GB528817A - Improvements in or relating to radio antenna - Google Patents

Abstract

528,817. Aerials. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 15, 1939, No. 14501. Convention date, May 18, 1938. [Class 40 (v)] A short-wave aerial system having a uniform impedance over a wide frequency band is characterized by the use of a quarter-wave ellipsoidal element having a transverse thickness which is a substantial fraction of the working wave-length, and an outer shell surrounding a part of that element. As shown in Fig. 8, the radiating element 51 projects beyond the outer conductor 53, the free end of which is connected to an earthed conducting surface 12. The element 51 is an expansion of the centre conductor 14 of the concentric feed-line, the outer conductor 13 being flared, as shown, for impedance matching. In Fig. 10 the outer conductor 52 is folded back to form a shell 53 of cylindrical or ellipsoidal shape which is connected to the earthed platform 10, 12 ; the ellipsoid may comprise a wooden former covered with copper foil. Irregularities in the aerial impedance due to phase differences between the extended portion 51 and the shell 53 may be prevented by the conductor 54 which may be a metallic strut also serving to provide increased support for the portion 51 and an earth connection for any DC voltages, e.g. lightning discharges. The increase in aerial impedance due to the conductor 54 may be compensated by increasing the length of portion 51 or by lowering the impedance of the feedline by decreasing the ratio of the conductor dimensions over the flared section. A " turnstile array may comprise such elements radially disposed about a central point in a common horizontal plane, the energizing voltages of adjacent elements being phase displaced by 360/N degrees whereby a rotating horizontally polarized field is obtained. In order to reduce vertical radiation the array may be supported a half wavelength above ground. Two independent transmissions may be radiated from two such arrays mounted on a common central support, cross-coupling being reduced by energizing the two arrays so that the fields are rotated in opposite directions ; when each array comprises four elements one array may be displaced axially by 45 degrees with respect to the other. Since in a combined sound and television transmitter a large band width is only required for the vision transmission, the sound transmission may be radiated from a horizontal loop aerial which is inherently decoupled from the "turnstile" aerial at all frequencies. The loop aerial, Fig. 20, comprises 4 folded elements 70, 71, each element being formed from a # or ¢ # length of conductor and being equivalent to two ¢# or “ # dipoles in'parallel ; the elements are supported at the voltage nodes 80 and the free ends are connected to the conductors of the coaxial line 73. Although the impedance of an isolated folded element is much greater than that of the coaxial feeder, the impedance of the loop is of the same magnitude as that of the feeder owing to the parallel connection of the elements and the interaction of adjacent elements. Fig. 21 shows an aerial system in which the sound transmission is radiated from such a loop 70, 71 mounted above a " turnstile " aerial comprising four “ # ellipsoidal elements from which the vision transmission is radiated. The outer conductor of the sound aerial feeder is connected to the pedestal 561 by a conductor 77 a quarter-wavelength below the aerial. Opposite elements of the " turnstile " are fed in opposition from a push-pull output, 90 degrees dephasing between adjacent elements being effected by a “# loop 81 which also serves to balance out any reactance in the radiating element. Additional support for the radiator 51 is provided by the insulator 59 ; weather screens 57, 58 of insulating material and electrical heating elements in the radiators 51, struts 55 and shells 53 are also provided. Alternatively the sound transmission is radiated from a vertical aerial of the type shown in Figs. 5 and 10 ; a number of horizontal ¢ # double ellipsoidal aerials can also be arranged round a central point. The band width of the " turnstile " aerial is so broad that the vision and sound transmissions for a single programme or for two different programmes could be radiated from the same aerial. Suitable dimensions for the aerials and corresponding impedance-frequency curves are given.