GB160251A - Improvements in or connected with radio-navigational apparatus and the like - Google Patents

Abstract

160,251. Murray, J. Erskine-, Robinson, J., and Crowther, H. L. Dec. 15, 1919. Bearings, determining; heterodyne receiving systems.-In a direction-finding system of the typo in which the frequency of the transmitted wave is a function of the direction of transmission, the receiving station is provided (1) with a tuned circuit to give a heterodyne effect of high or inaudible frequency to the continuous signalling wave, and (2) with another tuned circuit on which the heterodyne effect is impressed to give a superimposed heterodyne effect of low or audible frequency. The transmitting station comprises a directive transmitter adapted to emit successively along bearings between North and South continuous waves having a different frequency for each direction of propagation. Preferably the transmitter is adapted to transmit during 180‹ of rotation only. As the frequency for each direction of propagation is known at the receiving station, by listening for the minimum transmission of the signalling wave, that is, for the wave propagated in line with the receiving station but in the opposite direction, and recording its wavelength, the bearing of a line joining the transmitting and receiving stations can be found. The receiving station comprises two tuned circuits 4, 5, and 8, 9, 10 adapted to self-oscillate, as shown in Fig. 1, or having oscillations induced therein from an independent source. The circuit 4, 5, 6 is tunable between the range of the transmitting wave, for example 1000 to 2000 metres, whilst the circuit 8, 9, 10 is tunable to a low frequency, for example 20000 to 40000. When the circuit 4, 5, 6 is tuned to the wave-length corresponding to the direction of minimum transmission, two equal heterodyne '' swells " will be produced as the signalling wave rotates about the transmitting station, but if the circuit 4, 5, 6 is not tuned to the correct wave-length two heterodyne "swells" of unequal intensities are produced. As the limit of audibility is restricted, ordinary heterodyne between the signalling wave and the circuit 4, 5, 6 could only be compared over a very small angle on either side of the minimum, where signals may be very weak. For this reason, heterodyne effects in the circuit 4, 5, 6 are superimposed on the circuit 8, 9, 10 to obtain beats of audible frequency. An audible beat frequency will thus not be produced in the circuit 8, 9, 10 until the frequency of the signalling wave has varied sufficiently to produce beats in the circuit 4, 5, 6, which are of the order of the frequency of the oscillations in the circuit 8, 9, 10. The angle at which signals are heard may therefore be widened by increasing the frequency of the circuit 8, 9, 10. Specification 149,827 is referred to.