EurOCEAN 2000 - Vlaams Instituut voor de Zee

METHODOLOGY
The main activity of the research was to measure the propagation of EM waves through sea
water over the frequency range 1MHz to 30GHz with special preference being given to the
frequency range 1MHz to 3GHz because of the scarcity of experimental information. The
parameters to be measured was the EM wave velocity (ν m/s) and the EM wave attenuation
coefficient (α dB/m). A laboratory EM cavity, has been initially used for undertaking this
investigation. Measurements were firstly undertaken using a rectangular cavity and filled with
simulated sea water. This investigation was followed by further measurements using antennae
within a laboratory test tank. These experimental results have been rigorously compared with
EM wave theory for both the conduction zone and for the dielectric zone in order to understand
the transition between the zones and to provide satisfactory models for EM wave propagation.
A second series of experiments have been undertaken using transmitting and receiving aerials
within both a swimming pool and a dock complex.
Based upon the results, two application activities will be undertaken in the immediate future
namely:-
• A selection of the best operational frequency for short range transmission between 30m and
50m of the EM wave for use with imaging, communication, positioning and object tracking
systems. A demonstrator system will be produced.
• The best operational frequency range for using the EM cavity to measure such parameters
as σ, ε I , ε II , vσ and vε and thereby investigate the effect of pollutants and turbidity around an
outfall from sewerage and industrial waste water. A demonstrator system will be produced.
RESULTS
The test cavity was made from rectangular copper waveguide WG9A having a perspex lining
6mm thick. The internal dimensions of the cavity were 74mm x 31mm x 288mm. The perspex
had a dielectric constant of 3.18 with a ratio ε 11 /ε 1 of 0.0004 to 0.00080. The cavity used loop
antennae, one at each end wall to act as the transmitter and receiver.
The results over the frequency range 1 to 1000MHz are given in figures 2 and have shown that
propagation is possible in the presence of sea water at all frequencies over the entire range.
The signal strength in distilled water is of the order of 226mV whilst in sea water it drops to
23mV when σ varies between 1 S/m and 4 S/m but increases to 40mV when σ = 10 S/m.
Laboratory Tank Result
The laboratory tank has dimensions 2m(w) x 1.5m(h) x 2.5m(l) and was constructed from PVC
sheet within a welded metal frame. The tank was filled with either tap water or artificial sea
water having conductivity whose magnitude varies from 0 to 4S/m. Both loop and dipole
antennae were investigated with each transmitter and receiver being built upon a stand-alone
box. Each PVC box was electrically screened and the transmitter (Tx) box contained battery
operated electronics. Each antennae was fully immersed in the sea water in order to ensure that
the transmission path was entirely through the water.
527