8120
Installation and Operation Manual
35
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 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.
Sonar window of same area
Example of shadows
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
A
B
A
B
A
A
9-3 Single and Dual frequency fishfinding
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 displayed
as a single item, while any fish very close to
the bottom may not be found at all.
This frequency is useful for getting a deep,
wide overview of the area so that any areas of
interest can be identified and then examined
in detail with the 200 kHz frequency.
Sonar frequencies
The Instrument has two sonar frequencies,
200 kHz and 50 kHz. To select the sonar
frequency to use:
1 Go to a sonar window (see section 9).
2 Press
MENU
, select
Frequency
and select
200 kHz
,
50 kHz
,or
Mixed
.
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.
