay 41 of a local station may be replaced by a stand-by latching device, released mechanically on switching to Transmit. In a modification (Fig. 2, not shown) the stand-by switch 40 and relay 41 are not used and the switch 38 is in series between the tone device 17 and relay 21, with a stand-by, i.e. Receive position, a Transmit position operating the transmitter swich 39, and a Hold position connecting a local current source to the relay 21 through a further switch. The relay 21 is held pulled up by the local source in both transmit and hold positions and is connected up before either of these positions is reached in switching from stand-by so that the relay 21, once energized by a call signal, is held. In all cases there may be in series with the relay coil 21 a pair of contacts which open when a further relay is actuated by a device responsive to an overriding tone signal from the headquarters sent out before the call signal so that it is ensured that the relays 21 at all local stations are deenergized. The headquarters station may have a plurality of tone modulators and receivers to permit communication selectively with groups of local stations.