GB654498A - Improvements in and relating to aerial systems - Google Patents

Abstract

654,498. Aerials. BRITISH BROADCASTING CORPORATION, and PAGE, H. Sept. 23, 1948, No. 24953. [Classes 40 (v) and 40 (vii)] In an aerial system of the type comprising two groups of aerials fed respectively with currents of equal amplitudes but in phase quadrature with respect to each other and consisting of a plurality of horizontally disposed sets of aerials stacked vertically to form an array, aerials of one group in one set (e.g. aerials 10 and 12, Fig. 3) are connected with aerials of the other group and of another set (e.g. aerials 22 and 24) by a length of transmission line to which current is supplied over a line (e.g. line 15) connected to a point off-set an eighth-wavelength from its midpoint. In the Figures, the transmission lines (e.g. line 15) comprising either coaxial or balanced pairs of conductors are shown each as a single line. Fig. 2 illustrates an aerial system of the type to which the invention is applied. The system comprises two groups of aerials arranged in perpendicular vertical planes and fed in quadrature, i.e. groups 10, 12, 21, 23 and 11, 13, 22, 24. Groups 10, 12, 21, 23 is fed from source 14 over lines 15 and 28 and group 11, 13, 22, 24 is fed over lines 17 and 26 and a length of line 19 acting as a 90 degree phase-shifter. An aerial system of this type has a substantially uniform horizontal polar diagram so long as the spacing between opposite aerials of each pair does not exceed about half a wavelength and the operating band-width is kept relatively narrow. In the arrangement according to the invention, Fig. 3, line 15 supplies aerials 10, 12, 22 and 24 and line 17 similarly supplies aerials 11, 13, 21, 23. It is stated that by virtue of the fact that the length from the points 29 and 30 is a quarter-wavelength greater to aerials in the upper set than it is to aerials in the lower set, the variation in input impedance of the system with frequency is reduced and the band width characteristics of the aerial consequently improved. Fig. 3 may be modified to comprise an array of horizontal dipoles arranged in a plurality of planes each parallel to the axis 20. In Fig. 5 is shown an analagous arrangement using four sets of crossed horizontal dipoles 53, 57 ; 54, 58 ; 55, 59 ; and 56, 60. Fig. 4 (not shown), illustrates an extension of the system of Fig. 3, comprising eight sets of four dipoles which are divided into upper and lower portions each of four sets and joined by a transmission line which is coupled to the source at a point displaced from its midpoint by an eighth-wavelength. Each portion of four sets is divided into upper and lower portions of two sets which are similarly coupled together and fed and each portion of two sets is fed as before. Compensating for frequency-sensitivity of quarter-wavelength phase-changing device. To compensate for the change in phase-shift introduced by the device 19 with change in frequency an additional length of line 71, Fig. 6, is inserted in parallel with the line 17 at a distance of a quarter-wavelength at the midband frequency from the line 19. As shown, the line 71 is a quarter-wavelength long in which case it is short-circuited at its far end and has a characteristic impedance which is equal to that of the line 17. The line 71 may be made any integral multiple of a quarterwavelength long ; , in this case the characteristic impedance of the line is multiplied correspondingly and the line is short-circuited or open-circuited according as whether the multiple is odd or even.