Design and evaluation of a directional antenna for ocean buoys

Abstract

A system concept has been developed by Viasat, Inc. and Woods Hole Oceanographic Institution for improving the data telemetry bandwidth available on ocean buoys. This concept utilizes existing communications satellites as data relay stations and mechanically steered antenna arrays to achieve increased data rates and improved power efficiency needed for ocean applications. This report describes an initial feasibility and design study to determine if a mechanically steered antenna array can meet the requirements of open ocean buoy applications. To meet the system requirements, an 18-element microstrip antenna (9-element transmit, 9-element receive) was designed and fabricated under subcontract by Seavey Engineering Associates, Inc. It operates in the 4-6GHz frequency band (C-band) and provides 14 dB of gain. The 1/2 power beamwidth is +-t5° in azimuth and elevation. This antenna design, in conjunction with a simple rotating mount, was used to evaluate the potential of this approach to keep a geostationary satellite in view when mounted on an ocean buoy. The evaluation is based on laboratory measurements using a magnetic compass and a small stepper motor to maintain antenna orientation while the complete assembly was rotated and tilted at speeds similar to what would be expected on an offshore buoy equipped with a stabilizing wind vane. The results are promising, but less than conclusive because of limitations in the experimental test setup. The recent introduction of several commercially available mechanically steered antennas designed for use on small boats may provide a viable alternative to the approach described here with appropriate modification to operate at C-band.