Patent GB-526537-A (GB)
Title: Improvements in or relating to radio direction finders
IPC: G01S3/42
Filed: 1939-03-20
Abstract: 526,537. Automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 20, 1939, No. 8786. Convention date, March 19, 1938. [Class 38 (iv)] [Also in Group XL] Orientation. - Relates to means for automatically orientating a directional aerial so that it is kept pointing towards a distant transmitter, and consists in using for the purpose a polyphase motor one winding of which is fed with modulated side-band currents with suppressed carrier, and varying in phase and amplitude according to the orientation of the aerial, whilst the other winding is fed with currents of the same frequency but of fixed phase. The arrangement is such that the torque applied to the motor varies according to the direction of the incomdng signals. Automatic gain control is applied to maintain a driving torque in the motor until the zero position is reached. In Fig. 1 the pick-up from the shielded loop 1 passes through the R.F. amplifier 3 to the balanced modulator 7, 9 where it is modulated with an audio-frequency produced by the generator 41. The output of. the modulator, comprising the upper and lower sideband frequencies, is combined in the coil 13 with the pick-up from the open aerial 15, the necessary phase-adjustment being obtained by the circuit 6. The combined signals are detected in the receiver 19 to give an audio output of the same frequency as the modulating voltage, the amplitude and phase of which depends on the orientation of the frame aerial with respect to the direction of the received signals. This audio output is then fed via amplifiers 21. 25, 27 and 29 to one field-winding 33 of a two-phase motor 35, the other field-winding 37 of the motor being energized from the generator 41. The motor 35 is mechanically coupled to the loop aerial 1 which is thus rotated into the position of minimum pick-up, the rotational sense being governed by the phase of the audio-output current. For very large or very small pickups from the loop 1, the modulation components from the balanced modulator 7, 9 will tend to over-modulate or under-modulate the carrier frequency currents from the open aerial 15, either case resulting in a decreased audio-output and consequently a decreased motor-torque as shown in Fig. 2.' Over modulation is prevented by controlling the amplification of the loop pickup in accordance with the audio-output by taking part of the audio output from the amplifier 25 through the lead 47 to the diode portion 49 of valve 3, where it is rectified, the voltage across the load resistance 51 being applied as negative bias to the R.F. amplifying part of valve 3. A delay voltage is provided by the bias resistance 56, the effects of varying the delay being shown in Fig. 3. If A.V.C. is applied to the open aerial pick-up, an additional R.F. amplifier is provided in the input from the open aerial the amplification of which is inversely controlled with respect to the control applied to the loop input. The Specification as open to inspection under Sect. 91 also states that the invention can be applied to a " homing receiver in which a leftright indicator takes the place of the two-phase motor. This subject-matter does not appear in the Specification as accepted.