GB1297292A - - Google Patents

Abstract

1297292 Filters NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 11 Nov 1970 [11 Nov 1969 11 June 1970] 53593/70 Heading H1W [Also in Division H4] A frequency group branching filter device comprises a filter 57, Fig. 1, which is disposed in a beam forming region between the aperture of an aerial 53 and its feeder means 56. Said filter consists of a plurality of laminar dielectric elements, each having a thickness of a quarter wavelength at the central frequency of a highfrequency group of signals, which are to be separated from a low frequency group of signals. The elements of the filter have at least two different dielectric constants. As described, the aerial is of the Cassegrain type with a main reflector 51 and a sub-reflector 52. Waves of the low frequency group (which may, for example, comprise a 4GHz received band and a 6GHz transmitted band) pass through the filter 57 to or from a horn 55. Separation of the lowfrequency components is effected by suitable filters in the waveguide feeding the horn 55. Waves of the high-frequency group (which may, for example, comprise an 18GHz received band and a 26GHz transmitted wave) are reflected by the filter 57 to or from a horn 54. Separation of the high-frequency components is effected by suitable filters in the waveguide feeding the horn 54. The filter 57 comprises three laminµ each with a thickness substantially equal to a quarter wavelength at the central frequency of the high-frequency group, and having different dielectric constants. Materials which may be used are polystyrene, fused quartz, glass, and poly-2,5-dichlorostyrene. It is also possible to have laminµ of air (or "free space"), for example, five glass elements may be disposed with equal gaps between them (Fig. 3, not shown). In another filter three elements are integrally laminated, the central element being of different material from the outer elements. A plurality of ribbon-shaped dielectric elements of still another material are embedded at equal intervals in said outer elements (Fig. 6, not shown). A filter is also described in which a set of five air spaced glass elements 57b, Fig. 7, is supplemented on each side by a group of dielectric elements 91, 92 having thicknesses equivalent to a quarter wavelength at the central frequency of the low-frequency group, in order to improve impedance matching. The angle of incidence, at which beams enter a filter, is preferably less than 30 degrees. In other embodiments, reflectors additional to those shown in Fig. 1 are featured. In the arrangement shown in Fig. 11, the low-frequency group beam from the filter 57 is focused by a paraboloidal reflector 111 to a feed horn 55. The high-frequency group beam is reflected by a plane reflector 113 to a paraboloidal reflector 112 and is then focused to a feed horn 54. In another arrangement, a single paraboloidal reflector is used, between the sub-reflector and the filter (Fig. 10, not shown).