Patent GB-594704-A (GB)
Title: Radio direction finding
IPC: G01S11/10
Filed: 1944-10-31
Abstract: 594,704. Radio direction - finding. MARCONI'S WIRELESSTELEGRAPH CO., Ltd. Oct. 31, 1944, No. 21300. Convention date, May 31, 1943. [Class 40 (v)] In adirection-finding system, one or more aerials are adapted to be moved uniformly along a circular path whereby the phase of the signals received is modulated due to the Doppler effect, the phase of the modulation depending on the direction of the incoming signals. In Fig. 1, the dipole 1 is mounted at the end of a horizontal arm 4 rotating about the shaft 5 which is supported by the insulating tower 7, and the received signals are re-transmitted on a micro-wavelength over a path of constant length to a receiver 11 and indicator 17, Fig. 2, housed in the station 9 at the base of the tower. The output from the receiver 11 is applied to a communications receiver 13, the I.F. output from which is applied through the phasing circuit 15 to the X and Y plates of the cathoderay tube 17 in quadrature to produce a circular trace. Pulses from the generator 23 controlled by the oscillator 25 operating at the I.F. of receiver 13 are applied to the radial deflection electrodes 21 to produce a radial deflection 10, Fig. 3, for each rotation of the trace. During the half period of rotation when the dipole 1 is approaching the source of received signals, the frequency of the received signals is slightly increased due to the Doppler effect and during the other half period when the dipole is receding, the frequency is decreased so that the radial deflection oscillates between the limits 10<1> and 10". The cathode-ray tube is normally biassed so that the trace is only faintly visible, but twice during each revolution of the aerial the trace is brightened by means of the commutator 27 rotating on the shaft 5 which connects the voltage source 51 between the grid and cathode when each of the 180 degrees displaced conductive bars 29 and 31 comes under the brushes 35, 37. The brushes are rotated by the handle 43 until the two visible deflections are coincident and the corresponding bearing is read off the scale 49. The 180 degrees ambiguity is resolved by adding an auxiliary commutator, Fig. 2a (not shown), which reverses the polarity of the output from the pulse generator 23 for alternate periods of connection of the source 51 to the control grid. This differentiates between the two radial deflections 12<1> and 14<1>, Fig. 3a, a it may be determined whether the super imposed condition 12, 14 corresponds to the true or reciprocal bearing by noting which o the deflections follows the handle 43 when it is moved away from the superimposed condition. The length of the rotating arm should not exceed one wavelength since otherwise the excursions performed by the indicating mark will overlap and cause false indications. In a modification, Fig. 4, provision is made for receiving horizontally polarized radiation by replacing the counterweight of Fig. 1 by a horizontal loop aerial 69. The signals received by the dipole 1 and the loop 69 are then retransmitted at different frequencies and picked up by the receivers 79 and 77 located at the base of the tower. The two receiver outputs are alternately coupled to modulate the relay transmitter 83 operating at another frequency by means of the electronic switch 81 controlled by the oscillator 85 and the transmitter is also modulated by the switching oscillator 85 and by the shaft-rotation pick-up 87. The indicator (not shown) is remotely situated from the tower and operates as in Fig. 2 except that the low-frequency switching and rotation frequencies are first filtered out, the latter being used to operate a synchronous motor driving the commutator. The pulse generator is controlled either by an oscillator as in Fig. 2 or by the I.F. output from an auxiliary reference receiver having the same local oscillator as the communications receiver. The reference receiver is alternately fed from a dipole and horizontal loop similar to the ones on the rotating arm by means of an electronic switch controlled by the switching frequency.