GB1394334A - Cross-polarized parabolic antenna - Google Patents

Abstract

1394334 Aerials ANDREW CORP 9 March 1973 [24 March 1972] 11401/73 Heading H4A In a waveguide system for propagating crosspolarized waves comprising a circular waveguide and a pair of rectangular waveguides connected thereto, the central H-planes of both rectangular waveguides are aligned in a common radial plane at their points of connection to the circular waveguide. A 90 degrees polarization rotator is provided in the circular waveguide between said points of connection and comprises passive conductive means fixed in relation to said waveguide. As described, an aerial comprises a parabolic reflector or dish (Fig. 1, not shown) and a feed assembly 12, Fig. 2, consisting of a feed 14 having a radiating aperture at the focus of the parabola, and a pair of waveguides 16, 18 which support the feed and convey respective signals thereto. The waveguides extend through a vertex plate 20 at their inner ends and terminate in coupling flanges 22. They are formed with bends generally similar to conventional button-hook shaping but with their outer ends connecting transversely to the feed. Throughout their lengths their median H-planes are aligned in a common radial plane. The outer short side of each waveguide is covered with absorbent material. The feed 14 has a cylindrical body 28, Fig. 3, which is closed by a shorting cap 30 at its outer end and which has a radiating horn assembly 32 at its inner end. The horn mouth 34 is surrounded by a set-back choke and reflector 36 to improve the uniformity of illumination over the parabolic reflector. The polarization rotator between the waveguides 16, 18 is formed by a series of fixed closely spaced diametric conductive pins. A first pin 50 is inwardly adjacent to the waveguide 18, and is orthogonal to the polarization direction in that waveguide. Succeeding pins 52 are longitudinally spaced at small intervals and extend across the body 28 at successive small progressive angles until a pin 54, which is perpendicular to the pin 50. Further pins 56 are parallel to the pin 54. There is thus provided a polarization rotator which twists the plane of polarization of the signal from the waveguide 18 relative to that of the signal from the waveguide 16. The pins 56 serve to stabilize the rotation. There are also provided pins 42, 44 which are parallel to the pins 56, and which are located off-centre and away from the horn mouth 34 in relation to the respective waveguides 18, 16. The pins 42, 44 direct radiation towards the horn mouth 34. Tuning screws 58 are provided for optimization of performance. As an alternative to using pins in the polarization rotator a continuous vane or foil may be provided.