We investigate the propagation of radio waves underwater and between water and air to facilitate setting up hybrid wireless sensor networks with both surface and subsurface nodes. Our investigation includes signal attenuation, antenna radiation patterns, multipath due to reflections from the surface and substrate, noise, and reflection losses transmitting from one medium to another.
Researchers are working on using freshwater mussels as biological sensors. A sensor placed on the mussel detects the mussel's rhythmic opening and closing, or gape. Changes in the gape can indicate changes in the mussel's environment. We plan to attach gape sensors, microcontrollers, and radios to mussels and place them back in their natural environment. Small, inexpensive radios operating in the Industrial, Scientific and Medical (ISM) bands will provide the physical link of an underwater wireless sensor network (WSN). Despite the attenuation radio waves experience in water, the low cost of these radios should allow us to deploy enough to set up a reliable communications network. While commercially available radios can be used underwater with waterproofing, antennas designed for use in air are unsuitable for use in water, because of the different electromagnetic properties of water and air. We designed dipole, loop, and folded dipole antennas for use in water and attached these to transmitters. We measured the power transmitted by the antennas by immersing the transmitters in a tank of water and measuring the received power at different distances using a small dipole antenna attached to a spectrum analyzer. The distance between the antennas was precisely controlled with a motorized xy positioner.
Figure 1. Researchers atThe University of Iowa are creating underwater biological sensor networks, where mussels form the nodes in the network. In this paper, we focus on effective antennas for the ISM radios that provide the physical link between the nodes.
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