Patent GB-624562-A (GB)
Title: Improvements in direction finders
IPC: G01S3/52
Filed: 1945-10-04
Abstract: 624,562. Radio direction-finding. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 4, 1945, No. 25892. Convention date, May 29, 1943. Addition to 622,822. [Class 40 (vii)] In a direction-finding system in which an omnidirectional aerial, e.g. a vertical dipole, is rotated in a circular path in a horizontal plane, whereby the signals picked up are phasemodulated at the frequency of rotation, azimuth and elevation are determined by imposing upon the received signals a phase modulation which is adjusted to cancel the phase modulation produced by the rotation of the aerial so that the parameters of the applied modulation are indicative of the azimuth and elevation to be determined. The exact cancellation of the phase modulation is observed by comparing the modulated signals with an unmodulated oscillation of exact mean frequency and a given phase from an oscillator controlled by an A.F.C. system from the incoming signals. A dipole aerial 1, Fig. 1, supported on a housing 11 containing receiver equipment is carried on a counterbalanced boom 3 mounted on a vertical shaft 7 arranged within a tower 5, the shaft being driven from a motor 21 through suitable gearing. A crownwheel 17 fixed at the top of the tower drives a pinion 15 which in turn drives a power-supply generator within the housing 11. Signals received by the aerial 1 are fed to an R.F. amplifier 33, Fig. 3, the output of which is fed through one half of a delay line 43 to a modulator 31. An R.F. oscillator 25, ganged to the amplifier 33 and tuned closely to the signal carrier frequency, and a true I.F. oscillator 27 feed their respective energies to a balanced modulator 29. The output of the modulator 29 consisting of a beat frequency is fed as the local oscillation to the modulator 31 so that the beat frequency produced is the same or close to the true intermediate frequency. The I.F. output from the modulator 31 is fed through a filter 37 to an A.F.C. circuit 39 to which some of the energy of the true I.F. oscillator 27 is also fed by way of a 90 degrees phase-shifter 41. The A.F.C. circuit brings the oscillator 25 into exact synchronism with the signal carrier at the mid-point of line 43. Some of the energy of the R.F. oscillator 25 is fed to the vertical deflection system of a C.R. indicator 91. The delay line 43 introduces a phase delay of the signal modulation equal to the time required for radiation to travel a distance equal to the diameter of the circular path swept by the aerial 1. A feed is taken from either end of this line so that the phase in the respective feeds is delayed by magnitudes - # and + # with respect to the phase at the mid-point. The modulated energies are fed to a phase-adjusting circuit comprising two pairs of valves 47, 49 and 51, 53, each having two control grids. The energy from the input end of the line 43 is fed to the inner control grids of the valves 47 and 53 and the energy from the output end to the inner grids of valves 51 and 49. The outer grids of the valves 47 and 51 are connected together and to a source of bias potential comprising a potentiometer 55 and battery 59 while the outer grids of the valves 49 and 53 are similarly connected together and to a source of bias potential comprising a potentiometer 57 and battery 61, the arrangement being such that as the bias on the valves 47 and 51 is increased that on the valves 49 and 53 is correspondingly decreased. The pair of valves 47, 49 have a common output with a load resistance 69 and the pair 51, 53 have a common output with a load resistance 71. With this arrangement the deviation of the phase of the signal modulation may be varied by adjustment of the bias control, the valves 47 and 49 providing a phase delay variation between zero and - # and the valves 51, 53 providing the same magnitude of change between zero and + #. The bias control is connected to an . indicating system 65, 67 which, when deviation of phase is correctly matched, as later explained, indicates elevation of the incoming signals. A generator 81 housed in the housing 11 and driven by the shaft 13 provides a reference alternation of a frequency equal to one cycle for each revolution of the aerial. This alternation is fed through a 360 degrees adjustable phase-shifter 83 to a phase comparison or modulation device comprising two valves 77, 79, each having two control grids and connected with their outputs in opposition. The reference alternation is fed to the outer control grids while the two modulated carrier outputs from the phase-adjusting system are fed to the inner control grids. By adjusting the phase of the reference oscillation, the latter may be caused to produce phase modulation in the combined outputs from valves 77, 79 in such a manner that the output of the phase comparison device is pure carrier. This output is fed to the horizontal deflection system of the C.R. indicator 91. A straight-line indication indicates that both the phaseadjusting system and the phase comparison or modulation system have been correctly adjusted. The phase-shifter 83 is connected to an indicating system 85, 87 which indicates direction in azimuth of the arriving signal. Fig. 4 (not shown) illustrates a modification in which the C.R. indicator is dispensed with and in which the system is automatic. In this arrangement the R.F. reference oscillation from the oscillator 25 is rectified and two outputs in phase opposition are fed to the outer control grids of the valves 77 and 79 of the phase comparison or modulation device while the output of this device is fed to a phase-difference detector controlling a reversible motor in turn controlling the inverse bias control of the phase-adjusting system. The rectified outputs of the R.F. oscillator 25 are also fed together with the output of the reference alternation generator and phase adjuster 83 to a phase responsive servo system which brings the phase of the reference alternation into step with the modulation of the signal. Specification 592,570 is referred to.