A low-profile two-arm Archimedean spiral antenna radiating a circularly polarized normal beam or conical beam

A two-arm Archimedean spiral antenna (TAS) is proposed to provide a circularly polarized (CP) normal beam or conical beam. The antenna is fed by coaxial lines without a balun circuit. When it is excited out of phase, a CPnormal beam which is almost symmetric with respect to the antenna axis is produced. While excited in phase, it radiates a CP conical beam. The antenna height is chosen to be extremely small to realize a low-profile antenna. The simulated results show that the antenna has a good VSWR and a good axial ratio over the frequency range of 1.8GHz to 2.1GHz.

I. Introduction

A wide bandwidth and radiation pattern diversity make spiral antennas good candidates for mobile and satellite communication antennas [1]–[5]. A two-arm spiral antenna can radiate either a CP normal beam or a CP conical beam according to its feed phase relationships [5]. When excited out of phase, its maximum radiation is on or near the antenna axis over a wide frequency range [1]–[3]. A circularly polarized conical beam can be obtained when the spiral arms are excited in phase [4]–[6]. The conical beam has a null in the zenith direction and omnidirectional pattern with respect to the spiral antenna plane [4]. Therefore, this type of beam is suitable not only for the mobile and satellite communications but also for the wireless LAN of indoor communication [5].