GB813614A - Improvements relating to aerials - Google Patents

Abstract

813,614. Aerials. ANTIFERENCE Ltd., and BEST, N. M. May 15, 1957 [May 18, 1956], No. 15519/56. Class 40 (7). The invention relates to directive aerial systems for operation with radio waves in two distinct frequency bands and having a common polarization. According to the invention such a system comprises a dipole construction capable of operation in both bands and provided with feeder connections, a lower frequency director element spaced from the dipole construction, and a resonant element approximately a halfwavelength long in the higher band placed close to and symmetrically with respect to the director element in such a way that the combination is effective as a director in the higher frequency band. In a simple embodiment (Fig. 1) the dipole construction comprises a lower frequency dipole 11 connected to a feeder (not shown) in central insulator 12 and an element 15 adjacent thereto and approximately half a wavelength long in the higher frequency band. Such a two-band dipole construction is described in greater detail in Specification 813,613. On the opposite end of the cross-arm 13 supporting the dipole construction is mounted a director 14 for the lower frequency and beyond it, and supported by a short extension of the cross-arm, is an element 16 approximately half a wavelength long in the higher frequency band. Use of this element in conjunction with director 14 provides a combination effective as a director in the higher frequency band and prevents the director 14 from shielding the dipole construction at the higher frequencies. The element 16 is stated to have little effect on the operation in the lower frequency band. If required further simple directors resonant in the higher frequency band may be placed on the cross-arm between the dipole construction and the director 14 and also on a further extension of it beyond the element 16 (Fig. 2, not shown). In another modification (Fig. 3, not shown) the simple dipole 11 is replaced by a folded dipole and simple reflectors operative respectively in the higher and lower frequency bands placed behind it. Fig. 4 shows another embodiment incorporating three simple directors 18 placed as mentioned above and a different form of dipole construction in which elements 11a a quarter wavelength long in the higher frequency band and inwardly directed are attached to the arms of the lower frequency dipole 11 at points a half wavelength of the higher frequency from the centre. The dipole construction may also comprise closely adjacent dipoles resonant in the higher and lower frequency bands respectively with the inner ends of their elements connected together and to the feeder (Fig. 5, not shown). In any of the embodiments more than one lower frequency director may be used providing each one has adjacent to it a higher frequency band resonant element. To broaden the bandwidth any of the resonant elements may be replaced by a pair of rods, one on either side of the arm 13 or by an arcuate sheet of metal centred on the associated lower frequency element.