Patent GB-629948-A (GB)
Title: Radio direction finder
IPC: G01S3/46
Filed: 1946-04-06
Abstract: 629,948. Radio direction-finders. STANDARD TELEPHONES & CABLES, Ltd. April 6, 1946, No. 10668. Convention date, April 9, 1945. [Class 40 (vii)] A radio direction-finder, designed to eliminate pickup of horizontally polarized signals, comprises a central dipole 1, feeding a phase discriminating receiver 20, and two dipoles 3 and 5 space-coupled to the central dipole. The whole is mounted to be rotatable about a vertical axis and the two free aerials are alternately tuned to the frequency of reception thus modifying by re-radiation the phase of the signals received by aerial 1. The outputs from the receiver during each half of the tuning cycle are compared upon a cathode-ray tube. In the embodiment shown in Fig. 6, the aerials 3 and 5 are tuned to the same mean frequency by the fixed condensers 6 and 7. The ganged trimmer condensers 8, 9 are driven continuously by the motor 11 and the sweep circuit 19 generates a horizontal time-base voltage at twice the frequency of rotation of the condenser shafts. The tunings of the condensers are phased 180 degrees apart, i.e. the dipoles are tuned to resonance alternately and at equally spaced intervals, the resonance occurring at the same point on the time base for each dipole. Signals received from a distant transmitter are reradiated to the dipole 1 from the dipoles 3 and 5 with phase changes dependent on the values of the condensers 8 and 9. When the plane of the array is perpendicular to the direction of reception, the paths from the transmitter to the dipoles 3, 5 are equal so that the phases of both re-radiated signals vary with the settings of the trimmers in the same way. Consequently, the output from the phase discriminating receiver 20 is the same for both of the two time-base cycles corresponding to one complete revolution of the trimmers, and a single trace T3 is produced on the screen of the cathode-ray tube 14. When the array is not perpendicular to the direction of reception, a double trace, as shown by the broken lines, is produced. In the embodiment of Fig. 8, the dipoles 3, 5 are alternately short-circuited under the control of the circuit 26 which also controls the sweep circuit 19, and a switch 30. The sweep circuit produces two traces per complete cycle and these are caused to be spaced vertically as shown by the switch 30 which is operated at the same rate as the switches 24, 25. The output of receiver 20 is applied to the grid of the cathode-ray tube through a converter 32 which produces rectangular pulses of length proportional to the output of the receiver. Thus the lengths of the traces depend upon the phases of the received signals, and when these are equal, the traces are of equal length. In the correct operation of the system of Fig. 8, the aerials 3, 5 must be accurately tuned to the same frequency, close to the frequency of reception. This is performed roughly in the position shown and then, as shown in Fig. 7, the aerials are brought together by means of a hinge in the beam supporting them. The final adjustments are made in this position until the output from the dipole 1 is independent of the positions of the switches 24, 25, i.e. so that the dipoles 3, 5 affect the central dipole equally. Specification 444,005 is referred to.