Patent GB-357968-A (GB)
Title: Improvements in or relating to electric indicating systems
IPC: G01R29/08
Filed: 1930-04-23
Abstract: 357,968. Current and pressure meters. SMITH, C. H. and MEREDITH, F. W., Royal Aircraft Establishment, South Farnborough, Hampshire. April 23, 1930, No. 12545. [Class 37.] In an electrical system for comparing or indicating small voltage or current effects in two or more circuits connected to an amplifier, non-mechanical switching arrangements are provided at the input and output ends of the amplifier and are controlled by an alternating voltage so as to operate synchronously. The invention is shown applied to wireless direction-finding apparatus for giving a continuous and instantaneous indication of the bearing of a transmitting station, for example for use on aircraft. The sum and difference of the currents or voltages induced in a loop aerial, Fig. 1, and a non-directional aerial 2 are measured by instruments 19, 20. The loop 1 has one end connected to earth and the other end connected to the grids of two four-electrode valves V1, V1<1>, the anodes of which are connected to a coil 7. The control grids of the valves VI, V1<1> are connected to the output coil 4 of an alternating-current generator 3 of constant frequency, shown as a push-pull thermionic valve generator. The hightension supply to the valves V1, V1<1>, is connected to the mid-point of the coil 7, which is coupled to a coil 8 in series with two coils 9, 10 and a variable condenser 11. The aerial 2 is coupled to the coils 9, 10 by means of a coil 12. The condenser 11 is connected across the grid and filament of the first valve V2 of an amplifier consisting of valves V2 .. V5. An oscillation-generating valve V6 is coupled by means of the coil 13 to the coil 14 of the amplifier, the output frequency of which is adjusted to any desired value by heterodyne action. The anode circuit of the last valve V5 of the amplifier is connected to the grid of a valve V7, the anode circuit of which is connected to one diagonal of a Wheatstone bridge arrangement of copper-oxide rectifiers 15 .. 18, the other diagonal of the bridge being connected to the alternating-current output coil 4. The operation is as follows. The voltage generated in the coil 1 is applied to the grids of the valves V1, V1<1>, which are alternately in action due to the varying potential applied to the control grids by the generator 3. The effect of this voltage is alternately added to and subtracted from that of the voltage in the non- directional aerial 2, the resultant being amplified and applied to the grid of the valve V7. The indicating instruments 19, 20 are alternately in circuit in phase with the change from addition to subtraction and give readings proportional to these two effects. Provided the frequency of the generator 3 is low compared with that of the output from the amplifier, the ratio of the indications on the current meters 19, 20 is a measure of the angle between the perpendicular to the plane of the loop 1 and the direction of the transmitting station. The reading of one of these instruments may be adjusted to a fixed value by means of a potentiometer resistance 21, the other instrument then being direct reading. The two instruments 19, 20 may indicate on a common scale. Alternatively a crosscoil instrument may be used. In a modification, Fig. 2, the aerial 2 is connected to the mid-point of the loop 1 and through a phasing resistance 33 to earth. The ends of the loop 1 are connected to the grids of two three-electrode valves V8, V8<1>, the anodes of which are connected through coils 31, 32, to the ends of the coil 4. Fig. 3 shows a modification in which the anode coil of the alternating current generator is coupled to a coil 36 the ends of which are connected-through rectifiers 37, 38 and ammeters 39, 40 to the anode of the valve V7. In a further modification, Fig. 4, the valve V7 and the rectifiers are replaced by two valves V9, V10 connected as shown to the ammeters 41, 42 and a coil 43 coupled to the alternator coil. These two valves may be replaced by a composite valve with two anodes. Further modifications of the connections between the valve V7 and the alternating current generator are shown in Figs. 6 and 7. Two loop aerials may be employed instead of a loop aerial and a non-directional aerial. The Provisional Specification states that the invention may be used for multiplex telegraphy or telephony either with transmission over wires or by wireless.