Patent GB-987470-A (GB)
Title: Improvements in or relating to aircraft approach and landing systems
Inventor: JONES SPENCER SELTH DUNIAM
IPC: H01Q25/00
Filed: 1962-06-13
Abstract: 987,470. Overlapping beams landing system; directive aerial arrays. MINISTER OF AVIATION. June 7, 1963 [June 13, 1962], No. 22673/62. Headings H4A and H4D. An aircraft approach system comprises a transmitter aerial array of radiator elements spaced transversely of an aircraft runway centre line for transmitting two differently modulated signals and wherein the amplitude distribution across the array, of one of the modulated signals emitted by the radiator elements is asymmetrical in one sense about the centre-line of the array and the amplitude distribution across the array, of the other of the modulated signals emitted by the radiator elements is asymmetrical in the opposite sense about the centre line of the array, the two amplitude distributions being symmetrical with respect to one another about the centre-line of the array. The invention is concerned with the use of a localizer beam system at points within the Fresnel region of the aerial array, where prior art symmetrical excitation of the radiators for each modulation produces an insufficient rate of change of the difference of depth of modulation (D.D.M.). The array may be at the down wind end of the runway, have an aperture of 300 ft. and a Fresnel region extending out to one mile. A suitable circuit is shown in Fig. 6 wherein 21 is a transmitter producing a signal at between 108 and 112 mc/s. modulated at 90 c/s. for transmission via a coaxial line having a central conductor 23. A transmitter 25 similarly feeds a signal at the same carrier frequency, but modulated at 150 c/s. along a coaxial line having a central conductor 27. Each transmitter, however, feeds opposite ends of their respective coaxial lines. Radiating aerials A 1  . . . A n , are energized with both transmissions via hybrid ring circuits H 1  . . . H n , the various phases of the tranmissions at the aerials being determined by the lengths of loops L connected into the coupling lines, whilst the amplitudes are determined by the values of coupling capacitors C 1  . . . C n  and K 1  . . . K n . To produce a satisfactory rate of change of D.D.M. the following steps are taken: (i) An aperture amplitude distribution is selected to provide a satisfactory far-field polar diagram. Gaussian, Binomial and Chebishev are examples of such distributions. (ii) The resulting far-field polar diagram is computed from the Fourier transform of the aperture amplitude distribution. (iii) A far-field split angle between the beams is chosen as dictated by the I.C.A.O. specifications. (iv) The phase gradient across the array necessary to produce the split angle chosen in step (iii) is calculated. (v) A weighting function is applied to the selected amplitude distribution so as to give a suitable rate of change of D.D.M. at zero range as directed by the I.C.A.O. specifications. The process is then repeated until satisfactory characteristics are produced. Suitable amplitude and phase distributions are as follows. where x=distance along array from centre point thereof. #=wavelength of carrier frequency. k, L, m=design parameters. If the aperture of the array is so large that entire approach phase of the aircraft is within the Fresnel region, parameter L may be zero. A suitable radiation element for use in the circuit of Fig. 6 is shown in Fig. 7, and comprises a slot fed dipole 29, supported by the slotted metal tube 31 on a metal box 33 housing the hybrid ring, and having the two feed cables 23 and 27 running therethrough. Direction elements may be positioned on either side of the dipole each a quarter wavelength away in a direction perpendicular to the arms of the dipole. The complete array comprises a line of such elements separated by slightly less than one wavelength. The reception characteristics of the ground may be improved by resting the array on wire netting. A further suitable radiator element is shown in Fig. 4 and comprises a folded unipole 3 formed by folding the laterally extended outer conductor of the coaxial feed cable 1 to make contact with the inner conductor of the cable through an insulated orifice 5. Direction elements 7, may be positioned on either side of the unipole one quarter wavelength away. A third suitable radiation element is shown in Fig. 5 and comprises a horizontal dipole 1, fed via a supporting split tube with an open wire transmission line. Director elements 13 and 15 may be provided one quarter wavelength from the dipole.