Plane Equiangular 4-ARM Spiral Antenna with Conical Reflector Isolated or Fitted into a Structure.

We present an experimental study of the radiation characteristics of a plane equiangular 4-arm spiral antenna with a conical reflector, isolated in the first case. The centre of the spiral is confused with the apex of the cone (with varying angle). A compromise between gain and bandwidth is given. The optimized antenna is then fitted into a cylindrical cavity. When the spiral is on a level with the aperture of the cavity, the antenna is frequency-independant for a 1 : 2.66 bandwidth. When the spiral is included in the cavity, the antenna radiates as a circular aperture for a 1 : 1.71 bandwidth. E¿ and E¿ radiating patterns are equal. The antenna is no more frequency-independant. Isolated or embedded the realized antennas possess circular polarization and very good revolutionnary symmetry. Measurements of the current on the arms of the spiral allow the explanation of all results.