A slot-array antenna on a coaxial cylinder with a circularly polarized conical beam

As a vehicle-mounted antenna to be used for satellite communication and broadcasting without the need for capture and tracking, a circularly polarized conical beam with a tilt angle of 45 degrees and a beamwidth of 18 degrees is realized with a slot-array antenna on a coaxial cylinder. In the direction normal to the array axis, a circular polarization can in general be realized by using a pair of slots inclined by +/-45 degrees spaced by lambda(g)/4 (where lambda(g) is the effective guide wavelength including the effects of the slots cut on the external conductor) as a unit element. Since the projections of the horizontal and vertical components of the slot are not equal in the tilt direction, the axial ratio is degraded. Hence, a theoretical and experimental investigation is carried out for a method for improving the axial ratio by adjustment of the spacing of the slots comprising the pair and of the leaning angle of the slot. On amplitude and phase of the aperture distribution, they can be adjusted by the size and location of the slots. By shifting the slot, the optical path length in the tilt direction varies so that the antenna pattern and the axial ratio can be degraded. The correction of the slot shift is considered by taking into account the optical path length change. (C) 1999 Scripta Technica, Electron Comm Jpn.