Patent GB-1368926-A (GB)
Title: Radio direction-finding apparatus
IPC: G01S3/48
Filed: 1973-04-16
Abstract: 1368926 Radio direction-finding THOMSONCSF 16 April 1973 [17 April 1972] 18305/73 Heading H4D Direction finding apparatus comprises first and second spaced aerials, an f.m. receiver, a switching arrangement, including a switch coupled to the receiver input, and control means having an output coupled to this switch, for alternately coupling said receiver to the aerials, so that at the receiver input the signal from one aerial is gradually substituted by the signal from the other, a device being provided to measure the phase shift between the receiver output component having the same frequency as the aerial switching frequency. A diode switch 3, controlled by oscillator 5 alternately connects aerials A, B to f.m. receiver 4, the change over from A to B being gradual. In Fig. 3 (not shown) the resulting input to receiver 4 is illustrated as waveform b giving the phase of the receiver input signal with respect to the signal from aerial A (the dotted waveforms correspond to the situation when the incoming radio waves are of opposite sense). As the aerials are spaced their signals are out of phase, the phase difference being dependent upon the incoming direction of the radio waves X, Y (Fig. 1, not shown). The gradual change-over from aerial A to B produces a signal of gradually changing phase to be applied to the receiver discriminator. The discriminator produces an output signal U (Fig. 3) of magnitude corresponding to the phase difference. The signal is applied to a low pass filter 6 giving the fundamental of the output signal which is amplified in logarithmic amplifier 7. The signal e (Fig. 3, not shown) is passed to a synchronous detector 8 which gives the phase of the signal with respect to the switching waveform from oscillator 5. The phase difference (Œ#/2) gives the sense of the incoming waves. The information relating to phase difference and magnitude of the signal from the receiver is then passed to an indicating instrument 9 giving the magnitude and sense of the angle of incidence of the incoming waves. The information is passed to indicator 9 via a gate 21. Bandpass filters 19, 20 detect correct operation of oscillator 5 and noise levels respectively. If the output of filter 19 is sufficiently high and that of filter 20 is sufficiently low, gate 21 allows application of the information to indicator 9. A trigger circuit 10 having a threshold removes any small erratic fluctuations corresponding to when the angle of wave incidence is 0. Circuit 10 produces signals of positive, 0 or negative value depending on whether the waves arrive from the right, on the central axis or from the left of the antennas. Circuits 11, 12, 13 generating the Morse code for letter U, a continuous signal or Morse code for D respectively are correspondingly activated so that OR gate 14 activates AND gate 17 accordingly. An audio oscillator 15, via an adjustable attenuator 16, supplies a signal, modulated by gate 17 the headphone 18 thus indicating the sense of direction of the incoming waves to an operator. In addition, attenuator 16 is controlled by the signal amplitude from circuit 21 so that the volume of the signals in headphone 18 indicates the magnitude of the incidence angle.