An Ultracompact Omnidirectional Circularly Polarized Antenna

An ultracompact omnidirectional circularly polarized (OCP) antenna is presented in this paper. An innovative antenna configuration has been adopted that consists of a full-wavelength folded helical dipole radiator and a closely placed half-wavelength parasitic helical arm. Compared with the conventional single-arm half-wavelength helical OCP antenna, the current density has been significantly enhanced and thus the radiation resistance of this design has been improved substantially from 10 to 43 Ohms. As a result, a good impedance matching is realized. Moreover, very compact physical size has been maintained, i. e., only being
$\pi(0.044 \lambda_{0})^{2}\times 0.088 \lambda_{0}= 0.0005 \lambda_{0}^{3} (\lambda_{0}$
is the free-space wavelength at the center frequency). A prototype operating at 915 MHz ISM band has been designed. Simulated results demonstrate that the antenna has a good impedance matching around the targeted 915 MHz and achieves an excellent OCP radiation performance, e. g., the out-of-roundness value of the co-pol radiation pattern is less than 0.2 dB. It is an ideal candidate for IoT applications that requires very compact OCP antenna system.