Wideband Endfire CP Log-Periodic Array Based on Compact Broadband OCP Water Helical Resonator

This paper presents a wideband circularly polarized (CP) log-periodic antenna with endfire radiation based on normal-mode water helical resonator with omnidirectional circular polarization (OCP). The water resonator produces omnidirectional radiation of the horizontal and vertical electric field components, which are generated by the loop and short dipole components of the helix. There is a 90° phase difference between the two components, resulting in OCP radiation when the amplitudes of the two components are equal by simply adjusting the screw pitch of the helix. Owing to the larger aperture of a λ/2 water helical resonator compared to a λ/2 metallic helical resonator, a significantly improved radiation resistance of 48 Ω is achieved for the water helix (3.3 Ω for metallic helix). It is good for a wide impedance matching of the final array. The helical array is composed of 10 compact water helical elements obeying the logarithmic scaling of the operating frequency. A differential coupling-type feed line is employed to feed the array. Measured results show that a matching bandwidth from 635 to 1427 MHz (76.8%), an axial ratio bandwidth from 660 to 1275 MHz (63.6%), and a peak realized gain of 5.7 dBi are obtained. The proposed antenna has the potential to achieve a wider bandwidth by adding more elements, and can be applied to underwater communication or military communication with the advantage of high transparency.