GB680512A - Antenna assembly - Google Patents

Abstract

680,512. Aerials. STANDARD TELEPHONES & CABLES, Ltd. Dec. 29, 1950 [Jan. 12, 1950], No. 31584/50. Addition to 677,860. Class 40 (vii). The invention relates to an antenna system producing a rotary cardioid pattern by the use of a stationary aerial having an omnidirectional pattern and a rotating directive aerial having a figure-ofeight pattern. In such a system the figure-of-eight aerial is of small length relative to half a wavelength at the operating frequency and is enclosed in a resonator open to radiation about one periphery and the omnidirectional aerial is mounted symmetrically about the directive aerial. The directive aerial may be a folded dipole 1, a tenth of a wavelength long fed from coaxial line 4 through a rotary joint 3 and feeder 2. The dipole may be rotated at 1800 r.p.m. by means not shown. Plates 5 and 6 spaced apart by a distance slightly greater than #/2 are arranged above and below the dipole 1 and joined by a circle of rods 7 which ensure that only horizontally polarized energy passes and assists to match the impedance of the short dipole to free space. As the plates 5 and 6 and rods 7 are not adequate to completely match the dipole an inner circle of rods 8 is also provided. An adjustable plate 9 is mounted on the rods 8 and placed to compensate for the capacitive reactance of the dipole. Any slight remaining mismatch may be compensated for by a matching impedance bridged on the feeder and housed, for example, in rotary coupling 3. The omnidirectional aerial may be placed in any suitable position surrounding the dipole, but in order that the energy radiated may be solely horizontally polarized is preferably placed inside the rods 7. For example, some of the rods 8, e.g. those marked 10, 11, 12 and 13 may be hollow and act as extensions of coaxial feeders 18, 19, 20 and 21 which join in main feeder 22 and serve to feed, in phase, radiators 14, 15, 16 and 17 extending from the respective hollow rods to an adjacent rod 8. It is explained with reference to Figs. 4, 5 and 6 (not shown) that the cage resonator 5, 6, 7 does not entirely prevent the radiation of vertically polarized energy but that if the rods 7 be extended above plate 6 as by extensions 30 to a further plate 28 and preferably also below plate 5 by extensions 29 to a plate 27 vertically polarized signals may be reduced to negligible strength. As shown in Fig. 7, the rods 7 may be replaced by a cylinder of sheet metal 31 with slots 34 replacing the gaps between the rods; the extension rods 29 and 30 may also be replaced by extensions of this cylinder, no slots being required. The inner rods may be replaced by a further slotted cylinder 37 inside which a shorting plunger 39 replacing plate 9 may be adjusted. The omnidirectional antenna may be formed by conductors bridged across certain of the slots 38 in cylinder 37.