S-shaped planar antenna with 45° tilted bidirectional radiation pattern for motorcycle helmets

An S-shaped planar antenna (SPA) with a bidirectional radiation pattern and beam tilt characteristic is proposed to achieve maximum communication distance for helmet applications. The proposed SPA is comprised of an S-shaped radiation strip (consisting of a microstrip meander line and two main arms with two inverted L-shaped parasitic elements) and a rectangular ground plane, where a simplified microstrip power divider is introduced by modifying the feed structure in the center of the SPA, such that the S-shaped radiation strip works in the second resonant mode. The proposed SPA generates a tilted beam in the E-plane with an angle of 45°, which is attributed to the obliquely staggered arms of the antenna at a distance. By introducing inverted L-shaped parasitic elements at the end of the two arms, the directivity of the bidirectional radiation pattern can be further improved, thereby increasing the antenna gain. The working principle is analyzed theoretically, and the effects of the antenna structural parameters on the radiation pattern are also analyzed. The experimental results indicate that the 3:1 VSWR impedance bandwidth is 120 MHz, with a realized peak gain of 1.5 dBi at 2.45 GHz. The proposed antenna is designed for real-world applications, allowing the antenna to be obliquely installed while keeping the peak gain direction horizontal for maximum communication distance.