Measurements of underwater electromagnetic wave propagation

Acoustic wave is major medium for underwater communications, positioning, and telemetry. We have no alternative to acoustic wave in long range applications in spite of a few drawbacks which are multipath fading, narrow bandwidth, and susceptibility to propagation characteristics from back ground media. Applications of underwater electromagnetic wave (EMW) in range of hundreds meters have been studied for half a century but now some short range low frequency underwater modems have only been commercialized. If high frequency EMW can propagate through sea-water with low damping, it would compensate for shortcomings of acoustic wave. The previous conference we reported some amazing results that attenuation of electromagnetic wave measured in sea-water would be smaller than that of theoretically estimated from Maxwell's equations assumed an isotropic and homogenous medium. One of this result was obtained by using a Faraday cage filled by tap water and sea water in frequency range around 40 MHz. To verify these results we have developed new small loop antennae which has buffer layer made from tap water. We carried out a test measurement with the antennae at a pier. The obtained results were also hard to understand but there might be measurement errors. In November 2014, we started to design of a new complete measurement system for underwater EMW propagation utilizing channel sounding technique to avoid measurement errors. In the conference, the design concept of the new measurement system will be presented in addition to some test results obtained with the new antennae.