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.