NorthStar Navigation Multi-Function Unit M84 - Manual (Page 37)

Northstar M121/M84 Installation and Operation Manual 36
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.
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
9-3 Single and Dual frequency fishfinding
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.
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.
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.
A
B
A
A
B
A