Patent GB-726566-A (GB)
Title: Improvements in or relating to radio direction determining apparatus for use on veryhigh frequency waves
IPC: G01S1/02
Filed: 1953-03-03
Abstract: 726,566. Radio navigation; pulsing, carrier frequencies. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. March 3, 1953 [March 11, 1952], No. 5951/53. Addition to 717,651. Classes 40 (5) and 40 (7). In an axis defining or bearing determining overlapping beam radio system as described in the parent Specification, wherein two overlapping polar diagrams are produced alternately and their relative amplitudes varied in accordance with a periodic law to vary the direction of the equisignal line each of the different amplitudes is associated with a particular frequency. The aerial arrangements may be the same as in the parent Specification and, in the case of a transmitting beacon, magnetron modulation means may be used to modulate the signals from a variable frequency transmitter. The relationship between the transmission coefficient and frequency of the magnetron circuit associated with the wave-guide carrying energy for radiation in one polar diagram may be as shown at T1 and of the other associated with the second guide as shown at T2. Bi and B2 are the respective cut-off curves. Only square switching waveforms are applied to the magnetron cathodes in this embodiment and these serve to cut off the channels alternately. The transmitter frequency is varied periodically between the limits f1 and f2 and this changes the relative amplitude of the polar diagrams and at the same time each direction of the equisignal line is thereby characterized by a particular transmitted frequency. It is important that the maximum of one of the curves T1, T2 corresponds to the minimum of the other and that a linear response is obtained. When the frequency range of the transmitter is small it is difficult to obtain satisfactory curves T1, T2 and it is preferred to use two identical magnetrons of relatively large band-width for switching purposes only and to include in the guides beyond the magnetron coupling points attenuators having transmission characteristics varying with frequency according to straight line relationships corresponding to the operative part of curves T1 and T2. Fig. 10 illustrates such a system schematically; Fl and F2 being the attenuators and 10 and 20 the switching magnetrons controlled by switching generator 7. It is preferred that the period of the transmitter frequency variation shall be a simple multiple of the switching period and a synchronizing connection cd is provided for this purpose. For a receiving system a receiver may be substituted for the transmitter 8 and no link c,d is required. A constant intensity signal is obtained at the receiver only when the appropriate frequency is received from a particular direction and by measurement of the frequency when there is no switch frequency modulation of the received signal the direction of its arrival may be determined. It is necessary for the co-operating transmitter to be suitably frequency modulated. A receiver for a beacon according to the invention and details of the receiver mentioned above are described in connection with Fig. 11 (not shown). In these receivers a calibrated filter is adjusted in frequency until the signal passed carries no switching frequency modulation and the direction from which the signal is received can then be deduced from the filter pass frequency.