Patent GB-405018-A (GB)
Title: Improvements in or relating to radio signalling systems
IPC: G01S1/02
Filed: 1932-07-28
Abstract: 405,018. Electric selective signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and LINSELL, A. A., Electra House, Victoria Embankment, London. July 28, 1932, No. 21311. [Class 40 (i).] The relative amplitude of modulation note frequencies in a wireless system determines the information it is desired to convey, and produces a visual indication thereof at the receiver. The principle is applied to direction finding systems and systems for conveying meteorological &c. information to aircraft without the necessity for the use of a code. At the transmitter four different note-frequency generators G1 .. G4, Fig. 1, are bridged by potentiometers P1 .. P4, the outputs from which are connected in series in the grid circuit of the modulating valve V, and which can each be varied from zero to a predetermined maximum, while at the receiver, the demodulated output from the detector DV, Fig. 2, is applied to frequency discriminating transformers T1 .. T4 responsive to the four note frequencies and developing potentials across resistances r<1> .. r<4> after rectification at R1 ..... R4. The potentials across these resistances may be applied to the deflecting plates N, E, W, S, Fig. 4, of a cathode-ray tube C. In a direction indicating system the transmitter is coupled to a rotating beam aerial the sliding contacts of the potentiometers P1 .... P4 being actuated automatically as follows. When the beam is pointing due north south, east or west maximum potentials from the generators G1, G3, G2 or G4 respectively modulate the transmitter the outputs from the remaining generators being zero, while when the beam is between the north and east positions, for example, generators G1 and G2 both modulate the transmitter in an amplitude ratio depending on the tangent of the angle through which the beam has rotated from the north. Similarly when the beam is between east and south, generators G2 and G3 modulate the transmitter in the appropriate ratio and so on. At the receiver, the fluorescent screen FS of the cathode-ray tube is marked out with the compass points, Fig. 4, and under the action of the received note frequency amplitudes the cathode beam describes a curve similar to that shown in dotted lines which indicates the direction of the transmitting station. The identity of a non-directional beacon transmitter whose direction has been found by known methods may be determined if the carrier of each of a plurality of beacon stations is modulated with a combination of two audio frequencies in a fixed predetermined ratio, the fluorescent screen of the receiver cathode-ray tube being marked about its periphery with the names of the beacon stations in positions corresponding to the frequencies and the ratios characteristic thereof. In place of a fluorescent screen the cathode-ray tube may comprise a plurality of radially disposed target electrodes each in circuit with a relay which may for example be a neon tube giving a direct visual indication. An alternative form of tube rendering separate frequency selective circuits unnecessary comprises an electromagnet em, Fig. 6, one end of which is inserted into the re-entrant stem s of the tube C and whose winding w is fed with the demodulated received signals. Surrounding the electron- " gun " elements is a plurality of tuned reeds tr of magnetic material acting as deflector plates when vibrated by currents in the winding w to the frequency of which they are tuned. For conveying meteorological information to aircraft the fluorescent screen, Fig. 7, is divided into four quadrants marked with scales of temperature, strength of wind, visibility and base of cloud respectively, a peripheral scale being marked with compass directions to give the direction of the wind. Each of these quantities is transmitted as a combination of two note frequency modulations in a ratio depending on their magnitude, the quantities being transmitted in succession at such a rate as to give by persistence of vision, simultaneously readable indications at the receiver. Thus temperature indications may be transmitted as amplitude ratios between the note frequencies generated by sources G1, G2, Fig. 1, wind strength on note frequencies generated by sources, G2, G3, and so on, all the frequencies, taken two at a time, being necessary for indicating wind direction. The latter indication is distinguished from the others by wobbling the carrier amplitude while it is being transmitted to produce a line indication on the fluorescent screen rather than a spot. This is accomplished by varying the coupling between the transmitter and aerial or by applying an alternating potential to the grid of one of the transmitter power amplifying valves. The potentiometer sliders P1 .. P4 may be actuated automatically by a thermometer, wind vane, &c.