Introduction

Circularly Polarized (CP) antennas can mitigate the multipath interference, propose easier alignment for transmitting and receiving antennas as polarization matching, overcome the phasing issues, and resist the weather conditions. An omnidirectional CP radiation will result in 360° coverage desired for multipurpose wireless communications, which is relatively insensitive to transmitter and receiver orientations. Various omnidirectional CP antennas with conical radiation patterns have been designed over the past years, such as dielectric resonator antenna (DRA) [1], based on tilted dipoles [2], tightly coupled array mechanism [3], multiple-input multiple-output (MIMO) form [4], planar metamaterial structures [5],[6], and fractal formation [7]. Utilizing two electric and magnetic dipoles is a prevalent way to realize orthogonal quadrature fields leading an omnidirectional CP radiation [1],[7]. In order to realize a horizontally polarized magnetic dipole, Alford and Kandoian proposed a loop antenna in the 1950s. Different omnidirectional CP antennas based on the Alford’s loop have been proposed in [8],[9].
The use of omnidirectional CP antennas has the advantage that no specific orientations between the transmitter and the load terminal are required for operation. Hence, it can be expected that omnidirectional CP antennas should find great applications for the WLAN band. For such promising applications, a novel omnidirectional CP antenna with a transparent multi-purpose structure is proposed here. In this letter, a robust omnidirectional CP antenna for outdoor wireless communication at industrial, scientific and medical (ISM) 2.45 GHz band is presented. The design is developed based on the Alford loop (magnetic dipole) and the electric dipole antennas. Omnidirectional CP, relatively high bandwidth, considering harsh surrounding environments, and multidisciplinary structure for energy harvesting capabilities are the main findings of the reported design. The design has a multifunctional purpose that simultaneously functions as an antenna and exploits a possible feature of implementing the solar cell due to the use of transparent material. In the following, the antenna design and its performance are discussed.