NorthStar EXPLORER 443D, Installation And Operation Manual

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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 Explorer 443d/443df
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
Northstar Explorer 443d/443df Installation and Operation Manual 18