Patent GB-758139-A (GB)
Title: Radio direction finding system
Inventor: EARP CHARLES WILLIAM
IPC: G01S3/54
Filed: 1954-05-21
Abstract: 758,139. Radio direction-finding; phase discriminators. STANDARD TELEPHONES & CABLES, Ltd. May 21, 1954, No. 15080/54. Classes 40 (5) and 40 (7). A radio direction-finder comprises an array of aerials spaced uniformly round the circumference of a circle and coupled cyclically at a fixed frequency to a receiver so that there is derived a phase-modulated wave, means for producing quadrature reference waves isochronous with the cyclic coupling and an analysing arrangement for deriving two quadrature co-ordinate potentials for application to an indicator such as a cathode-ray oscillograph. In Fig. 1 the aerial array is shown schematically at 1, the coupling arrangement at 2, and the oscillator controlling the cyclic coupling at a repetition frequency P/2# at 4. There is also provided a central aerial 9 (or one offset several wavelengths) for obtaining a reference signal. The inputs from the aerial arrangements 1 and 9 are mixed with the output from a common local oscillator 6 to give I.F. signals. In the case of aerial 9 the I.F. signal is mixed with the output of local oscillator 13 of frequency P/2# and one set of sidebands selected by filter 14 and fed to mixer 8 which also receives the I.F. signals derived from aerial system 1. The output of mixer 8 is passed to filter 15 which selects the signal of frequency P2#. This, of course, is still phase-modulated due to the cyclic coupling of the elements of aerial array 1. The output of filter 15 is fed directly to the phase discriminator 17 forming part of analyzer 18 and also through delay device 16b introducing a delay substantially equal to the time taken to move one step round array 1. The output of the discriminator 17 is a wave of the form sin(pt + #) where # is the bearing angle to be determined. This wave is fed to two wave multiplying devices 20 and 21 where it is multiplied rsepectively with a Sin pt reference signal from unit 19 and a Cos pt reference signal from unit 19b to give signals of ¢{ Cos # - cos(2pt + #) and ¢{ sin # + sin(2 pt + #)} which when fed out over terminals X and Y to the deflecting systems of a cathode-ray oscillograph produce a circular indication swept out at twice the array scanning frequency and whose diameter passing through the origin lies in a direction #. If desired the signal of frequency 2 pt may be removed from the output at X and Y by filters and the resultant off set spot display produced transformed into a radial line display by known means. A balanced type of wave multiplier for use at 20 and 21 is described with reference to Fig. 3 (not shown) and modifications in the analyzing arrangement with reference to Figs. 4 and 5 (not shown). In the arrangement of Fig. 4 the output of the discriminator is fed to the wave multipliers through a balanced modulator fed with a high-frequency signal. The outputs of the multipliers are subsequently demodulated to provide the co-ordinate output potentials at X and Y. The analyzer of Fig. 5 has the direct input from filter 15 fed to each wave multiplier and two discriminators, each fed with the delayed signal, one following each multiplier and feeding respectively the X and Y outputs. Both modified forms of analyzer avoid distortion arising due to D.C. drift in the multipliers. Specification 594,530 is referred to.