Patent GB-1370669-A (GB)
Title: Antennae
IPC: H01Q15/00
Filed: 1971-10-11
Abstract: 1370669 Aerials NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 11 Oct 1971 [17 Oct 1970] 47162/71 Heading H4A An aerial device comprises two reflectors which are so disposed as to reflect an incident beam of radio waves sequentially. It also comprises a third reflector, which is disposed between the other reflectors, and is adjacent to one of them. The third reflector comprises dielectric elements and is capable of reflecting one of two frequency group components of the beam and of transmitting the other. This results in the two components of the beam being directed towards different points. As described, a Cassegrain type aerial for the ground station of a satellite communications system comprises a main paraboloidal reflector 601, Fig. 10, from which a paraboloidal sub-reflector 602 is supported by struts 604. Immediately below the sub-reflector 602 is another subreflector 603, which is of hyperboloidal shape, and which consists of a number of layers of dielectric material of two different kinds, each layer having a thickness substantially equal to a quarter-wavelength at the centre frequency of the higher of the aforesaid two frequency group components (Figs. 5a, 5b, not shown). The respective frequency ranges of the groups may be, for example, from 17 to 30 GHz, and from 4 to 6 GHz. The lower frequency group is directed by the reflector 601 towards its focus 623, passes through the sub-reflector 603, and is returned by the sub-reflector 602 as a parallel beam, which is converged by a hog-horn 605 towards axially mounted receiving (or transmitting) apparatus 607. The higher frequency group is also directed towards the focus 623, but is reflected by the sub-reflector 603 and is converged upon a horn 606, which feeds axially mounted apparatus 608 through a waveguide 611. The main reflector is supported on a mount 619, being rotatable about a horizontal axis 616 by a motor 614 through gears 612, 613, so that the beam may be steered in elevation. The mount 619 rides on a rail 620 on a foundation 621, and is rotatable about a vertical axis 622, so that the beam may be steered in azimuth. In another embodiment, the mechanically despun aerial of a satellite comprises a plane main reflector 709, Fig. 11, from which is supported a metal paraboloidal subreflector 701 inside which is a dielectric layer type sub-reflector 702. An incident beam is directed towards the sub-reflectors by the main reflector. The lower frequency component is transmitted by the sub-reflector 702 and is converged by the sub-reflector 701 to a point 706, from which a horn 708 leads to a transponder 717. The higher frequency component is converged by the sub-reflector 702 to a point 705, from which a horn 707 leads to a transponder 716. The beam may be directed to a particular latitude by tilting the main reflector (Fig. 12, not shown). In an alternative arrangement, there is a single paraboloidal sub-reflector and two main reflectors, one of which is of dielectric layer construction and is disposed just above the other and at a small angle to it (Fig. 9, not shown). In another embodiment, a paraboloidal main reflector is associated with two spaced apart feed horns and with two angularly displaced hyperboloidal sub-reflectors, one being of dielectric layer construction and disposed just inside the other (Fig. 7, not shown). A prior art Cassegrain type aerial is also described in which separation of two frequency group components is effected by a branching filter located between the receiving (or transmitting) apparatus and the single feed horn (Figs. 1, 2, 3, not shown). A reference to Specification 1,297,292 is included.