GB496239A - Improvements in wireless direction finding systems - Google Patents

Abstract

496,239. Directive wireless signalling. STANDARD TELEPHONES & CABLES, Ltd., and WAGSTAFFE, C. F. A. May 28,1937, No. 14849. [Class 40 (v)] In a direction finding receiving system having a plurality of fixed directional aerials angularly disposed relative to one another, the signals from each aerial are modulated at different sub-frequencies, the combined modulated outputs are amplified in a common high frequency amplifier, and the subfrequency components are selected to actuate a directional indicator, whereby the signal channels are more easily balanced and the application of automatic gain control is facilitated. The 180‹ ambiguity may be removed by combining the directional aerials with an open aerial and the direction may be indicated on a cathode ray oscillograph. In the Figure the pick-ups from two directional aerials, e.g. Adcock aerials, and an open aerial are coupled to the system through the aerial couplings A1, A2, A3. They are then doubly modulated in the modulators M1, M2, M6 by (i) a common heterodyne frequency from the oscillator BO, and (ii) different sub-frequencies of 1500, 2500, and 2000 c.p.s. The 2000 c.p.s. frequency is derived from the oscillator 0.2000 and the other two are intermodulation frequencies obtained by feeding the modulator M3 from the oscillators 0.500 and 0.2000 and filtering the output through filters F.1500 and F.2500. The componets of the output from modulators M1, M2, M6 consisting of an intermediate-frequency carrier and its side bands are selected and amplified in the common amplifier IFA and detected in the straight line detector D. The two output frequencies 1500 and 2500 c.p.s. are filtered out and modulated at M4 and M5 by a frequency of 500 c.p.s from oscillator 0.500. The sum and difference products of 2000 c.p.s are filtered out and applied through amplifiers Gland G3 to the deflection plates PX, PY of the oscillograph. The total phase shifts in the two paths are adjusted to equality by the network P1. The amplifiers G1 and G3 are adjusted to give a trace on the oscillograph inclined at 45‹ to the deflection axes when signals of equal magnitude and phase are applied to the modulators M1 and M2 from the oscillator LO. Since the amplitudes of the deflection voltages are proportional to the aerial pick-ups, the inclination of the oscillograph trace indicates the direction of the incoming signal. For the determination of sense an open aerial is used. The 2000 c.p.s. frequency in the output of the detector D corresponding to this signal is injected into the HT supply circuit of the oscillograph through a network P2 which is adjusted to synchronize the injected and deflection voltages. The sensitivity of the oscillograph is thus varied so that the amplitudes of the trace on each side of the origin are unequal. In a modification, Fig. 3, (not shown), the 180‹ ambiguity is removed by applying the 2000 c.p.s. frequency corresponding to the open aerial pick-up through a rectifier in series with a resistance to a control electrode of the oscillograph so that the electron beam is only in focus for half a cycle. If the signals from A1 and A2 are in anti-phase and equal, corresponding to a signal at a direction of 45‹ with one of the diagonals of the Adcock system, the intermediate-frequency carriers will cancel out and no indication will be observed. To avoid this the phase of one carrier is reversed by the switch S1 and a correct phase relationship between the oscillograph deflection voltages is obtained by a phase-reversal switch S2 in the corresponding low-frequency circuit. To avoid switching, the modulators M1 and M2 may be balanced modulators and the carrier is then supplied from the open aerial via modulator M6, In a modification, Fig. 4, (not shown), the modulators M1 and M2 are supplied with modulating frequencies of 1500 and 2500 c.p.s. from separate oscillators which also supply modulators M4 and M5, and modulator M6 is supplied with an intermodulation frequency of 1000 c.p.s. from modulator M3. The frequency of the oscillograph deflecting voltage is in this case 1000 c.p.s.