Datawell 45100, Руководство пользователя

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37
Using the directional antenna for the Reception of Waveriders over longer distances.
1. The received signal strength when the buoy is over the horizon decreases fast with
increasing distance. To receive a buoy at 75 km distance with the same signal strength as at
50 Km distance 10 times the transmitting power will be needed. So increasing transmitting
power brings little relief.
2. One solution is to place the receiving antenna at an height of approximately 20-30 meters.
3. A moderate gain can be achieved by placing a reflector behind the receiving antenna to
make it directional. The antenna as shown has a forward gain of 3 dB, that is equivalent to
doubling the transmitter power. More important is that it suppresses the noise received from
the landside by 6 dB. Usually this noise is the limiting factor, not absolute signal strength.
Therefore the “gain” is 3 + 6 = 9 dB, equivalent to having 8 times the transmitter power.
See Figure E3. .
The dimensions of the directional antenna can be derived from the graph below.
Directional antenna dimensions
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
f (MHz)
l (m
m)
L1
L2
A
B