Patent GB-595216-A (GB)
Title: Improvements relating to radio navigational systems
IPC: G01S1/02
Filed: 1945-05-16
Abstract: 595,216. Directive radio systems. STANDARD TELEPHONES & CABLES, Ltd. and WILLOUGHBY, E. O. May 16, 1945, No. 12372. [Class 40 (v)] A radio navigation system comprises a beacon having a rotating directive radiation pattern, e.g. a cardioid pattern, the energy radiated being frequency or phase modulated with a reference wave of a periodicity the same as that of the rotation of the directive pattern, and the bearing being obtained by the comparison of the phase of amplitude modulation of the energy produced at the receiver by the rotation of the radiation pattern with the phase of the reference wave. The beacon radiation pattern 1, Fig. 1, is preferably of a shape such as is obtained by the addition of a cardioid pattern to an omnidirectional pattern 2 having a common origin 3, the ratio of the front to back radiation being of the order of 2 : 1 or 3 : 1. The rotation of such a pattern produces at the receiver a modulation, Fig. 2, (not shown), the envelope of which need not necessarily be sinusoidal. Beacon, Fig. 3.--The aerial system 7 is mounted on a rotating shaft 11 driven by a motor 17, 18 and driving an A.C. generator 15, 16 which gives an output whose frequency is the same as the rotation of the shaft and is arranged to be a maximum when the radiation is directed due North. A system comprising a master oscillator 21, harmonic generator 26 and power amplifier 27 feed r.f. energy to the aerial 7 through a rotary coupling 20. The outputs of the A.C. generator 15, 16 and a microphone 23, after suitable amplification, are fed in series to a thermionic reactance 22 which swings the oscillator 21 in frequency in accordance with the rotation of the aerial and the speech signal wave. Alternatively an Armstrong frequency modulator may be used or the frequency modulation of the oscillator 21 may be effected by a condenser ganged to the shaft 11. A frequency modulation and carrier frequency stabiliser 29, which may be of known type, is provided. Electromagnetic means may be provided for rendering the aerial system omnidirectional for testing purposes, such means being controlled via slip rings 30 on the shaft 11. Receiving and indicating system, Fig. 12.- Energy received on an aerial 84 is converted after amplification to a suitable intermediate frequency by means of a frequency-changing system 86, 87, 88 and after further amplification is split between a low-frequency detector 99 and an amplitude limiter 90. The output of the limiter 90 is fed to a. frequency discriminator 91, the output of which is fed through the primary of a transformer 92 to a parallel tuned circuit 95, including a transformer 96, which accepts the low reference frequency and rejects the higher speech frequencies. The latter are fed over the transformer 92 to an amplifier and telephones 94, whilst the reference frequency is fed over the transformer 96 to a tuned amplifier 97 which includes a phase splitter and gives two outputs in phase quadrature. These outputs are fed to the fixed coils of a dynamometer-type indicator 98, producing a rotary field synchronous with the aerial rotation. The output of the amplifier 99 is fed through an amplifier 100, tuned to the beacon rotation frequency, to the fixed coil of the indicator, which takes up a position dependent upon the bearing of the receiver relative to the beacon. Alternatively a C.R. indicator may be used the outputs of the device 97 being fed in phase quadrature to the deflection system to produce a circular trace, and the output from the amplifier 100 being used to vary the intensity or to deflect the electron beam; for this purpose the output may be used to produce a pulse at its maximum giving, for example, a kink in the circular trace. In a modification, to obviate the possibility of mutual interference in the I.F. stages of the receiver between the amplitude and frequency modulations, the reference modulation may be transmitted as two frequency modulations which differ by the frequency of the beacon rotation. For example, if the beacon rotation frequency is 50 c/s. there may be used for generating the modulation frequency an A.C. generator giving two outputs of 150 and 200 c/s. In the receiver, the two demodulated frequencies of 150 and 200 c/s. are passed by the band-pass unit 95, 96, (Fig. 12), and the amplifier 97 is provided with a detector tuned to 50 c/s. to extract the reference frequency. Beacon aerial system, Fig. 4a, c.-The beacon is provided with an earth plate 8 having a circular aperture 9. A central cylindrical rotating portion 10 is mounted on the shaft 11 and carries the quarter-wave rod aerials 31, 32. The edges of the plate 8 are provided with a cylindrical wall 12 which together with the wall 13 of the member 10 forms a capacity ring, the depth of the walls being one quarter wavelength. The rod aerials 31, 33 are spaced one eighth-wave length apart and are fed in phase quadrature over a concentric feed which may form the shaft 11. To obtain the quadrature phasing, the aerials 31, 32 are connected to the main feeder over two equal feeder sections 35, 36 each much shorter than #/4 and are connected together by a quarter-wave-length section 34. The section 36 includes a stub 39 closed at its end 40. Contact elements 37, 38 are normally arranged in the positions opposite to those shown. Thus the parts 36, 39 form a quarter-wave short-circuited stub on the main feeder line and have no effect on the aerial feed. For the purpose of test, the contacts 37, 38 may be moved into the position shown, this movement being effected automatically by electromagnets controlled by the slip rings 30, Fig. 3. In a modification, Fig. 5, enabling a rotary feeder coupling to be dispensed with, one of the rod aerials 31 is fixed and the other, 32, which acts as a reflector, rotates around it. The central cylindrical portion 10 is provided with a central aperture and an inner cylindrical wall 44 one-quarter-wavelength long. The main feeder 28 which is enlarged at its end extends within the cylinder 44 and supports by means of insulating material 41 the aerial 31, .the. lower end of which is extended downwardly to form a central conductor 42 continuous with that of the main feeder 28. The enlarged outer conductor of the feeder is folded back as shown at 43 for a distance of #/4, thus forming with the wall 44 a quarterwave section open at both ends, forming a capacity ring which short-circuits the line conductor 43 to the earth plate 10. The rod 32 is spaced a suitable distance, e.g. #/4, from the rod 31 to serve as a reflector and is carried by the outer part of the cylindrical portion 10, which is rotated by the motor. The rod may be dropped into a casing 45 to give omnidirectional radiation. Preferably the earth plate 8 is in the form of a platform above ground level and of one or two wave-lengths diameter. Horizontal polarisation produced by currents in the earth plate may be minimised by the use of a horizontal compensating aerial tuned to halfwave resonance and end capacity coupled between the two vertical aerials, Fig. 10, (not shown). An alternative aerial, Fig. 11, comprises a drum 72 having a circumferential slot 73 in its cylindrical wall. The main feeder 28 is enlarged at 80 and a centre conductor 78 extends through this enlarged end. The upper half 74 of the drum is fed from the conductor 80 through a capacity coupling formed by a plate 79 and the upper end of the drum. The lower half 75 is connected to a sleeve 81 which forms a rotating support. The enlarged outer conductor 80 forms with the sleeve 81 a threequarter wavelength open circuited stub. The drum 72 is short-circuited to the earth plate 10 by a folded quarter-wave section comprising the base of the drum and the surface of the plate 10. The latter has a conical portion, as shown, which forms a quarter-wave opencircuited section, together with a short section formed by the cylindrical wall 11 and shaft 81. The drum halves are connected by a shortcircuiting strip 77 to form an unbalanced radiator giving the required directional pattern. This strip may be disconnected automatically by means controlled via the slip rings 30 when it is desired to render the beacon omnidirectional. Rotary r.f. coupler.-Fig. 6, 7 and 8 illustrate means for connecting the main feeder 28 to a rotating aerial system such as that shown in Fig. 4a. The shaft 11, Fig. 4a, is formed by an enlarged concentric feeder 46, 47, Figs. 6-8. In Fig. 6 the central conductor 47 of this feeder and the outer conductor 50 of the main feeder 28 are hollowed and form sleeves surrounding an intermediate central conductor 51. The overlap between the conductors 48, 51 and 50, 51 is such as to form in each case a quarter-wave short-circuited coupling sleeve. A sleeve 54 prevents leakage from the outer conductor of line 28. 14 is the bearing for the rotating feeder 47, 48 and 52 is a fixed support for the main feeder 28. In the arrangement of Fig. 7 the main feeder 28 is enlarged to form a fixed termination 57 having a central conductor 49 and an outer conductor 62 aligned respectively with the central conductor 47 and outer conductor 46 of the rotating feeder part. The central conductors 47, 49 terminate in condenser plates 55, 56 and the conductor 49 includes a series inductance 63 which together with the condensers forms a series tuned circuit. Inductances 61, 62 correct for stray capacities. The outer conductors 46, 62 terminate in condenser plates 59, 58 in the form of rings and an exterior cylinder 60 forms with the walls of the conductors quarter-wave short-circuited coupling sleeves. In the arrangement of Fig. 8 coupling is effected entirely by quarter-wave short-circuited sleeves, 64 for the inner conductors and 60 for the outer conductors. Tesling.-With the beacon radiating omnidirectionally the transmission is both frequency and amplitude modulated by the same wave so that at a receiver the indicator should give no