Focusing properties of fresnel zone-plate and its application to a helix radiating a circularly polarized wave

This paper discusses applications of the Fresnel zone-plate (FZP) made of metal foil to microwave antennas. First, the focusing effect and the far-field pattern for the FZP are analyzed using the Kirchhoff-Huygens diffraction integral. It is found that a phase-reversing FZP, in which the phase of either its even or odd zones is inverted, has a sharp beam with a low sidelobe level. On the basis of the fundamental analysis, an antenna syctem in which a reflector-type FZP fed by a backfire helix is proposed as an application of the phase-reversing FZP. It is pointed out that a sharp circularly polarized beam with a half-power beam-width of 9 deg is obtained when the outermost radius of the FZP is about 4 wavelengths. Next, the gain variation by the change in the ground plate radius of the axial-model helical antenna is explained by the Fresnel zone principle, and an axial-mode helical antenna with a zoned ground plate is proposed. An absolute gain of 12.2 dB is realized by a two-turn short helix with a zoned ground plate.