GB610002A - Improvements in or relating to radio direction finders - Google Patents

Abstract

610,002. Radio direction-finding modulating apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 27, 1946, No. 9547. Convention date, March 28, 1945. [Classes 40 (v) and 40 (vii)] Relates to direction finders of the type in which the angular position of the directive pattern is cyclically varied and the output from the receiver is applied to deflect the beam of an oscilloscope which is rotated in synchronism with the aerial scan to produce a circular trace in the absence of received signals. According to the invention, the directive pattern has one sharp null and a further broad null covering approximately 180 degrees and the angular position of the pattern is cyclically varied through an angle not exceeding 360 degrees, minus the angle covered by the broad null; according to another aspect of the invention, the directive pattern is produced by combining in opposition the signals received by two spaced aerials and the directivity is varied by differentially varying the amplitude of the two signals. The system is stated to be particularly suitable for the reception of pulse-type signals. As shown in Fig. 1, the two spaced vertical dipoles 1 and 3 are positioned #/4 or less from a corner reflector 5 so that the directive patterns produced by each dipole in conjunction with the reflector overlap and the two dipoles are coupled through line balance converters 7 and 9 and coaxial transmission lines 11 and 13 to push-pull amplifiers 25 and 27, the outputs from which are combined in opposition and applied to the receiver 29. When the gain of amplifiers 25 and 27 is balanced a resultant directive pattern is obtained with a sharp central null, Fig. 2, and the angular position of the null is cyclically varied through a range of Π50 degrees, Figs. 3 and 4, by applying a 100 Kc/s oscillation from 31 in opposite phase to the grids of amplifiers 25 and 27 to vary their respective gains differentially. The output from oscillator 31 is also split into two phasequadrature components at 39 which are applied through separate push-pull amplifiers to the X and Y plates of the cathode-ray tube 47 to produce a circular trace 41, Fig. 5, and the rectified receiver output is applied to bias these amplifiers so that their output is inversely proportional to the receiver output and the angular position of the sharp peak 45 in the resultant trace then indicates the bearing of the received signal on a scale on the screen of the tube. Since the aerial scan is non-linear the scale will not be linear but this is unimportant since in practice the aerial system is rotated by the manual control 21 until the peak 45 is vertical and the bearing is then read off the scale 19 attached to the aerial shaft. Fig. 8 (not shown), illustrates a schematic circuit corresponding to the arrangement of Fig. 1, but in a modification, Fig. 19, the amplifiers 25 and 27 are omitted and the attenuation of the signals received from the two dipoles is varied cyclically and differentially by varying impedances connected across the transmission lines 11 and 13. As shown, the point 89 on the line 13 is connected through a #/4 co-axial section to a point 91 on the inner conductor of a #/4 co-axial line stub 83 which is shortcircuited at its lower end and a point 93 near the upper end of the inner conductor of the stub is coupled through a capacitor 95 to the anode of the valve 97 to the grid of which is applied an output from oscillator 31. When valve 97 is non-conductive, the impedance between ground and points 93 and 91 is high and the high impedance at 91 is transformed to a low impedance at 89 to produce maximum attenuation in the line 13 but when valve 97 is conductive the reverse conditions obtain and there is minimum attenuation in the line 13 the change from one condition to the other being effected progressively. A similar arrangement is provided for the line 11 and the output from oscillator 31 is applied in push-pull to the grids of the two valves 97 so that attenuation in the two lines is varied differentially.