Patent GB-556476-A (GB)
Title: Improvements relating to radio beacons
IPC: G01S1/02
Filed: 1942-04-03
Abstract: 556,476. Radio beacons. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. April 3, 1942, No. 4507. [Class 40 (v)] In a system for determining the bearing of a mobile receiver with respect to a radio beacon, of the kind in which the beacon transmits a wave the phase of which varies progressively round it and the phase of the wave is determined at the receiver, the beacon emits an unmodulated carrier of frequency f1 and a second omnidirectional beacon emits an unmodulated carrier of frequency f2, thus producing a field containing the frequencies (f1 - f2) and (f1 + f2) whose phase varies progressively round the centre of the beacon systems one of these frequencies being applied at the receiver as a scanning frequency to a cathode ray oscillograph, and means being provided for indicating on the oscillograph screen when a predetermined value, for example zero of the rotary field of this frequency passes through a known geographical direction, for example north. In the beacon transmitter shown in Fig. 1, an oscillator 01 feeds energy of a frequency f1 to crossed loop antenna H1, H2 through an amplifier A1 and a phase-shifter PS, so that the phase of the radiated field varies progressively through 360 degrees in a circular path round the beacon. A second oscillator 02 supplies a frequency f2 through an amplifier A2 to an omnidirectional aerial V. Both oscillators feed a detector D, producing an output of frequency (f1 - f2) which may be amplified and filtered in a band-pass filter F1 before passing to an amplitude-limiter L1, the square-topped wave-form from which is fed to a high-pass filter F2. This produces a succession of sharp alternate positive and negative pulses these being (f1 - f2) per second of each, equally spaced in time, and occurring when the amplitude of the waves producing them passes through zero. The positive pulses are removed by a unidirectional amplitude limiter L2, and the negative pulses cause overbiasing of either or both amplifiers A1, A2 to interrupt the corresponding transmission or transmissions momentarily. At the receiver, the heat-frequency (f1 - f2) is fed from the detector through a filter and applied directly to one pair of deflecting plates of a cathode-ray tube, and by way of a 90 degrees phase shifter to the other pair, so producing a circular trace. The detector output is also fed to a high-pass filter which selects the pulse wave-form resulting from the momentary interruptions in the transmission, the pulses being applied to a control grid of the tube which either focuses the beam to produce a bright spot, or causes deflection of the spot to produce a kink in the circular trace, at the point representing the bearing. Errors introduced by the filter in the receiving circuit due to drift of the oscillator frequencies may be avoided by using an identical filter in the transmitter, in which case a variable beat-frequency may be employed to prevent unauthorized use of the transmission. In practice, the loop aerials shown are replaced by pairs of vertical aerials fed in phase opposition on medium and long waves, and on short waves by horizontal dipoles with a horizontal loop for the omnidirectional radiation.