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Shadows
Shadows are created around areas where the ultrasonic beam cannot ‘see’. These areas include
hollows on the bottom or beside rocks and ledges, where the strong echoes returned off the rocks
obscure the weak echoes of the fish and may also create a double bottom trace. See following for
an example of the sonar display in such an environment. A double bottom trace is shown on the
display.
When looking for fish with the wide angled 83 kHz frequency, be aware of increased shadows. Use
the high frequency 200 kHz in areas that have rocks and ledges because this frequency reduces the
shadow effect considerably.
The differences in the cone width affect what is displayed. See section 4-2 Single and Dual Frequency fishfinding.
Frequency and cone width
The pulse generated by the FISH 4432/4433
transducer travels down through the water,
spreading outwards to form a rough cone shape.
However, the cone width is dependent upon
the frequency of the pulse; at 83 kHz it is 20°,
whereas at 200 kHz it is 14°. The chart shows
how the cone width varies over depth for each
frequency used. Figures are approximate.
Water Cone width Cone width
Depth at 83 kHz at 200 kHz
10 4 2
20 7 5
30 11 7
40 15 10
50 18 12
60 22 15
70 25 17
80 29 20
90 33 22
100 36 25
150 55 37
200 73 50
300 109 75
400 146 100
500 182 125
600 218 149
700 255 174
800 291 199
900 328 224
1000 364 249
Depth 83 kHz 200 kHz
20°
14°
50
100
150
200
250
0
18
30
55
73
91
12
25
37
50
62
Sonar display of same area
Fish is visible on the display
Fish is hidden by the strong echoes off the
bottom and is not shown on the display
Example of shadows
Fish is visible on the display
FISH 4432/4433 Installation and Operation Manual 18
NAVMAN