Sea waves or a tidal current can cause unstable connectivity attitude between antennas in underwater radio frequency communication systems such as underwater wireless sensor networks. To compensate such instability and maximize the coverage area of communication systems, we propose a circularly polarized underwater antenna with wideband characteristics. This is realized through two vertically crossed half-sheath dipole antennas (HSDAs) and a phase shifter consisting of two inductors and two capacitors. They achieve wideband of over 50 % in the fractional bandwidth despite their inductors and capacitors. Down-scaled experiments using a water tank at 400 MHz verify that the proposed antenna performs as a circularly polarized omni-directional antenna with an axial ratio of less than 0.5 dB and can maximize the coverage area of underwater wireless sensor networks.
