Patent GB-592447-A (GB)
Title: Improvement in radio direction finders
IPC: G01S3/14
Filed: 1944-12-19
Abstract: 592,447. Radio direction-finding; modulating systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 19, 1944, No. 25523. Convention date, Dec. 31, 1942. [Class 40 (v)] [Also in Group XXXV] In a direction-finding system, a beat frequency output is produced by generating locally a signal differing in frequency from the incoming signal and radiating it from an auxiliary aerial positioned near to the DF aerial system. Means are also described for further preventing. errors due to horizontally polarised waves in an Adcock system by using vertical aerials each comprising a series of short conductive segments which are successively commutated so that the aerials are operative sequentially. As shown in Fig. 3, the locally-generated signal 'is radiated from the auxiliary aerial C and the combined incoming and local signals picked up by the four vertical aerials N, E, S, W of an Adcock system are fed to separate superheterodyne receivers 49, 51, 53 and 55, the beat frequency outputs from the N-S and E-W channels being combined in opposition in the amplifiers 22 and 24 to give outputs proportional to the components of the incoming signal resolved along the N-S and E-W axes which are applied to the perpendicular deflection plates of the cathode-ray indicator 31. The 180 degree ambiguity is removed by applying a signal received by the central sense aerial D located vertically above the auxiliary aerial C to the grid of the cathode-ray tube. A common local oscillator 75 of angular frequency alpha is provided for all the receivers and since the sense aerial D only receives the incoming signals of frequency w, the sense receiver 57 produces an IF of (alpha-w) which is fed through the IF amplifier 59, limiter 61 and filter 63 to the modulator 65 where it is combined with a low frequency # from the A.C. power supply 11. The modulator 65 gives a single side-band output (alpha-w-#) which is fed together with suitably phased components of frequency alpha from the oscillator 75 to a similar modulator 73 which gives a single side-band output (w+#) which is fed to the auxiliary aerial C. The DF aerials each receive the incoming signals w and the local signals (w+#) and their associated receivers produce two IF signals (alpha-w) and (alpha -w - #) which beat together in the second detector stages to give outputs of frequency # which bear the same phase relationship as the incoming signals w. In order to eliminate the reception of horizontally polarised waves caused by the mutual proximity effect of the DF aerials, each aerial comprises an insulating mast 1, Fig. 1, with a plurality of conductive 90 degree segments 3 co-operating with stationary brushes 5. As the mast is rotated the segments are connected together for one quarter of the rotation period to provide a continuous vertical conductor ; during the remaining period the segments are disconnected and are incapable of resonating to produce proximity effects. The shafts are driven by a motor 9 synchronised with the A.C. supply 11 and they are angularly displaced so as to be effective during successive periods P1-P4, Figs. 7-10; if the frequency of rotation is half that of the A.C. supply #, Fig. 4, then the beat frequency output from the receivers 49-55 will comprise half period portions of a sine wave of frequency #, Fig. 5, which are applied to filters 19-25 tuned to the frequency # giving continuous wave outputs, Fig. 6, with the same phase relationship as obtained for continuous reception. In a modification, Fig. 13 (not shown), the auxiliary aerial is off-set from its central position and compensating phase shifters are provided in the receiver channels ; alternatively, two auxiliary aerials are provided, Fig. 14 (not shown), located equi-distant from the centre point. In a further modification the sense voltage is derived from a diagonal pair of aerials operating simultaneously or from all four DF aerials operating simultaneously, the relation between the commutation frequency and the modulation frequency being adjusted accordingly. If required the receivers 49-55 may be replaced by a single receiver with its input and output switched in synchronism with the aerial switching. Modulating circuits, Fig 12 and Fig. 11 (not shown). A single side-band output from the modulator 65 is obtained by feeding two sets of signals in phase quadrature to the two balanced modulators 93, 93<1> and 95, 95<1> and combining the outputs in parallel. The IF signal from the filter 63 and the LF modulating signal from the source 11 are each applied in push-pull to the inner and outer control grids of the heptodes 93, 93' and the outputs are combined in parallel to suppress the carrier. The two signals are also, phase shifted 90 degrees by the circuits 99, 101 and applied in a similar manner to the valves 95, 95<1> giving a second suppressed carrier output which when combined with the first in a common load 113 gives a single sideband. The modulator 73, Fig. 11 (not shown), is similar except that the input from oscillator 75 is of variable frequency and a special phase shifter (see Group XXXV), is required to produce a constant 90 degrees phase shift over the frequency band.