Patent GB-652365-A (GB)
Title: Improvements in or relating to radio direction-finding systems
IPC: G01S1/02
Filed: 1947-04-25
Abstract: 652,365. Radio navigation; radio receiving circuits. SPERRY GYROSCOPE CO., Inc. April 25, 1947, No. 11129. Convention date, April 27, 1946. [Classes 40 (v) and 40 (vii)] In a radio direction-finding system the transmitter radiates a rotating beam constituted by two partially overlapping lobes distinguished from one another by different frequency characteristics, the rotation being in time relationships with a reference transmission radiated non-directionally, and the receiver, by comparing a signal derived from the beacon with one derived from the reference waves, provides a measure of the time relationships between reception of the directional and reference waves from which the bearing of the receiver from the transmitter can be determined. In the embodiment of Fig. 1 the transmitter 11 comprises the R.F. oscillator 21 which feeds the frequency multiplier amplifiers 22 and 23 to give two carriers of somewhat different frequencies, one to be radiated in the overlapping beams from reflector 12 and the other to be broadcast non- directionally by radiator 18, respectively. The modulator 26 serves to separate the R.F. energy into two substantially equal parts, amplitude modulate each part at two different frequencies and feed them respectively via wave-guides 35 and 36, the open ends of which are placed close together on either side of the focus of reflector 12 to provide partially overlapping beams. The modulator and reflector assembly are mounted on a rotatable platform 27 having a ring-gear 29 and pinion 31 driven by the motor 33. The modulator may be a mechanical wave-guide switching device (of the general type described in Specification 643,979, [Group XL (b)], driven by the hollow shaft 37 through gears 38, 39 from the motor 33, R.F. energy being fed to it via waveguides 24, 25 and the rotating joint 28. The reference signal radiated by the antenna 18 is modulated by a sine wave or impulse from the source 44 synchronized with the rotation of reflector 12 by means of the gearing 45, 46. The receiver 14 has an antenna 51 which receives waves from reflector 12 and radiator 18. The mixer 52 with local oscillator 53 provides two intermediate frequencies fed respectively to the I.F. amplifiers 54 and 61. A detector 55 reproduces the two modulation frequencies impressed by the modulator 26 and after amplification these are heterodyned and linearly detected in the heterodyne detector 57. Beat frequency filter 58 selects the difference Frequency which is fed via the amplifier 59 to an indicator 17. The reference signal from the I.F. amplifier 61 is demodulated by detector 62 and passes through amplifier 63 to the indicator 17 where it is compared, on a time basis, with the occurrence of maximum amplitude of the beat frequency signal. In another embodiment, indicated in Fig. 3 (not shown), the overlapping beams are of two radio frequencies supplied from separate V.H.F. sources and differing by some convenient audiofrequency. The receiver comprises a local oscillator and mixer, I.F. amplifier, heterodyne detector, and audio amplifier to feed the audio beat frequency to the indicator. In addition to the maximum beat frequency amplitude as the rotating beam equi-signal line sweeps past the receiver there is a possibility of weaker maxima due to side lobes of the directive beams and these may be discriminated against by rendering the receiver inoperative for signals less than some predetermined fraction of the strength of the true or primary equi-signal line. The detector 55 may be arranged as in Fig. 2 where diode 68 operates conventionally except that a bias, derived from diode 72, is applied thereto according to the voltage drop between tap 76 and earth. By making the time constant of the resistor 80 and capacitor 79 long in comparison with the time of a complete rotation of the beams this voltage will be proportional to the peak amplitude of the I.F. waves, and hence by adjustment of the tap 76 the diode 68 can be rendered operative only for signals greater than some predetermined fraction of the peak amplitudes occurring during a rotation of the beams.