An axially elevated spiral antenna for a high directive CP radiation

Spiral antennas[1~4] have good characteristics of broad bandwidth and circular polarization in operations. Regular spiral antenna structures radiate in both directions of upward and downward to the spiral plane with relatively broad patterns[2]. Thus in practical applications, the spiral antenna is placed on the top of a cavity filled with absorber materials and tends to loss a half of the power. To improve the efficiency and also make the radiation more directive, a broadband antenna design that extends the concepts of designing spiral antenna arms, is proposed in this paper for the potential applications of antenna measurements. It may achieve a broad frequency bandwidth of operation and radiates fields with a circular polarization as usually seen in the regular spiral structure. To enhance the directivity, the antenna structure is axially elevated on its spiral arms such that the fields tend to be radiated in the upward direction with a beamwidth narrower than that radiated from an analogous spiral antenna, which is found to be suitable for the applications of antenna measurement inside an anechoic chamber.