Patent GB-591668-A (GB)
Title: Improvements in radio direction finding systems
IPC: G01S1/02
Filed: 1944-08-15
Abstract: 591,668. Directive wireless systems. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. Aug. 15, 1944, No. 15586. [Class 40 (v)] In a direction finding system, signals from a directional and non-directional aerial are so combined that frequency or phase modulation of the resultant signal is produced, a characteristic of the modulation being indicative of the direction of reception. Frequency or phase modulation, technique is employed for reception with its attendent advantages and any amplitude modulation on the received signals may also be detected. In the applica. tion of the invention to a switched cardioid system, Fig. 3, of the type described in Specification 484,590, the signals from a loop aerial 8, which are phase reversed at a low frequency by valves V3, V4, are combined in phase quadrature in circuit 12 with the signals from a vertical aerial 10 which are amplified at V5. The combined signals which are thus phase modulated are fed to the receiver comprising frequency changer 13, I.F. amplifier 14, limiter 15, frequency discriminator 16 and lowfrequency amplifier 17. The modulation fre-, quency from amplifier 17 is fed-to the course meter 18, where it is compared in phase with the original switching oscillation fed from valves V3, V-4 via transformer 19 and a phase adjuster 20 which compensates for circuit delays and the fact that the low frequency output of discriminator 16 is in quadrature with the phase modulation. To prevent ambiguity due to incorrect tuning of the receiver a portionof the output from limiter 15 is fed to a selective filter 21 the output of which feeds a detector 22 arranged to unblock the low frequency amplifier 17 when signals'pass through the filter 21. Fig. 1 (not shown) illustrates the parallel feeding of thediscriminator . and selective filter, the output from a diode detector following the filter being used to overcome the high negative bias on the low frequency amplifier while a second diode prevents excessive reduction of the bias In another application of the invention, Fig. 4 (not shown), to a direction finding system of the type described in Specification 490,940, the signals from two diagonally spaced pairs of folded broad band dipoles, connected in phase opposition, are separately modulated with different low frequencies in balanced modulators, the resultant sidebands being combined with the signals from a folded monopole aerial, the transmission lines from all the aerials being of equal :length so that the combined signal is phase modulated. The modulation frequencies obtained from a frequency modulation receiver fed with the resultant signal are fed to the rotor coil of a dynamometer instrument the field. coils of which are fed with the original modulating waves, or may be employed to produce other types of bearing display such as that described in Specification 590,260, [Group XXXVI]. In order to produce an increase in the phase deviation obtained with the invention and also prevent incorrect tuning, the arrangement of Fig. 2 may be employed.- The limited output at frequency f from the receiver 3 is mixed with frequency F from a stable oscillator 1 in a modulator 2. One sideband (#-F) or (#+F) is selected by a filter 4 and is mixed with the receiver output fed directly from 3 in a second modulator 5, . and the resultant frequency F, which is selected by filter 6 and passed to the frequency or phase discriminator 7 has an increased phase deviation due to the differential delay in the two paths to the modulator 5 from receiver 3. Specification 533,782 also is referred to.