This invention is concerned with microwave signal receiving apparatus of the kind in which incident plane-polarized microwaves are collected at a focal point and fed through a wave guide to a receiving element of some suitable type.

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. Such apparatus may be used in a satellite television reception system.

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, collected microwaves being fed into the wave guide by means of a horn and passing along the wave guide to the polarizer before onward transmission through the wave guide to the receiving element.

According to the invention, in microwave signal receiving apparatus of the kind referred to a ferrite polarizer is positioned at the focal point to feed the microwaves from the focal point into the wave guide.

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 way of example.

In the accompanying drawings:

Figure 1 is a view ^* of the equipment in perspective; and

Figure 2 is a view in cross-section of a ferrite polarizer positioned within the entry to a wave guide.

Equipment for receiving microwave television signals from satellites comprises a dish reflector 10. A signal handling unit 12 is secured by means of stays 14 at a distance from the reflector and comprises a ferrite polarizer 16, a tubular wave guide 18, control electronics 20 and a low noise block down converter 22. A coaxial cable 24 extends between the signal handling unit 12 and a conventional receiver which processes the signal received.

The arrangement of the polarizer 16 and wave guide 18 is shown in detail in Figure 2 . The polarizer comprises an elongate ferrite body in the form of a rod 26 of generally square cross-section. A receiving end portion 28 and a transmitting end portion 30 comprise convergent tips at opposite ends of the body, each with an angle of taper of about 7°. The ferrite rod has a cross-section of approximately 15.21 sg mm and is approximately 4 cm long. It has a coefficient of resistivity m the order of 10 12 ohm cm and a permittivity in the range of 5 to 20. For example, it may be of yttrium aluminium iron garnet.

The ferrite rod 26 is mounted coaxially with the wave guide 18 by means of a plastics bobbin 32; the bobbin forms a housing of a dielectric material such as polypropylene. The bobbin comprises a portion 34 forming a plug which is located in an end portion of the wave guide 18 to mount the ferrite rod 26 in the wave guide. An electrical control coil 36 is wound around the portion 34, within the end portion of the wave guide, the windings being received in an external annular recess of the bobbin. Leads (not shown) from the coil extend along the wave guide to the control electronics 20.

The ferrite rod 26 is largely encased by the plastics bobbin 32 except that at its transmitting end ^'

(including its transmitting end portion 30) the rod projects beyond the plug-forming portion 34 of the bobbin into the wave guide.

At its opposite end, the ferrite rod 26 projects beyond the end of the wave guide, so that the receiving end portion 28 is positioned outside the wave guide. A nose portion 38 of the bobbin 32, which encases the

receiving end portion 28 of the ferrite body 26, is convergent in the direction extending away from the wave guide 18. The housing formed by the plastics bobbin 32 so protects the ferrite body where it projects from the wave guide, but furthermore it can give an improved performance in operation of the polarizer.

The signal handling unit 12 is so secured in relation to the dish reflector 10 that the nose portion 38 of the bobbin and the receiving end portion 28 of the ferrite body 26 are together positioned at the focal point of the dish reflector. Incident plane-polarized microwaves collected at the focal point of the dish reflector 10 so become directed along the length of the ferrite body and into the wave guide 18. The microwaves pass along the wave guide to a receiving element (not shown) associated with the low noise block down converter 22.

In known manner, the polarizer can be operated to isolate plane-polarized microwaves of a selected polarization. The coil 36, when energised, creates an axial magnetic field in relation to the ferrite body 26, and owing to such a magnetic field there occurs a rotation of the plane of polarization of microwaves passing along the body; such rotation occurs in accordance with the well known effect of Faraday rotation. The degree of rotation achieved can be controlled by controlling the electric current passed through the coil 36, and so the strength of the magnetic field. The polarizer is therefore operable to effect controlled rotation of the microwaves passing through, and so microwaves in any particular selected plane of polarization can be brought into suitable

alignment for reception by the receiving element of the converter 22.