Design of Low Profile Broadband Planar Spiral Antenna

This paper presents a low profile broadband planar spiral antenna fed by micro-strip exponentially gradual Balun. By studying the main factors affecting the bandwidth and profile of Archimedes helical antenna, the planar spiral radiator, micro-strip feed Balun and reflective back cavity are designed. A planar spiral antenna with good circular polarization in the frequency band from 2 to 6 GHz is designed by adding absorbing material to the reflecting back cavity to absorb the back radiation. In order to obtain the unidirectional radiation characteristics, a metal back cavity is loaded on one side of the antenna which is filled with absorbing material. The center feed is improved and the energy reflection at the feed point is reduced as much as possible by using the form of multi-section curve to have a smooth transition. The Balun length is reduced by adjusting the radiator impedance, thereby reducing the antenna profile thickness. The structural size of the broadband planar spiral antenna is 48 mm × 18.75 mm. The simulated results show that the Voltage Standing Wave Ratio (VSWR) is smaller than 2.5 and the Axial Ratio (AR) is less than 3 dB in the 2-6 GHz frequency range. The antenna has good broadband circular polarization in the bandwidth range and can be used in Sub-6G communication system.