Patent GB-235278-A (GB)
Title: Improvements in or relating to high frequency telephony or telegraphy
IPC: H04B3/56
Filed: 1924-03-07
Abstract: 235,278. Electrical Improvements, Ltd., Newcastle-upon-Tyne Electric Supply Co., Ltd., and Grant, L. C. March 7, 1924. Wired wireless systems.-A system of highfrequency telegraphy or telephony over power lines or other electric conductors comprises thermionic valve transmitting and receiving circuits at the ends of the line, and a three-part coupling device, consisting of (1) a line. coupler, (2) a filter for blocking the power currents, and (3) a tuning reactance for modifying the impedance of the line to the carrier currents. As shown in Fig. 1, the power line is connected to the oscillatory circuit C, C<1> of the signal transmitter through a condenser F, capable of withstanding high tension, a condenser H<2> forming with the earthed choke coil H<1> a filter for eliminating the power current and its harmonics, and a variable inductance or condenser G for adjusting the impedance of the line at the carrier frequency. The tuned plate circuit C, C<1> of the valve generator AA<1> is connected to the plates through a condenser B and at its opposite side is earthed and connected to the filaments. The tuned grid circuit D, D<1> is coupled to the plate coil C, and the grid circuit also includes a transformer secondary coil E, the primary coil E' being in contact with a battery E4 and microphone E<3>. The plate potential is supplied by a battery A<3> through a choke coil A<5>. The removal of the microphone from its switch hook may close the filament circuit of the valves A, A<1>. At a receiving station, the connection of the power line to earth is identical with that shown in Fig. 1, the coil C being coupled to a reactively-coupled detector valve followed by a low-frequency amplifier. When the receiving telephone is on its hook, the reaction is sufficient to cause the valve to oscillate, and so to give a call signal. The removal of the receiver reduces the reaction below the oscillating point. The coupling condenser F may comprise a horizontal wire lying beneath the overhead power lines. Alternatively, it may. as shown in Fig. 4, comprise two copper plates M<1>, M<2> immersed in oil in a tank M and separated by an insulating slab M<3> ; or the power main M<4> may be surrounded by a series of insulated concentric cylinders, the outermost being connected to the signal transmitter or receiver. In another form, an insulating cylinder surrounds the power line, 'and the wire from the signalling apparatus is wound round the cylinder. An existing insulator P<1> may be utilized in this way, as shown in Fig. 7. In the form shown in Fig. 8, an open-ended coil Q connected to the power line Q<1> is coupled to a coil Q<2> which may be connected on one side to the filter H, Fig. 1, the other side being left open or connected to earth. For connecting the signalling-apparatus to a three-phase power line, the secondary coils of line transformers may be connected together at one end and connected at their other ends through condensers or choke coils to the signalling-apparatus. Alternatively, the primary coii of a transformer may be connected across two phases of the power line, the secondary being connected to the switch-board instruments, and the signalling line connected through condensers to the opposite ends of the secondary coil.