title: Low-Profile Floating Antennas for Coastal Communications
Creators: name: Loni, Zia M.
Publisher: name: Griffith University
Year: 2019
Subjects: subject: Antenna
subject: Radio communication
subject: Seawater environment
subject: Floating buoy
subject: Coastal communications

description: The size, orientation and features of an antenna are critically important in sea surface radio communications. The performance of the communication link is dependent on antenna height, radiation pattern and antenna-seawater interaction. This research focusses on the design and development of a short-range radio communication system with lowprofile, low cost antennas that ensure reliable data transmission in a shallow seawater environment. Given the high conductivity and relative permittivity of seawater, the system used the seawater as a conducting ground plane of infinite extent. This thesis reports the system challenges resulting from a non-planar sea surface and an imperfect ground conductivity. Given the high propagation loss through seawater at Ultra High Frequencies (UHF), the approach undertaken was a hybrid architecture consisting of a subsurface transmitter close to the ocean bottom (depth &lt; 5 m) with a transmission line to the surface antenna, or a tethered but floating transmitter with an integrated antenna. Different techniques aimed at establishing communication links between the antenna and shore, another antenna and an Unmanned Air Vehicle (UAV) hovering above it, are reported. In all cases, the antenna moves laterally and rotates in all three axes with the wind and ocean currents, and vertically with waves and tides. In the first technique, a communications link from a subsurface transmitter to a floating monopole antenna through an insulated wire surrounded by seawater was designed, constructed and tested. The attenuation is dependent on the wire and insulation thickness and this influences the characteristic impedance of the transmission line. A quarter wave monopole antenna was formed by placing the wire through a spherical polystyrene float with a length of approximately one quarter wavelength. The attenuation through the insulated wire was approximately 38 dB/m. The vertically polarized radiation is omnidirectional in the plane of the water surface. Using a 10 dBm transmitter and a 1.5 m long subsurface transmission line, the maximum horizontal communications distance was less than 5 m between floating transceivers and up to 35 m to a slightly elevated shore receiver when the receiver sensitivity was -116 dBm. The packet loss for a 30m separation between transceivers was 1.73%. When the insulated wire was replaced by a flexible coaxial cable, the transmission line attenuation and the propagation range across the surface to a slightly elevated receiver, extended to more than 140 m in light wind conditions. The second technique was to use a floating transmitter with an integrated antenna. The antenna with the transceiver was located in a sealed 9 cm diameter Polyvinyl Chloride (PVC) pipe with a flexible rope tethered to the ocean floor. Two antenna designs were constructed and tested: a monopole and a modified spiral (hemispherical). The latter was assessed using a small conductive ground plane together with the sea surface. This antenna was optimized using Computer Simulation Technology (CST) and fabricated using 3D printing and vacuum forming. While the hemispherical antenna radiation beam is principally vertical, the horizontal propagation distance was measured as greater than 135 m for the receiver sensitivity of -116 dBm. The final part of the research was the design and construction of a real-world prototype (floating buoy). Vacuum forming and 3D printing techniques were used to construct a waterproof enclosure to contain both the hemispherical antenna and the electronic circuitry. These techniques give a low-cost solution and the buoy can be modified to integrate ocean sensing elements. The buoy can be opened, and batteries can easily be replaced to increase the longevity of the transmitter. Due to the transparent structure of the buoy, the transceiver circuit could be partly powered with solar energy. The longest transmission distance across the ocean surface was greater than 140 m using a low-profile hemispherical antenna integrated into the plastic enclosure containing the electronics given a transmitter power of +10 dBm and a receiver sensitivity of -116 dBm. The unit had a diameter of 20 cm and a height of less than 15 cm.