Patent GB-600195-A (GB)
Title: Improvements in radio navigational systems
IPC: G01S1/02
Filed: 1944-08-11
Abstract: 600,195. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd., and WILLOUGHBY, E, O. Aug. 11, 1944, No. 15396. [Class 40 (vii)] In an approach or glide path system for aircraft in which two overlapping radiations are commutated to produce a common equi-signal zone, two fields of different radiations are produced, one being the side-bands of an amplitude modulation of a carrier wave, and the other the unmodulated carrier wave, the different radiations being in phase quadrature. By this means phase modulation is created at the receiver station. Fig. 1 shows the radiation patterns of an approach beacon according to the invention and Fig. 2 a suitable antenna system for producing such patterns. The unmodulated carrier gives a circular radiation pattern 1, whilst the radiated sidebands give a cardioid pattern 2 which is switched about the course line 3, as shown in full and dotted lines, two equi-signal zones being produced extending in opposite directions from the point of radiation O. The antenna system comprises antenna elements 4, 5 and 6 in alignment located above a metal sheet 7. The centre antenna 5 is energized with sideband energy whilst the outer elements 4 and 6 are arranged to operate as reflectors alternately. Below the sheet 7 is arranged a single antenna element 8 which radiates the carrier. Fig. 3 (not shown) illustrates a radiation pattern having no 180 degrees ambiguity, the unmodulated carrier being radiated to give a stationary lobe of relatively wide angle whilst the side-bands are radiated as a lobe of relatively narrow angle which is oscillated within the envelope of the carrier pattern and about the course line. The corresponding antenna system (Fig. 4, not shown) comprises two antennae mounted above a metal sheet which are energized alternately with the side-band. energy and an antenna below the sheet which is energized with carrier energy, all the antennae being arranged in front of a reflecting device. Specifications 579,873, 580,481 and 580,484 are referred to in which devices for feeding antennae in dot-dash rhythm are disclosed. Fig. 6 shows the radiation patterns of a glide path produced by an antenna arrangement, Fig. 5 (not shown), disclosed in Specifications 577,276 and 577,527. In Fig. 6 the unmodulated carrier pattern is represented by 23, whilst the side-band patterns comprise the single lobe 24 and the multiple lobes 25, the form of the side-band patterns being changed by aerial switching. The equisignal course line 3 is formed by the overlap of the lowest lobe 25 with the lobe 24, this overlap being, as before, within the envelope of the carrier pattern. In a modification, the sideband energy and the carrier energy may be switched simultaneously, each side-band pattern being allotted a corresponding carrier pattern, this being particularly suitable when using wide aperture radiation systems, In a system for achieving this, Fig. 7 (not shown), the sideband antennae and the carrier antennae are arranged in pairs, each pair comprising a sideband antenna and a carrier antenna with a common reflector, the side-band antennae being offset from the focus occupied by the carrier antennae to obtain overlapping lobes. In this case the changing depth of modulation in any direction within the radiation envelope is produced by ensuring that the side-band energy is lower than the carrier energy. A narrow beam of side-band energy or carrier energy may be concentrated along the course line and the said energies switched respectively if highly distorted phase modulation at locations well off the course may be tolerated. The invention also extends to the radiation of the carrier and even harmonic frequency sidebands of the modulation frequency from one antenna system and odd harmonic side-bands from the other antenna system. A relatively large phase excursion may be created at the receiver station by the use of frequency multiplication or a high modulating frequency giving a phase excursion of the order of 930 degrees. Fig. 8 (not shown) shows an interlaced antenna system with horizontal and vertical reflecting sheets, and comprising upper and lower series of antenna elements. Swinging of side-band radiation lobes produced by the upper series is effected by adjusting the phase difference between adjacent elements in known manner with alternate elements undergoing a phase switch. The centre element of the upper series is maintained at 90 degrees phase displacement from the centre element of the lower series which produce the unmodulated carrier radiation, or the centres of the two series may be displaced with respect to one another along the course such a distance that a 90 degrees phase difference will result. The carrier radiation may be employed as an intelligence bearing channel, the path definition modulation and the intelligence modulation being separated by filters. Fig. 9 shows a circuit arrangement for producing separately side-band frequencies and a carrier wave. The output from a master oscillator 33 and a radio frequency amplifier 34 is divided, a part being supplied through a coupling unit 35 to the input of a radio frequency amplifier 37, the output of which feeds, through a tuned transformer coupling 38 and transmission line 39, the requisite antenna system for radiating the carrier wave. A modulator 50 may be provided if the carrier is to bear intelligence. 40 is a line length adjusting unit. The other part of the radio frequency is applied through a coupling unit 36 to a balanced modulator 41 comprising amplifiers 42, 43 in push-pull into the anode circuits of which the side-band modulating frequency is introduced by the transformer 44. The output from the balanced modulator 41 is obtained from the tuned transformer coupling 45 and is fed to the requisite antenna for radiating the side-bands either directly or via an aerial commutating device 47 according to the system employed. Various advantages of the system are referred to, the principal of which is that interference by other radiations at the same frequencies is overcome due to the fact that a receiver employing a frequency or phase discriminating circuit discriminates completely between two frequency modulation signals of similar excursion provided that the interfering signals have more than 6 db. difference in energy level at the receiver station.