Single And Dual Frequency Fishfinding - Simrad NX40 Installation & Operation Manual

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 echosounder window
in such an environment. A double bottom trace is shown
on the window.
When looking for fish with the wide angled 50 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.
A Fish is visible on the window
B Fish is hidden by the strong echoes off the bottom
and is not shown on the window

9-3 Single and Dual frequency fishfinding

Sounder frequencies
The unit has two echosounder frequencies, 200 kHz and
50 kHz..
To select the echosounder frequency
>
Frequency
When to use 200 kHz
The 200 kHz frequency is especially suitable for use in
shallow and medium depth water, typically less than 500
ft (150 m) and while running at speed.
At 200 kHz, the narrow cone reduces any noise caused
by air bubbles.
The 200 kHz frequency generates a higher definition pulse
which produces little shadow and returns excellent detail
over a small area of bottom. Therefore, it gives excellent
bottom discrimination capability and is particularly good
at showing individual fish, including bottom dwellers.
When to use 50 kHz
The 50 kHz frequency is particularly suitable for use in
deep water, typically greater than 500 ft (150 m).
At 50 kHz, the cone covers an area of water about four
times wider than the 200 kHz cone and penetrates to a
greater depth with minimal loss of the return signal.
However, it produces a lower definition display with more
shadow compared to the 200 kHz frequency. This means
that a group of small fish, for example, could be shown
as a single item, while any fish very close to the bottom
> or
200 kHz 50 kHz
Simrad NX40/45 Installation and Operation Manual
Example of shadows
A
Sounder window of same area
or
Mixed
A
B
B
A
A
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