Near sea-surface line-of-sight (LoS) radiowave
propagation at 5 GHz was investigated through narrowband
measurements in this paper. Results of the received signal
strength with a transmission distance of up to 10 km
were examined against free space loss model and 2-ray path
loss model. The experimental results have good agreement
with the predicted values using the 2-ray model. However,
the prediction ability of 2-ray model becomes poor when
the propagation distance increases. Our results and analysis
show that an evaporation duct layer exists and therefore,
a 3-ray path loss model, taking into consideration both
the reflection from sea surface and the refraction caused by
evaporation duct could predict well the trend of LoS signal
strength variations at relatively large propagation distances