Patent GB-586750-A (GB)
Title: Improvements in or relating to radio beacon systems for bearing determination
IPC: G01S3/14
Filed: 1942-05-11
Abstract: 586,750. Wireless beacons; transmitting phase differences. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. May 11, 1942, No. 6387. [Class 4'0 (v)] [Also in Group XXXVIII] A system for communicating a phase difference existing between two waves, which difference may represent a quantitative measure such as the bearing of a receiver in relation to a radial beacon, comprises means for transmitting over a single channel two or more waves in succession, and preferably in a predetermined rhythm, at least two of the waves having the same frequency and differing in phase at the receiver by the amount to be communicated, means for passing the waves after reception through two paths having different delay values such that at their outputs the periods of the differently phased waves of equal frequency overlap for at least a part of their duration, and means for measuring the phase difference between the waves during this overlap. A radio beacon system for bearing determination according to the invention comprises an omnidirectional aerial system the phase of the field of which varies radially but not circumferentially, and a system such as the Adcock, in which the phase of the field varies progressively circumferentially, the two systems being co-axial and keyed to the same transmitter alternately or in other rhythmic sequence. It is arranged that both systems give the same phase in a particular direction, for example, north. Thus, a receiver receives in successive periods waves of the same frequency and differing in phase by an amount representing the bearing of the receiver.. Since the phase of each transmission changes through 360 degrees for a movement of the receiver towards or from the beacon of one wavelength, the resulting Doppler effect on the received frequency would at high radio frequencies be sufficient to affect the phase shift produced by the delay circuits in the receiver. To avoid this, a second oscillator radiates omnidirectionally a heterodyning frequency, producing a low-frequency beat signal, the phase of which varies with bearing in the same way as does the original high-frequency signal, but which changes through 360 degrees for a change in distance from the beacon equal to a wavelength of the low-frequency signal, so that the Doppler effect is negligeable. When a lower radio-frequency is used it may be heterodyned at the receiver, instead of at the transmitter. In either case, in the receiver shown in Fig. 2, the low-frequency signals from LF are supplied directly to a path 1 and through a delay network DN to a path 2. The signals in path 1 are applied in phase quadrature to the deflecting plates of a cathode-ray oscillograph, producing a circular trace of the beam. The delayed signals in path 2 pass through a limiter L1 to produce a square-topped wave whose leading edge occurs at the instant the signal passes upwardly through zero. This wave is applied to a highpass filter F, which produces short positive and negative pulses at the trailing and leading edges respectively. Alternate pulses are removed by a unidirectional limiter L2, the others being applied to a control electrode of the oscillograph to produce a bright spot or a kink in the circular trace of the beam, the angular distance of which from the top of the trace indicates the difference in phase between the waves in the two paths. In Fig. 4 (b) the top row represents the signals in path 1 resulting from alternate transmissions A and B of equal duration and frequency, but differing in phase, as received ; the second row shows the signals after delay in path 2 ; and the third row shows the indication obtained, i.e. the angle # of phase difference during the periods when the A and B signals in the two paths overlap, and an indication N during the periods when the same signals overlap. This last indication corresponds to zero phase difference and therefor indicates the north point on the oscillograph trace, which may be brought to the correct point of the scale by operation of a phase adjuster. Another keying rhythm is indicated at ( f ) in which C represents no transmission or transmission on another frequency. The keying is periodically stopped to give a prolonged A transmission which may be used for the switch adjustment. Other keying rhythms and delay periods are described. In another form of receiver the output from one path is .fed in phase quadrature as parallel inputs to two differential detector arrangements of the kind described in Specification 586,115, the delayed output from the other path being applied to the same detectors in push-pull. The outputs from the detectors are applied to corresponding pairs of deflecting elements of the cathode-ray oscillograph, causing the spot to move along a radial line indicating the phase difference. Resistance-capacity time-delay circuits in the output circuits of the detectors may be switched out when rapid response of the oscillograph is desired to indicate north and bearing points, or switched in when bearing indications only are received in order to give a slow response with " noise " suppression and a well-defined beam trace.