Patent GB-1428614-A (GB)
Title: Automatic radio compass
IPC: G01S3/42
Filed: 1974-02-26
Abstract: 1428614 Radio direction finding A B BUKHMAN I L YANUSHKEVICH V D KALASHNIKOV M I LEVITINA J B ROZENBERG V V ELKIN and E M PONOMAREVA 26 Feb 1974 8574/74 Heading H4D An automatic radio compass of the kind in which the radiation pattern of a directional aerial is periodically reversed at a low switching frequency so that the signal received from a transmittting station is modulated at a depth which is dependent upon the angular displacement of that station from the equi-signal axis which results when both radiation patterns are considered in superposition, and in which a control signal is derived corresponding to said modulation depth for rotating the aerial until the equi-signal axis points towards the station, is characterized as follows. A second aerial is provided which has an effective length at least 25 times the effective length of the first aerial and from which a detected signal corresponding to an audio-frequency tone or pulse modulation present in the transmitted signal is derived for operating an indicator means. Furthermore, the receiver means for the second aerial is coupled to the receiver means for the first aerial by a common unit whereby it is ensured that the control signal is actually derived from signals transmitted by the transmitting station, and not by spurious signals such as noise. As described, in an embodiment for use where the transmitted signal is modulated with an audiofrequency tone, a directional aerial 1, Fig. 1, is a combination of a loop aerial 2 and an omnidirectional aerial 3, giving a cardioid radiation pattern. Said pattern is reversed at, for example 30 Hz, by a switch 4 which is controlled by an oscillator 26. The aerial is rotatable by a motor 30. Output from the switch 4 is coupled through a wide-band amplifier 5 to a receiver 6 comprising a channel unit 7, in which is a first mixer, receiving input also from a local oscillator 23, a second mixer 8, receiving input also from a controlled local oscillator 22, a narrow bandpass filter 9, and a detector 10. Output from the detector 10 is taken to an aerial rotation controlling circuit 11 which develops a control signal which is applied to the excitation winding of the motor 30. The second aerial 12 is omnidirectional and is connected to the input of a receiver 14 which comprises a channel unit 13, in which is a first mixer, receiving input also from the local oscillator 23, a second mixer 8, receiving input from the controlled local oscillator 22, and a narrow band pass filter 17 which provides two outputs for controlling the frequency of the local oscillator 22. The first of said outputs is connected to the input of an amplitude detector 18 which is connected to a stop or threshold circuit 20, having its output connected to the input of a local oscillator frequency control unit 21 provided a sawtooth voltage at its output for application to the local oscillator 22. The second of the said outputs from the filter 17 is connected through a frequency discriminator 19 to the local oscillator 22, and provides a fine control of the frequency thereof. Output from the channel unit 13 is also coupled through a detector 16 to the input of an indicator means 31. Said indicator means comprises an amplifier 32 with output divided between a filter 33 and an integrator 34. A difference amplifier 35 receives respective inputs from the said filter and integrator and output therefrom is coupled through an amplifier 36, an integrator 37, a threshold circuit 38, and an amplifier 39 to an indicator lamp A. In another embodiment, for use where the transmitted signal is pulsed, a coincidence circuit 44 is provided in the receiver 6 which only gives an input to the aerial rotation controlling unit 11, in response to an output from the switch 4, when enabled by signals received on the aerial 12. The receiver 6 includes a detector 45 following the channel unit 7 and providing one input to the coincidence circuit 44. The receiver 14 includes a detector 43 following the channel unit 13 and providing input to an amplitude selector 42. An amplifier 46 in said amplitude selector provides one input directly to a signal difference circuit 48 and another input thereto via an integrator 47. Output from the circuit 48 is applied via an amplifier 49 to a threshold circuit 50 having two outputs, one connected to the second input of the coincidence circuit 44, and the other connected to an indicating means 51. Said indicating means comprises an integrator 52, a threshold circuit 53, an amplifier 54, and an indicator lamp 55.