Spiral antenna above a composite HIS reflector

A spiral antenna radiates a circularly polarized (CP) wave in the two directions normal to the spiral plane (bidirectional radiation) [1]. For more effective utilization of the input power, this bidirectional radiation is transformed into unidirectional radiation using a conducting plane reflector [2]. Usually, the antenna height above the conducting plane is set to be one-quarter wavelength of the design frequency, to obtain an in-phase condition between the wave directly radiated into free space from the spiral and the wave reflected from the conducting plane. When installation space limitations require that the antenna height be less than one-quarter wavelength, an absorbing strip is often placed under the outermost spiral arms [3][4] to allow the height to be reduced. This placement of the absorbing strip contributes to mitigating the deterioration in the antenna characteristics observed when a small antenna height is used. By virtue of the presence of the absorbing strip, the spiral exhibits wideband characteristics with respect to the input impedance, axial ratio, and gain [1]. However, the use of the absorbing strip reduces the radiation efficiency; as the frequency decreases, the radiation efficiency becomes smaller [5]. This decrease in the radiation efficiency is a concern for those who design low-profile spiral antennas. This paper presents a new solution to this concern, where a reflector having a high impedance surface (HIS) is adopted. The radiation characteristics of a spiral with a uniform HIS reflector (where same-sized patches are used) are discussed. Subsequently, the radiation characteristics of a spiral with a composite HIS reflector (where two sizes of patches are used) are revealed. This composite HIS reflector clearly differs from the HIS reflectors in [6][7].