CYLINDRICAL AND CONICAL SHROUDS FOR THE REDUCTION OF GROUND NOISE OF PARABOLOIDAL ANTENNAS

The spillover noise of a reflector antenna can be reduced by attaching a shroud extending forward from the edge of the reflector; the shroud prevents ground radiation from entering the feed. Symmetrical parabol oidal antennas of diameter 40 wavelengths, equipped with cylindrical a nd conical shrouds, are analyzed using the method of moments. A cylind rical shroud, parallel to the reflector axis, may reduce antenna noise , but it raises the sidelobe level in the front hemisphere substantial ly and can also reduce antenna gain. These drawbacks can be overcome b y using a conical shroud, flared outward. Such a shroud reduces the sp illover lobes in the back hemisphere, thus lowering the antenna noise temperature, but generates a conical sidelobe in the front hemisphere. The peak level of this sidelobe can be reduced by building the shroud using two cones of different flare angles or by curving its cross sec tion. The decrease in noise temperature, as well as the location and l evel of the conical sidelobes in the front hemisphere, can be predicte d to useful accuracy using geometrical optics. The addition of a shrou d increases the level of cross polarization near the main beam. Howeve r, this effect is reduced if the sharp corner where the shroud joins t he reflector rim is replaced by a smooth transition. The level of cros s polarization is then at a level comparable to that produced by scatt ering from feed-support struts.