27
A.3.5 Wave height
The height of the waves around the transmitting buoy influences the received signal strength.
The higher the waves the higher the attenuation on the transmission path, thus the lower the
received signal strength. Two mechanisms exist, the first being active in the region between
H
s
= 0 and H
s
= 2 meters
8
and a second mechanism which determines the attenuation above an
H
s
of 2 meters.
Note that higher waves are often coupled with increased atmospheric activity thus a higher
atmospheric noise level.
Technical details:
The attenuation slope for the region between an H
s
= 0 and H
s
= 2 meter amounts to 3 dB per
meter Hs. The attenuation of waves with an H
s
of more than 2 meters can be approximated with
6 dB + H
s
/2.
Examples:
An H
s
of 1.5 meter means an extra attenuation of 1.5 x 3 dB = 4.5 dB when compared to an
H
s
of 0 meters.
An H
s
of 15 meters accounts to an extra attenuation of 6 + 15/2 dB = 13.5 dB.
Possible error sources and solutions:
Error: the reception becomes worse when measuring high waves
9
.
Solutions: use a directional antenna, place the antenna higher and/or closer to the buoy,
combat interferers as described above.
A.3.6 Existence of other buoy transmitters on the same frequency
It is possible that buoys from your or from another organisation are being used on the same
frequency
10
in each others neighbourhood.
This situation might endanger the receive quality of either :
Your buoy to your receiver communication.
The other buoy to the other receiver communication.
Or both.
8
Not coincidentally the height of the HF antenna on the buoy!
9
At the Datawell laboratory in Haarlem we are able to receive buoys from Petten errorless at a low H
s
,
but at an H
s
of greater than 4 meter receiving acceptable data becomes impossible! The distance from
Haarlem to Petten is 50 km and for the largest part over land, thus a very unfavorable condition.
10
For instance France allows only 3 frequencies to be used for databuoy telemetry equipment, thus not
only for Datawell wave height measurement buoys but those 3 frequencies have to be shared with other,
e.g. meteorological buoys in the seas around France. In fact, in the case of an overcrowded spectrum,
which is the case in several parts of the world, frequency coordination is necessary in order to guarantee
the data link qualities of all individual frequency users.
Summary of Contents for 45100
Page 2: ...2 Check if mains supply matches with the specifications of the device...
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Page 8: ...8...
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Page 36: ...36 E 2 Directional Antenna 25 45 MHz Figure E3 Directional quarter wave antenna...
