Communications: radio wave antennas – Antennas – With polarization filter or converter
Patent
1991-05-13
1993-11-09
Hajec, Donald T.
Communications: radio wave antennas
Antennas
With polarization filter or converter
343840, 333 21A, H01Q 1900, H01Q 1524
Patent
active
052607135
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with a microwave signal receiving apparatus for receiving satellite television transmissions comprising a receiving element, focusing means for focusing incident plane-polarized microwaves at a focal point, a waveguide for guiding the focused microwaves to the receiving element, and a ferrite polarizer operable to effect controlled rotation of the microwaves in order to isolate those microwaves of a selected polarization which become duly rotated for onward transmission through the waveguide and reception by the receiving element.
Ordinarily a dish reflector has been utilised in such apparatus to focus the incident microwaves at the focal point, and a horn has been used to feed the collected microwaves into the wave guide. The receiving element may, for example, be a wave guide probe associated with a low noise block down converter.
In order to isolate microwaves of a selected polarization, the apparatus comprises a polarizer by means of which controlled rotation of the planes of polarization of collected microwaves can be effected. By means of such rotation, microwaves in any particular selected plane of polarization can be brought to be duly aligned for reception by the receiving element, and so isolated from microwaves in other planes of polarization.
Whilst different types of polarizer have been used in such circumstances, one known type which has been used with advantage is that usually known as a ferrite polarizer. This solid state device comprises a ferrite body, through which microwave energy can be passed, and around which an electrically conductive coil extends. By means of the coil an axial magnetic field can be created in relation to the ferrite body. In the presence of such a magnetic field, there occurs rotation of the plane of polarization of plane-polarized microwave energy passing through the body, such rotation being referred to as Faraday rotation, or being said to exhibit the Faraday effect. Variation of the strength of the magnetic field results in variation of the degree of rotation achieved, and accordingly the rotation effected during passage of the microwaves through the ferrite body can be controlled.
It is known to mount such a ferrite polarizer within the wave guide, for example as described in EP-A1-0237 988. The collected microwaves are fed into the wave guide by means of a horn and pass along the wave guide to the polarizer before onward transmission through the wave guide to the receiving element.
Apparatus according to the invention is characterised in that the polarizer is positioned at the focal point to receive microwaves from the focusing means and to transmit the isolated microwaves into the waveguide and so to the receiving element.
In a preferred construction, a ferrite body of the polarizer comprises a receiving end portion positioned at the focal point and a transmitting end portion positioned within the wave guide. Both the receiving and transmitting end portions of the body may comprise convergent tips.
Preferably, at least the receiving end portion of the ferrite body is encased by a housing of a dielectric material. This can be not only for protection of the body but may also give an improved performance. The dielectric material may be, for example, a plastics material such as polypropylene. Where the dielectric housing encases the receiving end portion of the body it may, like the body, be beneficial for it to be convergent in the direction extending away from the wave guide. Whilst the housing may encase most of the ferrite body, the body should preferably project (with its transmitting end portion) beyond the housing into the wave guide.
The dielectric housing may comprise a mounting portion forming a plug which is located in an end portion of the wave guide. The controlling coil of the polarizer may be wound about the housing within the end portion of the wave guide.
There now follows a detailed description, to be read with reference to the accompanying drawings, of signal receiving equipment which illustrates the invention by wa
REFERENCES:
patent: 2863144 (1958-12-01), Herscovici et al.
patent: 3216017 (1965-11-01), Moore
patent: 3445851 (1969-05-01), Sheldon
patent: 4498061 (1985-02-01), Morz et al.
Hajec Donald T.
Le Huanganh
Motson & Company Limited
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