THIS INVENTION relates to an improved antenna. In particular, the invention is directed to an improved offset parabolic reflector antenna suitable for use in the microwave frequency range assignment for "C" Band and "Ku" Band satellite communications.

BACKGROUND ART The use of satellite communication has increased significantly in recent years, and there has been a corresponding increase in the number of earth stations for communicating with such satellites. These earth stations normally comprise an antenna having a parabolic or paraboloidal reflector dish and a receiver/feed at the focus of the reflector. Many earth stations are now dedicated to a single end consumer, and must therefore be of relatively simple and economic construction.

The paraboloidal reflectors used in known earth stations are usually of integral or one-piece construction, being made of metal or alternatively, plastic or fibreglass with a metallic reflective layer. As such reflectors may be 2 to 3 metres in diameter, they are difficult and cumbersome to handle and transport. In order to overcome these difficulties, composite reflector dishes have been developed. Such known composite reflectors typically are assembled from two halves to form a paraboloidal reflective surface. The reflector halves however, are still relatively cumbersome to

handle and transport.

The majority of paraboloidal reflector antennas are symmetrical antennas. However, offset antennas, in which the antenna feed is. located to the side of the projected beam of the reflector rather than in front of it, are sometimes used. These antennas have an advantage over symmetrical antennas in that blocking of the antenna aperture by the antenna feed is avoided. (Similar considerations apply for reception) . Elimination of such blockage results in better control of unwanted sidelobes when transmitting or receiving. However, the asymmetry so formed can degrade the cross polarisation performance accordingly.

Known paraboloidal reflector antennas are normally designed to operate at a particular frequency or within a particular band of frequencies, for example the satellite C- band or Ku-band (C-band is normally understood to be between 3.65 and 6.5 GHz, while Ku-band is normally understood to be between 10.5 and 14.5 GHz).

Since the known antennas are designed to operate in a single band, they are often limited to communicating with a single satellite, or a limited number of satellites. Cons quently, when access to alternate satellite communications is desired, it is necessary to install a different reflector dish as well as a different feed/receiver. Although multi-band antennas are known, such antennas are usually of complex and consequently expensive

construction and hence unsuitable for most single consumer earth stations .

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved offset paraboloidal reflector antenna of versatile performance yet of compact portability and of relatively simple and inexpensive construction.

In one broad form, the present invention provides an antenna operable in at least two microwave frequency bands comprising: a reflector of paraboloidal shape; and support means connected to said reflector for mounting an antenna feed and/or receiver so that the antenna feed and/or receiver aperture is located offset relative to the aperture of the antenna reflector; characterised in that the ratio F/D is within the range of about 1 to about 1.12 and the ratio d/D is within the range of about 0.052 to about 0.11, where F is the focal length of the reflector, D is the diameter of the projected beam of the reflector and d is the displacement of the projected beam edge referenced to the focal plane of the axisy metric parent paraboloid of the reflector. (See Fig. 5).

For prime focus fed offset parabolic antennas, the minimisation of F/D is desirable for economic and practical reasons, but on the contrary is undesirable on the grounds

that the axial symmetry is improved with increasing F/D ratio. The minimisation of d/D is desirable for the purpose, of minimising the order of beam asymmetry, but undesirable on the basis that increasing d/D allows feed horns to be accommodated without obstructing the projected aperture. The specific design configuration of the offset parabolic reflector antenna of this invention results in an antenna which is suitable for satellite communication in a number of bands, such as the C and Ku bands, yet is of simple and economical construction.

In the preferred embodiment, F/D is about 1 while ^' d/D is about 0.052.

Suitably, the reflector is of composite construction, being assembled from four quadrant portions. The reflector quadrants are preferably constructed of moulded plastics material, provided with a thin conductive layer. Surface distortion should be less than 0.25mm r.m.s. The four quadrants can be dismantled and packaged in a relatively small container for ease of handling and transport. In the preferred embodiment, each of the four quadrants of the reflector has a flange along each radial edge, the flanges of adjacent quadrants being in abutment when the quadrants are assembled, and held together by a U-shaped rib member to form the composite reflector. Advantageously, complementary protrusions and recesses are provided in the mating faces of the edge flanges.

In the preferred embodiment, the projected diameter of the antenna is 2.6 metres, which is also equal to the focal length. This antenna is suitable for transmitting and receiving operations in both the C-band and Ku-band. In use, the antenna is suitably fed by low flare angle horn feeds, typically corrugated horns, having a flare semi-angle of about 10 degrees, appropriate for the band of application.

The combination of versatile performance over a number of frequency bands, and compact portability when dissembled, give the antenna of this invention useful application in interactive communications with satellites yet the antenna can be erected on site at consumer level. Moreover, the simple construction of the antenna reduces the cost thereof and renders it suitable for single user earth stations.

In order that the invention may be more clearly understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view from the front of the antenna of the preferred embodiment of the invention,

Fig. 2 is a side elevation view of the antenna of Fig. 1,

Fig. 3 is a perspective view from the rear of the

antenna of Fig. 1 ,

Fig. 4 is an exploded view of enlarged detail, of- abutting flanges of the reflector of Fig. 1, and

Fig. 5 is a schematic diagram illustrating configuration parameters of the antenna of Fig. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

As shown in the drawings, the antenna of the preferred embodiment comprises a reflector 10 of paraboloidal shape. The reflector 10 is of composite construction, being assembled from four different, ribbed reflector quadrants,

10A, 10B, 10C, 10D. The reflector quadrants 10A, 10B, 10C,

10D are provided with flanges 20A, 20B, 20C, 20D along their radial edges, and are clamped together at their abutting flanges by steel U-shaped ribs 16 and threaded fasteners 17. Advantageously, the mating faces of the abutting flanges are provided with complementary slots 19 and protrusions 18 in order to ensure correct alignment of the quadrants, and to prevent relative movement thereof. The reflector quadrant panels can be shipped in a package of small size, and assembled on site without the need for formal realignment as the jointing process minimises the possibility of assembly errors.

The steel ribs 16 form the main support structure for the reflector 10. The support structure of the assembled reflector is pivotally mounted at 11 on a mounting pole 12

(Fig. 3). An adjustable link 15 is connected between a rib

16 and the mounting post 12 to vary the elevation of the antenna, as can be seen more clearly in Fig. 2.

Preferably, the reflector 10 is of pressed SMC (sheet moulding compound) moulded at temperatures in the order of 150 degrees C and at pressures in the order of 1500 psi. However, it will be apparent to those skilled in the art that any suitable plastics material, including fibreglass, may be used. The constituents of the SMC suitably incorporate doping to minimise the effects of distortion of the required quadrant profile. Such distortion is minimised in this process to a root mean square surface accuracy of 0.25mm. The four dissimilar quadrant segments are typically manufactured separately in four discreet and dedicated moulds. If reinforced plastic is used as the material for the reflector, the weight of a 2.6-2.7 metre reflector plus backing structure is approximately 12-0 Kg. Such a reflector has a wind survivability of 67 metres per second.

An antenna feed and/or receiver 13, as appropriate to its application, is mounted on an arm 14 which is connected to the steel backing ribs 16 of the antenna. Hereafter, for simplicity, item 13 will be referred to as an antenna feed. However, it will be apparent to those skilled in the art that it may in use be an antenna feed and/or receiver, as required.

The antenna feed 13 is mounted offset relative to

the aperture of the reflector 10. The feed support arm 14 is designed to position the antenna feed 13 so as to locate the feed component phase centre at the precise focal point. However, some adjustment potential is preferably included in the design to allow minimisation of crosspolar interference and optimisation of focal position.

The antenna is modelled using conventional techniques associated with offset paraboloidal antennas. However, the principal parameters of focal length and angle of offset of the feed axis with respect to the parent paraboloid have been chosen to allow the antenna to be fed from low flare angle horns which do not obstruct the reflected wavefront and that also allow clearance of a minimum of a half wavelength to thereby minimise horn diffraction at the lower frequency constraint of the C- band.

The focal length (F) to projected .diameter (D) ratio is nominated as approximately unity, and the ratio of vertical displacement of projected aperture (d) -to projected diameter (D) is correspondingly about 0.052 (See Fig. 5).

However, by using low flare angle horns, performance compliance would be possible at least within the appropriate range of 1<F/D <1.12, and correspondingly 0.052 <d/D <0.11.

The projected diameter of the antenna reflector of the preferred embodiment is 2.6m. This diameter has been chosen after evaluation of mean satellite operating

parameters employed over the wider surface areas worldwide and with particular consideration to use in the thinner route applications of voice and data. The antenna can be applied affordably in the TVRO arena for applications for Ku-band broadcast fringe and lower power video information and entertainment services. The antenna is particularly suitable for earth stations in relatively underserved areas having low levels of technical expertise and limited financial resources . The antenna is profiled to be fed, in use, from corrugated horns having a small flare semi-angle of about 10 degrees in order that the focal length can be retained within practical limits while maintaining the crosspolar performance within internationally acceptable levels. The use of small angle horns enables the offset angle to be minimised while still establishing the required removal of aperture blockage. Using low flare angle horns and illumination tapers of lOdB, an aperture efficiency minimum of 70% can be achieved. Polarisation is adjustable in the feed plane by feed rotation. Crosspolar discrimination exceeds 35dB on boresight assuming that the horns used provide no further degradation.

The technical result of this design selection is a versatile reflector antenna that can be used with appropriate feed horns to provide transmit and receive communications capability yet complying with the international standards of

CCIR 580-1 with due regard for the performance of satellite earth station antennas after 1991.

That is, the unique combination of (1) offset feed position (2) versatility of performance, i.e. compliance with the CCIR recommendation 580-1 at both C-band and Ku-band, by virtue of the following

(a) selection of focal length to projected diameter ratio (F/D) and selection of displacement of the projected beam aperture to projected diameter ratio (d/D) .

(b) minimisation of surface errors of the reflector to a root mean square of less than 0.25mm. - (3) compact portability due to the dismantable composite construction, results in a novel antenna with optimum design features.

The foregoing describes only one embodiment of the invention and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention. For example, the antenna feed system shown in the drawings is given by way of example only, and other feed and prime focus mounted devices may alternatively be used.