´Soft´ surface linear array feed for a frequency scanned antenna

The antenna for a 3-D pulsed radar system described comprises a parabolic cylinder reflector with a linear array feed at its focus. The feed takes the form of a corrugated waveguide slow-wave structure with offset crossed-slot radiators in its broadface. A comprehensive analysis of this arrangement established that by correctly locating these X-slots relative to the center line of the slow waveguide, a circularly polarized radiated wave is obtainable in the broadside direction. The reactive surface formed by the choked groove structure provides a surface which is ´soft´ to the horizontally polarized radiation emitted by the X-slot, while the exposed edges of the metallic fins behave as a soft ground plane for the vertically polarized radiated wave. In principle, if the surfaces are equally soft, the beamwidths for both radiation components would be harmonized. The fact that this is not achieved in practice can be attributed partly to the limited width of the soft surface, and to the fact that the surface is separated from the slot by a short parallel plate region. Ideally, a soft surface into which the slot is embedded is required.<>