Patent GB-631172-A (GB)
Title: Radio beacon system
IPC: G01S1/02
Filed: 1946-07-06
Abstract: 631,172. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. July 6, 1946, Nos. 20249 and 20250. Convention dates, July 31, 1945. [Classes 40 (vi) and 40 (vii)] A long-range radio beacon system operating on a carrier frequency below 200 kc/s. and having its side-bands confined within a band 300 c/s. to each side of the carrier frequency, comprises a pair of aerials 1, 2 spaced apart by half a wavelength and fed from a carrier source 3, the aerial 1 being successively coupled directly and through 90, 180 and 270 degrees phase-shifting networks so that four different radiation patterns X, M, Y, N, Fig. 3, are successively radiated, the relative amplitudes of successive signals received at a receiver being compared to enable its azimuth with respect to the beacon to be determined. Commutator 5 is driven by motor 14 to effect the successive phase-changes in the energy supplied to aerial 1 and commutator 6 is driven synchronously therewith to de-energize both aerials and then radiate a synchronizing pulse from aerial 2 after the completion of each cycle of phase-changes. The complete cycle may take one or two minutes so that substantially the whole of the side band energy may be confined to a very narrow band about the carrier frequency. The signals received at a receiver may be as shown at c, Fig. 2, the output of the receiver which is of very narrow band-width being as shown at d. This is limited to give a constant amplitude signal e, whence the synchronizing pulses 26 are suitably selected and used to generate sine waves j, the phases of which can be controllably varied and which are used to generate gate-pulses k. These are applied together with the received signals to a gating-circuit which consequently delivers, in the example illustrated, X and Y pulses m. The X and Y signals are applied to a centre-zero meter, the X signals, in this case, through an adjustable attenuator which is set until a central indication is obtained. By means of suitable switching, any one of the ratios X/X, Y/X, M/N, N/M, can be measured and it can be seen from Fig. 3 that one of these quantities lies in the range 0.4-1 at all positions around the beacon. To remove the symmetrical ambiguity about the 0-180 degree axis means are provided for switching the display to give a rough comparison of the amplitudes of the other pair of signals ; thus, in the example illustrated, the meter indicates whether M or N is the greater. The value of Y/X measured then indicates the direction from the beacon with only one ambiguity having an axis of symmetry along the line joining the aerials 1, and 2. Should the synchronizing signals be so obscured by noise as to render the operation of the receiver unsatisfactory, a cathode-ray display may be used. The received signal synchronizes a circular time-base to which the output of the receiver is applied. From the display produced the direction from the beacon can be estimated. A radio direction-finder is also incorporated in the receiver for use when reception conditions are very poor. The output from a spinning goniometer fed from a crossed-loop aerial system is compared with a signal of the same frequency derived from the goniometer driving motor in a phase-meter, whereby the direction of reception may be determined. Selection of synchronizing pulses. The signal, curve e, Fig. 2, is applied to a differentiating circuit which delivers pulses f. From the positive pulses of curve f, pulse signals g are generated and these are combined with the inverted negative pulses of curve f to produce the signals of curve h which shows the selection of pulse signals corresponding to the synchronizing pulses 26. These pulse signals are then used to synchronize the generation of the sine waves, curve j.