EurOCEAN 2000 - Vlaams Instituut voor de Zee

Figure 2 Spectral response for waveguide filled with 4 S/m conductivity
528
0
-10
-20
-30
dBm
-40
-50
-60
-70
TapWater
SaltWater (1S/m)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Distance in m
Figure 3 Dipole antenna (0.85m length), with Tx frequency at 5MHz
10
0
-10
-20
dBm
-30
-40
-50
-60
-70
0 0.2 0.4 0.6 0.8 1 1.2
Distance in m
TapW ater
SaltWater (1S/m)
Figure 4 Loop antenna (0.35m diam), with Tx frequency at 5MHz
The result for 0.85m dipole antennae is given in figure 3 for a frequency of 5MHz and having a
sea water conductivity of 1S/m at 20°C. The results have been compared with measurements
taken in tap water. The results in both cases clearly show a near and far field having similar
rates of attenuation. The strength of the EM wave electric field is about –15dB lower than in
tap water and this effect was also observed in the results of the cavity experiments.
The results for the loop antennae are given in figure 4 are also for a frequency of 5MHz. The
loop diameter is 0.35m. For this configuration in which the loop antennae are parallel there is
less evidence of the existence of a long range field and hence these conditions can only be used
for shorter propagation distances, unless the frequency is reduced.
CONCLUSION
The results obtained so far clearly indicate that it is possible to transmit EM waves through sea
water. It is unlikely that transmission at high frequencies (∼10’sMHz) can be obtained over
long distances but short range transmission should be possible at a frequency of about 1MHz.
The band width at this frequency should be sufficient for the transmission of digital pictures
using image compression and slow update rate.
As regards the monitoring of pollution the high dielectric constant of sea water (∼72) enable
pollutants to be observed when contained in a cavity structure because they generally have a
much lower dielectric constant such as oil (ε = 2.3).